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Sign reversal of magnetization in Sm2CrMnO6 perovskites
Journal of Magnetism and Magnetic Materials 483 (2019) 89–94
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Sign reversal of magnetization in Sm2CrMnO6 perovskites Kaipamangalath Aswathi ⁎ Manoj R. Varmaa,b,
a,b
b,c
T b
, Jasnamol Pezhumkattil Palakkal , Ramany Revathy ,
a Academy of Scientific and Innovative Research (AcSIR), CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST) Campus, Trivandrum 695 019, India b Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology, CSIR, Trivandrum 695 019, India c Technische Universität Darmstadt, Department of Materials and Earth Sciences, Alarich-Weiss-Strasse 2, 64287 Darmstadt, Germany
A R T I C LE I N FO
A B S T R A C T
Keywords: Sm2CrMnO6 Negative magnetization Compensation temperature Magnetic properties
A sign reversal of magnetization is observed in Sm2CrMnO6 synthesized via the citrate-gel method. The material crystallizes in monoclinic crystal structure with P121/n1 space group. The presence of Cr3+ and Mn3+ cations is confirmed by X-ray photoelectron spectroscopy analysis. The transition temperature (TC) is found to be at 50 K from the thermomagnetic data. The magnetization reverses its sign below a particular temperature under field cooled condition and the effect is reduced and disappeared at higher applied magnetic fields. The combined interaction between different magnetic components due to the presence of Sm3+, Cr3+, and Mn3+ leads to the sign reversal of magnetization.
1. Introduction Magnetization reversal (MR) is observed in many materials such as spinel ferrites, perovskites, molecular magnets, garnets, intermetallic alloys, and multilayers. Many intrinsic properties like crystal structure, magnetic anisotropy, magnetic exchange interactions, and temperature dependence of sublattice magnetization etc. are related to the sign reversal behavior. In many perovskites, negative magnetization was reported and was found to be due to different interactions [1–3]. MR observed in epitaxial Gd0.67Ca0.33MnO3 thin films is due to the antialigned sub lattices of Gd against Mn in the presence of an applied magnetic field [1]. In Gd0.67Ca0.33MnO3 perovskite, sign reversal of magnetization is observed due to the presence of different magnetic frame work of frozen ferromagnetic and paramagnetic ions with strong magnetic moments present in the system that behave differently under external magnetic field. In the case of ErMexMn(1−x)O3 (Me = Ni, Co) perovskite, the reason for negative magnetization is that the transition metal ordering at Tc creates a strong local field on the rare earth ion opposite to the applied field [4]. Negative magnetization was observed in Sr2YbRuO6 due to the presence of different magnetic components present in the system due to Ru5+and Yb3+ moments which align in the opposite direction with the application of external magnetic field [5]. A sign reversal of magnetization is also observed in a comparative study of bulk and thin films of ferromagnetic ErCo0.5Mn0.5O3 perovskite. Here the reason for this phenomena is the negative exchange interaction
⁎
between the uncompensated spins of Er ions and ferromagnetic Co and Mn ions [6,7]. Core-shell type La0.2Ce0.8CrO3 nanoparticles show negative magnetization, and in this configuration, the antiferromagnetic core of Cr3+ and Ce3+ and a disordered shell with the uncompensated spins in the core induces the sign reversal of magnetization [8]. Disorder in the occupation of Co and Ru sites induced negative magnetization in LaSrCoRuO6 [9]. Paramagnetic effect of the rare earth (RE) ions whose moments align opposite to the canted moment of transition metal ions leads to negative magnetization in YbCrO3chromates [10]. The interplay of different sublattice interaction leads to the sign reversal of magnetization in Er(CoMn)O3 perovskites [11]. Due to magnetocrystalline anisotropy, the net moment aligned in the direction opposite to the applied field gives spin reversal of magnetization in BiFe0.5Mn0.5O3 synthesized at high pressure [12]. In Al3+ substituted copper ferrite CuFe2O4, the reason for negative magnetization is the temperature dependence of magnetic moments of Cu2+ and Fe3+ that results in the antiparallel arrangement of spins [13]. Mao et al. have reported temperature and magnetic field induced negative magnetization in YFe0.5Cr0.5O3 due to antisymmetric Dzyaloshinskii–Moriya (DM) interaction and magnetocrystalline anisotropy [14]. D-M interaction and magnetostrictive distortion induced by orbital moments are the reasons for negative magnetization in Cr-doped manganite Bi0.3Ca0.7Mn0.75Cr0.25O3 as reported by Zhang et al. [15]. In YFe1−xCrxO3, the magnetization reversal is explained based on isotropic super exchange and the antisymmetric DM interactions between
Corresponding author. E-mail address: [email protected] (M.R. Varma).
https://doi.org/10.1016/j.jmmm.2019.03.094 Received 27 January 2019; Received in revised form 15 March 2019; Accepted 21 March 2019 Available online 22 March 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 483 (2019) 89–94
K. Aswathi, et al.
there is a possibility to occupy the cations Cr and Mn in same crystallographic site. Hence the compound is considered as a disordered monoclinic Sm2CrMnO6 or half doped SmCr0.5Mn0.5O3 [20]. The XPS spectra of Cr 2p and Mn 2p in the compound Sm2CrMnO6 are measured. The binding energy for C 1s was taken as a reference for correcting the binding energy of the obtained data. The Shirley function is used for the background fitting. The fitted curves for the spectra (Fig. 2(a) and (b)) show a binding energy peak at 575.12 eV and 640.45 eV corresponding to Cr3+and Mn3+cations respectively [21].
