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Modulation of magnetic and dielectric property of LaFeO3 by simultaneous doping with Ca2+ and Co2+-ions
Accepted Manuscript Modulation of magnetic and dielectric property of LaFeO3 by simultaneous doping 2+ 2+ with Ca and Co -ions A.S. Mahapatra, A. Mitra, A. Mallick, A. Shaw, J.M. Greneche, P.K. Chakrabarti PII:
S0925-8388(18)30208-1
DOI:
10.1016/j.jallcom.2018.01.207
Reference:
JALCOM 44671
To appear in:
Journal of Alloys and Compounds
Received Date: 1 September 2017 Revised Date:
22 December 2017
Accepted Date: 14 January 2018
Please cite this article as: A.S. Mahapatra, A. Mitra, A. Mallick, A. Shaw, J.M. Greneche, P.K. Chakrabarti, Modulation of magnetic and dielectric property of LaFeO3 by simultaneous doping with 2+ 2+ Ca and Co -ions, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.01.207. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Modulation of magnetic and dielectric property of LaFeO3 by simultaneous doping with Ca2+ and Co2+-ions A.S. Mahapatra1, A. Mitra1, A. Mallick1, A. Shaw1, J.M. Greneche2, P.K. Chakrabarti1* 1
Solid State Research Laboratory, Department of Physics, Burdwan University, Burdwan-
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713104, India Institut des Molécules et Matériaux du Mans, IMMM UMR CNRS 6283, Le Mans Université, 72085, Le Mans Cedex 9, France
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Abstract
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Structural, magnetic and dielectric properties of Ca and Co co-doped LaFeO3 [La0.9Ca0.1Fe0.9Co0.1O3, LCFCO] sample are investigated. Nanoparticles of pure LaFeO3 (LFO) are prepared using sol-gel wet chemical method. Here, Ca2+ and Co2+ dopants are chosen to improve the dielectric and magnetic properties of LFO, respectively. The
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refinement of X-ray diffraction patterns is consistent with the formation of distorted orthorhombic phase of LFO. The analysis of Raman and Mössbauer spectra allow to extract different structural and magnetic information. The observed field dependency of
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magnetization confirms the introduction of ferromagnetism in the antiferromagnetic structure of LFO due to the doping. Thermal variation of the magnetization suggests a strong domination of ferromagnetic part over antiferromagnetic at low temperatures,
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2
which is not found in LFO. The temperature variation of the dielectric constant gives rise the dielectric transition temperature of the doped sample. The dielectric spectroscopy of the samples is found to follow the Cole-Cole relaxation with improved dielectric and magneto-dielectric property of the doped sample. Keywords: Nano-structures;
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Fe Mössbauer Spectrometry; Magnetic properties;
Electrical properties.
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Corresponding author. Tel. /fax: +91 342 2657800/2634200.
E-mail address: [email protected] (P.K. Chakrabarti). 1. Introduction
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Rare-earth orthoferrites, with general formula ABO3 (A = lanthanide, B = Fe) have enormous potential for applications in electronic devices like non-volatile magnetic memory devices, ultrasensitive magnetic read heads of modern hard disc drives, as spin
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valves, solid oxide fuel cells, magneto-optic materials, environmental monitoring applications [1-6] etc. LnFeO3 (Ln = lanthanide) is one of the most promising perovskite
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material, which crystallizes in orthorhombic structure with space group Pbnm. Here the large cations Ln are located at the unit cell corners while the cation Fe occupies the centre of the distorted octahedron of oxygen anions of the ABO3 structure where, the degree of distortion depends on the radius of the rare earth ion [7]. Among these, LaFeO3
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(LFO) is a well-known wide-gapped antiferromagnetic insulator with high Néel temperature (TN ~740 K) [8]. The source of this magnetic ordering is the superexchange antiferromagnetic interaction among the Fe cations via oxygen anions where the angle
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Fe-O-Fe is nearly 180° [7]. The material also shows ferroelectric ordering introduced by the strong local electric field created due to a lattice distortion generated by the 3d
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electrons of the transition metal ion. In addition, the coupling between magnetic and electric orderings makes this material a potential candidate for applications in magnetoelectric devices [9]. The easiest route for the preparation of LFO is the solid state reaction where the precursor components of metal oxides or carbonates are calcined at a temperature higher than 1273 K. But this process contains several drawbacks such as, no control on the particle size, poor homogeneity and high porosity of the samples. To improve the homogeneity and also to lower the preparation temperature several wet chemical methods have been proposed by different authors which includes hydrothermal 2
ACCEPTED MANUSCRIPT synthesis [10], solution combustion synthesis [11], sol–gel [12], co-precipitation [13, 14] to prepare ultrafine pure powders. Various properties of LFO in the form of both bulk and thin films were investigated [15-18]: it is reported that LFO is a multiferroic at room temperature (RT) [9]. Though,
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LFO exhibits interesting properties but suffers from some limitations such as low value of RT susceptibility/magnetization, small electric polarization, weak coupling between the electric and magnetic orderings etc. To overcome these limitations several works are
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carried out in recent times and in this regard substitution of transition metal ions like, Al, Zn [13, 14, 19–21] and rare earth ion like Gd, Ho [7, 22] have shown some interesting
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enhancement in the aforementioned properties. Considering the same objective, we have considered in the present work co-doping of Ca and Co ion in the crystal lattice of LFO prepared by modified sol-gel technique. Here we have substituted Ca and Co ions partially replacing the La and Fe ions in the lattice of LFO respectively. Here, we have
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considered smaller doping concentration since, high concentration of doping may have adverse effect on the physical properties [23]. The magnetic properties of the samples are studied from the Field cooled (FC) and zero-field cooled (ZFC) magnetization and
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hysteresis loops obtained in the temperature range from 300 K down to 5 K. Their
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dielectric properties are investigated in the frequency range of 0.1 kHz to 5MHz at different temperatures from RT up to ~900 K. In addition, we have also studied the coupling of the magnetic and electric properties of the samples at RT. The observed data of the doped sample exhibit a strong modulation of magnetic property of LFO due to the substitution. 2. Materials and methods For the preparation of the nanocrystalline samples of LFO and Ca2+, Co2+substituted LFO [La0.9Ca0.1Fe0.9Co0.1O3] (labelled here as LCFCO) the sol-gel method was used with 3
ACCEPTED MANUSCRIPT use of high power ultra-sonication. The precursor materials used for this wet-chemical method were La(NO3)3· 6H2O (Aldrich, 99.9% purity), Ca(NO3)2· 4H2O (Merck, Germany), Fe(NO3)3· 9H2O (Merck, Germany) and Co(NO3)2· 6H2O (Merck, Germany). For the preparation of pure LFO stoichiometric amount of La(NO3)3· 6H2O and
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Fe(NO3)3· 9H2O were weighted and taken in a beaker to prepare a solution using 200 ml of deionized water where the stoichiometric ratio of La and Fe was 1:1. The solution was then kept in a water bath sonicator and sonication was continued for 1h at around 60 °C
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to make it homogeneous. During this interval, equimolar amount of citric acid was taken in another beaker and a homogeneous solution was prepared using 200 ml of deionized
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water. This citric acid solution was then mixed with the above salt solution dropwise under sonication where the temperature was maintained at ~60 °C. After completion of the mixing, the sonication was maintained for 1h and the mixed solution was vigorously stirred using magnetic stirrer keeping the temperature at ~80 ºC for about 6 h until the
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solution was converted into sol form. This sol was then put into an oven preheated at 80 ºC until a heavily dense form of the sol i.e. the gel was obtained. This gel is further dried at the same temperature for about two days to get the as-dried form of the gel. The as-
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dried gel was then heat treated at ~200 °C for 12 h to remove the citric acid as well as to
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oxidize the salts and finally we get the as-dried LFO. During this interval an auto combustion was generated in the gel where citric acid plays the role of fuel for the combustion. This as-dried sample was then sintered at 600 °C for 5 h in a programmable furnace with an accuracy of 1 °C to obtain the nanocrystalline sample of LFO. The sintered sample was grinded well using agate mortar for characterization. For the preparation of the doped sample (LCFCO) the same method mentioned above was applied, but in this case the required salts of Ca(NO3)2· 4H2O, and Co(NO3)2· 6H2O for
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ACCEPTED MANUSCRIPT doping were taken along with the other salts of La(NO3)3· 6H2O, Fe(NO3)3· 9H2O so that the ratio of La, Ca, Fe and Co in the solution becomes 0.9:0.1:0.9:0.1, respectively. The crystalline phase of the pure and doped samples were examined by X-ray
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diffraction pattern recorded in BRUKER AXS diffractometer (Model D8), with Cu target (Cu Kα, λ= 1.5405 Å) in the range of 2θ from 20 to 80°. To confirm the presence of the different cations EDX spectra of LCFCO were recorded. For further phase
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characterization Raman spectra of the samples were recorded by T64000 Raman Spectrophotometer (J. Y. HORIBA) with an excitation wavelength of 632 nm using
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HeNe laser. The static magnetic properties of LFO and LCFCO were carried out in Quantum design Vibrating Sample Magnetometer (VSM model, PPMS-16) where the maximum applied field was 10 T. The transmission
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Fe Mössbauer spectra were
obtained at 77 and 300 K using a conventional spectrometer with a 57Co source diffused
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into a Rh matrix. The isomer shift values are quoted to that of α-Fe foil at 300K, used as a standard for the transducer. Dielectric constant, dielectric loss etc. of the samples were measured with varying frequency in the range of 100 Hz to 5MHz using a HIOKI 3532-
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50 LCR HiTESTER at different temperatures from RT to 900 K. To determine the magneto-electric coupling, the magneto-capacitance (MC) co-efficient of the samples
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were calculated from dielectric constant measured in presence and absence of magnetic field at RT.
