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Magnetic irreversibility and spin dynamics in nanoparticles of iron-doped europium chromite
Accepted Manuscript Magnetic irreversibility and spin dynamics in nanoparticles of iron-doped europium chromite D.R. Ratkovski, J.M.Marín Ramírez, P.R.T. Ribeiro, H.V.S. Pessoni, A. Franco, Jr., F.L.A. Machado PII:
S0925-8388(17)32373-3
DOI:
10.1016/j.jallcom.2017.07.018
Reference:
JALCOM 42434
To appear in:
Journal of Alloys and Compounds
Received Date: 24 May 2017 Revised Date:
1 July 2017
Accepted Date: 3 July 2017
Please cite this article as: D.R. Ratkovski, J.M.Marí. Ramírez, P.R.T. Ribeiro, H.V.S. Pessoni, A. Franco Jr., F.L.A. Machado, Magnetic irreversibility and spin dynamics in nanoparticles of iron-doped europium chromite, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.018. 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.
ACCEPTED MANUSCRIPT
Magnetic irreversibility and spin dynamics in nanoparticles of iron-doped europium chromite D. R. Ratkovskia , J. M. Mar´ın Ram´ıreza , P. R. T. Ribeiroa , H. V. S. Pessonib , A. Franco Jrb , F. L. A. Machadoa,∗ a Departamento
de F´ısica, Universidade Federal de Pernambuco, 50670-901, Recife, Pernambuco, Brazil. de F´ısica, Universidade Federal de Goi´ as, C.P. 131, 74001-970 Goiˆ ania, Goi´ as, Brazil.
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b Instituto
Abstract
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The magnetic properties of nanoparticles of Eu1−x Fex CrO3 (0 ≤ x ≤ 1) prepared by a combustion reaction technique were investigated. An irreversible behavior in the magnetization was observed for a sample with x = 0.10 below the N´eel temperature (TN ) yielding a complex phase diagram. The irreversibility data was fitted to a de Almeida-Thouless line by using φ = 3.0 for the critical exponent and a glassy temperature TG of 174.4 K for H ≤ 40 kOe. The dynamics of the spins was investigated by measuring the ac magnetic susceptibility (χac ) near TN for frequencies (f ) in the range 10 − 104 Hz. The maximum in χac was found to shift to higher values of T for increasing values of f , yielding ' 0.003 for the shift per decade of f parameter. The Voguel-Fulcher law and a power-law were used for analyzing χac yielding τ0 = 1.8 × 10−9 s (= 1.6 × 10−15 s) for the characteristic relaxation time, Ea /kB = 39.23 K for the activation energy and TG = 165.9 K (= 167.5 K) for the Voguel-Fulcher (power-law) model. Moreover, the power-law yielded zν = 5.62 for the product of the dynamical critical exponent (ν) with the one associated to the correlation length (z). The dependence of TN with the bonding angles Cr3+ −O2− −Cr3+ was investigated for as-prepared samples yielding a trend contrary to the expected. The overall results were accounted for by taking into consideration the microstrain introduced by the sample preparation technique and by the ionic dopping.
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1. Introduction
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The rare-earth orthochromites [RE]CrO3 are orthorhombic distorted perovskites that have been attracting considerable attention because of their potential in application such as multifunctional materials [1, 2]. These systems are isostructural with the rare-earth orthoferrites [RE]FeO3 and they show physical and chemical properties that are strongly influenced by the rare-earth ion lying on the A-site of a perovskite structure [3, 4, 5]. [RE]CrO3 presents a rich variety of magnetic spin interactions, namely Cr3+ − Cr3+ , Cr3+ − [RE]3+ and [RE]3+ − [RE]3+ that are highly temperature dependent [2, 6]. The easy-axis for the Cr3+ magnetic moments is along the c-axis direction of an orthorhombic cell while the magnetic structure is represented by the Γ4 (Gx Ay Fz ) configuration, with Gx , Ay , Fz being the components of the Cr3+ spins along the crystallographic directions a, b and c, respectively [5, 7, 8]. Magnetic data has indicated that the weak ferromagnetic moment of the Cr3+ spins below the N´eel temperatures (TN ) arises from a Dzialoshinski-Moriya (DM) type antisymmetric exchange interaction [1, 9]. Furthermore, the N´eel temperature was found to be mainly determined by the Cr3+ − Cr3+ antiferromagnetic coupling, increasing monotonically with the size of the ionic ∗ Corresponding
author: F. L. A. Machado Email address: [email protected] (F. L. A. Machado)
Preprint submitted to Journal of Alloys and Compounds
radius in the lanthanide series. This behavior has been associated to the decreasing in the lattice distortions and to an increasing in the Cr3+ −O2− −Cr3+ distance which in turn varies the inter-cationic super-exchange interaction [6]. More recently, exchange-bias and spin-glass-like properties have also been reported in pure and in Ca-doped europium chromites [10, 11, 12].
Among the rare-earth orthochromites, the europium orthochromite (EuCrO3 ) exhibits a weak spontaneous ferromagnetic moment below the N´eel temperature that has been attributed to the ordering of the localized Cr3+ magnetic moments in a canted-antiferromagnetic (CAFM) phase [3, 7, 8]. It is also known that a net exchange interaction is produced in optically excited Eu3+ ions yielding a long-lived magnetically ordered state [13]. Thus EuCrO3 have also been considered as an active medium for highdensity optical storage and optical processing devices. Despite of its potential for applications, the magnetic properties of EuCrO3 has not been fully investigated yet. For instante, it is believed that TN is influenced by the bonding angle Cr3+ −O2− −Cr3+ (θB ) but up to now there is no detailed investigation showing that this occurs indeed. Moreover, the nature of the spin-glass-like phase in these materials is not fully understood yet. In this work, the magnetic properties of nanopowders of pure and Fe-doped europium chromite (Eu1−x Fex CrO3 ) samples prepared by a combustion reaction technique [14] are investigated. It is expected, for instance, that the difJune 30, 2017
ference in the ionic radii of the Fe3+ and Eu3+ ions in the A-site of the perovskite structure would modify the bonding angle θB which, in turn, would modify the N´eel transition temperature. Thus, TN was measured for a set of Fe-doped samples and their correlation with the θB determined. Moreover, a de Almeida-Thouless line [15] (AT line) was fit to the magnetic field dependence of the crossing over transition temperature from the CAFM to a spin-glass like phase for x = 0.10. The glassy temperature (TG ) and the critical exponente φ were determined from the A-T line. The dynamics of the magnetic moment was studied by means of ac magnetic susceptibility (χac ) measurements. The χac data were analyzed by considering using the Voguel-Fulcher law [16] and the scaling power-law function [17] yielding a value for the product of the correlation length critical exponent (z) with the dynamical critical exponent (ν) zν close to the one obtained for an undoped sample of europium chromite. We noticed that some structural and magnetic properties of iron-doped europium chromite prepared by different synthesis routes were reported in the past [18, 19, 20]. However, the goals and techniques employed in those works are different from the ones of the present work.
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EuCrO3
Fe3O4
x = 1.0
x = 0.6 x = 0.5 x = 0.4
33
(021)
(200)
(020)
32
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x = 0.2 x=0
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Intensity (a.u.)
x = 0.8
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x=0.10 x=0.08
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Intensity (a.u.)
x=0.15
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| EuCrO3
x=0.20
Transmittance (a.u.)
