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Structural, magnetic and magnetocaloric properties of Co-doped nanocrystalline La0.7Te0.3Mn0.7Co0.3O3
Accepted Manuscript Structural, magnetic and magnetocaloric properties of Co-doped nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 Meenakshi, Amit Kumar, Rabindra Nath Mahato PII: DOI: Reference:
S0304-8853(17)30499-7 http://dx.doi.org/10.1016/j.jmmm.2017.07.048 MAGMA 62979
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
10 February 2017 8 July 2017 12 July 2017
Please cite this article as: Meenakshi, A. Kumar, R.N. Mahato, Structural, magnetic and magnetocaloric properties of Co-doped nanocrystalline La0.7Te0.3Mn0.7Co0.3O3, Journal of Magnetism and Magnetic Materials (2017), doi: http://dx.doi.org/10.1016/j.jmmm.2017.07.048
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.
Structural, magnetic and magnetocaloric properties of Co-doped nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 Meenakshi, Amit Kumar and Rabindra Nath Mahato*
* Corresponding Author: School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India Email: [email protected]
Abstract Structural,
magnetic
and
magnetocaloric
properties
of
the
nanocrystalline
La0.7Te0.3Mn0.7Co0.3O3 perovskite manganite were investigated. X-ray diffraction pattern indicated that the nanocrystalline sample crystallized in orthorhombic crystal structure with Pbnm space group. The average particle size was calculated using scanning electron microscope and it was found to be ~ 150 nm. Temperature dependence magnetization measurements revealed ferromagnetic - paramagnetic phase transition and the Curie temperature (TC) was found to be ~ 201 K. Field dependence magnetization showed the hysteresis at low temperature with a coercive field of ~ 0.34 T and linear dependence at high temperature corresponds to paramagnetic region. Based on the magnetic field dependence magnetization data, the maximum entropy change and relative cooling power (RCP) were estimated and the values were 1.002 J kg-1 K-1 and 90 J kg-1 for a field change of 5 T respectively. Temperature dependent resistivity ρ(T) data exhibited semiconducting-like behavior at high temperature and the electrical transport was well explained by Mott’s variable-range hopping (VRH) conduction mechanism in the temperature range of 250 K -
1
300 K. Using the VRH fit, the calculated hoping distance (Rh) at 300 K was 54.4 Å and density of states N(EF) at room temperature was 7.04 x 10 18 eV-1cm-3. These values were comparable to other semiconducting oxides.
Keywords: Nanocrystalline; perovskite; manganites; X-ray diffraction; magnetic properties; magnetocaloric properties.
2
1. Introduction Currently, doped manganite perovskite with a general formula A1-xBxMnO3 (A= rare earth trivalent ions and B = divalent or tetravalent ions) have attracted considerable attention due to many intriguing features, such as colossal magnetoresistance, charge ordering, metal-insulator transition and magnetocaloric effect (MCE). A considerable amount of research has been carried out on electron doped compounds such as La1-xBxMnO3 where B = Te, Ce, Sb etc. [1, 2]. The parent compound LaMnO3 follows dual magnetic interactions depending on the coupling between trivalent manganese. It follows antiferromagnetic (AFM) coupling for inter-layer Mn3+ ions and ferromagnetic (FM) coupling for intra-layer Mn3+ ions [3, 4]. The doping on La-site by divalent or tetravalent ions on the parent compound drives Mn into mixed Mn2+, Mn3+ and Mn4+ states. The divalent ions such as Sr2+, Ba2+ etc. doped on La-site in the parent manganite system drives Mn into Mn3+ and Mn4+ states. However, the rare-earth tetravalent ions such as Te4+, Ce4+ converts manganese ion into Mn2+ and Mn3+ states [4]. Since the trivalent manganese ions in LaMnO3 changes to mixed Mn3+ and Mn2+ states in the electron doped La1-xBxMnO3 system, coexistence of FM and AFM magnetic phase separated ground states for light electron doping is observed. In case of heavy-electron doping, the AFM ground state is observed [5-8]. Electrondoped manganites have also gained attention for the magnetocaloric properties; such compounds show large magnetocaloric effect (MCE) under moderate applied filed which is very useful for magnetic refrigeration applications. Furthermore, the doping at Mn site is seen to be very effective for the La1-xBxMnO3 systems as all types of magnetic ground states are decided by Mn ion in manganites. Substitution of 3d cation on Mn site has been largely explored in such systems [9, 10]. The electrical transport properties of mixed valent manganites have shown interesting physical properties and can be manipulated the transport mechanism by partial doping
3
in the Mn-site. Thus, in order to induce particular properties in the perovskite LaMnO3, La3+ cation as well as Mn3+ must be partially doped by divalent or tetravalent cation. Most of the work in the doped La1-xBxMnO3 compound has been done in bulk form and very less work have been explored in nanocrystalline form. In the present work, we report the effect of Co substitution on structural, magnetic and magnecaloric properties of nanocrystalline La0.7Te0.3Mn0.7Co0.3O3. 2. Experimental details: The nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 compound was synthesized by sol-gel method [2]. Identification of the phase and crystal structure was determined by X-ray diffraction measured with Rigaku-made Miniflex600 powder diffractometer operated at 40 kV and 30 mA at room temperature for angles in the range 2θ=10° to 90° with a step size of 0.02°. The morphology, particle sizes and composition of the sample were examined by scanning electron microscopy (SEM, Carl Zeiss Evo-40) attached with the energy dispersive X-ray analysis (Bruker X-flash detector-4010). The magnetic measurements were performed as a function of temperature and magnetic field using vibrating sample magnetometer (PPMS, Cryogenic Ltd.) up to a maximum field of 5 T in the temperature range 2 K - 300 K. The magnetic entropy change |∆Smax M | was estimated from the magnetization data using a Maxwell relation. The electrical resistivity measurements of the prepared sample were done using standard four probe in the temperature range 150 K – 300 K. 3. Results and Discussion 3.1 Structural analysis The X-ray diffraction (XRD) pattern of the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample at room temperature is shown in Fig. 1. The XRD data shows that the nanocrystalline compound crystallizes in orthorhombic crystal structure with Pbnm space group [11]. The XRD pattern of 4
