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Hysteresis in magnetization–temperature curves of the orthochromite La0.1Gd0.9CrO3
Journal of Alloys and Compounds 545 (2012) 50–52
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Hysteresis in magnetization–temperature curves of the orthochromite La0.1Gd0.9CrO3 Neha Sharma a,⇑, Bipin K. Srivastava a, Anjali Krishnamurthy a, A.K. Nigam b a b
Department of Physics, University of Rajasthan, Jaipur 302 004, India Tata Institute of Fundamental Research, Mumbai 400 005, India
a r t i c l e
i n f o
Article history: Received 23 May 2012 Received in revised form 9 August 2012 Accepted 10 August 2012 Available online 19 August 2012 Keywords: Perovskites Orthochromites Thermal hysteresis in magnetization
a b s t r a c t Magnetic behavior is reported for lanthanum doped gadolinium orthochromite La0.1Gd0.9CrO3 in the temperature range 5–300 K. Magnetic hysteresis has been observed in field cooling magnetization– temperature curves measured by cooling the sample from 300 K down to 5 K in the presence of magnetic field and recording the data while cooling the sample (FCC mode) and also while heating it (FCH mode). The observation is attributed to a spin reorientation of the Cr3+ moments from high temperature GxFz configuration to an FxGz configuration at 25 K in FCC mode and subsequent inability of the system to revert back to GxFz configuration in FCH mode. Also reversal of magnetization is seen in the field cooling curves FCC while both zero field cooling and field cooling curves taken in heating mode FCH remain positive. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The rare earth orthochromites RCrO3 crystallize in distorted orthorhombic perovskite structure (space group Pnma) with four formula units per unit cell [1]. Owing to different magnetic interactions between the pairs of the two kinds of magnetic ions, R3+ and Cr3+, these systems undergo complex magnetic transitions at different temperatures. Magnetic structure of GdCrO3 has been reported in detail by Cooke et al. [2]. A reversal of magnetization is observed in the magnetization–temperature (M–T) curves of GdCrO3 (both nano and bulk particle samples) recorded in field cooled mode for low applied fields [3,4]. The lanthanum orthochromite LaCrO3 obtains antiferromagnetic structure below 285 K with Cr3+ moments aligning in canted orientation. We have undertaken study of the effect of substitution of lanthanum for gadolinium in GdCrO3 on the magnetic behavior. In this paper, we are reporting our study on a system with 10 atomic% of La substituted for Gd, viz., La0.1Gd0.9CrO3. The sample shows a magnetic thermal hysteresis in M–T curves recorded in field cooling mode. This is apart from the observation of magnetization reversal in M–T curves in low fields as was seen in our study on La0.5Gd0.5CrO3 [5]. There is considerable interest in observations on magnetic thermal hysteresis. Mollah et al. [6] reported thermal hysteresis in measurements of resistivity as well as magnetization in the polycrystalline manganites Pr0.65Ca0.35 xSrxMnO3 (x = 0–0.35). They explained it on the basis of interplay of charge and spin ordering. ⇑ Corresponding author. Tel./fax: +91 141 2706378. E-mail address: [email protected] (N. Sharma). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.08.051
An observation of hysteresis in strontium substituted system La1 xSrxMnO3 has been explained in terms of competing interactions between three dimensional metallic ferromagnetic and two dimensional metallic A-type antiferromagnetic phases [7]. Thermal hysteresis in magnetization and conductivity has been reported in the polycrystalline La0.55Dy0.12Ca0.33MnO3 and attributed to the presence of the canted ferromagnetic structure [8]. An inverse thermal hysteresis observed in field-cooled magnetization for polycrystalline La0.87Mn0.98Fe0.02Ox in the temperature range of 85–200 K has been ascribed to the inhomogeneous structural phase separation [9]. Thermal hysteresis of about 8 K in the magnetization and resistivity seen in (Sm0.65Sr0.35)MnO3 around the Curie temperature (Tc = 120 