Fe-O-Fe, Cr-O-Cr, and Fe-O-Cr [16]. Multiple temperature induced magnetization reversal is observed in SmCr1-xFexO3 system reported by Yin et al. [17]. The interplay of different magnetic components like ferromagnetic and paramagnetic present in SmCr0.85Mn0.15O3 leads to negative magnetization [18]. Chromates with negative magnetization are reported in several perovskite systems and in the present investigation we report the magnetization reversal in a distorted perovskite Sm2CrMnO6. 2. Experimental procedure
3.2. Magnetic characterization
Citrate-gel combustion method was used for the synthesis of Sm2CrMnO6 samples where citric acid was used as the fuel. The stoichiometric amount of high purity chemicals Sm(NO3)3.6H2O, Cr (NO3)2.6H2O, and Mn(NO3)2.4H2O from Sigma Aldrich (99.99%) were used for the synthesis. The precursor powder obtained after the combustion was pre-calcined at 600 °C for 2 h in the air. Then the sample was heat treated at high-temperatures ranging from 800 °C to 1000 °C in the air for 12 h with intermediate grinding. The obtained phase-pure sample was pelletized and sintered at 1000 °C for 12 h. The X-ray diffraction pattern of the phase pure sample was measured by using a PANalytical X’pert pro powder X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) and structure refinement was done with GSASEXPUI software [19]. Elemental composition was analyzed with X-ray photo electron spectrometer (XPS) PHI 5000 Versa probe scanning esca microscope with scanning monochromatic Al Kα X-ray source (Energy = 1486.6 eV) and peak fitting was done with XPSPEAK41 software. Magnetic moment as a function of temperature and field were measured by using a Vibration sample magnetometer (VSM) attached to the Physical property measurement system (PPMS Quantum Design, USA). Different field and temperature sweep-protocols were employed during VSM measurements.
3.2.1. M-H loop M-H hysteresis loops measured at different temperatures in the range 2–300 K are shown in Fig. 3(a). Here the hysteresis loops look like ferromagnetic but not saturated even in the applied field of 90 k Oe. It indicates the presence of antiferromagnetic interaction along with ferromagnetic interaction in the material [22–24]. The coercivity (HC) and remanence (Mr) obtained at different temperatures are depicted in Fig. 3 (b). At 2 K, HC is 4155 Oe, and Mr is 6.492 emu/g, which decreases as the temperature is increased and finally a linear M-H behavior (zero values for HC and Mr) corresponding to the paramagnetic state is observed at 100 K. 3.2.2. Sign reversal of magnetization The temperature dependent magnetization in zero field cooled (ZFC) and field cooled (FC) conditions with different external magnetic fields (20 Oe, 100 Oe, 500 Oe, and1000 Oe) are shown in Fig. 4(a)–(d). Under ZFC condition, the sample was cooled from 300 K to 2 K without applying an external magnetic field and, the measurement was performed on the warming from 2 to 300 K by applying an external magnetic field. Further, the FC measurement was carried out on cooling from 300 to 2 K under an external magnetic field. In ZFC condition the moment is gradually increasing with the increase in temperature and attain maximum value at 50 K then reach zero value as in different applied fields. Under FC condition at 20 Oe, the magnetic moment (MFC) initially increases slowly on decreasing temperature. At a temperature T1 ∼ 135 K, MFC reaches a maximum value Mmax = 1.51 emu/ mol as plotted in the inset of Fig. 4(a) and gradually decreases on further cooling. The MFC crosses zero at a temperature (the compensation temperature, Tcomp = 91 K) and changes to negative below Tcomp. Afterward, MFC attains a maximum negative value Mmin = −67.23 emu/mol near 17 K. Below this temperature, the magnetization starts increasing with decrease in temperature. The same trend is following in the applied fields of 100 Oe and 500 Oe. As the applied magnetic field is increased to 1000 Oe, the MFC crosses the zero value twice as indicated as Tcomp1 = 69 K and Tcomp2 = 8 K in Fig. 4(d) and then the MFC becomes positive i.e., the studied composition has two