(b) x=0.20
x=0.15
x=0.05
x=0
x=0
2. Sample preparation and techniques
32.0 32.4 32.8 33.2 33.6 34.0
Nanoparticles of Fe-doped europium orthochromites (Eu1−x Fex CrO3 ) were synthesized by using a combustion reaction method that is based on thermochemical concepts of propellant chemistry [21]. Details of the sample preparation process can be found elsewhere [14]. The samples were analyzed by X-ray diffraction (XRD) using the CuKα radiation (λ = 1.5418 ˚ A) in a Shimadzu XRD-7000 diffractometer. The XRD spectra were refined by employing the Rietveld method and by using the MAUD software. Pellets of as-prepared powders mixed with KBr were characterized by Fourier transform infrared spectroscopy (FT-IR) in the range 400-4000 cm−1 using a Shimadzu IRTracer-100. The magnetization and ac magnetic susceptibility (χac ) measurements were performed by using the AC Measurement System modulus (ACMS) of a PPMS (Physical Property Measurement System made by Quantum Design) for applied magnetic fields (H) and temperatures (T ) in the ranges ± 85 kOe and 5 ≤ T ≤ 300 K, respectively. The magnetization was measured in the warming up mode and for a fix value of H following two procedures: (1) the sample was first cooled under no applied magnetic field (zero-field-cooled or ZFC) before applying H and (2) the sample was first cooled with the H already applied (field-cooled or FC). χac was measured for frequencies in the range 10 ≤ f ≤ 104 Hz while the amplitude of the ac-magnetic field of kept constant (= 10 Oe) throughout the measurements. Finally, room temperature hysteresis loops were also obtained by using a VSM magnetometer for −20 ≤ H ≤ 20 kOe. The measurements were performed in as-prepared (AP) powder samples and in samples thermal annealed (TT)
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Figure 1: (Color online) (a) X-ray diffraction pattern for Eu1−x Fex CrO3 for 0 ≤ x ≤ 1. A peak clearly seen near 36◦ for x ≥ 0.4 was identified to be due to magnetite present in highly doped samples. The diffraction peak near 33◦ is plotted in a higher angle resolution for x ≤ 0.2 in (b). This peak was found to shift monotonically to higher values of diffraction angles for increasing values of x. (c) FTIR absorption band spectra assigned to the stretching mode of Eu−O for x ≤ 0.2 showing that the maximum in the absorption band shifts to low values of energy for increasing values of x.
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at 1073 K according the following protocole: the samples were first warmed up to 1073 K at a rate of 10 K/min; remained at this temperature for 24 h, it was then cooled down to room temperature under the same temperature rate (10 K/min). This procedure was particularly important for the sample with x = 0.1 for determining the influence of microstrains introduced along the preparation of samples in the spin-glass-like properties observed in this sample composition. The average particle size (d) varied little with thermal treatment. For instance, for the sample with x = 0.1 which was investigated in detail d was found to increase from 39.11 nm to 40.29 nm only. 3. Experimental results and discussions Fig. 1 (a) shows the XRD spectra for a narrow range of diffraction angles (31 ≤ 2θ ≤ 37 degrees) for x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0. The peaks were properly identified to the europium chromite crystalline structure 2
ACCEPTED MANUSCRIPT Table 1: Structural data obtained by using the XRD data for x ≤ 0.20
Density ρ (g/cm3 ) 7.4058 7.3787 7.3110 7.2304 7.1784 7.0281 6.8981
4 x=0 x=0.2 x=0.4 x=0.5 x=0.6 x=0.8 x=1.0
3 2 1
Reliability χ2 1.422 1.089 1.119 1.089 1.076 1.096 1.071
0 -1
T = 300 K
M (emu/g)
0.8
-2
-20
0.4 0.0
-0.4
-3 -4
-0.8 -100
-10
0
M(Hmax) (emu/g)
150
Hc (Oe)
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H (Oe)
50
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Eu(1-x)FexCrO3
0.8
T = 300 K 2
0.6
1 0
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1.0
0.4
x
100
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50 0
0.0 0.0
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H (kOe)
4
250
Crystallite size d (nm) 28.70 40.99 39.60 39.86 39.11 36.11 38.23
(a)
Eu(1-x)FexCrO3
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M (emu/g)
θB (degree) 156.30 149.74 152.88 147.43 146.45 147.91 145.77
Mr (emu/g)
[22, 12]. However, it was also observed a peak indexed to magnetite (Fe3 O4 ) crystalline structure near 36◦ for x ≥ 0.4. Thus, to avoid the influence of the magnetite we focused the main studies in the samples with x ≤ 0.2. For this Fe concentration range the corresponding XRD spectra are shown in Fig. 1 (b) for a narrower diffraction angle range. Notice that the diffraction peak (112) near 33 ◦ shifts monotonically to higher diffraction angles for increasing values of x. This is a clear indication that Fe3+ ions are indeed being incorporated into the orthochromite structure. The Rietveld analysis was employed for analyzing the XRD spectra indicating that the samples are orthochromites and that they belong to the P nma structure (space group 62). The corresponding lattice parameters (a, b and c), the bonding angle θB and the average particle size (d) determined by using the Scherrer equation are summarized in Table 1 for x ≤ 0.2. It is interesting to notice that the size of the lattice parameters diminishes with increasing values of x. Even so, the overall mass density (ρ) diminishes because the atomic mass for Fe (= 55.85 u) is only about 1/3 the one for Eu (= 151.96 u). The mismatch in the ionic atomic radii (Eu3+ = 120.6 pm and Fe3+ = 92 pm) was found to produce additional micro-strains () that was determined to increase monotonically with x. The values of are listed in Table 1 as well. Fig. 1 (c) shows Fourier transform infrared spectroscopy (FTIR) data for x ≤ 0.2 measured at room temperature in the wavenumber range 400-500 cm−1 . The minima observed in the transmission data correspond to absorption band associated to the stretching mode of Eu−O [6, 14, 23, 24]. It was found that the addition of iron shifts in the band towards lower values of energy. The shiftings are in agreement with what it is expect from the variation in the mass density ρ listed in Table 1: the sample become less dense for increasing values x diminishing the effective elastic constant and the corresponding energy of the Eu-O vibrational mode [25]. This result is also a clear indication that Fe3+ ions are indeed being incorporated into the chromite structure. FTIR spectra for a broader wavenumber range were reported elsewhere [14]. Fig. 2 (a) shows hysteresis loops measured at 300 K for samples in the full iron concentration range (0 ≤ x ≤ 1.0). Europium chromite is known to be paramagnetic at room temperature (RT) and to exhibit a CAFM ordering at
Strain ε 0.00158 0.00195 0.00189 0.00235 0.00223 0.00243 0.00295
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EuCrO3 Eu0.98 Fe0.02 CrO3 Eu0.95 Fe0.05 CrO3 Eu0.92 Fe0.08 CrO3 Eu0.90 Fe0.10 CrO3 Eu0.85 Fe0.15 CrO3 Eu0.80 Fe0.20 CrO3
Cell parameters a (nm) b (nm) c (nm) 0.5529(2) 0.7643(3) 0.5348(2) 0.5520(7) 0.7635(1) 0.5340(6) 0.5515(1) 0.7629(2) 0.5337(1) 0.5514(7) 0.7627(1) 0.5336(7) 0.5511(6) 0.7627(1) 0.5335(7) 0.5512(1) 0.7629(2) 0.5338(1) 0.5507(9) 0.7628(2) 0.5335(9)
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Sample
0.2
0.4
0.6
0.8
1.0
x Figure 2: (Color online) (a) Room temperature hysteresis loops for Eu1−x Fex CrO3 for 0 ≤ x ≤ 1.0. The lower right inset is a blowup of the data for small values of H. For x ≥ 0.4 it was found non-vanishing values for Mr and Hc . A plot of Mr and Hc as a function of x is shown in (b). Notice that both Mr and Hc vanishes below about x = 0.2. The upper right inset in (b) is a plot of the magnetization measured at H = 20 kOe as a function of x. The solid, dotted and dashed lines are guide to the eyes.