the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample is indexed using JCPDS. The lattice parameters are found to be a= 5.5322 Å, b= 5.4930 Å and c= 7.7788 Å [12]. The crystal symmetry and lattice parameters are similar to its bulk counterpart [10]. The average crystallite size (D) is estimated using Scherrer relation [12],
D =
λ β θ
(1)
where k is the grain shape factor, λ, θ and β are the X-ray wavelength, Bragg diffraction angle, and full width at half maxima (FWHM) of the diffraction peak respectively. The value of average crystallite size is found to be ~ 24 nm for La0.7Te0.3Mn0.7Co0.3O3 sample. The SEM images show that the particles are spherical in shape (shown in Fig. 2) and the particle size varies from tens of nanometers to few hundreds of nanometers. The average particle size is calculated using particle size distribution and is found to be ~ 150 nm (see inset of Fig. 2). The particles sizes observed by SEM are several times larger than those calculated by XRD, which shows that each particle observed by SEM consist of several crystallized grains. 3.2 Magnetic measurements The temperature dependence magnetization measurements M(T) are carried out under an applied magnetic field of 500 Oe. The field cooled (FC) and zero field cooled (ZFC) magnetization data (denoted as MFC and MZFC) shows ferromagnetic (FM) to paramagnetic (PM) phase transition and reaches a maximum value of magnetization (MFC) ~ 0.54 µ B.
Below TC, there is a
divergence in MFC and MZFC curves, which commonly seen in the perovskite manganite samples [13, 14]. The large bifurcation in MFC and MZFC curves originates from the anisotropic field generated from FM clusters. A cusp like peak observed in MZFC data and it could be random
5
alignment of the thermally activated magnetic moments. The Curie temperature (TC) is determined from the derivative of the MFC(T) curve (see inset of Fig. 3) and it is found to be ~ 201 K. The temperature variation of the inverse magnetic susceptibility in the paramagnetic region follows Curie-Weiss law [15],
χ=
(2)
where C is the Curie constant and θP is the paramagnetic curie temperature. The inverse susceptibility (χ-1) versus temperature plot is shown in the left inset of Fig. 3. The χ-1 vs. T data shows that the Curie-Weiss law obeys in the temperature range of 210 K - 300 K and the paramagnetic Curie temperature is found to be ~ 205 K. The positive value of θP indicates that the FM exchange interaction exists between the nearest neighbours and the obtained θP value. The experimentally observed value of effective magnetic moment (μ = 2.83√C) is 4.13 μ /f.u. and the theoretically calculated spin – only moment (μ = g S(S + 1)) value is 4.10 μ /f.u. considering Co3+ in its low state and Mn3+ in its high spin state. Both theoretical and experimentally observed μ value are comparable to each other. The effective paramagnetic moment calculated from the Curie-Weiss law approximately corresponds to the trivalent spin state configurations of cobalt. Field dependence magnetization measurements at different temperatures are shown in Fig. 4. The field dependence magnetization data shows the ferromagnetic loop below TC with a coercive field of 0.34 T at 2 K and the linear M(H) corresponds to paramagnetic region. Fig. 5 shows the magnetic isotherms for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 compound measured near magnetic ordering temperature. It is clearly seen that the magnetization increases
6
sharply at lower applied magnetic fields and it does not saturate up to 5 T. In equilibrium condition, the magnetic equation of state can be expressed as,
H = A + BM ' (3) M
where A and B are the temperature dependent parameters. The Arrott plots (M2 versus H/M) are used to identify the nature of the ferromagnetic phase transition. According to Banerjee’s criterion [16], the Arrott plots with positive slopes correspond to a second order magnetic phase transition and a negative slope indicates first – order magnetic transition. Fig. 6 shows the Arrott plots for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample and are found to be positive slopes, confirming the second order magnetic phase transition. The linear regions (at high field) of Arrot plots are numerically fitted to Eq. (3) to derive the temperature dependence parameters. The obtained A parameter has been plotted as a function of temperature as shown in the inset of Fig. 6. The Curie temperature of La0.7Te0.3Mn0.7Coe0.3O3 nanocrystalline sample is determined by the sign change of parameter A and it is found to be 200 K which is similar to the TC value determined from the low field magnetization measurements. 3.3 Magnetocaloric effect According to a thermodynamic Maxwell’s relationship: (∂S/∂H)T = (∂M/∂T)H, the magnetic entropy change |∆SM| produced by the variation of a magnetic field from 0 to H is given by [17, 18], 2 /(
|∆S( (T, H)| = S( (T, H) − S( (T, 0) = -3 . / 0 dH
(4)
7
The magnetic entropy change (|∆SM|) as a function of temperature is shown in
Fig. 7. The
456 maximum entropy change |∆S( | is found to be 1.002 Jkg-1K-1 for a magnetic field change of 5