compensation temperature in the applied field of 1000 Oe. Increasing the applied field to 2500 Oe, the MFC is positive throughout the measurement temperature range, as shown in Fig. 5(a). A ferromagnetic transition is found at TC = 50 K for the material, which is calculated from the minimum of dM/dT vs. temperature plot in ZFC condition (Fig. 5(b)). Thus, the negative magnetic moment is observed in the perovskite Sm2CrMnO6 under field cooled condition in low positive magnetic fields below compensation temperature, as reported in several other perovskite systems [5,7,14,17,18,25]. Different explanations are reported for this unusual property and, the main cause for the occurrence of a sign reversal magnetization is the presence of different magnetic components in the material and the interaction between them [1–3,26–29]. For the confirmation of the occurrence of sign reversal of magnetization in the material, we have carried out a field polarity test as shown in Fig. 6. During this test, we measured the MFC for the temperature range of 300–2 K under both positive and negative magnetic
3. Results and discussion 3.1. Structural characterization and X-ray photoelectron spectroscopy The powder X-ray diffraction (XRD) data obtained from the measurement was refined by using the Rietveld refinement method for extracting the structural information of the compound and is shown in Fig. 1. It reveals that the material has a monoclinic structure with P121/n1 space group. The calculated lattice parameters are a (Å) = 5.631(3), b(Å) = 5.831(7), c(Å) = 7.991(4), α (°) = γ (°) = 90.000(0), and β (°) = 90.014(8). The quality factors of the refinement are χ2 = 2.147, Rp = 8.07%, and wRp = 10.72%. It is expected to be a stoichiometry A2BB’O6/AB0.5B’0.5O3 in simple perovskite ABO3 structure with (50:50 ratio) B site cationic substitution, such that
Fig. 1. The powder XRD data of Sm2CrMnO6 refined by using Rietveld method. 90
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Fig. 2. XPS spectra of (a) Cr 2p3/2 and (b) Mn 2p3/2 of Sm2CrMnO6 along with the fitted curves.
Fig. 3. (a) M-H loops at different temperatures (b) Temperature variation of HC and Mr.
Fig. 4. Temperature dependence of FC and ZFC magnetization at (a) 20 Oe (b) 100 Oe (c) 500 Oe (d)1000 Oe. 91
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Fig. 5. Temperature dependence of FC and ZFC magnetization at 2500 Oe (b) Derivative of ZFC magnetization with respect to temperature at different applied fields. Table 1 Fitted parameters obtained from the modified Curie-Weiss law. External Field (Oe)
Tcomp (K)
MCr (emu/mol)
H1 (Oe)
Mmin/Mmax
20 100 500 1000
93 90 81 69, 8
308.55 691.00 7052.00 4206.00
−85217 −111187 −3.986 × 106 −2.622 × 106
−104.00 −101.98 12.94 29.20
sign reversal of magnetization is an intrinsic material property and not an artifact occurring as a result of some trapped currents or residual fields in the superconducting magnet of the PPMS. For explaining the sign reversal magnetization, a model with two magnetic components in the material is considered. One magnetic component is due to the canted ferromagnetic Cr3+ spin (MCr), and the other component is due to combined paramagnetic Sm3+ and Mn3+ spins (M[Sm+Mn]). In field cooled condition, the first component (MCr) align in the same direction to the second component (M[Sm+Mn]) upon the application of the external magnetic field above Tcomp. Below Tcomp, under low applied fields viz. 20 Oe, 100 Oe, 500 Oe, the magnetization become negative and reaches a maximum negative value and then
Fig. 6. Magnetization as a function of temperature measured at H = ± 500 Oe under FC condition.
field of 500 Oe. The MFC at +500 Oe is exactly equal and opposite to that at −500 Oe and the Tcomp coincides exactly at 83 K under both positive and negative applied field. This polarity test confirms that the
Fig. 7. FC Fitted curves by modified curie Weiss law at (a) 20 Oe (b) 100 Oe (c) 500 Oe (d) 1000 Oe. 92
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Fig. 8. Derivative of fc magnetization with respect to temperature at 10 kOe. (b) 1/χ versus T graph at 30 kOe.