3. Results, analysis and discussions 3.1 XRD Analysis. The X-ray diffractograms (XRD) of LFO and LCFCO are compared in Fig. 1. We successfully assigned the XRD peaks of each sample from the available JCPDS file no. 74-2203 corresponding to the LFO phase with space group Pbnm. The absence of any
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ACCEPTED MANUSCRIPT other peak confirm that LCFCO is of pure crystallographic phase and Ca2+, Co2+ ions are successfully incorporated in the crystal lattice of LFO. To justify the composition, we have recorded the EDX data of the doped sample (LCFCO) and the corresponding observation including the analysis are shown in the Fig. 2. Here the presence of all the
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cations are found in the observed spectra and the ratio of the La, Ca, Fe and Co cations calculated from the spectra is very close (~0.91:0.09:0.91:0.09) with our starting composition (0.9:0.1:0.9:0.1). The line broadening of the 100% intense peak of the XRD
well-known Debye-Scherrer equation [13], .
(1)
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< >=
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pattern (112/200) is used to calculate the average crystallite size of the samples using the
Here, is the average crystallite size, λ is the wavelength of the incident X-ray radiation, β1/2 is the full width at half maximum and θ is the corresponding Bragg angle.
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The estimated average crystallite sizes, as obtained from the above equation are ~30 nm for LFO and ~25 nm for LCFCO. Since, LFO crystallizes with distorted orthorhombic structure, some strains should be associated with the crystal lattice. Due to the
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incorporation of dopants of Ca2+ and Co2+ in LFO lattice, this strain should be modified in the doped sample. The lattice strains of the samples are estimated from the XRD line
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broadening using the Hall-Williamson (H-W) method from the following equation, β cos =
+ 4 sin
(2)
Here, β is the full width at half maximum (FWHM) after the instrumental broadening correction, K is a constant (= 0.89), D is the average crystallite size and is the lattice strain introduced inside the distorted orthorhombic lattice [24]. Here, the average crystallite size is obtained from the ordinate intercept of the straight line fit of the β cos 4 sin plot and the lattice strain is obtained from the slope of the straight 6
ACCEPTED MANUSCRIPT line. The above plot along with the straight line fit of LFO and LCFCO sample is shown in Fig. 3(a). The obtained values of the average crystallite size and lattice strain are ~50 nm, ~0.00171 for LFO and ~50 nm, ~0.00243 for LCFCO. Thus we see that the lattice strain has increased in LCFCO due to the co-doping. This increase in lattice strain may
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be due to the increase in distortion of the octahedron of oxygen anions of the lattice due to substitution of smaller Ca ion (ionic radius ~0.99 Å) replacing La ion (ionic radius ~1.06 Å).
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To extract the detail structural information of the samples, XRD patterns of LFO and LCFCO are successfully studied by Rietveld analysis using MAUD program, version
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2.33 [25]. For this, the theoretical diffraction pattern of the sample is generated with LaFeO3 phase having space group Pbnm and then we try to fit this with the observed XRD pattern. The theoretical pattern of each sample is then generated by repetitive modification of different parameters corresponding to structural, microstructural and
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background function until a good agreement between the observed and calculated pattern is reached. The reliability of the profile fitting is judged by the low values of goodness of fit (GoF) and the reliability parameters (Rexp, Rw, Rb) [26]. We have also refined the
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oxygen occupancy factor during the Rietveld analysis. From the fitting parameter we
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observe that in LCFCO some oxygen vacancy is generated and the value as obtained is ~0.066. The reason behind this oxygen vacancy may be the charge imbalance due to doping. Though it is very difficult to determine the oxygen vacancy accurately from the XRD data, still a basic idea can be obtained about this parameters from this analysis [2729]. The degree of orthorhombic distortion, is defined through the values of (c/a, √2c/b) [7]. The values of the different reliability parameters and structural lattice parameters obtained from the profile fitting are listed in Table 1, as well as those of the degree of orthorhombic distortion (through the values of c/a, √2c/b) [7]. The later ones slightly
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ACCEPTED MANUSCRIPT deviate from the respective values of LFO (c/a =1.41667, √2c/b =1.99991), reported in the JCPDS file no. 74-2203. From the table we also observe that the orthorhombic distortion has increased in LCFCO compared to pristine LFO. The arrangement of different ions in the unit cell of LCFCO as extracted from the Rietveld analysis is shown
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in Fig. 3(b). 3.2 Raman Analysis.
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Raman spectroscopy is an important tool to find different information regarding Raman active modes and to examine the presence of impurity or precipitates of dopants
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if any etc. Raman spectra of LFO and LCFCO were recorded at RT with an excitation wavelength of 632 nm using lower LASER power of 2 mW. The spectra of both the samples displayed in Fig. 4 are more or less similar nature and consistent with the earlier reported results of Raman spectra of LaFeO3 and other rare earth manganite belonging to
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the space group Pbnm of orthorhombic structure [3, 30, 31]. Here in the observed Raman pattern, we did not find any extra peak except LFO, which can confirm that both samples have the same crystal structure. It is well established that in LaFeO3 there are four
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formula units per unit cell, and the decomposition of the normal-mode is given by [3]: Γ = 7A# + 7B%# + 5B'# + 5B(# + 8A* + 10B%* + 8B'* + 10B(*
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Among these Ag, B1g, B2g and B3g modes are Raman active indicating that there are 24 Raman active modes in LFO. However, for complex oxides like LFO, detection of all the active Raman modes is not possible; instead, some dominant modes with broad features are found in the spectra. We have tried to describe the experimental spectra by means of Lorentzian functions and the peaks are in good agreement with the previous observations [3, 30, 31], as illustrated in Fig. 4. The peaks observed at around 144 cm-1 and 172 cm-1 resulting from La vibrations are assigned to B1g and Ag modes respectively. The mode at
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ACCEPTED MANUSCRIPT ~290 cm-1 is associated with different orientations of the FeO6 octahedral units and the mode at ~434 cm-1 originates from the vibration due to bending of the FeO6 octahedra. The peaks around 496 cm-1 and 637 cm-1 are associated to the asymmetric and symmetric stretching of different Fe-O bonds of these same polyhedra. For rare-earth with large
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ionic radius like La, the frequency of the mode at ~496 cm-1 is attributed to the Jahn– Teller distortion [3]. Also, the intensity of the modes below 200 cm-1 and ~433 cm-1 are found to be decreased in LCFCO as compared to those of LFO. This is attributed to the
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randomness of the substituted dopants/host cations like, Ca/La and Co/Fe in the lattice of LFO. Due to the substitution of dopants, the long range cooperative motion of La-O and
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Fe-O in LFO is broken by the random distribution of Ca and Co, respectively: consequently, the intensity of the Raman modes associated with these bonds decreases [3]. The frequency of the mode found at ~637 cm-1 has been found to decrease down to ~627 cm-1 in the doped sample which can be an effect of substitution of Fe with Co of
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larger atomic weight. The reverse case observed in substitution of La with Ca, where the modes at ~144 cm-1 and ~172 cm-1 are shifted towards higher frequency (to ~146 cm-1 and ~176 cm-1 respectively). These observations are quite consistent with those reported
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earlier on doped BiFeO3 by Huang et al. [32].
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3.3 Magnetic Properties.