T for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample. Relative cooling power (RCP) is an important parameter to evaluate the cooling efficiency of a magnetic refrigerant. RCP corresponds to the amount of heat that can be exchanged between the cold and hot reservoirs of the refrigerator in an ideal thermodynamic cycle. In other words, it reflects the cooling efficiency of the refrigerator. Relative cooling power (RCP) is calculated using the relation, 456 | RCP = |∆S( × δT:;2(
(5)
456 | where |∆S( is the maximum entropy change near TC and δTFWHM is the full-width at half
maxima. The calculated RCP value for La0.7Te0.3Mn0.7Co0.3O3 is 90 J/kg for a magnetic field 456 | change of 5 T. The observed |∆S( value for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 is
less than that of its bulk counterpart but the larger operating temperature window (i.e. δTFWHM = 89 K) than bulk (δTFWHM = 48 K) [10] sample shows the importance of nanocrystalline behavior. 456 | The moderate |∆S( value and wide operating temperature range compared to other
perovskite-type manganites [19, 20], making this nanocrystalline compound a promising candidate for magnetic refrigeration applications in the sub room temperature. 3.4 Critical exponent analysis In order to understand the correlation between critical exponents, magnetocaloric effect and nature of magnetic phase transitions, a detailed critical exponent analysis has been carried out. According to scaling hypothesis, the magnetic systems following second order phase transitions
8
are governed by a set of interrelated critical exponents β, γ and δ. These critical exponents are mathematical expressed as [21, 22], M< (T) = M3 (−t)> t < 0, T < T (6) χ3A (T) =
h3 (t)C t > 0, T > T (7) M3
F
M = DH G t = 0, T = T (8) where M0, h0/M0, and D are the critical amplitudes and t=(1-T/Tc) is the reduced temperature. The exponent β is associated with the spontaneous magnetization (MS), γ is related to the initial magnetic susceptibility (χ0), and δ is associated with the critical magnetization isotherm. The value of δ is determined from the slope of ln M versus ln H plot at T= TC which is shown in Fig. 8. The obtained value of δ is ~3.16. According to Oesterreicher et. al. [23], the field dependence of the magnetic entropy change for the materials following second order phase transitions can also be expressed as, ∆HI = JKL (9) where J is constant and the exponent n depends on the magnetic state of the sample. The value of n is calculated from the linear plot of ln(∆SM) versus ln(H), shown in inset of Fig. 8. The critical exponents β and γ are determined using the following expressions, A
A
n = 1 + O .1 − >0 n = 1 +
(10)
β−1 (11) β+γ
9
The obtained values of critical exponents of n, δ, β and γ are tabulated in Table II. The deduced values of the critical exponents are close to that of the mean free model [24, 25], though the value of γ is slightly higher than that predicted by the mean field model indicating long range magnetic interactions are present in La0.7Te0.3Mn0.7Co0.3O3 sample. 3.5 Electrical transport measurements Temperature dependent electrical resistivity data for nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample is shown in Fig. 9. The resistivity data ρ(T) shows the abrupt change in resistivity at ~ 200 K and these could be the signature of electrical transition which is also reflected in the M(T) data. To explain the electrical resistivity data, we have fitted the temperature dependence resistivity data. According to Mott’s VRH model, electrical resistivity in three-dimensional hopping is given by [27],
A/V
ρ(T) = ρ3 exp . T 0
(12)
where T0 is the Mott’s characteristic temperature, which can also be expressed in terms of density of states (N(EF)) in the vicinity of Fermi energy and the localization length “a” as follows [28]: T3 =
AW
\ X Y(Z[ )5
(13)
The mean hopping distance Rh(T) and hopping energy Eh(T) at a given temperature can be written as,
10
^
A/V
R ] (T) = W a . T 0 A
A/V
E] (T) = V k T ^/V T3
(14)
(15)
The Mott’s VRH data fit well with the VRH model (not shown) for the temperature range 250 K – 300 K, suggesting the electrical transport is dominated by Variable range hopping. From the intercept and slope of the fitted curve, we have calculated ρ0, T0 using equation (12) and are found to be 4.84 x 10-7 mΩ cm and 323.26 x 106 K respectively (shown in Table II). Using value of T0 in equations (13-15), we have calculated the values of N(EF), Rh(T) and Eh(T) at room temperature by considering the localization length a=4.5 Å and the values are shown in Table II. The value of N(EF) is nearly the same order of magnitude as in other known semiconducting oxides [N(E)~1017-1019 eV-1] [29]. 4. Conclusions The structural, magnetic and magnetocaloric properties of Co-doped nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample were investigated. The sample shows orthorhombic crystal structure at room temperature with Pbnm space group. The average crystallite size has been found to be ~ 24 nm. The sample undergoes paramagnetic to ferromagnetic transition and the Curie temperature is found to be ~ 201 K. A substantial magnetic entropy change and relative cooling power is obtained and the values are 1.002 Jkg-1K-1and 90 Jkg-1 for a magnetic field change of 5 T respectively. The electrical resistivity data shows the signature of electrical transition at 200 K and Mott’s variable-range hoping conduction mechanism is dominated in the temperature range of 250 K - 300 K. The hoping distance (Rh) and density of states (N(EF)) at
11
300 K are 54.4 Å and 7.04 x 1018 eV-1cm-3 respectively for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample.
Acknowledgement: RNM is thankful to University with Potential for Excellence –II at JNU (UPE II) grants and University Grant Commission for UGC-BSR start up grants respectively. The financial support from the project DST Purse is also acknowledgeable. Meenakshi and AK are thankful to Ms. Ruchita Pal for SEM measurements and AIRF, JNU for magnetic measurements.