much lower temperatures. However, it was observed nonvanishing remanent magnetizations (Mr ) and coercivities (Hc ) at 300 K for samples with x > 0.4. This parameters are typical of ferromagnets but in the present case it is due to the presence of magnetite, a ferrimagnetic material with a critical temperature of about ' 850 K as it was 3
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identified by XRD. The limit of solubility of iron in the europium chromite is better seen in Fig. 2 (b) were Mr and Hc are plotted as a function of x. Above x = 0.20 a non-vanishing coercivity is observed at room temperature. The inset in Fig. 2(b) shows that the slope of the magnetization data measured at the highest applied magnetic field M (Hmax ) increases monotonically with the iron concentration for x ≤ 0.4 and the slope changes for higher concentrations due to the presence of magnetite. The hysteresis loop shown in Fig. 3 was measured for a sample with x = 0.10 at T = 5.0 K. The magnetization was found not saturate up to the highest applied magnetic field (H = 85 kOe) as expected for a CAFM phase while it is paramagnetic at 300 K as shown in the upper left inset. The lower-right inset of Fig. 3 is a blow-up of hysteresis loops for small values of H for T = 5 K and for T = 300 K. It is important to notice the large values Hc (= 2.20 kOe) for Mr (= 0.22 emu/g) obtained for T = 5 K. Fig. 4 (a) shows ZFC (open symbols) and FC (closed symbols) data for the x = 0.10 for an as-prepared sample in the temperature range 5 − 300 K and for H = 1, 5, 10, 20, 30, 50 and 85 kOe. The main figure actually shows the ZFC and FC magnetizations minus the corresponding paramagnetic contributions measured at 300 K shown in the inset. One may notice that the temperature where the irreversible behavior (Tirr ) vanishes in the T -dependence of the ZFC-FC magnetizations varies with H. Such a behavior, for instance, is expected in spinglass-like systems. For spin-glasses, it is found that the Hdependence of Tirr can be accounted for by a de AlmeidaThouless line[15]: H = H0 [1 − Tirr (H)/TG ]φ/2 , where φ is a critical exponent and TG is the freezing temperature for H = 0. The A-T line was found to fit the data for H ≤ 40 kOe and by using the mean-field critical exponent[15, 26]
0.5
0.0
0
Figure 3: (Color online) Hysteresis loop mesured at T = 5 K for x = 0.10 and for H varying in the range ± 85 kOe. The lower right-side inset is a blow-up of the hysteresis loop for low values of H measured at 5 K and for T = 300 K. It is clearly seen that the sample is paramagnetic at 300 K and that large values for both Mr and Hc are obtained at 5 K. The upper left-side inset is a plot of the hysteresis loop for T = 300 K. The solid lines are guide to the eyes.
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2.0
M(300 K) (emu/g)
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M (emu/g)
M (emu/g)
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M (emu/g)
4
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T = 300 K
M(300 K) (emu/g)
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80
100
120
140
160
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Tirr (K)
Figure 4: (Color online) The ZFC-FC magnetization data measured for as-prepared (AP) and treated thermally (TT) samples with x = 0.10 and for different values of H are shown in (a) and (b), respectively. The corresponding paramagnetic contributions at 300 K were actually subtract from the ZFC and FC data and they are showed in the insets. Notice that the irreversibility temperature (Tirr ) shifts to low values for increasing values of H. The H vs. Tirr data for the AP (circles) and for the TT (pentagons) samples obtained from the ZFC-FC magnetization data are shown in (c). The solid lines are fit to A-T lines while the inset shows the M × T data for H = 100 Oe used for obtaining TN for the TT-sample (broken line).
φ = 3.0 yielding H0 = 177.6 kOe and TG = 174.4 K. For higher values of H the A-T line was found to deviate from the data. The influence of the microstrains introduced along the sample preparation procedures was investigated by studying a sample annealed as described in preceding section. The corresponding data and AT-line for this sample is shown in Fig. 4 (b) and (c). By using φ = 3.0 and for the same H interval used for the AP-sample the fitting yielded H0 = 2202 kOe and TG = 174.4 K. The fitting parameters for the AP-sample are compiled together with the values obtained for a pure europium chromite sample [11] in Table 2. In Fig. 4(c) the A-T lines are represented by full lines while the data for the AP and TT samples are represented by circles and pentagons , respectively. Below about 60 K the phase diagram become even 4
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(a)
bonding angle. For the annealed samples, on the other hand, the trend goes in the other way indicating that the microstrain created along the synthesis process modifies the inter-cationic super-exchange interaction altering the values of TN of the as-prepared samples [2, 6]. Plots for the x-dependence of the microstrains present in the asprepared and in the annealed samples are shown in Fig 5(c) and the data for the as-prepared samples are also listed in Table 1. The microstrain was found to be higher and to increase with increasing values of x for the as-prepared sample while it is greatly reduced and nearly constant in the annealed samples. The residual microstrain in the TTsample is more likely to be due to the Fe-doping while most of the excess of the microstrain in the AP-sample was introduced in the combustion reaction method. It is important to recall that TN depends on parameters sensitive to microstrains such as the exchange-interaction. So, by annealing the samples most of the microstrains were removed yielding the expected behavior for TN , e.g., TN decreased by decreasing the bonding angle.
(b)
156
156
180
180 154
154
TN (K)
TN (K)
152
176
178 152
176
150 174
B (degree)
B (degree)
178
150 174
148 172
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148 172
146 0.05
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TT
AP 170 0.00
170 0.00
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Figure 5: (Color online) TN and the Cr3+ − O2− − Cr3+ bonding angle (θB ) are ploted as a funciton of x for the iron concentration range 0 ≤ x ≤ 0.2 for both as prepared (a) and annealed (b) samples. In (c) it is shown the microstrain data determined for both set of samples. The inset in (a) is a representative drawing of the bonding angle while the solid lines are guide to the eyes only.
1.0
1.50
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162 164 166 168 170 172 174
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175
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T (K)
fit power law
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0 1.6x10
-9
0 = 1.8x10
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EA/kB = 39.23 K TG = 165.87 K
0.01
(s)
AC C
175
fit Vogel-Fulcher
0.1
s
0.01
1E-3
The bonding angle Cr3+ − O2− − Cr3+ is believed to have a direct influence in the value of the N´eel temperature in rare earth chromites [2, 27, 28]. For instance, TN is expected to increase for increasing values of the bonding angle. In order to verify how strongly θB influences TN this bonding angle was measured for samples with iron concentration in the range 0 ≤ x ≤ 0.2. As shown in Fig. 5 (a) and (b) the bonding angle decreases monotonically for increasing values of the Fe3+ ion concentration for both as-prepared and annealed samples. However, in contrast to what it was expected for the as-prepared samples, TN was found to increase with increasing values of x or, equivalently, to increase with decreasing values of the
170
T (K)
z5.62 TG = 167.5 K
more complex because the influence of the Cr3+ − Eu3+ interactions while below about 10 K the magnetic properties are also influenced by the Eu3+ − Eu3+ interactions. The influence of these additional interactions become even more evident in the FC magnetization data for for H ≥ 40 kOe.
(b)
10 Hz 30 Hz 50 Hz 0.1 kHz 0.3 kHz 0.5 kHz 1 kHz 3 kHz 5 kHz 10 kHz
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Microstrain (%)
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1E-4 168
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169
Tf (K)
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(d) 168.5
169
169.5
Tf (K)
Figure 6: (Color online) T -dependence of the in-phase component of the χac for as-prepared (a) and annealed (b) samples with x = 0.10 measured for several values of f . The measuring time (= 1/f ) was plotted vs. Tf and the data was fitted to the power-law and VogelFulcher-law models. The corresponding fittings are showed in (b) and (c), respectively.
Fig. 6 shows the temperature dependence for χac near the N´eel temperature for as-prepared (a) and annealed (b) 5
ACCEPTED MANUSCRIPT Table 2: Critical exponents and fitting parameters obtained by using the Voguel-Fulcher Law (VF), a power-law (PL) and the A-T line (AT). X is a measure of the relative shift in Tf per decade of f (= ∆T /TG ∆log(f )).
Sample
TN (K)
EuCrO3 ∗ Eu0.90 Fe0.10 CrO3 ∗ From Ref. 11.