K) has been related to the existence of two paramagnetic phases [10]. Apart from in the oxide systems, magnetic hysteresis has also been reported in some metallic alloys. In Ni50.4Mn34.0Sn15.6, Shamberger and Ohuchi [11] reported hysteresis in the magnetic field induced martensitic phase transition. This arises due to only a limited fraction of the alloy transforming cyclically between the martensite and the austenite phases. In CoGd alloys [12] and the multilayered Co–Gd system [13], hysteresis is observed between two stable magnetic states. At higher temperatures, Co moments are aligned along the external field and Gd moments are aligned anti-parallel, and at lower temperatures Gd moments are aligned along the external field and Co moments is aligned anti-parallel. This paper reports the experimental details on sample preparation and the results on structural characterization and magnetic behavior of orthochromite La0.1Gd0.9CrO3. Magnetization has been measured, in zero field cooling (ZFC) and field cooling (FC) modes, as a function of temperature in the range 5–300 K in the presence
N. Sharma et al. / Journal of Alloys and Compounds 545 (2012) 50–52
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of magnetic field up to 400 Oe. We have explained our observation on magnetic thermal hysteresis in the studied sample in terms of spin reorientation arising from the effect of competition between the anisotropy field acting on the Cr ions and their interaction with the Gd ions. 2. Experimental details Polycrystalline sample of La0.1Gd0.9CrO3 has been prepared following solid state ceramic method, as described in our paper [5] on La0.5Gd0.5CrO3, using La2O3, Cr2O3 and Gd2O3 (all of 3N or better purity) as the starting materials. Structural characterization is done using powder X-ray diffraction measurement on PanAlytical make X’Pert PRO MPD diffractometer, with CuKa radiation, in 2h range of 20°–110°. Magnetization measurements have been performed employing a Quantum Design MPMS SQUID magnetometer. Magnetization versus temperature (M–T) data have been recorded in the temperature range 5–300 K in both zero field cooled (ZFC) and field cooled (FC) modes under externally applied magnetic fields ranging between 20 and 400 Oe. For ZFC measurements the sample was first cooled down to 5 K in zero external magnetic field, an external magnetic field was applied and magnetization was recorded in the presence of this field in the warming cycle. For FC measurements the sample was cooled under the presence of an external field of the same strength and magnetization was recorded in both cooling and warming cycles (referred to as FCC and FCH modes).
3. Results and discussion Fig. 1 shows X-ray diffraction pattern of the prepared sample of La0.1Gd0.9CrO3. All the reflections get indexed in orthorhombic structure with the lattice parameters a = 5.5109(2) Å, b = 7.6223(3) Å and c = 5.3350(2) Å. Reported cell constants for GdCrO3 [4] are 5.314, 7.606 and 5.524 Å respectively. For La0.5Gd0.5CrO3 the values are a = 5.4880(3) Å, b = 7.7203(7) Å and c = 5.4100(4) Å [4]. Fig. 2 shows M–T measurements recorded in the presence of an external field of 100 Oe. The first magnetic transition, attributed to Cr3+–Cr3+ exchange, is observed at TN 177 K (taken as the minima in dM/dT vs T curve); also ZFC and FCH curves depart from each other at this temperature. In ZFC measurement, the moment below TN continuously increases with decreasing temperature till it exhibits a maximum of magnetization at 6 K. Since this is close to that for spin reorientation temperature of 7 K reported [3] in single crystal GdCrO3, the behavior can be ascribed to the rotation of magnetic moments or magnetic domains, each of which has a
Fig. 1. Powder X-ray diffraction pattern of the perovskite La0.1Gd0.9CrO3. The observed and calculated patterns are shown as the crossed markers and the top solid line respectively. The vertical markers denote the angles of calculated Bragg reflections. The lowest solid line represents the difference between the calculated and observed intensities. RBragg = 4.42 and v2 = 1.63.