References: [1] P. Raychaudhuri , C. Mitra, P. D. A. Mann, J. Appl. Phys. 93, 10, (2003) [2] R. N. Mahato, K. Sethupathi, V. Sankaranarayanan, R. Nirmala, J. Magn. Magn.Mater. 322, 17 (2010) [3] J. Zaanen, G. A. Sawatzky and J. W. Allen, Phys. Rev. Lett. 55, 418, 1985. [4] P. Mandal and S. Das, Phys. Rev. B 56, 15073, (1997). [5] A. Maignan, C. Martin, F. Damay and B. Raveau, Chem. Mater. 10, 950, (1998). [6] B. Raveau , A. Maignan, C. Martin and M. Hervieu, Chem. Mater. 10, 2641 (1998). [7] Cz. Kapusta, P. C. Riedi, M. Sikora and M. R. Ibarra, Phys. Rev. Lett. 84, 4216, (2000). [8] Y. Wang and H. J. Fan, Phys. Rev. B 83, 224409, (2011). [9] S.K. Barik, C. Krishnamoorthi, R. Mahendiran, J. Magn. Magn.Mater. 323, 7, (2011). [10] B. Uthaman, K.S. Anand, R. K. Rajan, H. H. Kyaw, S. Thomas, S. Al-Harthi, K. G. Suresh and M. R. Varma, RSC Adv. 5, 105, (2015). [11] Petrov, A. N.; Zuev, A. Yu.; Tikhonova, I. L.; Voronin, V. I.; Solid State Ionics, 129,179, (2000).
12
[12] Taylor, X-ray Metallography, Wiley, New York , 1961. [13] W. Tong, B. Zhang, S. Tan, Y. Zhang, Phys. Rev. B 70, 1, (2004). [14] L. Ghivelder, I. Abrego Castillo, M. A. Gusmão, J. A. Alonso, and L. F. Cohen, Phys. Rev. B 60, 17 (1999). [15] A. H. Morrish, The Physical Principles of Magnetism. Wiley-IEEE Press, New York, 2010 [16] S. K. Banerjee, Phys. Rev. Lett. 12, 1, (1964). [17] Z.M. Wang, G. Ni, Q.Y. Xu, H. Sang, Y.W. Du, J. Appl. Phys. 90, 11, (2001). [18] M. Foldeaki, R. Chahine, T. K. Bose, J. Appl. Phys. 77, 7, (1995). [19] Shengman Liu, Physica B 456, 227, (2015). [20] M. Pękała, V. Drozd, J.F. Fagnard, Ph. Vanderbemden, J. alloys and compounds, 507, 2, (2010). [21] V. Franco, J. S. Blazquez and A. Conde, Appl. Phys. Lett. 89, 222512, (2006). [22] V. Franco, C. F. Conde, A. Conde and L. F. Kiss, App. Phys. Lett. 90, 052509, (2007). [23] H. Oesterreicher and F. T. Parkar, Journal of applied Physics 55, 4334, (1984). [24] S. N. Kaul, J. Magn. Magn.Mater. 53, 5, (1985). [25] K. Huang, Statistical Mechanics, Wiley, New York, 2nd edition, 1987. [26] D. D. Sarma, A. Chainani, S. R. Krishnakumar, E. Vescovo, C. Carbone, W. Eberhardt, O. Rader, C. Jung, C. Hellwig, W. Gudat, H. Srikanth, and A. K. Raychaudhuri, Phys. Rev. Lett. 80, 4004 (1998). [27] N. F. Mott, Taylor and Francis, Metal Insulator Transitions, London, (1990). [28] S. Ravi and M. Kar, Physica B 348, 169 (2004). [29] M. Viret, L.Ranno, and J. M. D. Cory, Phys. Rev. B 55, 8067, (1997).
13
Fig. 1 Powder x-ray diffraction patterns for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample, measured at room temperature. Asterisk (*) symbol used for unreacted Co.
14
Fig. 2 SEM image for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 compound at room temperature. Particle size distribution is shown in the inset.
15
Fig. 3 Temperature dependence of the field cooled magnetization, M(T), for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample under an applied field of 0.05 T. Top right inset shows the derivatives of the M(T) curve and left inset is the Curie – Weiss fit.
16
Fig. 4 Magnetization as a function of magnetic field for nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample measured at different temperatures. The inset magnifies the region near origin, making the clear view for coercivity.
17
Fig. 5 A series of magnetization isotherms for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample.
18
Fig. 6 Arrott plots (M2 versus H/M) constructed from the magnetization data for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample. Inset shows temperature dependence of A parameter.
19
Fig. 7 Temperature dependence of magnetic entropy change (|∆SM|) at different magnetic fields for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3.
20
Fig. 8 ln M vs. ln H plot for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample. Inset shows ln (|∆SM|) vs.ln H plot.
21
Fig. 9 Temperature dependent resistivity plot for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample at room temperature. Inset shows the enlarge data of the electrical resistance changes around 200 K.
22
Structural, magnetic and magnetocaloric properties of Co-doped nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 Meenakshi, Amit Kumar, Rabindra Nath Mahato*
TABLE I: Comparison of critical parameters for nanocrystalline La0.7Te0.3Mn0.9Co0.1O3 sample with the various models
La0.7Te0.3Mn0.7Co0.3O3
Tc (K)
n
β
γ
δ
Ref.
201
0.795
0.607
1.313
3.161
This
±0.001
±0.001
±0.001
±0.001
work
Mean field model
-
-
0.5
1
3
20
3D Heisenberg model
-
-
0.365
1.336
4.80
20
±0.003
±0.004
±0.04
0.325
1.241
4.82
±0.002
±0.002
±0.02
0.25
1
5
3D Ising model
Tricritical mean field
-
-
-
-
20
21
theory
23
TABLE II. The values of ρ0, T0, density of states at the Fermi level N(EF), the hopping distance Rh and the hopping energy Eh at room temperature for nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample. Parameters
Values
ρ0 (mΩ cm)
4.84 x 10-7
T0 (x 106 K)
323.26
N(EF) x 1018 eV-1 cm-3)
7.04
Rh (Å) at 300 K
54.4
Eh (meV) at 300 K
209.53
24
Highlights: •
Nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 has been synthesized by sol-gel method
•
The above nanocrystalline sample undergo paramagnetic –ferromagnetic phase transition
•
|∆SMmax| of ~1.002 Jkg-1K-1 was observed at 200 K under field change of 5 T
•
The sample shows semiconducting-like behavior and observed signature of electrical transition
25
S0304-8853(17)30499-7 http://dx.doi.org/10.1016/j.jmmm.2017.07.048 MAGMA 62979
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
10 February 2017 8 July 2017 12 July 2017
Please cite this article as: Meenakshi, A. Kumar, R.N. Mahato, Structural, magnetic and magnetocaloric properties of Co-doped nanocrystalline La0.7Te0.3Mn0.7Co0.3O3, Journal of Magnetism and Magnetic Materials (2017), doi: http://dx.doi.org/10.1016/j.jmmm.2017.07.048
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.