180.0 175.7
(VF) 177.1 165.9
TG (K) (PL) 178.2 167.5
(AT) − 174.4
Ea /kB (K) (VF) 31.82 39.23
zν (VF) 5.56 5.62
φ (AT) − 3.0
H0 (kOe) (AT) − 177.6
X 0.001 0.003
the behavior obtained for annealed samples. Moreover, the irreversible behavior was investigated for a sample with x = 0.10 leading to a H-dependence for the crossing over temperature to a spin-glass-like regime that was accounted for by a de Almeida-Thouless line for a broad range of applied magnetic fields. It is important to mention that the AT-line expression used to fit the data is to be applied for values of Tirr close to TG only [15]. Furthermore, the dynamics of the spins was investigated by using ac magnetic susceptibility data and two theoretical models (VoguelFulcher and power-law models) currently used for analyzing the f -dependence of χac yielding fitting parameters close to the values obtained for other similar sample materials. The microstrain induced during the sample preparation was found to be the main source for the spin-glasslike behaviors found in these sample materials. Finally, it is interesting to remind that random anisotropy was found to produce spin-glass-like properties in amorphous FM materials [26] while magnetic field induced crossing over from a spin-glass-like phase to a random-field regime was also found to be a characteristic of dilute magnetic materials [31, 32]. Thus, it is likely that the microstrain in the CAFM phase of europium chromite plays the same rule as the random-anisotropy and the random-field, respectively, in amorphous FM and in diluted AF materials.
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samples with x = 0.10 and for f in the range 10 − 104 Hz. As indicated by the black arrow in Fig. 6(a), the temperature corresponding to the maxima (Tf ) in the in-phase component of χac was found to shift to higher temperatures for increasing values of f . The total shift in the temperature of the maximum ∆T for the range of frequencies investigated was found to be about 1.4 K yielding a shift per decade of frequency (X = ∆T /[TG ∆log(f )]) of about 0.003. The value found for X and the shiftings in the maxima are signature of spin-glass-like systems as well. Moreover, the dynamics of the spins near the freezing temperature has been analyzed by using two theoretical models namely the Voguel-Fulcher law (τ = τ0 exp[Ea /kB (Tf − TG )]) and a power-law critical dynamics (τ = τ0 [1 − Tf /TG ]−zν ). τ (= 1/f ) is the measuring time, τ0 is a characteristic relaxation time, Ea is the activation energy, kB the Boltzmann constant, TG the glassy temperature and −zν the product of the correlation length critical exponent (z) with the dynamical critical exponent (ν). Both models were found to nicely fit the data in the range of frequencies investigated yielding τ0 = 1.8 × 10−9 s (= 1.6 × 10−15 s), Ea /kB = 39.23 K, TG = 165.9 K (= 167.5 K) for the Voguel-Fulcher (power-law) model. Besides, the power law fitting yielded zν = 5.62 which is comparable to the values obtained for other spin glasses systems [29, 30] indicating that it belongs to a well defined class of universality. These data are summarized in Table 2. It is important to notice that the χac data were nicely fit to both Voguel-Fulcher and power-law for the frequency range investigated. However, it was recently found that for some spin-glass-like systems the power-law may be more appropriated for measurements performed by using much higher values of frequencies [30]. For the annealed sample, however, the peaks near TN were found to narrower and the corresponding shiftings smaller (X ' 0.001) than the ones obtained for the as-prepared sample. Thus, the microstrains were also found to induce the spin-glass-like behaviors in these materials.
τ0 (s) (VF) (PL) 4.5 × 10−12 1.0 × 10−17 1.8 × 10−9 1.6 × 10−15
Acknowledgments Research supported by the Brazilian Agencies CNPq, FACEPE, FAPEG and FINEP.
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References [1] J. R. Sahu, C. R. Serrao, N. Ray, U. V. Waghmare, C. N. R. Rao, Rare earth chromites: a new family of multiferroics, J. Mater. Chem. 17 (2007) 42. doi:10.1039/b612093h. ´ [2] J. Prado-Gonjal, R. Schmidt, J. J. Romero, D. Avila, U. Amador, E. Mor´ an, Microwave-assisted synthesis, microstructure, and physical properties of rare-earth chromites, Inorg. Chem. 52 (2013) 313. doi:10.1021/ic302000j. [3] K. Tsushima, T. I, O. S., Weak-ferromagnetism in EuCrO3 , Solid State Commun. 7 (1) (1969) 71. doi:10.1016/ 0038-1098(69)90695-4. [4] A. Dur´ an, A. Ar´ evalo-L´ opez, E. Castillo-Mart´ınez, M. Garc´ıaGuaderrama, E. Moran, M. Cruz, F. Fern´ andez, M. AlarioFranco, Magneto-thermal and dielectric properties of biferroic YCrO3 prepared by combustion synthesis, J. Solid State Chem. 183 (2010) 1863. doi:10.1016/j.jssc.2010.06.001. [5] R. Saha, A. Sundaresan, C. N. R. Rao, Novel features of multiferroic and magnetoelectric ferrites and chromites exhibiting magnetically driven ferroelectricity, Mater. Horiz. 1 (2014) 20. doi:10.1039/C3MH00073G.
4. Conclusions In summary, the structural and magnetic properties of Eu1−x Fex CrO3 prepared by a combustion reaction technique were investigated in detail. For low values of x (0.0 ≤ x ≤ 0.20) the samples were found to be singlephase. Surprisingly, it was found that the N´eel temperature decreases with increasing values of the bonding angle Cr3+ -O2− -Cr3+ for an as-prepared sample in opposite to 6
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doped ZnO nanoparticulate powders, Physica B 476 (2015) 12. doi:10.1016/j.physb.2015.07.004. E. Montarroyos, F. L. A. Machado, S. M. Rezende, J. R. L. de Almeida, M. V. P. Altoe, F. P. Missell, Irreversibility line in the ferromagnet Co70.4 Fe4.6 Si15 B10 alloy, J. Appl. Phys. 70 (10) (1991) 6169. doi:10.1063/1.356919. M. M. Rashad, S. M. El-Sheikh, Magnetic properties of nanoclusters lanthanum chromite powders doped with samarium and strontium ions synthesized via a novel combustion method, Mater. Res. Bull. 46 (2011) 469. doi:10.1016/j.materresbull. 2010.10.016. M. Taheri, F. S. Razavi, Z. Yamani, R. Flacau, P. G. Reuvekamp, A. Schulz, R. K. Kremer, Magnetic properties of nano-clusters lanthanum chromite powders doped with samarium and strontium ions synthesized via a novel combustion method, Phys. Rev. B 93 (2016) 104414. doi:10.1103/ PhysRevB.93.104414. F. L. A. Machado, F. C. Montenegro, S. M. Rezende, M. E., L. J. Azevedo, W. G. Clark, Spin-glass behavior in the A1:Mn quasicrystalline alloys, Solid State Commun. 79 (6) (1991) 469. doi:10.1016/0038-1098(91)90032-Q. P. R. T. Ribeiro, J. M. M. Ramirez, R. Vidyasagar, F. L. A. Machado, S. M. Rezende, E. D. Dahlberg, GMI in the reentrant spin-glass Fe90 Zr10 alloy: Investigation of the spin dynamics in the MHz frequency regime, Appl. Phys. Lett. 109 (10) (2016) 102404. doi:10.1063/1.4962534. F. C. Montenegro, S. M. Rezende, M. D. CoutinhoFilho, Crossover from random field to spinglass behavior in Fex Zn1x F2 , J. Appl. Phys. 63 (1988) 3755. doi:doi.org/10. 1063/1.340657. K. A. P. de Lima, J. B. Brito, P. H. R. Barbosa, E. P. Raposo, M. D. Coutinho-Filho, Magnetic field effect on the fractal cluster spin-glass phase of an ising antiferromagnet near the firstneighbor percolation threshold: Fe0.25 Zn0.75 F2 , Phys. Rev. B 85 (2012) 064416. doi:10.1103/PhysRevB.85.064416.