Fig. 2. M–T curves recorded for La0.1Gd0.9CrO3 in ZFC (filled circles) and FCH (open circles) modes under an applied field of 100 Oe. Inset shows the Neel temperature, i.e. the temperature of departure of FC and ZFC curves.
random orientation after zero field cooling. The profile of FCH curve is analogous to that of the ZFC curve except for the absence of low temperature peak in M–T curve. Now, as reported in our one earlier paper [5] FC curve for the sample with lower Gd concentration, viz., La0.5Gd0.5CrO3, shows reversal of magnetization under fields up to 500 Oe [5]. For the sample under study here, observation of FC mode magnetization under 100 Oe (Fig. 2) remaining positive throughout the temperature range of measurement merits special mention. In order to understand this, we recorded the FC mode M–T curves in different magnetic fields while cooling the sample down to 5 K and also while heating it up to 300 K. Fig. 3 shows the measurements under external fields 20, 100 and 400 Oe respectively. Under all the three applied fields, FC cooling curve (FCC) deviates considerably from the curve recorded in heating mode (FCH). All the curves thus exhibit thermal hysteresis. This shows that the transition proceeds as first order phase transition.
Fig. 3. FC data in both cooling (black filled circles) and warming (red open circles) modes for La0.1Gd0.9CrO3 taken at 20, 100 and 400 Oe. Insets show the behavior near Neel temperature (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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N. Sharma et al. / Journal of Alloys and Compounds 545 (2012) 50–52
In the un-substituted GdCrO3, Yoshii [4] and Sardar et al. [14] reported observations showing difference between M–T curves recorded in FC mode during cooling and warming of the sample. However, the authors did not suggest any explanation. In the sample under study, the profile of FCC curve can be explained by considering three types of magnetic interactions between two magnetic ions (Gd3+ and Cr3+) namely Cr3+–Cr3+, Cr3+–Gd3+ and Gd3+–Gd3+. Below Neel temperature TN (177 K), Cr3+ ions order in a GxFz configuration (following Bertaut notation [15], where Gx represents the basic antiferromagnetic spin arrangement of the Cr3+ ions along the a axis and Fz the ferromagnetic spin arrangement along the c axis due to the canting of the Gx spins). Cr3+ moments are canted out of true antiferromagnetic alignment due to the Dzialoshinski-Moriya type antisymmetric exchange interaction which arises due to the introduction of spin–orbit coupling as a perturbation on the quenched Cr3+ ground state. Below TN the moment initially increases with decreasing temperature and then decreases giving rise to a hump of positive magnetization before passing through zero at the compensation temperature Tcomp. As in the case of other analogous orthochromites, this can be attributed to the superposition of moments of the canted ordered sub-lattice of Cr3+ and the paramagnetic Gd3+ aligned under the combined external field and the internal field due to Cr3+ moments. Moment continues to decrease till a temperature Tmin before showing an upturn which is again a characteristic feature of the FCC curve [5,16–20]. As in GdCrO3 the upturn would be attributed to the zero field spin reorientation of the Cr3+ moments from the high temperature GxFz mode to an FxGz mode. As reported and explained in the case of GdCrO3 this would arise from the effect of competition between the anisotropy field acting on the Cr ions and their interaction with the Gd ions. The antisymmetric and the anisotropic-symmetric exchange interactions produce an effective field for Cr3+ up spins in the direction perpendicular to these spins and an effective field for the Cr3+ down spins in the direction opposite to the above [20]. As the temperature is lowered, these effective fields increase due to the increase of Gd3+ moment. When the absolute value of the interaction energy of Cr3+ spins with these effective fields exceeds the anisotropy energy of Cr3+ ion, these effective fields rotate the Cr3+ spins, keeping their original antiferromagnetic configuration. Then as the sample temperature is again raised, viz., in FCH measurement, spins do not reorient from FxGz mode to GxFz mode [21]. This is so because the Cr3+–Gd3+ magnetic interactions do not produce strong enough fields to rotate the Cr3+ spins. The fields direct along the moments of the original spin configuration, rather than in the direction perpendicular to the Cr3+ moment. The cause of magnetic hysteresis is thus different in the present case from those in the reported cases mentioned in Section 1. The fact that such a magnetic hysteresis was not observed in La0.5Gd0.5CrO3 system would owe to the fact that lower Gd concentration may not produce strong enough fields to re-orient the Cr3+ moments while cooling (FCC mode).