and Mn3+-Mn3+ interactions. This results in the reduction of transition temperature from 196 K to 50 K as shown in Fig. 5(b) and Fig. 8(a). The 1/χ–T graph at 30 kOe shown in Fig. 8(b) and the paramagnetic region is fitted with C-W law χ = C/(T-θ). The Curie constant (C) and the Weiss temperature (θ) were determined from the CW fit as 3.827 emu mol−1 K−1 and 2 K, respectively. The negative value of θ indicates the presence of antiferromagnetic interactions as reported by Bhame et al. for the system La2FeMnO6 [35]. The effective paramagnetic moment is calculated from the C-W analysis by using the
monotonically increase in the positive direction. The reason that below Tcomp, the ferromagnetic component MCr arising due to the canted antiferromagnetic state of Cr3+ align in the direction of field but the paramagnetic component M[Sm+Mn] due to the combined Sm3+ and Mn3+ align opposite to the direction of the field i.e. they couple antiferromagnetically resulting in a negative magnetization [30]. Nonetheless, the energy barrier created by magnetic anisotropy vanishes at high applied fields and the components of paramagnetic moments align in the direction of field since the applied field dominates the internal field and the net magnetic moment becomes positive [18,31]. The negative magnetization curves can be fitted by using the modified CurieC (H − H ) Weiss (C-W) law M = MCr + T −1 θ as shown in Fig. 7(a)–(d). Here M C is the total magnetization, MCr is the magnetic moment due to Cr moments, H is the applied magnetic field, H1 is the internal magnetic field and θC is the Curie-Weiss constant [3]. The obtained values of these components after fitting the negative FC moment below Tcomp by using the modified C-W law are tabulated in Table 1. From the results, one can notice that the internal magnetic field H1 (due to Cr3+ions) is in the opposite direction to the applied magnetic field, and its value depends upon the applied field as well. The observed increase in the MCr and H1 values (Table 1) can be understood in terms of an increase in AFM ordering with the applied field. Furthermore, a small negative value of θ indicates the prominent AFM interaction between the cantedCr3+and Mn3+/Sm3+ions in Sm2CrMnO6 under low applied magnetic field values [32,33,3]. The ratio of Mmin (negative) and the Mmax (positive) represents the quantitative measurement for magnetization reversal, pointing that the high external fields decrease the magnetization reversal which is also listed in Table 1. The existence of an antiferromagnetic coupling between the boundary of antiphase regions may result in the phenomenon sign reversal of magnetization. Such anti-phase regions due to Cr3+ and Mn3+ along with Sm3+ creates a barrier due to exchange anisotropy in low applied fields below compensation temperature. But at high applied fields the barrier created by exchange anisotropy vanishes and the net moment becomes positive [30]. “Neeraj et al. have reported the impact of Mn and Gd doping in SmCrO3on the magnetic properties of the material. It was observed that Mn and Gd substitution SmCrMO3 ∼ [SCMO] decreases the TN of the sample, SCMO due to the double exchange interaction between Mn3+and Cr3+ [18]. In the present investigation, the M-H loop of Sm2CrMnO6 looks like a ferromagnetic one and the TC obtained is 50 K, which indicates that the addition of Mn3+ and Cr3+ increases the ferromagnetic ordering. Also when the applied field is low, the negative magnetization in Sm2CrMnO6 disappears due to the suppression of magnetic anisotropy”. The compound SmCrO3 was reported with a magnetic transition TN = 196 K [34]. The substitution of Mn3+(t2g3eg1) on the Cr3+(t2g3eg0) site decrease the antiferromagnetic ordering. This can be explained in terms of dilution of Cr3+-Cr3+ interactions by Mn3+-Cr3+,
1/2
3k χ T , where µeff is measured in Bohr magequation, μeff = ⎛ B 2 ⎞ ⎝ Nμ0 μB ⎠ neton per formula units (µB/F.U.) C is the slope of the graph. Using this relation, the effective moment from experimental data is obtained as 5.53 µB/F.U., which is very close to the theoretical value of effective moment for spin only interaction 5.62 µB/F.U. Hence it can be concluded that Mn addition in SmCrO3 gives rise to minor domination of antiferromagnetic interaction and the combined effect of ferromagnetic and antiferromagnetic interactions between Cr3+, Sm3+ and Mn3+ leading to the interesting phenomenon sign reversal of magnetization.
4. Conclusion Monoclinic Sm2CrMnO6 is prepared by citrate–gel method. XPS analysis confirms the presence Cr3+ and Mn3+ cations in the compound. Sign reversal of magnetization is observed in the material. The reason for negative magnetization in low positive applied fields in this system is due to the interaction between different magnetic components. Combined moments of paramagnetic components Sm3+ and Mn3+ align in the opposite direction to the canted ferromagnetic Cr3+ because of the internal field. In short, the interplay of ferromagnetic and antiferromagnetic interaction by different components results a sign reversal of magnetization below compensation temperature at low fields. A ferromagnetic transition point is observed at 50 K and nonsaturating magnetic hysteresis loops gives a clear indication about the presence of AFM transition. Acknowledgment A.K. is acknowledging UGC (University Grant Commission) for providing senior research fellowship. R. R is thankful to KSCSTE (Kerala State Council for Science, Technology and Environment) for granting junior research fellowship. M.R.V. is thankful to council of Scientific and Industrial Research for financial support. References [1] J. Kim, N. Haberkorn, L. Civale, E. Nazaretski, P. Dowden, A. Saxena, R. Movshovich, A. Saxena, J.D. Thompson, R. Movshovich, Direct observation of magnetic phase coexistence and magnetization reversal in a Gd0.67Ca0.33MnO3 thin film, Appl. Phys. Lett. 100 (2012) 022407-1-022407-4.