Structural, magnetic and magnetocaloric properties of Co-doped nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 Meenakshi, Amit Kumar and Rabindra Nath Mahato*
* Corresponding Author: School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India Email: [email protected]
Abstract Structural,
magnetic
and
magnetocaloric
properties
of
the
nanocrystalline
La0.7Te0.3Mn0.7Co0.3O3 perovskite manganite were investigated. X-ray diffraction pattern indicated that the nanocrystalline sample crystallized in orthorhombic crystal structure with Pbnm space group. The average particle size was calculated using scanning electron microscope and it was found to be ~ 150 nm. Temperature dependence magnetization measurements revealed ferromagnetic - paramagnetic phase transition and the Curie temperature (TC) was found to be ~ 201 K. Field dependence magnetization showed the hysteresis at low temperature with a coercive field of ~ 0.34 T and linear dependence at high temperature corresponds to paramagnetic region. Based on the magnetic field dependence magnetization data, the maximum entropy change and relative cooling power (RCP) were estimated and the values were 1.002 J kg-1 K-1 and 90 J kg-1 for a field change of 5 T respectively. Temperature dependent resistivity ρ(T) data exhibited semiconducting-like behavior at high temperature and the electrical transport was well explained by Mott’s variable-range hopping (VRH) conduction mechanism in the temperature range of 250 K -
1
300 K. Using the VRH fit, the calculated hoping distance (Rh) at 300 K was 54.4 Å and density of states N(EF) at room temperature was 7.04 x 10 18 eV-1cm-3. These values were comparable to other semiconducting oxides.
Keywords: Nanocrystalline; perovskite; manganites; X-ray diffraction; magnetic properties; magnetocaloric properties.
2
1. Introduction Currently, doped manganite perovskite with a general formula A1-xBxMnO3 (A= rare earth trivalent ions and B = divalent or tetravalent ions) have attracted considerable attention due to many intriguing features, such as colossal magnetoresistance, charge ordering, metal-insulator transition and magnetocaloric effect (MCE). A considerable amount of research has been carried out on electron doped compounds such as La1-xBxMnO3 where B = Te, Ce, Sb etc. [1, 2]. The parent compound LaMnO3 follows dual magnetic interactions depending on the coupling between trivalent manganese. It follows antiferromagnetic (AFM) coupling for inter-layer Mn3+ ions and ferromagnetic (FM) coupling for intra-layer Mn3+ ions [3, 4]. The doping on La-site by divalent or tetravalent ions on the parent compound drives Mn into mixed Mn2+, Mn3+ and Mn4+ states. The divalent ions such as Sr2+, Ba2+ etc. doped on La-site in the parent manganite system drives Mn into Mn3+ and Mn4+ states. However, the rare-earth tetravalent ions such as Te4+, Ce4+ converts manganese ion into Mn2+ and Mn3+ states [4]. Since the trivalent manganese ions in LaMnO3 changes to mixed Mn3+ and Mn2+ states in the electron doped La1-xBxMnO3 system, coexistence of FM and AFM magnetic phase separated ground states for light electron doping is observed. In case of heavy-electron doping, the AFM ground state is observed [5-8]. Electrondoped manganites have also gained attention for the magnetocaloric properties; such compounds show large magnetocaloric effect (MCE) under moderate applied filed which is very useful for magnetic refrigeration applications. Furthermore, the doping at Mn site is seen to be very effective for the La1-xBxMnO3 systems as all types of magnetic ground states are decided by Mn ion in manganites. Substitution of 3d cation on Mn site has been largely explored in such systems [9, 10]. The electrical transport properties of mixed valent manganites have shown interesting physical properties and can be manipulated the transport mechanism by partial doping
3
in the Mn-site. Thus, in order to induce particular properties in the perovskite LaMnO3, La3+ cation as well as Mn3+ must be partially doped by divalent or tetravalent cation. Most of the work in the doped La1-xBxMnO3 compound has been done in bulk form and very less work have been explored in nanocrystalline form. In the present work, we report the effect of Co substitution on structural, magnetic and magnecaloric properties of nanocrystalline La0.7Te0.3Mn0.7Co0.3O3. 2. Experimental details: The nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 compound was synthesized by sol-gel method [2]. Identification of the phase and crystal structure was determined by X-ray diffraction measured with Rigaku-made Miniflex600 powder diffractometer operated at 40 kV and 30 mA at room temperature for angles in the range 2θ=10° to 90° with a step size of 0.02°. The morphology, particle sizes and composition of the sample were examined by scanning electron microscopy (SEM, Carl Zeiss Evo-40) attached with the energy dispersive X-ray analysis (Bruker X-flash detector-4010). The magnetic measurements were performed as a function of temperature and magnetic field using vibrating sample magnetometer (PPMS, Cryogenic Ltd.) up to a maximum field of 5 T in the temperature range 2 K - 300 K. The magnetic entropy change |∆Smax M | was estimated from the magnetization data using a Maxwell relation. The electrical resistivity measurements of the prepared sample were done using standard four probe in the temperature range 150 K – 300 K. 3. Results and Discussion 3.1 Structural analysis The X-ray diffraction (XRD) pattern of the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample at room temperature is shown in Fig. 1. The XRD data shows that the nanocrystalline compound crystallizes in orthorhombic crystal structure with Pbnm space group [11]. The XRD pattern of 4
the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample is indexed using JCPDS. The lattice parameters are found to be a= 5.5322 Å, b= 5.4930 Å and c= 7.7788 Å [12]. The crystal symmetry and lattice parameters are similar to its bulk counterpart [10]. The average crystallite size (D) is estimated using Scherrer relation [12],
D =
λ β θ
(1)
where k is the grain shape factor, λ, θ and β are the X-ray wavelength, Bragg diffraction angle, and full width at half maxima (FWHM) of the diffraction peak respectively. The value of average crystallite size is found to be ~ 24 nm for La0.7Te0.3Mn0.7Co0.3O3 sample. The SEM images show that the particles are spherical in shape (shown in Fig. 2) and the particle size varies from tens of nanometers to few hundreds of nanometers. The average particle size is calculated using particle size distribution and is found to be ~ 150 nm (see inset of Fig. 2). The particles sizes observed by SEM are several times larger than those calculated by XRD, which shows that each particle observed by SEM consist of several crystallized grains. 3.2 Magnetic measurements The temperature dependence magnetization measurements M(T) are carried out under an applied magnetic field of 500 Oe. The field cooled (FC) and zero field cooled (ZFC) magnetization data (denoted as MFC and MZFC) shows ferromagnetic (FM) to paramagnetic (PM) phase transition and reaches a maximum value of magnetization (MFC) ~ 0.54 µ B.