4. Conclusion Single phase gadolinium doped lanthanum chromite, La0.1Gd0.9CrO3 was prepared following solid state ceramic method. The system crystallizes in orthorhombic symmetry. As reported in the lesser Gd containing sample La0.5Gd0.5CrO3, reversal of magnetization is also observed in the sample under study in the field cooling curves (FCC) taken in cooling mode while both zero field cooling (ZFC) and field cooling curves taken in heating mode (FCH) remain positive. An interesting observation is of thermal hysteresis in FCC and FCH curves. This is explained by assuming reorientation of Cr3+ spins from GxFz to FxGz mode at a low temperature during cooling (FCC) and the same not reverting to GxFz during heating (FCH) on account of Cr3+–Gd3+ magnetic interactions not producing strong enough field as it is along the spin and not at right angle. Acknowledgements NS thanks the Council of Scientific and Industrial Research, New Delhi, for the award of a fellowship. Thanks are also due to University Grants Commission, New Delhi, for financial assistance. References [1] S. Geller, E.A. Wood, Acta Crystallogr. 9 (1956) 563–568. [2] A. Jaiswal, R. Das, K. Vivekanand, T. Maity, P.M. Abraham, S. Adyanthaya, P. Poddar, J. Appl. Phys. 107 (2010) 013912–013918. [3] A. Jaiswal, R. Das, K. Vivekanand, T. Maity, P.M. Abraham, S. Adyanthaya, P. Poddar, J. Appl. Phys. 107 (2010) 013912–013918. [4] K. Yoshii, J. Solid State Chem. 159 (2001) 204–208. [5] Neha Sharma, Bipin K. Srivastava, Anjali Krishnamurthy, A.K. Nigam, Solid State Sci. 12 (2010) 1464–1468. [6] S. Mollah, H.L. Huang, H.D. Yang, Mater. Lett. 61 (2007) 2329–2332. [7] Joonghoe Dho, W.S. Kim, N.H. Hur, Phys. Rev. Lett. 87 (1-4) (2001) 187201. [8] W. Zhong, W.P. Ding, Y.M. Zhou, W. Chen, Z.B. Guo, Y.W. Du, Q.J. Yan, Solid State Commun. 107 (1998) 55–58. [9] K. De, S. Majumdar, S. Giri, J. Appl. Phys. 101 (1–5) (2007) 103909. [10] R.P. Borges, F. Ott, R.M. Thomas, V. Skumryev, J.M.D. Coey, J.I. Arnaudas, L. Ranno, Phys. Rev. B 60 (1999) 12847–12851. [11] P.J. Shamberger, F.S. Ohuchi, Phys. Rev. B 79 (2009) 1–9. 144407. [12] S. Demirtas, R.E. Camley, A.R. Koymen, Appl. Phys. Lett. 87 (2005) 1–3. 202111. [13] S. Demirtas, M.R. Hossu, R.E. Camley, H.C. Mireles, A.R. Koymen, Phys. Rev. B 72 (1-7) (2005) 184433. [14] Kripasindhu Sardar, Martin R. Lees, Reza J. Kashtiban, Jeremy Sloan, Richard I. Walton, Chem. Mater. 23 (2011) 48–56. [15] E.F. Bertaut, in: G.T. Rado, H. Shul (Eds.), I. Magnetism, Academic Press, New York, 1963, p. 149. [16] Y. Ren, T.T.M. Palstra, D.I. Khomskii, E. Pellegrin, A.A. Nugroho, A.A. Menovsky, G.A. Sawatzky, Nature 396 (1998) 441–444. [17] K. Yoshii, A. Nakamura, Y. Ishii, Y. Morii, J. Solid State Chem. 162 (2001) 84–89. [18] Octavio Peña, Mona Bahout, Dicnisio Gutierrez, Pedro Duran, Carlos Moure, Solid State Sci. 5 (2003) 1217–1227. [19] R. Shukla, J. Manjanna, A.K. Bera, S.M. Yusuf, A.K. Tyagi, Inorg. Chem. 48 (2009) 11691–11696. [20] J. Hemberger, S. Lobina, H.A. Krug von Nidda, N. Tristan, V.Yu. Ivanov, A.A. Mukhin, A.M. Balbashov, A. Loidl, Phys. Rev. B. 70 (2004) 1–8. 024414. [21] T. Yamaguchi, J. Phys. Chem. Solids 35 (1974) 479–500.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Hysteresis in magnetization–temperature curves of the orthochromite La0.1Gd0.9CrO3 Neha Sharma a,⇑, Bipin K. Srivastava a, Anjali Krishnamurthy a, A.K. Nigam b a b
Department of Physics, University of Rajasthan, Jaipur 302 004, India Tata Institute of Fundamental Research, Mumbai 400 005, India
a r t i c l e
i n f o
Article history: Received 23 May 2012 Received in revised form 9 August 2012 Accepted 10 August 2012 Available online 19 August 2012 Keywords: Perovskites Orthochromites Thermal hysteresis in magnetization