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Research articles
Sign reversal of magnetization in Sm2CrMnO6 perovskites Kaipamangalath Aswathi ⁎ Manoj R. Varmaa,b,
a,b
b,c
T b
, Jasnamol Pezhumkattil Palakkal , Ramany Revathy ,
a Academy of Scientific and Innovative Research (AcSIR), CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST) Campus, Trivandrum 695 019, India b Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology, CSIR, Trivandrum 695 019, India c Technische Universität Darmstadt, Department of Materials and Earth Sciences, Alarich-Weiss-Strasse 2, 64287 Darmstadt, Germany
A R T I C LE I N FO
A B S T R A C T
Keywords: Sm2CrMnO6 Negative magnetization Compensation temperature Magnetic properties
A sign reversal of magnetization is observed in Sm2CrMnO6 synthesized via the citrate-gel method. The material crystallizes in monoclinic crystal structure with P121/n1 space group. The presence of Cr3+ and Mn3+ cations is confirmed by X-ray photoelectron spectroscopy analysis. The transition temperature (TC) is found to be at 50 K from the thermomagnetic data. The magnetization reverses its sign below a particular temperature under field cooled condition and the effect is reduced and disappeared at higher applied magnetic fields. The combined interaction between different magnetic components due to the presence of Sm3+, Cr3+, and Mn3+ leads to the sign reversal of magnetization.
1. Introduction Magnetization reversal (MR) is observed in many materials such as spinel ferrites, perovskites, molecular magnets, garnets, intermetallic alloys, and multilayers. Many intrinsic properties like crystal structure, magnetic anisotropy, magnetic exchange interactions, and temperature dependence of sublattice magnetization etc. are related to the sign reversal behavior. In many perovskites, negative magnetization was reported and was found to be due to different interactions [1–3]. MR observed in epitaxial Gd0.67Ca0.33MnO3 thin films is due to the antialigned sub lattices of Gd against Mn in the presence of an applied magnetic field [1]. In Gd0.67Ca0.33MnO3 perovskite, sign reversal of magnetization is observed due to the presence of different magnetic frame work of frozen ferromagnetic and paramagnetic ions with strong magnetic moments present in the system that behave differently under external magnetic field. In the case of ErMexMn(1−x)O3 (Me = Ni, Co) perovskite, the reason for negative magnetization is that the transition metal ordering at Tc creates a strong local field on the rare earth ion opposite to the applied field [4]. Negative magnetization was observed in Sr2YbRuO6 due to the presence of different magnetic components present in the system due to Ru5+and Yb3+ moments which align in the opposite direction with the application of external magnetic field [5]. A sign reversal of magnetization is also observed in a comparative study of bulk and thin films of ferromagnetic ErCo0.5Mn0.5O3 perovskite. Here the reason for this phenomena is the negative exchange interaction
⁎
between the uncompensated spins of Er ions and ferromagnetic Co and Mn ions [6,7]. Core-shell type La0.2Ce0.8CrO3 nanoparticles show negative magnetization, and in this configuration, the antiferromagnetic core of Cr3+ and Ce3+ and a disordered shell with the uncompensated spins in the core induces the sign reversal of magnetization [8]. Disorder in the occupation of Co and Ru sites induced negative magnetization in LaSrCoRuO6 [9]. Paramagnetic effect of the rare earth (RE) ions whose moments align opposite to the canted moment of transition metal ions leads to negative magnetization in YbCrO3chromates [10]. The interplay of different sublattice interaction leads to the sign reversal of magnetization in Er(CoMn)O3 perovskites [11]. Due to magnetocrystalline anisotropy, the net moment aligned in the direction opposite to the applied field gives spin reversal of magnetization in BiFe0.5Mn0.5O3 synthesized at high pressure [12]. In Al3+ substituted copper ferrite CuFe2O4, the reason for negative magnetization is the temperature dependence of magnetic moments of Cu2+ and Fe3+ that results in the antiparallel arrangement of spins [13]. Mao et al. have reported temperature and magnetic field induced negative magnetization in YFe0.5Cr0.5O3 due to antisymmetric Dzyaloshinskii–Moriya (DM) interaction and magnetocrystalline anisotropy [14]. D-M interaction and magnetostrictive distortion induced by orbital moments are the reasons for negative magnetization in Cr-doped manganite Bi0.3Ca0.7Mn0.75Cr0.25O3 as reported by Zhang et al. [15]. In YFe1−xCrxO3, the magnetization reversal is explained based on isotropic super exchange and the antisymmetric DM interactions between
Corresponding author. E-mail address: [email protected] (M.R. Varma).
https://doi.org/10.1016/j.jmmm.2019.03.094 Received 27 January 2019; Received in revised form 15 March 2019; Accepted 21 March 2019 Available online 22 March 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 483 (2019) 89–94
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there is a possibility to occupy the cations Cr and Mn in same crystallographic site. Hence the compound is considered as a disordered monoclinic Sm2CrMnO6 or half doped SmCr0.5Mn0.5O3 [20]. The XPS spectra of Cr 2p and Mn 2p in the compound Sm2CrMnO6 are measured. The binding energy for C 1s was taken as a reference for correcting the binding energy of the obtained data. The Shirley function is used for the background fitting. The fitted curves for the spectra (Fig. 2(a) and (b)) show a binding energy peak at 575.12 eV and 640.45 eV corresponding to Cr3+and Mn3+cations respectively [21].