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[6] A. Jaiswal, R. Das, K. Vivekanand, T. Maity, P. M. Abraham, S. Adyanthaya, P. Poddar, Magnetic and dielectric properties and Raman spectroscopy of GdCrO3 nanoparticles, J. Appl. Phys. 107 (2010) 013912. doi:10.1063/1.3275926. [7] R. Hornreich, Magnetic interactions and weak ferromagnetism in the rare-earth orthochromites, J. Magn. Magn. Mater. 7 280. doi:10.1016/0304-8853(78)90203-2. [8] K. Yoshii, Magnetic properties of perovskite GdCrO3 , J. Solid State Chem. 159 (2001) 204. doi:10.1006/jssc.2000.9152. [9] I. A. Sergienko, E. Dagotto, Role of the dzyaloshinskii-moriya interaction in multiferroic perovskites, Phys. Rev. B 73 (2006) 094434. doi:10.1103/PhysRevB.73.094434. [10] Y. Su, J. Zhang, L. Li, B. Li, Y. Zhou, D. Deng, Z. Chen, S. Cao, Temperature dependence of magnetic properties and change of specific heat in perovskite ErCrO3 chromites, Appl. Phys. A 100 (1) (2010) 73. doi:10.1007/s00339-010-5839-1. [11] M. Taheri, R. K. Kremer, S. Trudel, F. S. Razavi, Exchange bias effect and glassy-like behavior of EuCrO3 and CeCrO3 nanopowders, J. Appl. Phys. 118 (12) (2015) 124306. doi:10.1063/ 1.4931613. [12] D. Deng, X. Wang, J. Zheng, X. Qian, D. Yu, D. Sun, C. Jing, B. Lu, B. Kang, S. Cao, J. Zhang, Phase separation and exchange bias effect in Ca doped EuCrO3 , J. Magn. Magn. Mater. 395 (2015) 283. doi:10.1016/j.jmmm.2015.07.075. [13] E. Golovenchits, G. Kwei, V. Sanina, Long-lived, magnetically ordered state in EuCrO3 excited by optical pumping, JETP Lett. 31 (1980) 223. [14] J. M. M. Ram´ırez, H. V. S. Pessoni, A. Franco, F. L. A. Machado, Synthesis of europium orthochromites (EuCrO3 ) nanoparticles by a combustion reaction method, J. Alloys Compd. 690 (2017) 315. doi:10.1016/j.jallcom.2016.08.105. [15] J. R. L. de Almeida, D. J. Thouless, Stability of the SherringtonKirkpatrick solution of a spin glass model, J. Phys. A: Math. Gen 11 (1978) 983. doi:10.1088/0305-4470/11/5/028. [16] J. Tholence, On the frequency dependence of the transition temperature in spin glasses, Solid State Commun. 35 (2) (1980) 113. doi:10.1016/0038-1098(80)90225-2. [17] P. C. Hohenberg, B. I. Halperin, Theory of dynamic critical phenomena, Rev. Mod. Phys. 49 (1977) 435. doi:10.1103/ RevModPhys.49.435. [18] T. C. Gibb, Magnetic exchange interactions in perovskite solid solutions. Part 2. Iron-57 and 151 Eu M¨ ossbauer spectra of EuFe1−x Crx O3 (0 ≤ x ≤ 1), J. Chem. Soc., Dalton Trans. 9 (1983) 2031. doi:10.1039/DT9830002031. [19] H. M. Widatallah, T. M. H. Al-Shahumi, A. M. Gismelseed, Z. Klencs´ ar, A. D. Al-Rawas, I. A. Al-Omari, M. E. Elzain, A. A. Yousif, M. Pekala, Structural and 57 Fe M¨ ossbauer study of EuCr1−x Fex O3 nanocrystalline particles, Hyperfine Interact. 205 (2011) 101. doi:10.1007/s10751-011-0484-4. [20] H. M. Widatallah, T. M. Al-Shahumi, Z. Klencs´ ar, M. Pekala, A. M. Gismelseed, I. A. Al-Omari, A. D. Al-Rawas, D. Seifu, Structural, magnetic and 151Eu M¨ ossbauer studies of mechanosynthesized nanocrystalline EuCr1−x Fex 03 particles, Acta Materialia 61 (12) (2013) 4461. doi:10.1016/j.actamat. 2013.04.015. [21] S. Jain, K. Adiga, V. P. Verneker, A new approach to thermochemical calculations of condensed fuel-oxidizer mixtures, Combust. Flame 40 (1981) 71. doi:10.1016/0010-2180(81)90111-5. [22] Y. Su, J. Zhang, B. Li, B. Kang, Q. Yu, C. Jing, S. Cao, The dependence of magnetic properties on temperature for rare earth ErCrO3 chromites, Ceram. Int. 38 (2012) S421. doi:10.1016/j.ceramint.2011.05.024. [23] I. Parkin, A. Komarov, Q. Fang, Alternative solid state routes to mixed metal oxides (LnCrO3 , LnFeO3 ), Polyhedron 15 (1996) 3117. doi:10.1016/0277-5387(95)00576-5. [24] R. M. Patil, P. B. Shete, N. D. Thorat, S. V. Otari, K. C. Barick, A. Prasad, R. S. Ningthoujam, B. M. Tiwale, S. H. Pawar, Nonaqueous to aqueous phase transfer of oleic acid coated iron oxide nanoparticles for hyperthermia application, RSC Adv. 4 (2014) 4515. doi:10.1039/C3RA44644A. [25] A. Franco, H. Pessoni, Enhanced dielectric constant of Co-
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Fe-doped europium chromite samples were prepared by a combustion reaction technique;
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Spin-glass-like properties in iron-doped europium chromite samples are investigated; Microstrain was found to be the main source of the spin-glasslike properties;
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The correlation of the Cr-O-Cr angle and the Néel temperature was also investigated;
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Critical exponents and sample parameters were obtained.
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DOI:
10.1016/j.jallcom.2017.07.018
Reference:
JALCOM 42434
To appear in:
Journal of Alloys and Compounds
Received Date: 24 May 2017 Revised Date:
1 July 2017
Accepted Date: 3 July 2017
Please cite this article as: D.R. Ratkovski, J.M.Marí. Ramírez, P.R.T. Ribeiro, H.V.S. Pessoni, A. Franco Jr., F.L.A. Machado, Magnetic irreversibility and spin dynamics in nanoparticles of iron-doped europium chromite, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.018. 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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Magnetic irreversibility and spin dynamics in nanoparticles of iron-doped europium chromite D. R. Ratkovskia , J. M. Mar´ın Ram´ıreza , P. R. T. Ribeiroa , H. V. S. Pessonib , A. Franco Jrb , F. L. A. Machadoa,∗ a Departamento
de F´ısica, Universidade Federal de Pernambuco, 50670-901, Recife, Pernambuco, Brazil. de F´ısica, Universidade Federal de Goi´ as, C.P. 131, 74001-970 Goiˆ ania, Goi´ as, Brazil.
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Abstract
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The magnetic properties of nanoparticles of Eu1−x Fex CrO3 (0 ≤ x ≤ 1) prepared by a combustion reaction technique were investigated. An irreversible behavior in the magnetization was observed for a sample with x = 0.10 below the N´eel temperature (TN ) yielding a complex phase diagram. The irreversibility data was fitted to a de Almeida-Thouless line by using φ = 3.0 for the critical exponent and a glassy temperature TG of 174.4 K for H ≤ 40 kOe. The dynamics of the spins was investigated by measuring the ac magnetic susceptibility (χac ) near TN for frequencies (f ) in the range 10 − 104 Hz. The maximum in χac was found to shift to higher values of T for increasing values of f , yielding ' 0.003 for the shift per decade of f parameter. The Voguel-Fulcher law and a power-law were used for analyzing χac yielding τ0 = 1.8 × 10−9 s (= 1.6 × 10−15 s) for the characteristic relaxation time, Ea /kB = 39.23 K for the activation energy and TG = 165.9 K (= 167.5 K) for the Voguel-Fulcher (power-law) model. Moreover, the power-law yielded zν = 5.62 for the product of the dynamical critical exponent (ν) with the one associated to the correlation length (z). The dependence of TN with the bonding angles Cr3+ −O2− −Cr3+ was investigated for as-prepared samples yielding a trend contrary to the expected. The overall results were accounted for by taking into consideration the microstrain introduced by the sample preparation technique and by the ionic dopping.