a b s t r a c t Magnetic behavior is reported for lanthanum doped gadolinium orthochromite La0.1Gd0.9CrO3 in the temperature range 5–300 K. Magnetic hysteresis has been observed in field cooling magnetization– temperature curves measured by cooling the sample from 300 K down to 5 K in the presence of magnetic field and recording the data while cooling the sample (FCC mode) and also while heating it (FCH mode). The observation is attributed to a spin reorientation of the Cr3+ moments from high temperature GxFz configuration to an FxGz configuration at 25 K in FCC mode and subsequent inability of the system to revert back to GxFz configuration in FCH mode. Also reversal of magnetization is seen in the field cooling curves FCC while both zero field cooling and field cooling curves taken in heating mode FCH remain positive. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The rare earth orthochromites RCrO3 crystallize in distorted orthorhombic perovskite structure (space group Pnma) with four formula units per unit cell [1]. Owing to different magnetic interactions between the pairs of the two kinds of magnetic ions, R3+ and Cr3+, these systems undergo complex magnetic transitions at different temperatures. Magnetic structure of GdCrO3 has been reported in detail by Cooke et al. [2]. A reversal of magnetization is observed in the magnetization–temperature (M–T) curves of GdCrO3 (both nano and bulk particle samples) recorded in field cooled mode for low applied fields [3,4]. The lanthanum orthochromite LaCrO3 obtains antiferromagnetic structure below 285 K with Cr3+ moments aligning in canted orientation. We have undertaken study of the effect of substitution of lanthanum for gadolinium in GdCrO3 on the magnetic behavior. In this paper, we are reporting our study on a system with 10 atomic% of La substituted for Gd, viz., La0.1Gd0.9CrO3. The sample shows a magnetic thermal hysteresis in M–T curves recorded in field cooling mode. This is apart from the observation of magnetization reversal in M–T curves in low fields as was seen in our study on La0.5Gd0.5CrO3 [5]. There is considerable interest in observations on magnetic thermal hysteresis. Mollah et al. [6] reported thermal hysteresis in measurements of resistivity as well as magnetization in the polycrystalline manganites Pr0.65Ca0.35 xSrxMnO3 (x = 0–0.35). They explained it on the basis of interplay of charge and spin ordering. ⇑ Corresponding author. Tel./fax: +91 141 2706378. E-mail address: [email protected] (N. Sharma). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.08.051
An observation of hysteresis in strontium substituted system La1 xSrxMnO3 has been explained in terms of competing interactions between three dimensional metallic ferromagnetic and two dimensional metallic A-type antiferromagnetic phases [7]. Thermal hysteresis in magnetization and conductivity has been reported in the polycrystalline La0.55Dy0.12Ca0.33MnO3 and attributed to the presence of the canted ferromagnetic structure [8]. An inverse thermal hysteresis observed in field-cooled magnetization for polycrystalline La0.87Mn0.98Fe0.02Ox in the temperature range of 85–200 K has been ascribed to the inhomogeneous structural phase separation [9]. Thermal hysteresis of about 8 K in the magnetization and resistivity seen in (Sm0.65Sr0.35)MnO3 around the Curie temperature (Tc = 120 K) has been related to the existence of two paramagnetic phases [10]. Apart from in the oxide systems, magnetic hysteresis has also been reported in some metallic alloys. In Ni50.4Mn34.0Sn15.6, Shamberger and Ohuchi [11] reported hysteresis in the magnetic field induced martensitic phase transition. This arises due to only a limited fraction of the