Fig. 5 compares the magnetization vs. magnetic field (M-H) loops of the pristine and
doped sample recorded at 5 K and at RT and the maximum applied field for these loops is 100 kOe (~10 T). Relevant magnetic properties viz. maximum magnetization (Mmax), coercive field (Hc), remnant magnetization (Mr) obtained from the M-H loops are listed in Table 2. The M-H loops of LFO shows weak nonlinearity in magnetization vs. magnetic field data even at 5 K, whereas the corresponding variation for LCFCO deviates strongly from linearity up to the applied field of ~30 kOe, beyond which the 9
ACCEPTED MANUSCRIPT magnetization remains linearly dependent. This type of variation in M-H loop is generally observed in samples with weak ferromagnetic contribution. Thus the weak non-linearity in M-H data i.e., the irreversibility of the magnetization between the increasing and decreasing field and the value of Hc confirms the presence of weak
where
ferromagnetic
exchange
interaction
occurs
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ferromagnetism in LFO. Antiferromagnetic material comprises of two spin sublattices in
each
sublattice
and
antiferromagnetic superexchange interaction favours between the two [33]. In the long
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range ordering, due to the complete compensation of the spin magnetic moments of these sublattices the two-sublattice system results in zero net magnetic moment. However, in
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case of nanoparticles, this long-range collinear antiparallel spin structure is disturbed at the surface of the particles originating a non-compensation and thus a small spontaneous magnetization. This effect is further enhanced when the surface-to-volume ratio increases, i.e. the size nanoparticle is decreasing. This type of behaviour of LFO is
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consistent with the previous observation by Ghosh et al. and Köferstein et al. [34, 35]. In addition, the value of the coercive field of LFO at 5 K (2.9 kOe) is smaller than that at RT (9.8 kOe). This feature is also consistent with the antiferromagnetic nature of the
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sample. This canted antiferromagnetic nature of LFO is found to be substantially
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enhanced in the Co-doped sample. Indeed, as observed in Fig. 5 the M-H loops of LCFCO behaves as a weak ferromagnet. One can distinguish two main contributions to the enhancement of magnetization in LCFCO. The first one results from the doping of Ca ion replacing the La ion: indeed, the Fe3+–O2-–Fe3+ superexchange angle becomes closer to 180° (see Table 1), which in turn strengthens the superexchange interaction leading to the enhancement of magnetization. The second one is the introduction of ferromagnetism due to the substitution of Co ion in place of Fe. Such interaction has been already observed in Co-doped BiFeO3 sample, as reported by Xu et al. [36]. The
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ACCEPTED MANUSCRIPT first contribution is expected to dominate near the RT while the second one becomes more effective in the low temperature region. The value of Hc of LCFCO is found thus to have increased at low temperature, that is clearly evidenced from the 5 K M-H loop, as shown in Fig. 5. Now to estimate quantitatively the ferromagnetic and antiferromagnetic
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components in the observed magnetic property of the doped sample, a description of the M-H loops is carried out using the following relation [23, 33]; 6 345
7
89:;% <=
>±>@A >@A
73C
B 89: ='345 6 BDE + F/ 45
(3)
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-(/) = 12
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This equation gives the magnetization, M(H), at an applied field magnetic H where the first term represents the irreversible component of magnetization coming from the ferromagnetic phase and the second linear component is due to the antiferromagnetic H I and/or paramagnetic contribution. Here, -G3 , -G3 , /JK are the ferromagnetic saturation
magnetization, remnant magnetization, intrinsic coercive field, respectively while χ is the
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antiferromagnetic susceptibility. The experimental magnetization recorded at RT and 5 K are compared in Fig. 5 along with the theoretically fitted curves. The simulated
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ferromagnetic and antiferromagnetic contributions are also illustrated in Fig. 5. The extracted fitting parameters are listed in table 3. From the fitting parameter, we observe
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that the ferromagnetic part contributes ~33% of the total magnetization of LCFCO at RT, and increases up to ~41% at 5 K. The increase in the ferromagnetic part at low temperature can be attributed to the decrease in the thermal excitation which increases the ferromagnetic moment and decreases the antiferromagnetic moment. To get a detailed insight of the magnetic nature of the co-doped sample zero-field cooled (ZFC) and field cooled (FC) temperature dependent magnetization curves are measured from 5 K to 300 K where the field applied for the FC measurement is 500 Oe and the corresponding curves are displayed in Fig. 6. The observed ZFC and FC curves 11
ACCEPTED MANUSCRIPT show a bifurcation near the RT. This type of clear difference between the ZFC and FC curves is associated with the magnetic blocking observed when cooling from high temperature the samples with superparamagnetic or spin-glass type of behaviour. However, the observed large magnetic coercivity at RT (shown in Table 2) excludes the
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presence of superparamagnetism in the sample. The source of the spin-glass behaviour in LCFCO is attributed to the struggle between ferromagnetic and antiferromagnetic interaction in the sample. The presence of the spin-glass like behaviour is consistent with
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the earlier report by Xu et al and Liang et al. [36, 37]. The ZFC magnetization continues to decrease with the decrease in temperature down to 20 K (Fig. 6) and below this,
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magnetization begins to increase with decreasing temperature. In case of FC, the magnetization is found to decrease with lowering temperature in the range from 300 K down to 291 K, which indicates the presence of the antiferromagnetic ordering in the sample. After this the magnetization continues to increase with further lowering
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temperature. This increment in the magnetization indicates that the ferromagnetic part, due to the Co substitution starts to dominate over the antiferromagnetic nature below 291 K. Below ~ 20 K it is found that both ZFC and FC magnetization starts to increase
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rapidly up to the lowest temperature of 5 K. At this temperature, the antiferromagnetic
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part becomes negligible leaving the ferromagnetic part to dominate. The clear bifurcation of the ZFC and FC magnetization curve indicates presence of magnetic blocking phenomena in LCFCO. However, the presence of large coercive field at RT and the absence of cusp in the ZFC magnetization indicate that the blocking arises due to the spin glass behaviour of the sample. Such a variation of the ZFC and FC magnetization is consistent with the Co substituted BiFeO3 particles [36]. 3.4 57Fe Mössbauer Spectrometry.
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ACCEPTED MANUSCRIPT To know the charge state of Fe ions, the hyperfine behaviour of the samples are investigated using Mössbauer spectrometry. The Mössbauer spectra of LFO and LCFCO observed at RT and 77 K are compared in Fig. 7. Analysis of the Mössbauer spectrum can provide different information regarding, electronic density at the nuclei through the
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value of the isomer shift (IS), possible electric field gradient from quadrupole splitting (QS) value, the magnetic environment from magnetic hyperfine splitting (BHF) etc. Detailed analyses of the observed spectra are carried out and the obtained results are
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listed in Table 4. The values of different parameters like IS, QS and BHF are consistent with the earlier observations [33, 38]. The isomer shift, as estimated from the analysis is
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found to be in the range of 0.33-0.37 and 0.44-0.48 for both samples at 300 and 77K, respectively. This indicates that the samples are mainly dominated by Fe3+ ionic state excluding the possibility of any other charge state. The superexchange interaction among these Fe3+ ions via oxygen gives rise to the antiferromagnetic nature of the samples,
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which is consistent with the hyperfine magnetic splitting (sextet) and with the magnetic observations discussed in the earlier section. Also, both the spectra recorded at RT and at 77 K are fitted with two sextets having small value of quadrupole shift 2ε and almost
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equal IS value. These sextets correspond to the two non-equivalent positions of the Fe3+
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ions due to weak distortion of the octahedral oxygen environment. These observations are quite consistent with the earlier observation by Chauhan et al. and Han et al. [33, 38]. In addition to the magnetic sextet, the fitting of the RT data of LCFCO shows the presence of a weak quadrupolar doublet with IS value of 0.37. This tiny doublet in LCFCO originates from the paramagnetic behaviour of the fraction of spin-glass system present in the sample. Since there is no charge state other than Fe3+ in the sample, this spin glass behaviour is believed to come from the competition between ferromagnetic and antiferromagnetic interaction in the sample [33, 38]. Because of its origin, the
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ACCEPTED MANUSCRIPT doublet was not found either in case of LFO, where the antiferromagnetism is strong or in the 77 K data of LCFCO, where ferromagnetism dominates the antiferromagnetic interaction. 3.5 Dielectric Properties.
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Temperature and frequency dependence of the dielectric constant of LFO and LCFCO are measured where the frequency range of 50 Hz to 5 MHz and temperature range of 300 K to 825 K are considered. The thermal variation of the real part of the dielectric
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constant ( L ) of both LFO and LCFCO measured at 100 kHz in the mentioned temperature range are depicted in Fig. 8. Values of L are found to increase with the
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increase in temperature until a critical temperature (Tm) is reached. Above Tm, L is found to decrease rapidly with further rise in temperature. Tm where the value of L is L maximum (M ) is called the transition temperature. Initially with the rise in temperature
the electric dipoles gains thermal energy to be aligned with the applied electric field and
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consequently, the dielectric constant increases. In addition to this the interfacial polarization increases with the rise in temperature which also increases the dielectric constant. But at Tm the thermal energy becomes sufficiently large and due to this the
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dipoles start to be randomly oriented and the sample becomes paraelectric. Due to this
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fact the dielectric constant decreases above Tm. The transition temperature in LFO (Tm ~715 K) has been significantly shifted towards higher temperature in LCFCO (Tm ~748 K), as clearly observed in Fig. 8. Interestingly, LCFCO shows a sharp phase transition in comparison to the broad phase transition of LFO. The reason behind this sharp nature of the phase transition may be due to the reduction in the overlapping of the ferroelectric and non-ferroelectric regions in the sample [39]. The frequency variation of the real part of the dielectric constant ( L ) and dielectric loss (tan δ) of both the pure and doped samples are measured in the frequency range of 50 Hz to 5 MHz (Fig. 9). Values of L of 14
ACCEPTED MANUSCRIPT both sample show strong frequency dependency. The real part of the dielectric constant is found to have a sigmoidal type of variation in the low frequency region and a tendency towards saturation is observed in the high frequency region. In the high frequency region the dipoles are unable to follow the ac field due to which the value of L tends to saturate
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at a low value. There are several types of polarization which contribute to the dielectric constant. Among them, the dipolar and interfacial polarization contribute maximum in the low frequency region while the electronic polarization dominates in the high
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frequency region [40]. The frequency variation of L of LFO and LCFCO deviates from the ideal Debye-like relaxation and is found to follow the Cole-Cole relaxation consisting
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of dipoles with a distribution of relaxation time. According to the Cole-Cole relaxation the real part of the dielectric constant is described by the following equation; (N) = O + L
WZ B[
WZ %S'(TU@@ ) VW XY= BS(TU@@ ) ( VW)
∆Q R%S(TU@@ ) VW XY=
(4)
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Here, N is the angular frequency, ∆ = H − O is the dielectric relaxation strength, H and O are respectively the high and low frequency permittivity limits, ]JJ is the relaxation time or time constant and ^ is the shape parameter [41]. High value of the
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dielectric permittivity at low frequency is due to the electrode polarization effect while it approaches to a limiting value (O ) at high frequency. For this to avoid the effect of
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electrode polarization we have considered L data above 15 kHz for the determination of O and ∆ using equation 4. The observed values of L of LFO and LCFCO along with its Cole-Cole fitting are compared in Fig. 9(a) and 9(b). The values of O , ∆ obtained from the fitting ~91, ~457 and ~150, ~3314 for LFO and LCFCO, respectively. However, the effect of enhanced conductivity is clearly established from Fig. 9(b) in case of LCFCO. From the frequency dependency of dielectric loss i.e. ‘tan δ’ of LFO and LCFCO we observe that in both the samples the loss factor is prominent in the low
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ACCEPTED MANUSCRIPT frequency region and decreases with the increase of the frequency. From Fig. 9(a) and 9(b), the dielectric loss in LCFCO has been reduced in the low frequency region but remains greater in the high frequency region as compared to pure LFO.