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1. Introduction
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The rare-earth orthochromites [RE]CrO3 are orthorhombic distorted perovskites that have been attracting considerable attention because of their potential in application such as multifunctional materials [1, 2]. These systems are isostructural with the rare-earth orthoferrites [RE]FeO3 and they show physical and chemical properties that are strongly influenced by the rare-earth ion lying on the A-site of a perovskite structure [3, 4, 5]. [RE]CrO3 presents a rich variety of magnetic spin interactions, namely Cr3+ − Cr3+ , Cr3+ − [RE]3+ and [RE]3+ − [RE]3+ that are highly temperature dependent [2, 6]. The easy-axis for the Cr3+ magnetic moments is along the c-axis direction of an orthorhombic cell while the magnetic structure is represented by the Γ4 (Gx Ay Fz ) configuration, with Gx , Ay , Fz being the components of the Cr3+ spins along the crystallographic directions a, b and c, respectively [5, 7, 8]. Magnetic data has indicated that the weak ferromagnetic moment of the Cr3+ spins below the N´eel temperatures (TN ) arises from a Dzialoshinski-Moriya (DM) type antisymmetric exchange interaction [1, 9]. Furthermore, the N´eel temperature was found to be mainly determined by the Cr3+ − Cr3+ antiferromagnetic coupling, increasing monotonically with the size of the ionic ∗ Corresponding
author: F. L. A. Machado Email address: [email protected] (F. L. A. Machado)
Preprint submitted to Journal of Alloys and Compounds
radius in the lanthanide series. This behavior has been associated to the decreasing in the lattice distortions and to an increasing in the Cr3+ −O2− −Cr3+ distance which in turn varies the inter-cationic super-exchange interaction [6]. More recently, exchange-bias and spin-glass-like properties have also been reported in pure and in Ca-doped europium chromites [10, 11, 12].
Among the rare-earth orthochromites, the europium orthochromite (EuCrO3 ) exhibits a weak spontaneous ferromagnetic moment below the N´eel temperature that has been attributed to the ordering of the localized Cr3+ magnetic moments in a canted-antiferromagnetic (CAFM) phase [3, 7, 8]. It is also known that a net exchange interaction is produced in optically excited Eu3+ ions yielding a long-lived magnetically ordered state [13]. Thus EuCrO3 have also been considered as an active medium for highdensity optical storage and optical processing devices. Despite of its potential for applications, the magnetic properties of EuCrO3 has not been fully investigated yet. For instante, it is believed that TN is influenced by the bonding angle Cr3+ −O2− −Cr3+ (θB ) but up to now there is no detailed investigation showing that this occurs indeed. Moreover, the nature of the spin-glass-like phase in these materials is not fully understood yet. In this work, the magnetic properties of nanopowders of pure and Fe-doped europium chromite (Eu1−x Fex CrO3 ) samples prepared by a combustion reaction technique [14] are investigated. It is expected, for instance, that the difJune 30, 2017
ference in the ionic radii of the Fe3+ and Eu3+ ions in the A-site of the perovskite structure would modify the bonding angle θB which, in turn, would modify the N´eel transition temperature. Thus, TN was measured for a set of Fe-doped samples and their correlation with the θB determined. Moreover, a de Almeida-Thouless line [15] (AT line) was fit to the magnetic field dependence of the crossing over transition temperature from the CAFM to a spin-glass like phase for x = 0.10. The glassy temperature (TG ) and the critical exponente φ were determined from the A-T line. The dynamics of the magnetic moment was studied by means of ac magnetic susceptibility (χac ) measurements. The χac data were analyzed by considering using the Voguel-Fulcher law [16] and the scaling power-law function [17] yielding a value for the product of the correlation length critical exponent (z) with the dynamical critical exponent (ν) zν close to the one obtained for an undoped sample of europium chromite. We noticed that some structural and magnetic properties of iron-doped europium chromite prepared by different synthesis routes were reported in the past [18, 19, 20]. However, the goals and techniques employed in those works are different from the ones of the present work.
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EuCrO3
Fe3O4
x = 1.0
x = 0.6 x = 0.5 x = 0.4
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Transmittance (a.u.)
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x=0.15
x=0.05
x=0
x=0
2. Sample preparation and techniques
32.0 32.4 32.8 33.2 33.6 34.0
Nanoparticles of Fe-doped europium orthochromites (Eu1−x Fex CrO3 ) were synthesized by using a combustion reaction method that is based on thermochemical concepts of propellant chemistry [21]. Details of the sample preparation process can be found elsewhere [14]. The samples were analyzed by X-ray diffraction (XRD) using the CuKα radiation (λ = 1.5418 ˚ A) in a Shimadzu XRD-7000 diffractometer. The XRD spectra were refined by employing the Rietveld method and by using the MAUD software. Pellets of as-prepared powders mixed with KBr were characterized by Fourier transform infrared spectroscopy (FT-IR) in the range 400-4000 cm−1 using a Shimadzu IRTracer-100. The magnetization and ac magnetic susceptibility (χac ) measurements were performed by using the AC Measurement System modulus (ACMS) of a PPMS (Physical Property Measurement System made by Quantum Design) for applied magnetic fields (H) and temperatures (T ) in the ranges ± 85 kOe and 5 ≤ T ≤ 300 K, respectively. The magnetization was measured in the warming up mode and for a fix value of H following two procedures: (1) the sample was first cooled under no applied magnetic field (zero-field-cooled or ZFC) before applying H and (2) the sample was first cooled with the H already applied (field-cooled or FC). χac was measured for frequencies in the range 10 ≤ f ≤ 104 Hz while the amplitude of the ac-magnetic field of kept constant (= 10 Oe) throughout the measurements. Finally, room temperature hysteresis loops were also obtained by using a VSM magnetometer for −20 ≤ H ≤ 20 kOe. The measurements were performed in as-prepared (AP) powder samples and in samples thermal annealed (TT)
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Figure 1: (Color online) (a) X-ray diffraction pattern for Eu1−x Fex CrO3 for 0 ≤ x ≤ 1. A peak clearly seen near 36◦ for x ≥ 0.4 was identified to be due to magnetite present in highly doped samples. The diffraction peak near 33◦ is plotted in a higher angle resolution for x ≤ 0.2 in (b). This peak was found to shift monotonically to higher values of diffraction angles for increasing values of x. (c) FTIR absorption band spectra assigned to the stretching mode of Eu−O for x ≤ 0.2 showing that the maximum in the absorption band shifts to low values of energy for increasing values of x.