alloy transforming cyclically between the martensite and the austenite phases. In CoGd alloys [12] and the multilayered Co–Gd system [13], hysteresis is observed between two stable magnetic states. At higher temperatures, Co moments are aligned along the external field and Gd moments are aligned anti-parallel, and at lower temperatures Gd moments are aligned along the external field and Co moments is aligned anti-parallel. This paper reports the experimental details on sample preparation and the results on structural characterization and magnetic behavior of orthochromite La0.1Gd0.9CrO3. Magnetization has been measured, in zero field cooling (ZFC) and field cooling (FC) modes, as a function of temperature in the range 5–300 K in the presence
N. Sharma et al. / Journal of Alloys and Compounds 545 (2012) 50–52
51
of magnetic field up to 400 Oe. We have explained our observation on magnetic thermal hysteresis in the studied sample in terms of spin reorientation arising from the effect of competition between the anisotropy field acting on the Cr ions and their interaction with the Gd ions. 2. Experimental details Polycrystalline sample of La0.1Gd0.9CrO3 has been prepared following solid state ceramic method, as described in our paper [5] on La0.5Gd0.5CrO3, using La2O3, Cr2O3 and Gd2O3 (all of 3N or better purity) as the starting materials. Structural characterization is done using powder X-ray diffraction measurement on PanAlytical make X’Pert PRO MPD diffractometer, with CuKa radiation, in 2h range of 20°–110°. Magnetization measurements have been performed employing a Quantum Design MPMS SQUID magnetometer. Magnetization versus temperature (M–T) data have been recorded in the temperature range 5–300 K in both zero field cooled (ZFC) and field cooled (FC) modes under externally applied magnetic fields ranging between 20 and 400 Oe. For ZFC measurements the sample was first cooled down to 5 K in zero external magnetic field, an external magnetic field was applied and magnetization was recorded in the presence of this field in the warming cycle. For FC measurements the sample was cooled under the presence of an external field of the same strength and magnetization was recorded in both cooling and warming cycles (referred to as FCC and FCH modes).
3. Results and discussion Fig. 1 shows X-ray diffraction pattern of the prepared sample of La0.1Gd0.9CrO3. All the reflections get indexed in orthorhombic structure with the lattice parameters a = 5.5109(2) Å, b = 7.6223(3) Å and c = 5.3350(2) Å. Reported cell constants for GdCrO3 [4] are 5.314, 7.606 and 5.524 Å respectively. For La0.5Gd0.5CrO3 the values are a = 5.4880(3) Å, b = 7.7203(7) Å and c = 5.4100(4) Å [4]. Fig. 2 shows M–T measurements recorded in the presence of an external field of 100 Oe. The first magnetic transition, attributed to Cr3+–Cr3+ exchange, is observed at TN 177 K (taken as the minima in dM/dT vs T curve); also ZFC and FCH curves depart from each other at this temperature. In ZFC measurement, the moment below TN continuously increases with decreasing temperature till it exhibits a maximum of magnetization at 6 K. Since this is close to that for spin reorientation temperature of 7 K reported [3] in single crystal GdCrO3, the behavior can be ascribed to the rotation of magnetic moments or magnetic domains, each of which has a
Fig. 1. Powder X-ray diffraction pattern of the perovskite La0.1Gd0.9CrO3. The observed and calculated patterns are shown as the crossed markers and the top solid line respectively. The vertical markers denote the angles of calculated Bragg reflections. The lowest solid line represents the difference between the calculated and observed intensities. RBragg = 4.42 and v2 = 1.63.