Fe-O-Fe, Cr-O-Cr, and Fe-O-Cr [16]. Multiple temperature induced magnetization reversal is observed in SmCr1-xFexO3 system reported by Yin et al. [17]. The interplay of different magnetic components like ferromagnetic and paramagnetic present in SmCr0.85Mn0.15O3 leads to negative magnetization [18]. Chromates with negative magnetization are reported in several perovskite systems and in the present investigation we report the magnetization reversal in a distorted perovskite Sm2CrMnO6. 2. Experimental procedure
3.2. Magnetic characterization
Citrate-gel combustion method was used for the synthesis of Sm2CrMnO6 samples where citric acid was used as the fuel. The stoichiometric amount of high purity chemicals Sm(NO3)3.6H2O, Cr (NO3)2.6H2O, and Mn(NO3)2.4H2O from Sigma Aldrich (99.99%) were used for the synthesis. The precursor powder obtained after the combustion was pre-calcined at 600 °C for 2 h in the air. Then the sample was heat treated at high-temperatures ranging from 800 °C to 1000 °C in the air for 12 h with intermediate grinding. The obtained phase-pure sample was pelletized and sintered at 1000 °C for 12 h. The X-ray diffraction pattern of the phase pure sample was measured by using a PANalytical X’pert pro powder X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) and structure refinement was done with GSASEXPUI software [19]. Elemental composition was analyzed with X-ray photo electron spectrometer (XPS) PHI 5000 Versa probe scanning esca microscope with scanning monochromatic Al Kα X-ray source (Energy = 1486.6 eV) and peak fitting was done with XPSPEAK41 software. Magnetic moment as a function of temperature and field were measured by using a Vibration sample magnetometer (VSM) attached to the Physical property measurement system (PPMS Quantum Design, USA). Different field and temperature sweep-protocols were employed during VSM measurements.
3.2.1. M-H loop M-H hysteresis loops measured at different temperatures in the range 2–300 K are shown in Fig. 3(a). Here the hysteresis loops look like ferromagnetic but not saturated even in the applied field of 90 k Oe. It indicates the presence of antiferromagnetic interaction along with ferromagnetic interaction in the material [22–24]. The coercivity (HC) and remanence (Mr) obtained at different temperatures are depicted in Fig. 3 (b). At 2 K, HC is 4155 Oe, and Mr is 6.492 emu/g, which decreases as the temperature is increased and finally a linear M-H behavior (zero values for HC and Mr) corresponding to the paramagnetic state is observed at 100 K. 3.2.2. Sign reversal of magnetization The temperature dependent magnetization in zero field cooled (ZFC) and field cooled (FC) conditions with different external magnetic fields (20 Oe, 100 Oe, 500 Oe, and1000 Oe) are shown in Fig. 4(a)–(d). Under ZFC condition, the sample was cooled from 300 K to 2 K without applying an external magnetic field and, the measurement was performed on the warming from 2 to 300 K by applying an external magnetic field. Further, the FC measurement was carried out on cooling from 300 to 2 K under an external magnetic field. In ZFC condition the moment is gradually increasing with the increase in temperature and attain maximum value at 50 K then reach zero value as in different applied fields. Under FC condition at 20 Oe, the magnetic moment (MFC) initially increases slowly on decreasing temperature. At a temperature T1 ∼ 135 K, MFC reaches a maximum value Mmax = 1.51 emu/ mol as plotted in the inset of Fig. 4(a) and gradually decreases on further cooling. The MFC crosses zero at a temperature (the compensation temperature, Tcomp = 91 K) and changes to negative below Tcomp. Afterward, MFC attains a maximum negative value Mmin = −67.23 emu/mol near 17 K. Below this temperature, the magnetization starts increasing with decrease in temperature. The same trend is following in the applied fields of 100 Oe and 500 Oe. As the applied magnetic field is increased to 1000 Oe, the MFC crosses the zero value twice as indicated as Tcomp1 = 69 K and Tcomp2 = 8 K in Fig. 4(d) and then the MFC becomes positive i.e., the studied composition has two