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at 1073 K according the following protocole: the samples were first warmed up to 1073 K at a rate of 10 K/min; remained at this temperature for 24 h, it was then cooled down to room temperature under the same temperature rate (10 K/min). This procedure was particularly important for the sample with x = 0.1 for determining the influence of microstrains introduced along the preparation of samples in the spin-glass-like properties observed in this sample composition. The average particle size (d) varied little with thermal treatment. For instance, for the sample with x = 0.1 which was investigated in detail d was found to increase from 39.11 nm to 40.29 nm only. 3. Experimental results and discussions Fig. 1 (a) shows the XRD spectra for a narrow range of diffraction angles (31 ≤ 2θ ≤ 37 degrees) for x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0. The peaks were properly identified to the europium chromite crystalline structure 2
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Density ρ (g/cm3 ) 7.4058 7.3787 7.3110 7.2304 7.1784 7.0281 6.8981
4 x=0 x=0.2 x=0.4 x=0.5 x=0.6 x=0.8 x=1.0
3 2 1
Reliability χ2 1.422 1.089 1.119 1.089 1.076 1.096 1.071
0 -1
T = 300 K
M (emu/g)
0.8
-2
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Crystallite size d (nm) 28.70 40.99 39.60 39.86 39.11 36.11 38.23
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Eu(1-x)FexCrO3
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θB (degree) 156.30 149.74 152.88 147.43 146.45 147.91 145.77
Mr (emu/g)
[22, 12]. However, it was also observed a peak indexed to magnetite (Fe3 O4 ) crystalline structure near 36◦ for x ≥ 0.4. Thus, to avoid the influence of the magnetite we focused the main studies in the samples with x ≤ 0.2. For this Fe concentration range the corresponding XRD spectra are shown in Fig. 1 (b) for a narrower diffraction angle range. Notice that the diffraction peak (112) near 33 ◦ shifts monotonically to higher diffraction angles for increasing values of x. This is a clear indication that Fe3+ ions are indeed being incorporated into the orthochromite structure. The Rietveld analysis was employed for analyzing the XRD spectra indicating that the samples are orthochromites and that they belong to the P nma structure (space group 62). The corresponding lattice parameters (a, b and c), the bonding angle θB and the average particle size (d) determined by using the Scherrer equation are summarized in Table 1 for x ≤ 0.2. It is interesting to notice that the size of the lattice parameters diminishes with increasing values of x. Even so, the overall mass density (ρ) diminishes because the atomic mass for Fe (= 55.85 u) is only about 1/3 the one for Eu (= 151.96 u). The mismatch in the ionic atomic radii (Eu3+ = 120.6 pm and Fe3+ = 92 pm) was found to produce additional micro-strains () that was determined to increase monotonically with x. The values of are listed in Table 1 as well. Fig. 1 (c) shows Fourier transform infrared spectroscopy (FTIR) data for x ≤ 0.2 measured at room temperature in the wavenumber range 400-500 cm−1 . The minima observed in the transmission data correspond to absorption band associated to the stretching mode of Eu−O [6, 14, 23, 24]. It was found that the addition of iron shifts in the band towards lower values of energy. The shiftings are in agreement with what it is expect from the variation in the mass density ρ listed in Table 1: the sample become less dense for increasing values x diminishing the effective elastic constant and the corresponding energy of the Eu-O vibrational mode [25]. This result is also a clear indication that Fe3+ ions are indeed being incorporated into the chromite structure. FTIR spectra for a broader wavenumber range were reported elsewhere [14]. Fig. 2 (a) shows hysteresis loops measured at 300 K for samples in the full iron concentration range (0 ≤ x ≤ 1.0). Europium chromite is known to be paramagnetic at room temperature (RT) and to exhibit a CAFM ordering at
Strain ε 0.00158 0.00195 0.00189 0.00235 0.00223 0.00243 0.00295
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EuCrO3 Eu0.98 Fe0.02 CrO3 Eu0.95 Fe0.05 CrO3 Eu0.92 Fe0.08 CrO3 Eu0.90 Fe0.10 CrO3 Eu0.85 Fe0.15 CrO3 Eu0.80 Fe0.20 CrO3
Cell parameters a (nm) b (nm) c (nm) 0.5529(2) 0.7643(3) 0.5348(2) 0.5520(7) 0.7635(1) 0.5340(6) 0.5515(1) 0.7629(2) 0.5337(1) 0.5514(7) 0.7627(1) 0.5336(7) 0.5511(6) 0.7627(1) 0.5335(7) 0.5512(1) 0.7629(2) 0.5338(1) 0.5507(9) 0.7628(2) 0.5335(9)
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0.4
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x Figure 2: (Color online) (a) Room temperature hysteresis loops for Eu1−x Fex CrO3 for 0 ≤ x ≤ 1.0. The lower right inset is a blowup of the data for small values of H. For x ≥ 0.4 it was found non-vanishing values for Mr and Hc . A plot of Mr and Hc as a function of x is shown in (b). Notice that both Mr and Hc vanishes below about x = 0.2. The upper right inset in (b) is a plot of the magnetization measured at H = 20 kOe as a function of x. The solid, dotted and dashed lines are guide to the eyes.
much lower temperatures. However, it was observed nonvanishing remanent magnetizations (Mr ) and coercivities (Hc ) at 300 K for samples with x > 0.4. This parameters are typical of ferromagnets but in the present case it is due to the presence of magnetite, a ferrimagnetic material with a critical temperature of about ' 850 K as it was 3
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identified by XRD. The limit of solubility of iron in the europium chromite is better seen in Fig. 2 (b) were Mr and Hc are plotted as a function of x. Above x = 0.20 a non-vanishing coercivity is observed at room temperature. The inset in Fig. 2(b) shows that the slope of the magnetization data measured at the highest applied magnetic field M (Hmax ) increases monotonically with the iron concentration for x ≤ 0.4 and the slope changes for higher concentrations due to the presence of magnetite. The hysteresis loop shown in Fig. 3 was measured for a sample with x = 0.10 at T = 5.0 K. The magnetization was found not saturate up to the highest applied magnetic field (H = 85 kOe) as expected for a CAFM phase while it is paramagnetic at 300 K as shown in the upper left inset. The lower-right inset of Fig. 3 is a blow-up of hysteresis loops for small values of H for T = 5 K and for T = 300 K. It is important to notice the large values Hc (= 2.20 kOe) for Mr (= 0.22 emu/g) obtained for T = 5 K. Fig. 4 (a) shows ZFC (open symbols) and FC (closed symbols) data for the x = 0.10 for an as-prepared sample in the temperature range 5 − 300 K and for H = 1, 5, 10, 20, 30, 50 and 85 kOe. The main figure actually shows the ZFC and FC magnetizations minus the corresponding paramagnetic contributions measured at 300 K shown in the inset. One may notice that the temperature where the irreversible behavior (Tirr ) vanishes in the T -dependence of the ZFC-FC magnetizations varies with H. Such a behavior, for instance, is expected in spinglass-like systems. For spin-glasses, it is found that the Hdependence of Tirr can be accounted for by a de AlmeidaThouless line[15]: H = H0 [1 − Tirr (H)/TG ]φ/2 , where φ is a critical exponent and TG is the freezing temperature for H = 0. The A-T line was found to fit the data for H ≤ 40 kOe and by using the mean-field critical exponent[15, 26]
0.5
0.0
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Figure 3: (Color online) Hysteresis loop mesured at T = 5 K for x = 0.10 and for H varying in the range ± 85 kOe. The lower right-side inset is a blow-up of the hysteresis loop for low values of H measured at 5 K and for T = 300 K. It is clearly seen that the sample is paramagnetic at 300 K and that large values for both Mr and Hc are obtained at 5 K. The upper left-side inset is a plot of the hysteresis loop for T = 300 K. The solid lines are guide to the eyes.
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Figure 4: (Color online) The ZFC-FC magnetization data measured for as-prepared (AP) and treated thermally (TT) samples with x = 0.10 and for different values of H are shown in (a) and (b), respectively. The corresponding paramagnetic contributions at 300 K were actually subtract from the ZFC and FC data and they are showed in the insets. Notice that the irreversibility temperature (Tirr ) shifts to low values for increasing values of H. The H vs. Tirr data for the AP (circles) and for the TT (pentagons) samples obtained from the ZFC-FC magnetization data are shown in (c). The solid lines are fit to A-T lines while the inset shows the M × T data for H = 100 Oe used for obtaining TN for the TT-sample (broken line).
φ = 3.0 yielding H0 = 177.6 kOe and TG = 174.4 K. For higher values of H the A-T line was found to deviate from the data. The influence of the microstrains introduced along the sample preparation procedures was investigated by studying a sample annealed as described in preceding section. The corresponding data and AT-line for this sample is shown in Fig. 4 (b) and (c). By using φ = 3.0 and for the same H interval used for the AP-sample the fitting yielded H0 = 2202 kOe and TG = 174.4 K. The fitting parameters for the AP-sample are compiled together with the values obtained for a pure europium chromite sample [11] in Table 2. In Fig. 4(c) the A-T lines are represented by full lines while the data for the AP and TT samples are represented by circles and pentagons , respectively. Below about 60 K the phase diagram become even 4
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(a)
bonding angle. For the annealed samples, on the other hand, the trend goes in the other way indicating that the microstrain created along the synthesis process modifies the inter-cationic super-exchange interaction altering the values of TN of the as-prepared samples [2, 6]. Plots for the x-dependence of the microstrains present in the asprepared and in the annealed samples are shown in Fig 5(c) and the data for the as-prepared samples are also listed in Table 1. The microstrain was found to be higher and to increase with increasing values of x for the as-prepared sample while it is greatly reduced and nearly constant in the annealed samples. The residual microstrain in the TTsample is more likely to be due to the Fe-doping while most of the excess of the microstrain in the AP-sample was introduced in the combustion reaction method. It is important to recall that TN depends on parameters sensitive to microstrains such as the exchange-interaction. So, by annealing the samples most of the microstrains were removed yielding the expected behavior for TN , e.g., TN decreased by decreasing the bonding angle.