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For the confirmation of the magneto-dielectric property of the samples, the magnetic field variation of dielectric constant is studied. This is one of the important way to investigate the coupling effect, as used by many authors [9, 13, 42]. The magnetoelectric coupling of the sample is studied by measuring the dielectric constant in
MC =
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the sample is calculated from the following formula,
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presence and absence of the magnetic field. The magnetocapacitance coefficient (MC) of
[ε′ (b,d);ε′ (,d)] ε′ (,d)
(5)
Here, ′ (H, T) and ′ (0, T) represent the dielectric constants in presence and absence of external magnetic field (H), respectively [9, 13]. The variations of magnetocapacitance
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coefficient with external magnetic field for the samples are displayed in Fig. 10: it is noted a strong dependency of the dielectric constant of the samples by the external magnetic field. Material with coupled magnetic and electric orderings when placed in an
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external magnetic field, a strain is induced in it. This strain introduces a stress in the
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samples which in turn produces an electric field in the material. The ferroelectric domains are oriented by this electric field causing an increase in the polarization value [42]. From the figure we see that the maximum value of MC of LFO (~0.055 % @ 0.8 T) is drastically enhanced in LCFCO (~1.83 % @ 0.8 T). This is probably due to the doping of smaller Ca atom in place of La atom in LFO lattice. In addition, the enhancement of the magnetic property of LCFCO increases the value of the strain due to external magnetic field which in turn increases the MC value in the sample. Conclusion
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ACCEPTED MANUSCRIPT Nanoparticles of pure and Ca, Co co-doped LFO are successfully prepared by the solgel route. The crystalline structures of the pure and doped samples are confirmed from the analysis of the XRD data using the Rietveld analysis. Different structural parameters are also determined from the above analysis. The information extracted from the detailed
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analysis of the Raman spectroscopy is in good agreement with the relevant findings of LFO and related systems, reported earlier. The change in intensity of some particular Raman modes in LCFCO substantiated the findings of Rietvield analysis i.e. the
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substitution of Ca and Co ions in place of La and Fe ions, respectively. Strain is found to be enhanced in the crystal lattice of LFO due to the Co-doping in LCFCO. Strong
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modulation of the magnetic property is observed due to the choice of co-dopants of Ca and Co in the host. Presence of the mixed state of antiferromagnetic and ferromagnetic phase is confirmed from the detail analysis of the M-H loops of the doped sample. However, the ferromagnetic phase dominates over the antiferromagnetic phase over a
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large range of temperature in LCFCO. The temperature variation of the magnetization shows an increase in strength of the ferromagnetic property of the doped sample with the decrease in temperature whereas in case of pure LFO this feature is absent. Also, a
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fraction of spin-glass phase is also found at the RT data of the doped sample. The
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Mössbauer spectra at 300 K and 77 K confirm the presence of Fe3+ cations in both samples. In addition, Mössbauer Spectra of the doped sample ruled out the formation of any magnetic impurity like α-Fe2O3. Analysis of temperature and frequency variation of dielectric constant shows improved dielectric property of the doped sample compared to that of LFO. The coupling of the magnetic and dielectric property of LCFCO is found to be enhanced compared to LFO. Thus, fruitful modification of magnetic, dielectric and magneto-dielectric behaviour in doped sample compared to that of LFO would be interesting for applications in electronic devices.
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ACCEPTED MANUSCRIPT Acknowledgement The authors wish to acknowledge the UGC-DAE CSR, Kolkata and Mumbai center, Govt. of India for the financial assistance of the present work (Project file no. UGC-
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DAE-CSRKC/CRS/13/MS04/Ext/2016, Dated: 22.12.2016 and Project file no. UDCSR/MUM/CD/CRS-M-265/2017/1043, Dated: 20.01.2017). Authors also wish to acknowledge Dr. A. Banerjee, UGC-DAE CSR, Indore centre for the magnetic measurement facility provided. Authors also wish to acknowledge the financial support
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provided by the UGC, Govt. of India, through the CAS program (Fie no.
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F.530/5/CAS/2011(SAP-I)) and FIST program (Fie no. SR/FST/PSI-170/2011(C)). REFERENCES
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ACCEPTED MANUSCRIPT [8] G.R. Hearne, M.P. Pasternak, Phys. Rev. B 51 (1995) 11495–11500. [9] S. Acharya, J. Mondal, S. Ghosh, S.K. Roy, P.K. Chakrabarti, Mater. Lett. 64 (2010) 415–418.
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Table 1
Structural parameters and reliability factors obtained from Rietveld analysis of XRD pattern of LFO and LCFCO Cell Parameters a (Å)
b (Å)
c (Å)
LFO
5.5589(3)
5.5626(6)
7.8566(8)
LCFCO
5.5295(6)
5.5337(7)
7.8196(3)
OD [(c/a), √2×(c/b)] 1.41334, 1.99742 1.41415, 1.99838
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Reliability Factors Rexp Rb GoF (%) (%)
Fe-O1-Fe ° (deg)
Fe-O2-Fe ° (deg)
150.1(5)
160.7(3)
6.47
5.64
5.14
1.14
152.5(5)
164.3(3)
5.99
5.26
4.69
1.13
Rwp (%)
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Table 2 Magnetic parameters extracted from the M-H loops of LFO and LCFCO.
LFO LCFCO
T (K)
Mmax (emu/g)
Hc (kOe)
Mr (emu/g)
Mmax/ Mr
300
1.07
9.89
0.11
9.73
5 300
1.11 1.89
3.03 1.51
0.04
27.75
0.21
9.00
5
2.19
14.10
0.56
3.91
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Sample
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Table 3
Magnetic properties extracted from the analysis of M-H loops of LCFCO.
1.904 2.194
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300 5
Paramagnetic Contribution F × 10g (emu/g Oe) 1.325 ±0.007 1.311 ±0.009
Ferromagnetic Contribution
H -G3 (emu/g) 0.631 ±0.005 0.905 ±0.008
I -G3 (emu/g) 0.221 ±0.002 0.562 ±0.003
/JK (kOe) 1.757 ±0.031 17.696 ±0.109
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T (K)
Observed Magnetization (emu/g)
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Table 4
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Hyperfine parameters of LFO and LCFCO obtained from analysis of Mössbauer Spectra recorded at 300 and 77 K T (K) 300 LFO 77 300 LCFCO 77
Type Sextet 1 Sextet 2 Sextet 1 Sextet 2 Doublet Sextet Sextet 1 Sextet 2
IS (mm/s) ± 0.01 0.37 0.33 0.48 0.46 0.37 0.36 0.48 0.47
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2ε/QS (mm/s) ± 0.01 -0.04 -0.12 -0.05 -0.07 0.88 -0.06 -0.06 -0.06
Bhyp (T) ± 0.5 52.1 49.5 56.1 53.5 48.4 55.7 53.4
Area ratio (%) ±2 82 18 87 13 3 97 56 44
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Figure Captions:
Figure 1. Observed and generated (from Rietveld analysis) X-ray diffraction patterns of (a)
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LFO and (b) LCFCO. Figure 2: EDX spectra of LCFCO sample.
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Figure 3. β cosθ vs. 4 sinθ plot (Hall–Williamson plot) of LFO and LCFCO. The solid lines show the linear fit.
Figure 4. Raman spectra of LFO and LCFCO at room temperature together with spectral deconvolutions using Lorentzian functions. The ticks in the top panel indicate the positions of
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different Raman modes.
Figure 5. Magnetic hysteresis loops (M-H loops) of LFO and LCFCO observed at (a) 300 K, (b) 5K. Fitting with antiferromagnetic and ferromagnetic contribution in M-H loops of
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LCFCO recorded at (c) 300 K, (d) 5K. Inset shows the enlarged view of the fitting.
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Figure 6. FC and ZFC magnetization curves of LCFCO. Inset shows the enlarged view of FC magnetization.
Figure 7. Mössbauer spectra including their analysis of LFO at (a) 300 K, (b) 77 K and LCFCO at (c) 300 K, (d) 77 K. Figure 8. Thermal variation of L (@ 100 kHz) of LFO and LCFCO. Figure 9. Frequency variation of L and tanδ of (a) LFO, (b) LCFCO observed at RT along with the Cole-Cole fit of L data. Figure 10. Magnetic field variation of MC of LFO and LCFCO.