compensation temperature in the applied field of 1000 Oe. Increasing the applied field to 2500 Oe, the MFC is positive throughout the measurement temperature range, as shown in Fig. 5(a). A ferromagnetic transition is found at TC = 50 K for the material, which is calculated from the minimum of dM/dT vs. temperature plot in ZFC condition (Fig. 5(b)). Thus, the negative magnetic moment is observed in the perovskite Sm2CrMnO6 under field cooled condition in low positive magnetic fields below compensation temperature, as reported in several other perovskite systems [5,7,14,17,18,25]. Different explanations are reported for this unusual property and, the main cause for the occurrence of a sign reversal magnetization is the presence of different magnetic components in the material and the interaction between them [1–3,26–29]. For the confirmation of the occurrence of sign reversal of magnetization in the material, we have carried out a field polarity test as shown in Fig. 6. During this test, we measured the MFC for the temperature range of 300–2 K under both positive and negative magnetic
3. Results and discussion 3.1. Structural characterization and X-ray photoelectron spectroscopy The powder X-ray diffraction (XRD) data obtained from the measurement was refined by using the Rietveld refinement method for extracting the structural information of the compound and is shown in Fig. 1. It reveals that the material has a monoclinic structure with P121/n1 space group. The calculated lattice parameters are a (Å) = 5.631(3), b(Å) = 5.831(7), c(Å) = 7.991(4), α (°) = γ (°) = 90.000(0), and β (°) = 90.014(8). The quality factors of the refinement are χ2 = 2.147, Rp = 8.07%, and wRp = 10.72%. It is expected to be a stoichiometry A2BB’O6/AB0.5B’0.5O3 in simple perovskite ABO3 structure with (50:50 ratio) B site cationic substitution, such that
Fig. 1. The powder XRD data of Sm2CrMnO6 refined by using Rietveld method. 90
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Fig. 2. XPS spectra of (a) Cr 2p3/2 and (b) Mn 2p3/2 of Sm2CrMnO6 along with the fitted curves.
Fig. 3. (a) M-H loops at different temperatures (b) Temperature variation of HC and Mr.
Fig. 4. Temperature dependence of FC and ZFC magnetization at (a) 20 Oe (b) 100 Oe (c) 500 Oe (d)1000 Oe. 91
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Fig. 5. Temperature dependence of FC and ZFC magnetization at 2500 Oe (b) Derivative of ZFC magnetization with respect to temperature at different applied fields. Table 1 Fitted parameters obtained from the modified Curie-Weiss law. External Field (Oe)
Tcomp (K)
MCr (emu/mol)
H1 (Oe)
Mmin/Mmax
20 100 500 1000
93 90 81 69, 8
308.55 691.00 7052.00 4206.00
−85217 −111187 −3.986 × 106 −2.622 × 106
−104.00 −101.98 12.94 29.20
sign reversal of magnetization is an intrinsic material property and not an artifact occurring as a result of some trapped currents or residual fields in the superconducting magnet of the PPMS. For explaining the sign reversal magnetization, a model with two magnetic components in the material is considered. One magnetic component is due to the canted ferromagnetic Cr3+ spin (MCr), and the other component is due to combined paramagnetic Sm3+ and Mn3+ spins (M[Sm+Mn]). In field cooled condition, the first component (MCr) align in the same direction to the second component (M[Sm+Mn]) upon the application of the external magnetic field above Tcomp. Below Tcomp, under low applied fields viz. 20 Oe, 100 Oe, 500 Oe, the magnetization become negative and reaches a maximum negative value and then
Fig. 6. Magnetization as a function of temperature measured at H = ± 500 Oe under FC condition.
field of 500 Oe. The MFC at +500 Oe is exactly equal and opposite to that at −500 Oe and the Tcomp coincides exactly at 83 K under both positive and negative applied field. This polarity test confirms that the
Fig. 7. FC Fitted curves by modified curie Weiss law at (a) 20 Oe (b) 100 Oe (c) 500 Oe (d) 1000 Oe. 92
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Fig. 8. Derivative of fc magnetization with respect to temperature at 10 kOe. (b) 1/χ versus T graph at 30 kOe.