Below TC, there is a
divergence in MFC and MZFC curves, which commonly seen in the perovskite manganite samples [13, 14]. The large bifurcation in MFC and MZFC curves originates from the anisotropic field generated from FM clusters. A cusp like peak observed in MZFC data and it could be random
5
alignment of the thermally activated magnetic moments. The Curie temperature (TC) is determined from the derivative of the MFC(T) curve (see inset of Fig. 3) and it is found to be ~ 201 K. The temperature variation of the inverse magnetic susceptibility in the paramagnetic region follows Curie-Weiss law [15],
χ=
(2)
where C is the Curie constant and θP is the paramagnetic curie temperature. The inverse susceptibility (χ-1) versus temperature plot is shown in the left inset of Fig. 3. The χ-1 vs. T data shows that the Curie-Weiss law obeys in the temperature range of 210 K - 300 K and the paramagnetic Curie temperature is found to be ~ 205 K. The positive value of θP indicates that the FM exchange interaction exists between the nearest neighbours and the obtained θP value. The experimentally observed value of effective magnetic moment (μ = 2.83√C) is 4.13 μ /f.u. and the theoretically calculated spin – only moment (μ = g S(S + 1)) value is 4.10 μ /f.u. considering Co3+ in its low state and Mn3+ in its high spin state. Both theoretical and experimentally observed μ value are comparable to each other. The effective paramagnetic moment calculated from the Curie-Weiss law approximately corresponds to the trivalent spin state configurations of cobalt. Field dependence magnetization measurements at different temperatures are shown in Fig. 4. The field dependence magnetization data shows the ferromagnetic loop below TC with a coercive field of 0.34 T at 2 K and the linear M(H) corresponds to paramagnetic region. Fig. 5 shows the magnetic isotherms for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 compound measured near magnetic ordering temperature. It is clearly seen that the magnetization increases
6
sharply at lower applied magnetic fields and it does not saturate up to 5 T. In equilibrium condition, the magnetic equation of state can be expressed as,
H = A + BM ' (3) M
where A and B are the temperature dependent parameters. The Arrott plots (M2 versus H/M) are used to identify the nature of the ferromagnetic phase transition. According to Banerjee’s criterion [16], the Arrott plots with positive slopes correspond to a second order magnetic phase transition and a negative slope indicates first – order magnetic transition. Fig. 6 shows the Arrott plots for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample and are found to be positive slopes, confirming the second order magnetic phase transition. The linear regions (at high field) of Arrot plots are numerically fitted to Eq. (3) to derive the temperature dependence parameters. The obtained A parameter has been plotted as a function of temperature as shown in the inset of Fig. 6. The Curie temperature of La0.7Te0.3Mn0.7Coe0.3O3 nanocrystalline sample is determined by the sign change of parameter A and it is found to be 200 K which is similar to the TC value determined from the low field magnetization measurements. 3.3 Magnetocaloric effect According to a thermodynamic Maxwell’s relationship: (∂S/∂H)T = (∂M/∂T)H, the magnetic entropy change |∆SM| produced by the variation of a magnetic field from 0 to H is given by [17, 18], 2 /(
|∆S( (T, H)| = S( (T, H) − S( (T, 0) = -3 . / 0 dH
(4)
7
The magnetic entropy change (|∆SM|) as a function of temperature is shown in
Fig. 7. The
456 maximum entropy change |∆S( | is found to be 1.002 Jkg-1K-1 for a magnetic field change of 5
T for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample. Relative cooling power (RCP) is an important parameter to evaluate the cooling efficiency of a magnetic refrigerant. RCP corresponds to the amount of heat that can be exchanged between the cold and hot reservoirs of the refrigerator in an ideal thermodynamic cycle. In other words, it reflects the cooling efficiency of the refrigerator. Relative cooling power (RCP) is calculated using the relation, 456 | RCP = |∆S( × δT:;2(
(5)
456 | where |∆S( is the maximum entropy change near TC and δTFWHM is the full-width at half
maxima. The calculated RCP value for La0.7Te0.3Mn0.7Co0.3O3 is 90 J/kg for a magnetic field 456 | change of 5 T. The observed |∆S( value for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 is
less than that of its bulk counterpart but the larger operating temperature window (i.e. δTFWHM = 89 K) than bulk (δTFWHM = 48 K) [10] sample shows the importance of nanocrystalline behavior. 456 | The moderate |∆S( value and wide operating temperature range compared to other
perovskite-type manganites [19, 20], making this nanocrystalline compound a promising candidate for magnetic refrigeration applications in the sub room temperature. 3.4 Critical exponent analysis In order to understand the correlation between critical exponents, magnetocaloric effect and nature of magnetic phase transitions, a detailed critical exponent analysis has been carried out. According to scaling hypothesis, the magnetic systems following second order phase transitions
8
are governed by a set of interrelated critical exponents β, γ and δ. These critical exponents are mathematical expressed as [21, 22], M< (T) = M3 (−t)> t < 0, T < T (6) χ3A (T) =