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Figure 3
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Figure 6
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Figure 9 (a)
Figure 9 (b)
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Figure 10
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ACCEPTED MANUSCRIPT Highlights: • LaFeO3 and Ca, Co co-doped therein are successfully prepared by sol-gel technique. • Analyses of XRD and Raman spectra confirmed the complete substitution of dopants. • Ferromagnetic property is introduced in the antiferromagnetic host by doping.
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• Analysis of Mössbauer spectra confirms the magnetic phases of the samples.
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• Doping has improved the magneto-electric property of LaFeO3 for application.
S0925-8388(18)30208-1
DOI:
10.1016/j.jallcom.2018.01.207
Reference:
JALCOM 44671
To appear in:
Journal of Alloys and Compounds
Received Date: 1 September 2017 Revised Date:
22 December 2017
Accepted Date: 14 January 2018
Please cite this article as: A.S. Mahapatra, A. Mitra, A. Mallick, A. Shaw, J.M. Greneche, P.K. Chakrabarti, Modulation of magnetic and dielectric property of LaFeO3 by simultaneous doping with 2+ 2+ Ca and Co -ions, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.01.207. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Modulation of magnetic and dielectric property of LaFeO3 by simultaneous doping with Ca2+ and Co2+-ions A.S. Mahapatra1, A. Mitra1, A. Mallick1, A. Shaw1, J.M. Greneche2, P.K. Chakrabarti1* 1
Solid State Research Laboratory, Department of Physics, Burdwan University, Burdwan-
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713104, India Institut des Molécules et Matériaux du Mans, IMMM UMR CNRS 6283, Le Mans Université, 72085, Le Mans Cedex 9, France
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Abstract
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Structural, magnetic and dielectric properties of Ca and Co co-doped LaFeO3 [La0.9Ca0.1Fe0.9Co0.1O3, LCFCO] sample are investigated. Nanoparticles of pure LaFeO3 (LFO) are prepared using sol-gel wet chemical method. Here, Ca2+ and Co2+ dopants are chosen to improve the dielectric and magnetic properties of LFO, respectively. The
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refinement of X-ray diffraction patterns is consistent with the formation of distorted orthorhombic phase of LFO. The analysis of Raman and Mössbauer spectra allow to extract different structural and magnetic information. The observed field dependency of
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magnetization confirms the introduction of ferromagnetism in the antiferromagnetic structure of LFO due to the doping. Thermal variation of the magnetization suggests a strong domination of ferromagnetic part over antiferromagnetic at low temperatures,
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2
which is not found in LFO. The temperature variation of the dielectric constant gives rise the dielectric transition temperature of the doped sample. The dielectric spectroscopy of the samples is found to follow the Cole-Cole relaxation with improved dielectric and magneto-dielectric property of the doped sample. Keywords: Nano-structures;
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Fe Mössbauer Spectrometry; Magnetic properties;
Electrical properties.
1
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Corresponding author. Tel. /fax: +91 342 2657800/2634200.
E-mail address: [email protected] (P.K. Chakrabarti). 1. Introduction
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Rare-earth orthoferrites, with general formula ABO3 (A = lanthanide, B = Fe) have enormous potential for applications in electronic devices like non-volatile magnetic memory devices, ultrasensitive magnetic read heads of modern hard disc drives, as spin
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valves, solid oxide fuel cells, magneto-optic materials, environmental monitoring applications [1-6] etc. LnFeO3 (Ln = lanthanide) is one of the most promising perovskite
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material, which crystallizes in orthorhombic structure with space group Pbnm. Here the large cations Ln are located at the unit cell corners while the cation Fe occupies the centre of the distorted octahedron of oxygen anions of the ABO3 structure where, the degree of distortion depends on the radius of the rare earth ion [7]. Among these, LaFeO3
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(LFO) is a well-known wide-gapped antiferromagnetic insulator with high Néel temperature (TN ~740 K) [8]. The source of this magnetic ordering is the superexchange antiferromagnetic interaction among the Fe cations via oxygen anions where the angle
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Fe-O-Fe is nearly 180° [7]. The material also shows ferroelectric ordering introduced by the strong local electric field created due to a lattice distortion generated by the 3d
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electrons of the transition metal ion. In addition, the coupling between magnetic and electric orderings makes this material a potential candidate for applications in magnetoelectric devices [9]. The easiest route for the preparation of LFO is the solid state reaction where the precursor components of metal oxides or carbonates are calcined at a temperature higher than 1273 K. But this process contains several drawbacks such as, no control on the particle size, poor homogeneity and high porosity of the samples. To improve the homogeneity and also to lower the preparation temperature several wet chemical methods have been proposed by different authors which includes hydrothermal 2
ACCEPTED MANUSCRIPT synthesis [10], solution combustion synthesis [11], sol–gel [12], co-precipitation [13, 14] to prepare ultrafine pure powders. Various properties of LFO in the form of both bulk and thin films were investigated [15-18]: it is reported that LFO is a multiferroic at room temperature (RT) [9]. Though,
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LFO exhibits interesting properties but suffers from some limitations such as low value of RT susceptibility/magnetization, small electric polarization, weak coupling between the electric and magnetic orderings etc. To overcome these limitations several works are
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carried out in recent times and in this regard substitution of transition metal ions like, Al, Zn [13, 14, 19–21] and rare earth ion like Gd, Ho [7, 22] have shown some interesting
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enhancement in the aforementioned properties. Considering the same objective, we have considered in the present work co-doping of Ca and Co ion in the crystal lattice of LFO prepared by modified sol-gel technique. Here we have substituted Ca and Co ions partially replacing the La and Fe ions in the lattice of LFO respectively. Here, we have
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considered smaller doping concentration since, high concentration of doping may have adverse effect on the physical properties [23]. The magnetic properties of the samples are studied from the Field cooled (FC) and zero-field cooled (ZFC) magnetization and
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hysteresis loops obtained in the temperature range from 300 K down to 5 K. Their
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dielectric properties are investigated in the frequency range of 0.1 kHz to 5MHz at different temperatures from RT up to ~900 K. In addition, we have also studied the coupling of the magnetic and electric properties of the samples at RT. The observed data of the doped sample exhibit a strong modulation of magnetic property of LFO due to the substitution. 2. Materials and methods For the preparation of the nanocrystalline samples of LFO and Ca2+, Co2+substituted LFO [La0.9Ca0.1Fe0.9Co0.1O3] (labelled here as LCFCO) the sol-gel method was used with 3
ACCEPTED MANUSCRIPT use of high power ultra-sonication. The precursor materials used for this wet-chemical method were La(NO3)3· 6H2O (Aldrich, 99.9% purity), Ca(NO3)2· 4H2O (Merck, Germany), Fe(NO3)3· 9H2O (Merck, Germany) and Co(NO3)2· 6H2O (Merck, Germany). For the preparation of pure LFO stoichiometric amount of La(NO3)3· 6H2O and
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Fe(NO3)3· 9H2O were weighted and taken in a beaker to prepare a solution using 200 ml of deionized water where the stoichiometric ratio of La and Fe was 1:1. The solution was then kept in a water bath sonicator and sonication was continued for 1h at around 60 °C
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to make it homogeneous. During this interval, equimolar amount of citric acid was taken in another beaker and a homogeneous solution was prepared using 200 ml of deionized
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water. This citric acid solution was then mixed with the above salt solution dropwise under sonication where the temperature was maintained at ~60 °C. After completion of the mixing, the sonication was maintained for 1h and the mixed solution was vigorously stirred using magnetic stirrer keeping the temperature at ~80 ºC for about 6 h until the
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solution was converted into sol form. This sol was then put into an oven preheated at 80 ºC until a heavily dense form of the sol i.e. the gel was obtained. This gel is further dried at the same temperature for about two days to get the as-dried form of the gel. The as-
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dried gel was then heat treated at ~200 °C for 12 h to remove the citric acid as well as to
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oxidize the salts and finally we get the as-dried LFO. During this interval an auto combustion was generated in the gel where citric acid plays the role of fuel for the combustion. This as-dried sample was then sintered at 600 °C for 5 h in a programmable furnace with an accuracy of 1 °C to obtain the nanocrystalline sample of LFO. The sintered sample was grinded well using agate mortar for characterization. For the preparation of the doped sample (LCFCO) the same method mentioned above was applied, but in this case the required salts of Ca(NO3)2· 4H2O, and Co(NO3)2· 6H2O for
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ACCEPTED MANUSCRIPT doping were taken along with the other salts of La(NO3)3· 6H2O, Fe(NO3)3· 9H2O so that the ratio of La, Ca, Fe and Co in the solution becomes 0.9:0.1:0.9:0.1, respectively. The crystalline phase of the pure and doped samples were examined by X-ray
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diffraction pattern recorded in BRUKER AXS diffractometer (Model D8), with Cu target (Cu Kα, λ= 1.5405 Å) in the range of 2θ from 20 to 80°. To confirm the presence of the different cations EDX spectra of LCFCO were recorded. For further phase
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characterization Raman spectra of the samples were recorded by T64000 Raman Spectrophotometer (J. Y. HORIBA) with an excitation wavelength of 632 nm using
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HeNe laser. The static magnetic properties of LFO and LCFCO were carried out in Quantum design Vibrating Sample Magnetometer (VSM model, PPMS-16) where the maximum applied field was 10 T. The transmission
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Fe Mössbauer spectra were
obtained at 77 and 300 K using a conventional spectrometer with a 57Co source diffused
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into a Rh matrix. The isomer shift values are quoted to that of α-Fe foil at 300K, used as a standard for the transducer. Dielectric constant, dielectric loss etc. of the samples were measured with varying frequency in the range of 100 Hz to 5MHz using a HIOKI 3532-
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50 LCR HiTESTER at different temperatures from RT to 900 K. To determine the magneto-electric coupling, the magneto-capacitance (MC) co-efficient of the samples
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were calculated from dielectric constant measured in presence and absence of magnetic field at RT.