Fig. 2. M–T curves recorded for La0.1Gd0.9CrO3 in ZFC (filled circles) and FCH (open circles) modes under an applied field of 100 Oe. Inset shows the Neel temperature, i.e. the temperature of departure of FC and ZFC curves.
random orientation after zero field cooling. The profile of FCH curve is analogous to that of the ZFC curve except for the absence of low temperature peak in M–T curve. Now, as reported in our one earlier paper [5] FC curve for the sample with lower Gd concentration, viz., La0.5Gd0.5CrO3, shows reversal of magnetization under fields up to 500 Oe [5]. For the sample under study here, observation of FC mode magnetization under 100 Oe (Fig. 2) remaining positive throughout the temperature range of measurement merits special mention. In order to understand this, we recorded the FC mode M–T curves in different magnetic fields while cooling the sample down to 5 K and also while heating it up to 300 K. Fig. 3 shows the measurements under external fields 20, 100 and 400 Oe respectively. Under all the three applied fields, FC cooling curve (FCC) deviates considerably from the curve recorded in heating mode (FCH). All the curves thus exhibit thermal hysteresis. This shows that the transition proceeds as first order phase transition.
Fig. 3. FC data in both cooling (black filled circles) and warming (red open circles) modes for La0.1Gd0.9CrO3 taken at 20, 100 and 400 Oe. Insets show the behavior near Neel temperature (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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N. Sharma et al. / Journal of Alloys and Compounds 545 (2012) 50–52
In the un-substituted GdCrO3, Yoshii [4] and Sardar et al. [14] reported observations showing difference between M–T curves recorded in FC mode during cooling and warming of the sample. However, the authors did not suggest any explanation. In the sample under study, the profile of FCC curve can be explained by considering three types of magnetic interactions between two magnetic ions (Gd3+ and Cr3+) namely Cr3+–Cr3+, Cr3+–Gd3+ and Gd3+–Gd3+. Below Neel temperature TN (177 K), Cr3+ ions order in a GxFz configuration (following Bertaut notation [15], where Gx represents the basic antiferromagnetic spin arrangement of the Cr3+ ions along the a axis and Fz the ferromagnetic spin arrangement along the c axis due to the canting of the Gx spins). Cr3+ moments are canted out of true antiferromagnetic alignment due to the Dzialoshinski-Moriya type antisymmetric exchange interaction which arises due to the introduction of spin–orbit coupling as a perturbation on the quenched Cr3+ ground state. Below TN the moment initially increases with decreasing temperature and then decreases giving rise to a hump of positive magnetization before passing through zero at the compensation temperature Tcomp. As in the case of other analogous orthochromites, this can be attributed to the superposition of moments of the canted ordered sub-lattice of Cr3+ and the paramagnetic Gd3+ aligned under the combined external field and the internal field due to Cr3+ moments. Moment continues to decrease till a temperature Tmin before showing an upturn which is again a characteristic feature of the FCC curve [5,16–20]. As in GdCrO3 the upturn would be attributed to the zero field spin reorientation of the Cr3+ moments from the high temperature GxFz mode to an FxGz mode. As reported and explained in the case of GdCrO3 this would arise from the effect of competition between the anisotropy field acting on the Cr ions and their interaction with the Gd ions. The antisymmetric and the anisotropic-symmetric exchange interactions produce an effective field for Cr3+ up spins in the direction perpendicular to these spins and an effective field for the Cr3+ down spins in the direction opposite to the above [20]. As the temperature is lowered, these effective fields increase due to the increase of Gd3+ moment. When the absolute value of the interaction energy of Cr3+ spins with these effective fields exceeds the anisotropy energy of Cr3+ ion, these effective fields rotate the Cr3+ spins, keeping their original antiferromagnetic configuration. Then as the sample temperature is again raised, viz., in FCH measurement, spins do not reorient from FxGz mode to GxFz mode [21]. This is so because the Cr3+–Gd3+ magnetic interactions do not produce strong enough fields to rotate the Cr3+ spins. The fields direct along the moments of the original spin configuration, rather than in the direction perpendicular to the Cr3+ moment. The cause of magnetic hysteresis is thus different in the present case from those in the reported cases mentioned in Section 1. The fact that such a magnetic hysteresis was not observed in La0.5Gd0.5CrO3 system would owe to the fact that lower Gd concentration may not produce strong enough fields to re-orient the Cr3+ moments while cooling (FCC mode).