h3 (t)C t > 0, T > T (7) M3
F
M = DH G t = 0, T = T (8) where M0, h0/M0, and D are the critical amplitudes and t=(1-T/Tc) is the reduced temperature. The exponent β is associated with the spontaneous magnetization (MS), γ is related to the initial magnetic susceptibility (χ0), and δ is associated with the critical magnetization isotherm. The value of δ is determined from the slope of ln M versus ln H plot at T= TC which is shown in Fig. 8. The obtained value of δ is ~3.16. According to Oesterreicher et. al. [23], the field dependence of the magnetic entropy change for the materials following second order phase transitions can also be expressed as, ∆HI = JKL (9) where J is constant and the exponent n depends on the magnetic state of the sample. The value of n is calculated from the linear plot of ln(∆SM) versus ln(H), shown in inset of Fig. 8. The critical exponents β and γ are determined using the following expressions, A
A
n = 1 + O .1 − >0 n = 1 +
(10)
β−1 (11) β+γ
9
The obtained values of critical exponents of n, δ, β and γ are tabulated in Table II. The deduced values of the critical exponents are close to that of the mean free model [24, 25], though the value of γ is slightly higher than that predicted by the mean field model indicating long range magnetic interactions are present in La0.7Te0.3Mn0.7Co0.3O3 sample. 3.5 Electrical transport measurements Temperature dependent electrical resistivity data for nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample is shown in Fig. 9. The resistivity data ρ(T) shows the abrupt change in resistivity at ~ 200 K and these could be the signature of electrical transition which is also reflected in the M(T) data. To explain the electrical resistivity data, we have fitted the temperature dependence resistivity data. According to Mott’s VRH model, electrical resistivity in three-dimensional hopping is given by [27],
A/V
ρ(T) = ρ3 exp . T 0
(12)
where T0 is the Mott’s characteristic temperature, which can also be expressed in terms of density of states (N(EF)) in the vicinity of Fermi energy and the localization length “a” as follows [28]: T3 =
AW
\ X Y(Z[ )5
(13)
The mean hopping distance Rh(T) and hopping energy Eh(T) at a given temperature can be written as,
10
^
A/V
R ] (T) = W a . T 0 A
A/V
E] (T) = V k T ^/V T3
(14)
(15)
The Mott’s VRH data fit well with the VRH model (not shown) for the temperature range 250 K – 300 K, suggesting the electrical transport is dominated by Variable range hopping. From the intercept and slope of the fitted curve, we have calculated ρ0, T0 using equation (12) and are found to be 4.84 x 10-7 mΩ cm and 323.26 x 106 K respectively (shown in Table II). Using value of T0 in equations (13-15), we have calculated the values of N(EF), Rh(T) and Eh(T) at room temperature by considering the localization length a=4.5 Å and the values are shown in Table II. The value of N(EF) is nearly the same order of magnitude as in other known semiconducting oxides [N(E)~1017-1019 eV-1] [29]. 4. Conclusions The structural, magnetic and magnetocaloric properties of Co-doped nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample were investigated. The sample shows orthorhombic crystal structure at room temperature with Pbnm space group. The average crystallite size has been found to be ~ 24 nm. The sample undergoes paramagnetic to ferromagnetic transition and the Curie temperature is found to be ~ 201 K. A substantial magnetic entropy change and relative cooling power is obtained and the values are 1.002 Jkg-1K-1and 90 Jkg-1 for a magnetic field change of 5 T respectively. The electrical resistivity data shows the signature of electrical transition at 200 K and Mott’s variable-range hoping conduction mechanism is dominated in the temperature range of 250 K - 300 K. The hoping distance (Rh) and density of states (N(EF)) at
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300 K are 54.4 Å and 7.04 x 1018 eV-1cm-3 respectively for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample.
Acknowledgement: RNM is thankful to University with Potential for Excellence –II at JNU (UPE II) grants and University Grant Commission for UGC-BSR start up grants respectively. The financial support from the project DST Purse is also acknowledgeable. Meenakshi and AK are thankful to Ms. Ruchita Pal for SEM measurements and AIRF, JNU for magnetic measurements.
References: [1] P. Raychaudhuri , C. Mitra, P. D. A. Mann, J. Appl. Phys. 93, 10, (2003) [2] R. N. Mahato, K. Sethupathi, V. Sankaranarayanan, R. Nirmala, J. Magn. Magn.Mater. 322, 17 (2010) [3] J. Zaanen, G. A. Sawatzky and J. W. Allen, Phys. Rev. Lett. 55, 418, 1985. [4] P. Mandal and S. Das, Phys. Rev. B 56, 15073, (1997). [5] A. Maignan, C. Martin, F. Damay and B. Raveau, Chem. Mater. 10, 950, (1998). [6] B. Raveau , A. Maignan, C. Martin and M. Hervieu, Chem. Mater. 10, 2641 (1998). [7] Cz. Kapusta, P. C. Riedi, M. Sikora and M. R. Ibarra, Phys. Rev. Lett. 84, 4216, (2000). [8] Y. Wang and H. J. Fan, Phys. Rev. B 83, 224409, (2011). [9] S.K. Barik, C. Krishnamoorthi, R. Mahendiran, J. Magn. Magn.Mater. 323, 7, (2011). [10] B. Uthaman, K.S. Anand, R. K. Rajan, H. H. Kyaw, S. Thomas, S. Al-Harthi, K. G. Suresh and M. R. Varma, RSC Adv. 5, 105, (2015). [11] Petrov, A. N.; Zuev, A. Yu.; Tikhonova, I. L.; Voronin, V. I.; Solid State Ionics, 129,179, (2000).