3. Results, analysis and discussions 3.1 XRD Analysis. The X-ray diffractograms (XRD) of LFO and LCFCO are compared in Fig. 1. We successfully assigned the XRD peaks of each sample from the available JCPDS file no. 74-2203 corresponding to the LFO phase with space group Pbnm. The absence of any
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cations are found in the observed spectra and the ratio of the La, Ca, Fe and Co cations calculated from the spectra is very close (~0.91:0.09:0.91:0.09) with our starting composition (0.9:0.1:0.9:0.1). The line broadening of the 100% intense peak of the XRD
well-known Debye-Scherrer equation [13], .
(1)
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< >=
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pattern (112/200) is used to calculate the average crystallite size of the samples using the
Here,
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The estimated average crystallite sizes, as obtained from the above equation are ~30 nm for LFO and ~25 nm for LCFCO. Since, LFO crystallizes with distorted orthorhombic structure, some strains should be associated with the crystal lattice. Due to the
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incorporation of dopants of Ca2+ and Co2+ in LFO lattice, this strain should be modified in the doped sample. The lattice strains of the samples are estimated from the XRD line
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broadening using the Hall-Williamson (H-W) method from the following equation, β cos =
+ 4 sin
(2)
Here, β is the full width at half maximum (FWHM) after the instrumental broadening correction, K is a constant (= 0.89), D is the average crystallite size and is the lattice strain introduced inside the distorted orthorhombic lattice [24]. Here, the average crystallite size is obtained from the ordinate intercept of the straight line fit of the β cos 4 sin plot and the lattice strain is obtained from the slope of the straight 6
ACCEPTED MANUSCRIPT line. The above plot along with the straight line fit of LFO and LCFCO sample is shown in Fig. 3(a). The obtained values of the average crystallite size and lattice strain are ~50 nm, ~0.00171 for LFO and ~50 nm, ~0.00243 for LCFCO. Thus we see that the lattice strain has increased in LCFCO due to the co-doping. This increase in lattice strain may
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be due to the increase in distortion of the octahedron of oxygen anions of the lattice due to substitution of smaller Ca ion (ionic radius ~0.99 Å) replacing La ion (ionic radius ~1.06 Å).
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To extract the detail structural information of the samples, XRD patterns of LFO and LCFCO are successfully studied by Rietveld analysis using MAUD program, version
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2.33 [25]. For this, the theoretical diffraction pattern of the sample is generated with LaFeO3 phase having space group Pbnm and then we try to fit this with the observed XRD pattern. The theoretical pattern of each sample is then generated by repetitive modification of different parameters corresponding to structural, microstructural and
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background function until a good agreement between the observed and calculated pattern is reached. The reliability of the profile fitting is judged by the low values of goodness of fit (GoF) and the reliability parameters (Rexp, Rw, Rb) [26]. We have also refined the
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oxygen occupancy factor during the Rietveld analysis. From the fitting parameter we
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observe that in LCFCO some oxygen vacancy is generated and the value as obtained is ~0.066. The reason behind this oxygen vacancy may be the charge imbalance due to doping. Though it is very difficult to determine the oxygen vacancy accurately from the XRD data, still a basic idea can be obtained about this parameters from this analysis [2729]. The degree of orthorhombic distortion, is defined through the values of (c/a, √2c/b) [7]. The values of the different reliability parameters and structural lattice parameters obtained from the profile fitting are listed in Table 1, as well as those of the degree of orthorhombic distortion (through the values of c/a, √2c/b) [7]. The later ones slightly
7
ACCEPTED MANUSCRIPT deviate from the respective values of LFO (c/a =1.41667, √2c/b =1.99991), reported in the JCPDS file no. 74-2203. From the table we also observe that the orthorhombic distortion has increased in LCFCO compared to pristine LFO. The arrangement of different ions in the unit cell of LCFCO as extracted from the Rietveld analysis is shown
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in Fig. 3(b). 3.2 Raman Analysis.
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Raman spectroscopy is an important tool to find different information regarding Raman active modes and to examine the presence of impurity or precipitates of dopants
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if any etc. Raman spectra of LFO and LCFCO were recorded at RT with an excitation wavelength of 632 nm using lower LASER power of 2 mW. The spectra of both the samples displayed in Fig. 4 are more or less similar nature and consistent with the earlier reported results of Raman spectra of LaFeO3 and other rare earth manganite belonging to
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the space group Pbnm of orthorhombic structure [3, 30, 31]. Here in the observed Raman pattern, we did not find any extra peak except LFO, which can confirm that both samples have the same crystal structure. It is well established that in LaFeO3 there are four
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formula units per unit cell, and the decomposition of the normal-mode is given by [3]: Γ = 7A# + 7B%# + 5B'# + 5B(# + 8A* + 10B%* + 8B'* + 10B(*
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Among these Ag, B1g, B2g and B3g modes are Raman active indicating that there are 24 Raman active modes in LFO. However, for complex oxides like LFO, detection of all the active Raman modes is not possible; instead, some dominant modes with broad features are found in the spectra. We have tried to describe the experimental spectra by means of Lorentzian functions and the peaks are in good agreement with the previous observations [3, 30, 31], as illustrated in Fig. 4. The peaks observed at around 144 cm-1 and 172 cm-1 resulting from La vibrations are assigned to B1g and Ag modes respectively. The mode at
8
ACCEPTED MANUSCRIPT ~290 cm-1 is associated with different orientations of the FeO6 octahedral units and the mode at ~434 cm-1 originates from the vibration due to bending of the FeO6 octahedra. The peaks around 496 cm-1 and 637 cm-1 are associated to the asymmetric and symmetric stretching of different Fe-O bonds of these same polyhedra. For rare-earth with large
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ionic radius like La, the frequency of the mode at ~496 cm-1 is attributed to the Jahn– Teller distortion [3]. Also, the intensity of the modes below 200 cm-1 and ~433 cm-1 are found to be decreased in LCFCO as compared to those of LFO. This is attributed to the
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randomness of the substituted dopants/host cations like, Ca/La and Co/Fe in the lattice of LFO. Due to the substitution of dopants, the long range cooperative motion of La-O and
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Fe-O in LFO is broken by the random distribution of Ca and Co, respectively: consequently, the intensity of the Raman modes associated with these bonds decreases [3]. The frequency of the mode found at ~637 cm-1 has been found to decrease down to ~627 cm-1 in the doped sample which can be an effect of substitution of Fe with Co of
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larger atomic weight. The reverse case observed in substitution of La with Ca, where the modes at ~144 cm-1 and ~172 cm-1 are shifted towards higher frequency (to ~146 cm-1 and ~176 cm-1 respectively). These observations are quite consistent with those reported
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earlier on doped BiFeO3 by Huang et al. [32].
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3.3 Magnetic Properties.