4. Conclusion Single phase gadolinium doped lanthanum chromite, La0.1Gd0.9CrO3 was prepared following solid state ceramic method. The system crystallizes in orthorhombic symmetry. As reported in the lesser Gd containing sample La0.5Gd0.5CrO3, reversal of magnetization is also observed in the sample under study in the field cooling curves (FCC) taken in cooling mode while both zero field cooling (ZFC) and field cooling curves taken in heating mode (FCH) remain positive. An interesting observation is of thermal hysteresis in FCC and FCH curves. This is explained by assuming reorientation of Cr3+ spins from GxFz to FxGz mode at a low temperature during cooling (FCC) and the same not reverting to GxFz during heating (FCH) on account of Cr3+–Gd3+ magnetic interactions not producing strong enough field as it is along the spin and not at right angle. Acknowledgements NS thanks the Council of Scientific and Industrial Research, New Delhi, for the award of a fellowship. Thanks are also due to University Grants Commission, New Delhi, for financial assistance. References [1] S. Geller, E.A. Wood, Acta Crystallogr. 9 (1956) 563–568. [2] A. Jaiswal, R. Das, K. Vivekanand, T. Maity, P.M. Abraham, S. Adyanthaya, P. Poddar, J. Appl. Phys. 107 (2010) 013912–013918. [3] A. Jaiswal, R. Das, K. Vivekanand, T. Maity, P.M. Abraham, S. Adyanthaya, P. Poddar, J. Appl. Phys. 107 (2010) 013912–013918. [4] K. Yoshii, J. Solid State Chem. 159 (2001) 204–208. [5] Neha Sharma, Bipin K. Srivastava, Anjali Krishnamurthy, A.K. Nigam, Solid State Sci. 12 (2010) 1464–1468. [6] S. Mollah, H.L. Huang, H.D. Yang, Mater. Lett. 61 (2007) 2329–2332. [7] Joonghoe Dho, W.S. Kim, N.H. Hur, Phys. Rev. Lett. 87 (1-4) (2001) 187201. [8] W. Zhong, W.P. Ding, Y.M. Zhou, W. Chen, Z.B. Guo, Y.W. Du, Q.J. Yan, Solid State Commun. 107 (1998) 55–58. [9] K. De, S. Majumdar, S. Giri, J. Appl. Phys. 101 (1–5) (2007) 103909. [10] R.P. Borges, F. Ott, R.M. Thomas, V. Skumryev, J.M.D. Coey, J.I. Arnaudas, L. Ranno, Phys. Rev. B 60 (1999) 12847–12851. [11] P.J. Shamberger, F.S. Ohuchi, Phys. Rev. B 79 (2009) 1–9. 144407. [12] S. Demirtas, R.E. Camley, A.R. Koymen, Appl. Phys. Lett. 87 (2005) 1–3. 202111. [13] S. Demirtas, M.R. Hossu, R.E. Camley, H.C. Mireles, A.R. Koymen, Phys. Rev. B 72 (1-7) (2005) 184433. [14] Kripasindhu Sardar, Martin R. Lees, Reza J. Kashtiban, Jeremy Sloan, Richard I. Walton, Chem. Mater. 23 (2011) 48–56. [15] E.F. Bertaut, in: G.T. Rado, H. Shul (Eds.), I. Magnetism, Academic Press, New York, 1963, p. 149. [16] Y. Ren, T.T.M. Palstra, D.I. Khomskii, E. Pellegrin, A.A. Nugroho, A.A. Menovsky, G.A. Sawatzky, Nature 396 (1998) 441–444. [17] K. Yoshii, A. Nakamura, Y. Ishii, Y. Morii, J. Solid State Chem. 162 (2001) 84–89. [18] Octavio Peña, Mona Bahout, Dicnisio Gutierrez, Pedro Duran, Carlos Moure, Solid State Sci. 5 (2003) 1217–1227. [19] R. Shukla, J. Manjanna, A.K. Bera, S.M. Yusuf, A.K. Tyagi, Inorg. Chem. 48 (2009) 11691–11696. [20] J. Hemberger, S. Lobina, H.A. Krug von Nidda, N. Tristan, V.Yu. Ivanov, A.A. Mukhin, A.M. Balbashov, A. Loidl, Phys. Rev. B. 70 (2004) 1–8. 024414. [21] T. Yamaguchi, J. Phys. Chem. Solids 35 (1974) 479–500.