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[12] Taylor, X-ray Metallography, Wiley, New York , 1961. [13] W. Tong, B. Zhang, S. Tan, Y. Zhang, Phys. Rev. B 70, 1, (2004). [14] L. Ghivelder, I. Abrego Castillo, M. A. Gusmão, J. A. Alonso, and L. F. Cohen, Phys. Rev. B 60, 17 (1999). [15] A. H. Morrish, The Physical Principles of Magnetism. Wiley-IEEE Press, New York, 2010 [16] S. K. Banerjee, Phys. Rev. Lett. 12, 1, (1964). [17] Z.M. Wang, G. Ni, Q.Y. Xu, H. Sang, Y.W. Du, J. Appl. Phys. 90, 11, (2001). [18] M. Foldeaki, R. Chahine, T. K. Bose, J. Appl. Phys. 77, 7, (1995). [19] Shengman Liu, Physica B 456, 227, (2015). [20] M. Pękała, V. Drozd, J.F. Fagnard, Ph. Vanderbemden, J. alloys and compounds, 507, 2, (2010). [21] V. Franco, J. S. Blazquez and A. Conde, Appl. Phys. Lett. 89, 222512, (2006). [22] V. Franco, C. F. Conde, A. Conde and L. F. Kiss, App. Phys. Lett. 90, 052509, (2007). [23] H. Oesterreicher and F. T. Parkar, Journal of applied Physics 55, 4334, (1984). [24] S. N. Kaul, J. Magn. Magn.Mater. 53, 5, (1985). [25] K. Huang, Statistical Mechanics, Wiley, New York, 2nd edition, 1987. [26] D. D. Sarma, A. Chainani, S. R. Krishnakumar, E. Vescovo, C. Carbone, W. Eberhardt, O. Rader, C. Jung, C. Hellwig, W. Gudat, H. Srikanth, and A. K. Raychaudhuri, Phys. Rev. Lett. 80, 4004 (1998). [27] N. F. Mott, Taylor and Francis, Metal Insulator Transitions, London, (1990). [28] S. Ravi and M. Kar, Physica B 348, 169 (2004). [29] M. Viret, L.Ranno, and J. M. D. Cory, Phys. Rev. B 55, 8067, (1997).
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Fig. 1 Powder x-ray diffraction patterns for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample, measured at room temperature. Asterisk (*) symbol used for unreacted Co.
14
Fig. 2 SEM image for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 compound at room temperature. Particle size distribution is shown in the inset.
15
Fig. 3 Temperature dependence of the field cooled magnetization, M(T), for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample under an applied field of 0.05 T. Top right inset shows the derivatives of the M(T) curve and left inset is the Curie – Weiss fit.
16
Fig. 4 Magnetization as a function of magnetic field for nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample measured at different temperatures. The inset magnifies the region near origin, making the clear view for coercivity.
17
Fig. 5 A series of magnetization isotherms for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample.
18
Fig. 6 Arrott plots (M2 versus H/M) constructed from the magnetization data for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample. Inset shows temperature dependence of A parameter.
19
Fig. 7 Temperature dependence of magnetic entropy change (|∆SM|) at different magnetic fields for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3.
20
Fig. 8 ln M vs. ln H plot for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample. Inset shows ln (|∆SM|) vs.ln H plot.
21
Fig. 9 Temperature dependent resistivity plot for the nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample at room temperature. Inset shows the enlarge data of the electrical resistance changes around 200 K.
22
Structural, magnetic and magnetocaloric properties of Co-doped nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 Meenakshi, Amit Kumar, Rabindra Nath Mahato*
TABLE I: Comparison of critical parameters for nanocrystalline La0.7Te0.3Mn0.9Co0.1O3 sample with the various models
La0.7Te0.3Mn0.7Co0.3O3
Tc (K)
n
β
γ
δ
Ref.
201
0.795
0.607
1.313
3.161
This
±0.001
±0.001
±0.001
±0.001
work
Mean field model
-
-
0.5
1
3
20
3D Heisenberg model
-
-
0.365
1.336
4.80
20
±0.003
±0.004
±0.04
0.325
1.241
4.82
±0.002
±0.002
±0.02
0.25
1
5
3D Ising model
Tricritical mean field
-
-
-
-
20
21
theory
23
TABLE II. The values of ρ0, T0, density of states at the Fermi level N(EF), the hopping distance Rh and the hopping energy Eh at room temperature for nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 sample. Parameters
Values
ρ0 (mΩ cm)
4.84 x 10-7
T0 (x 106 K)
323.26
N(EF) x 1018 eV-1 cm-3)
7.04
Rh (Å) at 300 K
54.4
Eh (meV) at 300 K
209.53
24
Highlights: •
Nanocrystalline La0.7Te0.3Mn0.7Co0.3O3 has been synthesized by sol-gel method
•
The above nanocrystalline sample undergo paramagnetic –ferromagnetic phase transition
•
|∆SMmax| of ~1.002 Jkg-1K-1 was observed at 200 K under field change of 5 T
•
The sample shows semiconducting-like behavior and observed signature of electrical transition
25