Fig. 5 compares the magnetization vs. magnetic field (M-H) loops of the pristine and
doped sample recorded at 5 K and at RT and the maximum applied field for these loops is 100 kOe (~10 T). Relevant magnetic properties viz. maximum magnetization (Mmax), coercive field (Hc), remnant magnetization (Mr) obtained from the M-H loops are listed in Table 2. The M-H loops of LFO shows weak nonlinearity in magnetization vs. magnetic field data even at 5 K, whereas the corresponding variation for LCFCO deviates strongly from linearity up to the applied field of ~30 kOe, beyond which the 9
ACCEPTED MANUSCRIPT magnetization remains linearly dependent. This type of variation in M-H loop is generally observed in samples with weak ferromagnetic contribution. Thus the weak non-linearity in M-H data i.e., the irreversibility of the magnetization between the increasing and decreasing field and the value of Hc confirms the presence of weak
where
ferromagnetic
exchange
interaction
occurs
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ferromagnetism in LFO. Antiferromagnetic material comprises of two spin sublattices in
each
sublattice
and
antiferromagnetic superexchange interaction favours between the two [33]. In the long
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range ordering, due to the complete compensation of the spin magnetic moments of these sublattices the two-sublattice system results in zero net magnetic moment. However, in
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case of nanoparticles, this long-range collinear antiparallel spin structure is disturbed at the surface of the particles originating a non-compensation and thus a small spontaneous magnetization. This effect is further enhanced when the surface-to-volume ratio increases, i.e. the size nanoparticle is decreasing. This type of behaviour of LFO is
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consistent with the previous observation by Ghosh et al. and Köferstein et al. [34, 35]. In addition, the value of the coercive field of LFO at 5 K (2.9 kOe) is smaller than that at RT (9.8 kOe). This feature is also consistent with the antiferromagnetic nature of the
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sample. This canted antiferromagnetic nature of LFO is found to be substantially
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enhanced in the Co-doped sample. Indeed, as observed in Fig. 5 the M-H loops of LCFCO behaves as a weak ferromagnet. One can distinguish two main contributions to the enhancement of magnetization in LCFCO. The first one results from the doping of Ca ion replacing the La ion: indeed, the Fe3+–O2-–Fe3+ superexchange angle becomes closer to 180° (see Table 1), which in turn strengthens the superexchange interaction leading to the enhancement of magnetization. The second one is the introduction of ferromagnetism due to the substitution of Co ion in place of Fe. Such interaction has been already observed in Co-doped BiFeO3 sample, as reported by Xu et al. [36]. The
10
ACCEPTED MANUSCRIPT first contribution is expected to dominate near the RT while the second one becomes more effective in the low temperature region. The value of Hc of LCFCO is found thus to have increased at low temperature, that is clearly evidenced from the 5 K M-H loop, as shown in Fig. 5. Now to estimate quantitatively the ferromagnetic and antiferromagnetic
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components in the observed magnetic property of the doped sample, a description of the M-H loops is carried out using the following relation [23, 33]; 6 345
7
89:;% <=
>±>@A >@A
73C
B 89: ='345 6 BDE + F/ 45
(3)
SC
-(/) = 12
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This equation gives the magnetization, M(H), at an applied field magnetic H where the first term represents the irreversible component of magnetization coming from the ferromagnetic phase and the second linear component is due to the antiferromagnetic H I and/or paramagnetic contribution. Here, -G3 , -G3 , /JK are the ferromagnetic saturation
magnetization, remnant magnetization, intrinsic coercive field, respectively while χ is the
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antiferromagnetic susceptibility. The experimental magnetization recorded at RT and 5 K are compared in Fig. 5 along with the theoretically fitted curves. The simulated
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ferromagnetic and antiferromagnetic contributions are also illustrated in Fig. 5. The extracted fitting parameters are listed in table 3. From the fitting parameter, we observe
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that the ferromagnetic part contributes ~33% of the total magnetization of LCFCO at RT, and increases up to ~41% at 5 K. The increase in the ferromagnetic part at low temperature can be attributed to the decrease in the thermal excitation which increases the ferromagnetic moment and decreases the antiferromagnetic moment. To get a detailed insight of the magnetic nature of the co-doped sample zero-field cooled (ZFC) and field cooled (FC) temperature dependent magnetization curves are measured from 5 K to 300 K where the field applied for the FC measurement is 500 Oe and the corresponding curves are displayed in Fig. 6. The observed ZFC and FC curves 11
ACCEPTED MANUSCRIPT show a bifurcation near the RT. This type of clear difference between the ZFC and FC curves is associated with the magnetic blocking observed when cooling from high temperature the samples with superparamagnetic or spin-glass type of behaviour. However, the observed large magnetic coercivity at RT (shown in Table 2) excludes the
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presence of superparamagnetism in the sample. The source of the spin-glass behaviour in LCFCO is attributed to the struggle between ferromagnetic and antiferromagnetic interaction in the sample. The presence of the spin-glass like behaviour is consistent with
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the earlier report by Xu et al and Liang et al. [36, 37]. The ZFC magnetization continues to decrease with the decrease in temperature down to 20 K (Fig. 6) and below this,
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magnetization begins to increase with decreasing temperature. In case of FC, the magnetization is found to decrease with lowering temperature in the range from 300 K down to 291 K, which indicates the presence of the antiferromagnetic ordering in the sample. After this the magnetization continues to increase with further lowering
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temperature. This increment in the magnetization indicates that the ferromagnetic part, due to the Co substitution starts to dominate over the antiferromagnetic nature below 291 K. Below ~ 20 K it is found that both ZFC and FC magnetization starts to increase
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rapidly up to the lowest temperature of 5 K. At this temperature, the antiferromagnetic
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part becomes negligible leaving the ferromagnetic part to dominate. The clear bifurcation of the ZFC and FC magnetization curve indicates presence of magnetic blocking phenomena in LCFCO. However, the presence of large coercive field at RT and the absence of cusp in the ZFC magnetization indicate that the blocking arises due to the spin glass behaviour of the sample. Such a variation of the ZFC and FC magnetization is consistent with the Co substituted BiFeO3 particles [36]. 3.4 57Fe Mössbauer Spectrometry.
12
ACCEPTED MANUSCRIPT To know the charge state of Fe ions, the hyperfine behaviour of the samples are investigated using Mössbauer spectrometry. The Mössbauer spectra of LFO and LCFCO observed at RT and 77 K are compared in Fig. 7. Analysis of the Mössbauer spectrum can provide different information regarding, electronic density at the nuclei through the
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value of the isomer shift (IS), possible electric field gradient from quadrupole splitting (QS) value, the magnetic environment from magnetic hyperfine splitting (BHF) etc. Detailed analyses of the observed spectra are carried out and the obtained results are
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listed in Table 4. The values of different parameters like IS, QS and BHF are consistent with the earlier observations [33, 38]. The isomer shift, as estimated from the analysis is
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found to be in the range of 0.33-0.37 and 0.44-0.48 for both samples at 300 and 77K, respectively. This indicates that the samples are mainly dominated by Fe3+ ionic state excluding the possibility of any other charge state. The superexchange interaction among these Fe3+ ions via oxygen gives rise to the antiferromagnetic nature of the samples,
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which is consistent with the hyperfine magnetic splitting (sextet) and with the magnetic observations discussed in the earlier section. Also, both the spectra recorded at RT and at 77 K are fitted with two sextets having small value of quadrupole shift 2ε and almost
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equal IS value. These sextets correspond to the two non-equivalent positions of the Fe3+
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ions due to weak distortion of the octahedral oxygen environment. These observations are quite consistent with the earlier observation by Chauhan et al. and Han et al. [33, 38]. In addition to the magnetic sextet, the fitting of the RT data of LCFCO shows the presence of a weak quadrupolar doublet with IS value of 0.37. This tiny doublet in LCFCO originates from the paramagnetic behaviour of the fraction of spin-glass system present in the sample. Since there is no charge state other than Fe3+ in the sample, this spin glass behaviour is believed to come from the competition between ferromagnetic and antiferromagnetic interaction in the sample [33, 38]. Because of its origin, the
13
ACCEPTED MANUSCRIPT doublet was not found either in case of LFO, where the antiferromagnetism is strong or in the 77 K data of LCFCO, where ferromagnetism dominates the antiferromagnetic interaction. 3.5 Dielectric Properties.
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Temperature and frequency dependence of the dielectric constant of LFO and LCFCO are measured where the frequency range of 50 Hz to 5 MHz and temperature range of 300 K to 825 K are considered. The thermal variation of the real part of the dielectric
SC
constant ( L ) of both LFO and LCFCO measured at 100 kHz in the mentioned temperature range are depicted in Fig. 8. Values of L are found to increase with the
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increase in temperature until a critical temperature (Tm) is reached. Above Tm, L is found to decrease rapidly with further rise in temperature. Tm where the value of L is L maximum (M ) is called the transition temperature. Initially with the rise in temperature
the electric dipoles gains thermal energy to be aligned with the applied electric field and
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consequently, the dielectric constant increases. In addition to this the interfacial polarization increases with the rise in temperature which also increases the dielectric constant. But at Tm the thermal energy becomes sufficiently large and due to this the
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dipoles start to be randomly oriented and the sample becomes paraelectric. Due to this
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fact the dielectric constant decreases above Tm. The transition temperature in LFO (Tm ~715 K) has been significantly shifted towards higher temperature in LCFCO (Tm ~748 K), as clearly observed in Fig. 8. Interestingly, LCFCO shows a sharp phase transition in comparison to the broad phase transition of LFO. The reason behind this sharp nature of the phase transition may be due to the reduction in the overlapping of the ferroelectric and non-ferroelectric regions in the sample [39]. The frequency variation of the real part of the dielectric constant ( L ) and dielectric loss (tan δ) of both the pure and doped samples are measured in the frequency range of 50 Hz to 5 MHz (Fig. 9). Values of L of 14
ACCEPTED MANUSCRIPT both sample show strong frequency dependency. The real part of the dielectric constant is found to have a sigmoidal type of variation in the low frequency region and a tendency towards saturation is observed in the high frequency region. In the high frequency region the dipoles are unable to follow the ac field due to which the value of L tends to saturate
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at a low value. There are several types of polarization which contribute to the dielectric constant. Among them, the dipolar and interfacial polarization contribute maximum in the low frequency region while the electronic polarization dominates in the high
SC
frequency region [40]. The frequency variation of L of LFO and LCFCO deviates from the ideal Debye-like relaxation and is found to follow the Cole-Cole relaxation consisting
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of dipoles with a distribution of relaxation time. According to the Cole-Cole relaxation the real part of the dielectric constant is described by the following equation; (N) = O + L
WZ B[
WZ %S'(TU@@ ) VW XY= BS(TU@@ ) ( VW)
∆Q R%S(TU@@ ) VW XY=
(4)
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Here, N is the angular frequency, ∆ = H − O is the dielectric relaxation strength, H and O are respectively the high and low frequency permittivity limits, ]JJ is the relaxation time or time constant and ^ is the shape parameter [41]. High value of the
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dielectric permittivity at low frequency is due to the electrode polarization effect while it approaches to a limiting value (O ) at high frequency. For this to avoid the effect of
AC C
electrode polarization we have considered L data above 15 kHz for the determination of O and ∆ using equation 4. The observed values of L of LFO and LCFCO along with its Cole-Cole fitting are compared in Fig. 9(a) and 9(b). The values of O , ∆ obtained from the fitting ~91, ~457 and ~150, ~3314 for LFO and LCFCO, respectively. However, the effect of enhanced conductivity is clearly established from Fig. 9(b) in case of LCFCO. From the frequency dependency of dielectric loss i.e. ‘tan δ’ of LFO and LCFCO we observe that in both the samples the loss factor is prominent in the low
15
ACCEPTED MANUSCRIPT frequency region and decreases with the increase of the frequency. From Fig. 9(a) and 9(b), the dielectric loss in LCFCO has been reduced in the low frequency region but remains greater in the high frequency region as compared to pure LFO.