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Figure 5: (Color online) TN and the Cr3+ − O2− − Cr3+ bonding angle (θB ) are ploted as a funciton of x for the iron concentration range 0 ≤ x ≤ 0.2 for both as prepared (a) and annealed (b) samples. In (c) it is shown the microstrain data determined for both set of samples. The inset in (a) is a representative drawing of the bonding angle while the solid lines are guide to the eyes only.
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The bonding angle Cr3+ − O2− − Cr3+ is believed to have a direct influence in the value of the N´eel temperature in rare earth chromites [2, 27, 28]. For instance, TN is expected to increase for increasing values of the bonding angle. In order to verify how strongly θB influences TN this bonding angle was measured for samples with iron concentration in the range 0 ≤ x ≤ 0.2. As shown in Fig. 5 (a) and (b) the bonding angle decreases monotonically for increasing values of the Fe3+ ion concentration for both as-prepared and annealed samples. However, in contrast to what it was expected for the as-prepared samples, TN was found to increase with increasing values of x or, equivalently, to increase with decreasing values of the
170
T (K)
z5.62 TG = 167.5 K
more complex because the influence of the Cr3+ − Eu3+ interactions while below about 10 K the magnetic properties are also influenced by the Eu3+ − Eu3+ interactions. The influence of these additional interactions become even more evident in the FC magnetization data for for H ≥ 40 kOe.
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Figure 6: (Color online) T -dependence of the in-phase component of the χac for as-prepared (a) and annealed (b) samples with x = 0.10 measured for several values of f . The measuring time (= 1/f ) was plotted vs. Tf and the data was fitted to the power-law and VogelFulcher-law models. The corresponding fittings are showed in (b) and (c), respectively.
Fig. 6 shows the temperature dependence for χac near the N´eel temperature for as-prepared (a) and annealed (b) 5
ACCEPTED MANUSCRIPT Table 2: Critical exponents and fitting parameters obtained by using the Voguel-Fulcher Law (VF), a power-law (PL) and the A-T line (AT). X is a measure of the relative shift in Tf per decade of f (= ∆T /TG ∆log(f )).
Sample
TN (K)
EuCrO3 ∗ Eu0.90 Fe0.10 CrO3 ∗ From Ref. 11.
180.0 175.7
(VF) 177.1 165.9
TG (K) (PL) 178.2 167.5
(AT) − 174.4
Ea /kB (K) (VF) 31.82 39.23
zν (VF) 5.56 5.62
φ (AT) − 3.0
H0 (kOe) (AT) − 177.6
X 0.001 0.003
the behavior obtained for annealed samples. Moreover, the irreversible behavior was investigated for a sample with x = 0.10 leading to a H-dependence for the crossing over temperature to a spin-glass-like regime that was accounted for by a de Almeida-Thouless line for a broad range of applied magnetic fields. It is important to mention that the AT-line expression used to fit the data is to be applied for values of Tirr close to TG only [15]. Furthermore, the dynamics of the spins was investigated by using ac magnetic susceptibility data and two theoretical models (VoguelFulcher and power-law models) currently used for analyzing the f -dependence of χac yielding fitting parameters close to the values obtained for other similar sample materials. The microstrain induced during the sample preparation was found to be the main source for the spin-glasslike behaviors found in these sample materials. Finally, it is interesting to remind that random anisotropy was found to produce spin-glass-like properties in amorphous FM materials [26] while magnetic field induced crossing over from a spin-glass-like phase to a random-field regime was also found to be a characteristic of dilute magnetic materials [31, 32]. Thus, it is likely that the microstrain in the CAFM phase of europium chromite plays the same rule as the random-anisotropy and the random-field, respectively, in amorphous FM and in diluted AF materials.
EP
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samples with x = 0.10 and for f in the range 10 − 104 Hz. As indicated by the black arrow in Fig. 6(a), the temperature corresponding to the maxima (Tf ) in the in-phase component of χac was found to shift to higher temperatures for increasing values of f . The total shift in the temperature of the maximum ∆T for the range of frequencies investigated was found to be about 1.4 K yielding a shift per decade of frequency (X = ∆T /[TG ∆log(f )]) of about 0.003. The value found for X and the shiftings in the maxima are signature of spin-glass-like systems as well. Moreover, the dynamics of the spins near the freezing temperature has been analyzed by using two theoretical models namely the Voguel-Fulcher law (τ = τ0 exp[Ea /kB (Tf − TG )]) and a power-law critical dynamics (τ = τ0 [1 − Tf /TG ]−zν ). τ (= 1/f ) is the measuring time, τ0 is a characteristic relaxation time, Ea is the activation energy, kB the Boltzmann constant, TG the glassy temperature and −zν the product of the correlation length critical exponent (z) with the dynamical critical exponent (ν). Both models were found to nicely fit the data in the range of frequencies investigated yielding τ0 = 1.8 × 10−9 s (= 1.6 × 10−15 s), Ea /kB = 39.23 K, TG = 165.9 K (= 167.5 K) for the Voguel-Fulcher (power-law) model. Besides, the power law fitting yielded zν = 5.62 which is comparable to the values obtained for other spin glasses systems [29, 30] indicating that it belongs to a well defined class of universality. These data are summarized in Table 2. It is important to notice that the χac data were nicely fit to both Voguel-Fulcher and power-law for the frequency range investigated. However, it was recently found that for some spin-glass-like systems the power-law may be more appropriated for measurements performed by using much higher values of frequencies [30]. For the annealed sample, however, the peaks near TN were found to narrower and the corresponding shiftings smaller (X ' 0.001) than the ones obtained for the as-prepared sample. Thus, the microstrains were also found to induce the spin-glass-like behaviors in these materials.
τ0 (s) (VF) (PL) 4.5 × 10−12 1.0 × 10−17 1.8 × 10−9 1.6 × 10−15
Acknowledgments Research supported by the Brazilian Agencies CNPq, FACEPE, FAPEG and FINEP.
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References [1] J. R. Sahu, C. R. Serrao, N. Ray, U. V. Waghmare, C. N. R. Rao, Rare earth chromites: a new family of multiferroics, J. Mater. Chem. 17 (2007) 42. doi:10.1039/b612093h. ´ [2] J. Prado-Gonjal, R. Schmidt, J. J. Romero, D. Avila, U. Amador, E. Mor´ an, Microwave-assisted synthesis, microstructure, and physical properties of rare-earth chromites, Inorg. Chem. 52 (2013) 313. doi:10.1021/ic302000j. [3] K. Tsushima, T. I, O. S., Weak-ferromagnetism in EuCrO3 , Solid State Commun. 7 (1) (1969) 71. doi:10.1016/ 0038-1098(69)90695-4. [4] A. Dur´ an, A. Ar´ evalo-L´ opez, E. Castillo-Mart´ınez, M. Garc´ıaGuaderrama, E. Moran, M. Cruz, F. Fern´ andez, M. AlarioFranco, Magneto-thermal and dielectric properties of biferroic YCrO3 prepared by combustion synthesis, J. Solid State Chem. 183 (2010) 1863. doi:10.1016/j.jssc.2010.06.001. [5] R. Saha, A. Sundaresan, C. N. R. Rao, Novel features of multiferroic and magnetoelectric ferrites and chromites exhibiting magnetically driven ferroelectricity, Mater. Horiz. 1 (2014) 20. doi:10.1039/C3MH00073G.
4. Conclusions In summary, the structural and magnetic properties of Eu1−x Fex CrO3 prepared by a combustion reaction technique were investigated in detail. For low values of x (0.0 ≤ x ≤ 0.20) the samples were found to be singlephase. Surprisingly, it was found that the N´eel temperature decreases with increasing values of the bonding angle Cr3+ -O2− -Cr3+ for an as-prepared sample in opposite to 6
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[26]
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Fe-doped europium chromite samples were prepared by a combustion reaction technique;
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Spin-glass-like properties in iron-doped europium chromite samples are investigated; Microstrain was found to be the main source of the spin-glasslike properties;
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The correlation of the Cr-O-Cr angle and the Néel temperature was also investigated;
M AN U
Critical exponents and sample parameters were obtained.
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