and Mn3+-Mn3+ interactions. This results in the reduction of transition temperature from 196 K to 50 K as shown in Fig. 5(b) and Fig. 8(a). The 1/χ–T graph at 30 kOe shown in Fig. 8(b) and the paramagnetic region is fitted with C-W law χ = C/(T-θ). The Curie constant (C) and the Weiss temperature (θ) were determined from the CW fit as 3.827 emu mol−1 K−1 and 2 K, respectively. The negative value of θ indicates the presence of antiferromagnetic interactions as reported by Bhame et al. for the system La2FeMnO6 [35]. The effective paramagnetic moment is calculated from the C-W analysis by using the
monotonically increase in the positive direction. The reason that below Tcomp, the ferromagnetic component MCr arising due to the canted antiferromagnetic state of Cr3+ align in the direction of field but the paramagnetic component M[Sm+Mn] due to the combined Sm3+ and Mn3+ align opposite to the direction of the field i.e. they couple antiferromagnetically resulting in a negative magnetization [30]. Nonetheless, the energy barrier created by magnetic anisotropy vanishes at high applied fields and the components of paramagnetic moments align in the direction of field since the applied field dominates the internal field and the net magnetic moment becomes positive [18,31]. The negative magnetization curves can be fitted by using the modified CurieC (H − H ) Weiss (C-W) law M = MCr + T −1 θ as shown in Fig. 7(a)–(d). Here M C is the total magnetization, MCr is the magnetic moment due to Cr moments, H is the applied magnetic field, H1 is the internal magnetic field and θC is the Curie-Weiss constant [3]. The obtained values of these components after fitting the negative FC moment below Tcomp by using the modified C-W law are tabulated in Table 1. From the results, one can notice that the internal magnetic field H1 (due to Cr3+ions) is in the opposite direction to the applied magnetic field, and its value depends upon the applied field as well. The observed increase in the MCr and H1 values (Table 1) can be understood in terms of an increase in AFM ordering with the applied field. Furthermore, a small negative value of θ indicates the prominent AFM interaction between the cantedCr3+and Mn3+/Sm3+ions in Sm2CrMnO6 under low applied magnetic field values [32,33,3]. The ratio of Mmin (negative) and the Mmax (positive) represents the quantitative measurement for magnetization reversal, pointing that the high external fields decrease the magnetization reversal which is also listed in Table 1. The existence of an antiferromagnetic coupling between the boundary of antiphase regions may result in the phenomenon sign reversal of magnetization. Such anti-phase regions due to Cr3+ and Mn3+ along with Sm3+ creates a barrier due to exchange anisotropy in low applied fields below compensation temperature. But at high applied fields the barrier created by exchange anisotropy vanishes and the net moment becomes positive [30]. “Neeraj et al. have reported the impact of Mn and Gd doping in SmCrO3on the magnetic properties of the material. It was observed that Mn and Gd substitution SmCrMO3 ∼ [SCMO] decreases the TN of the sample, SCMO due to the double exchange interaction between Mn3+and Cr3+ [18]. In the present investigation, the M-H loop of Sm2CrMnO6 looks like a ferromagnetic one and the TC obtained is 50 K, which indicates that the addition of Mn3+ and Cr3+ increases the ferromagnetic ordering. Also when the applied field is low, the negative magnetization in Sm2CrMnO6 disappears due to the suppression of magnetic anisotropy”. The compound SmCrO3 was reported with a magnetic transition TN = 196 K [34]. The substitution of Mn3+(t2g3eg1) on the Cr3+(t2g3eg0) site decrease the antiferromagnetic ordering. This can be explained in terms of dilution of Cr3+-Cr3+ interactions by Mn3+-Cr3+,
1/2
3k χ T , where µeff is measured in Bohr magequation, μeff = ⎛ B 2 ⎞ ⎝ Nμ0 μB ⎠ neton per formula units (µB/F.U.) C is the slope of the graph. Using this relation, the effective moment from experimental data is obtained as 5.53 µB/F.U., which is very close to the theoretical value of effective moment for spin only interaction 5.62 µB/F.U. Hence it can be concluded that Mn addition in SmCrO3 gives rise to minor domination of antiferromagnetic interaction and the combined effect of ferromagnetic and antiferromagnetic interactions between Cr3+, Sm3+ and Mn3+ leading to the interesting phenomenon sign reversal of magnetization.
4. Conclusion Monoclinic Sm2CrMnO6 is prepared by citrate–gel method. XPS analysis confirms the presence Cr3+ and Mn3+ cations in the compound. Sign reversal of magnetization is observed in the material. The reason for negative magnetization in low positive applied fields in this system is due to the interaction between different magnetic components. Combined moments of paramagnetic components Sm3+ and Mn3+ align in the opposite direction to the canted ferromagnetic Cr3+ because of the internal field. In short, the interplay of ferromagnetic and antiferromagnetic interaction by different components results a sign reversal of magnetization below compensation temperature at low fields. A ferromagnetic transition point is observed at 50 K and nonsaturating magnetic hysteresis loops gives a clear indication about the presence of AFM transition. Acknowledgment A.K. is acknowledging UGC (University Grant Commission) for providing senior research fellowship. R. R is thankful to KSCSTE (Kerala State Council for Science, Technology and Environment) for granting junior research fellowship. M.R.V. is thankful to council of Scientific and Industrial Research for financial support. References [1] J. Kim, N. Haberkorn, L. Civale, E. Nazaretski, P. Dowden, A. Saxena, R. Movshovich, A. Saxena, J.D. Thompson, R. Movshovich, Direct observation of magnetic phase coexistence and magnetization reversal in a Gd0.67Ca0.33MnO3 thin film, Appl. Phys. Lett. 100 (2012) 022407-1-022407-4.
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