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For the confirmation of the magneto-dielectric property of the samples, the magnetic field variation of dielectric constant is studied. This is one of the important way to investigate the coupling effect, as used by many authors [9, 13, 42]. The magnetoelectric coupling of the sample is studied by measuring the dielectric constant in
MC =
M AN U
the sample is calculated from the following formula,
SC
presence and absence of the magnetic field. The magnetocapacitance coefficient (MC) of
[ε′ (b,d);ε′ (,d)] ε′ (,d)
(5)
Here, ′ (H, T) and ′ (0, T) represent the dielectric constants in presence and absence of external magnetic field (H), respectively [9, 13]. The variations of magnetocapacitance
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coefficient with external magnetic field for the samples are displayed in Fig. 10: it is noted a strong dependency of the dielectric constant of the samples by the external magnetic field. Material with coupled magnetic and electric orderings when placed in an
EP
external magnetic field, a strain is induced in it. This strain introduces a stress in the
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samples which in turn produces an electric field in the material. The ferroelectric domains are oriented by this electric field causing an increase in the polarization value [42]. From the figure we see that the maximum value of MC of LFO (~0.055 % @ 0.8 T) is drastically enhanced in LCFCO (~1.83 % @ 0.8 T). This is probably due to the doping of smaller Ca atom in place of La atom in LFO lattice. In addition, the enhancement of the magnetic property of LCFCO increases the value of the strain due to external magnetic field which in turn increases the MC value in the sample. Conclusion
16
ACCEPTED MANUSCRIPT Nanoparticles of pure and Ca, Co co-doped LFO are successfully prepared by the solgel route. The crystalline structures of the pure and doped samples are confirmed from the analysis of the XRD data using the Rietveld analysis. Different structural parameters are also determined from the above analysis. The information extracted from the detailed
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analysis of the Raman spectroscopy is in good agreement with the relevant findings of LFO and related systems, reported earlier. The change in intensity of some particular Raman modes in LCFCO substantiated the findings of Rietvield analysis i.e. the
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substitution of Ca and Co ions in place of La and Fe ions, respectively. Strain is found to be enhanced in the crystal lattice of LFO due to the Co-doping in LCFCO. Strong
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modulation of the magnetic property is observed due to the choice of co-dopants of Ca and Co in the host. Presence of the mixed state of antiferromagnetic and ferromagnetic phase is confirmed from the detail analysis of the M-H loops of the doped sample. However, the ferromagnetic phase dominates over the antiferromagnetic phase over a
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large range of temperature in LCFCO. The temperature variation of the magnetization shows an increase in strength of the ferromagnetic property of the doped sample with the decrease in temperature whereas in case of pure LFO this feature is absent. Also, a
EP
fraction of spin-glass phase is also found at the RT data of the doped sample. The
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Mössbauer spectra at 300 K and 77 K confirm the presence of Fe3+ cations in both samples. In addition, Mössbauer Spectra of the doped sample ruled out the formation of any magnetic impurity like α-Fe2O3. Analysis of temperature and frequency variation of dielectric constant shows improved dielectric property of the doped sample compared to that of LFO. The coupling of the magnetic and dielectric property of LCFCO is found to be enhanced compared to LFO. Thus, fruitful modification of magnetic, dielectric and magneto-dielectric behaviour in doped sample compared to that of LFO would be interesting for applications in electronic devices.
17
ACCEPTED MANUSCRIPT Acknowledgement The authors wish to acknowledge the UGC-DAE CSR, Kolkata and Mumbai center, Govt. of India for the financial assistance of the present work (Project file no. UGC-
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DAE-CSRKC/CRS/13/MS04/Ext/2016, Dated: 22.12.2016 and Project file no. UDCSR/MUM/CD/CRS-M-265/2017/1043, Dated: 20.01.2017). Authors also wish to acknowledge Dr. A. Banerjee, UGC-DAE CSR, Indore centre for the magnetic measurement facility provided. Authors also wish to acknowledge the financial support
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provided by the UGC, Govt. of India, through the CAS program (Fie no.
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F.530/5/CAS/2011(SAP-I)) and FIST program (Fie no. SR/FST/PSI-170/2011(C)). REFERENCES
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Table 1
Structural parameters and reliability factors obtained from Rietveld analysis of XRD pattern of LFO and LCFCO Cell Parameters a (Å)
b (Å)
c (Å)
LFO
5.5589(3)
5.5626(6)
7.8566(8)
LCFCO
5.5295(6)
5.5337(7)
7.8196(3)
OD [(c/a), √2×(c/b)] 1.41334, 1.99742 1.41415, 1.99838
21
Reliability Factors Rexp Rb GoF (%) (%)
Fe-O1-Fe ° (deg)
Fe-O2-Fe ° (deg)
150.1(5)
160.7(3)
6.47
5.64
5.14
1.14
152.5(5)
164.3(3)
5.99
5.26
4.69
1.13
Rwp (%)
ACCEPTED MANUSCRIPT
Table 2 Magnetic parameters extracted from the M-H loops of LFO and LCFCO.
LFO LCFCO
T (K)
Mmax (emu/g)
Hc (kOe)
Mr (emu/g)
Mmax/ Mr
300
1.07
9.89
0.11
9.73
5 300
1.11 1.89
3.03 1.51
0.04
27.75
0.21
9.00
5
2.19
14.10
0.56
3.91
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Sample
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Table 3
Magnetic properties extracted from the analysis of M-H loops of LCFCO.
1.904 2.194
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300 5
Paramagnetic Contribution F × 10g (emu/g Oe) 1.325 ±0.007 1.311 ±0.009
Ferromagnetic Contribution
H -G3 (emu/g) 0.631 ±0.005 0.905 ±0.008
I -G3 (emu/g) 0.221 ±0.002 0.562 ±0.003
/JK (kOe) 1.757 ±0.031 17.696 ±0.109
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T (K)
Observed Magnetization (emu/g)
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Table 4
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Hyperfine parameters of LFO and LCFCO obtained from analysis of Mössbauer Spectra recorded at 300 and 77 K T (K) 300 LFO 77 300 LCFCO 77
Type Sextet 1 Sextet 2 Sextet 1 Sextet 2 Doublet Sextet Sextet 1 Sextet 2
IS (mm/s) ± 0.01 0.37 0.33 0.48 0.46 0.37 0.36 0.48 0.47
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2ε/QS (mm/s) ± 0.01 -0.04 -0.12 -0.05 -0.07 0.88 -0.06 -0.06 -0.06
Bhyp (T) ± 0.5 52.1 49.5 56.1 53.5 48.4 55.7 53.4
Area ratio (%) ±2 82 18 87 13 3 97 56 44
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Figure Captions:
Figure 1. Observed and generated (from Rietveld analysis) X-ray diffraction patterns of (a)
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LFO and (b) LCFCO. Figure 2: EDX spectra of LCFCO sample.
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Figure 3. β cosθ vs. 4 sinθ plot (Hall–Williamson plot) of LFO and LCFCO. The solid lines show the linear fit.
Figure 4. Raman spectra of LFO and LCFCO at room temperature together with spectral deconvolutions using Lorentzian functions. The ticks in the top panel indicate the positions of
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different Raman modes.
Figure 5. Magnetic hysteresis loops (M-H loops) of LFO and LCFCO observed at (a) 300 K, (b) 5K. Fitting with antiferromagnetic and ferromagnetic contribution in M-H loops of
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LCFCO recorded at (c) 300 K, (d) 5K. Inset shows the enlarged view of the fitting.
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Figure 6. FC and ZFC magnetization curves of LCFCO. Inset shows the enlarged view of FC magnetization.
Figure 7. Mössbauer spectra including their analysis of LFO at (a) 300 K, (b) 77 K and LCFCO at (c) 300 K, (d) 77 K. Figure 8. Thermal variation of L (@ 100 kHz) of LFO and LCFCO. Figure 9. Frequency variation of L and tanδ of (a) LFO, (b) LCFCO observed at RT along with the Cole-Cole fit of L data. Figure 10. Magnetic field variation of MC of LFO and LCFCO.
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Figure 5
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Figure 9 (a)
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Figure 10
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ACCEPTED MANUSCRIPT Highlights: • LaFeO3 and Ca, Co co-doped therein are successfully prepared by sol-gel technique. • Analyses of XRD and Raman spectra confirmed the complete substitution of dopants. • Ferromagnetic property is introduced in the antiferromagnetic host by doping.
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• Analysis of Mössbauer spectra confirms the magnetic phases of the samples.
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• Doping has improved the magneto-electric property of LaFeO3 for application.