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Effect of oxygen deficit on magnetic properties of LaCo0.5Fe0.5O3
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 298 (2006) 19–24 www.elsevier.com/locate/jmmm
Effect of oxygen deficit on magnetic properties of LaCo0.5Fe0.5O3 I.O. Troyanchuka,, D.V. Karpinskya,, R. Szymczakb, H. Szymczakb a
Institute of Solids and Semiconductors Physics, NAS of Belarus, Minsk, Belarus b Institute of Physics PAS, Warsaw, Poland Received 12 October 2004; received in revised form 14 February 2005 Available online 25 March 2005
Abstract LaCo0.5Fe0.5O3+d system has been prepared using different thermal treatments by conventional ceramic technology. The samples prepared at 1570 K in air and below this temperature are mixtures of orthorhombic and rhombohedral phases. Increasing temperature up to 1670 K stabilizes the rhombohedral phase whereas annealing in vacuumed quartz tube with a metallic getter results in orthorhombic single-phase structure. All the samples exhibit the ferromagnetic component at temperatures up to 360 K as a result of Dzialoshinsky–Moriya interactions between the Fe3+ ions. The orthorhombic samples have a giant magnetic anisotropy, mostly associated with Co2+ ions. Co3+ ions are supposed to adopt a low spin state. r 2005 Elsevier B.V. All rights reserved. PACS: 75.30.m; 75.30.Gw; 75.60.d Keywords: Magnetically ordered materials; Magnetic anisotropy; Domain effects; Magnetization and hysteresis
1. Introduction In LaCoO3 tri-valent Co3+ ions exhibit a transition of spin state around 90 K: from the low spin (LS) non-magnetic ground state ðS ¼ 0Þ to the intermediate one (IS, S ¼ 1). A diffuse Corresponding author. Tel.: +375 017 284 03 36;
fax: +375 017 284 08 88. E-mail addresses: [email protected] (I.O. Troyanchuk), [email protected] (D.V. Karpinsky).
metal–insulator (MI) transition is observed in this compound at 500 K [1–4]. The resistivity is characterized with an activated character at low temperatures, decreases by some orders of the value at 500 K and increases with temperature at higher temperatures [1]. For Sr-doped system La1xSrxCoO3 the transition of the spin state at 90 K disappears, and cobalt ions remain magnetic down to the lowest temperature. Samples with Sr content x40.2 exhibit a spontaneous magnetization at low temperatures [5]. Some researchers
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.03.009
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have supposed a coexistence of both cluster spinglass state and ferromagnetism in the region 0.3oxo0.5 on the base of the data on magnetization and neutron diffraction [6]. For less Sr content value (xo0.2) the magnetization measurements indicate the spin-glass ground state [6,7] where strong ferromagnetic short-order correlations are observed according to experimental data on neutron diffuse scattering [7]. The crystal lattice expansion as a result of Sr-substitution was supposed to stabilize the intermediate spin state of Co3+ [7], whereas the ferromagnetic short order could be caused by charge carrier’s introduction also changing the electrotransport properties. For L1xSrxCoO3 at xo0.2 the r value is less by some orders than for LaCoO3, though it retains the semiconductor behavior [8]. Above 500 K, r increases with temperature, indicating insulator– metal (IM) transition at 500 K, as in the case of LaCoO3. In [9] the possibility was shown of changing the magnetic state of Co3+ ions in LaCoO3 by substitution not only for Sr ions but also for isovalent Ni3+ ions. The La(Co1xNix)O3 samples with x ¼ 0:5 exhibit ferromagnetic component at low temperatures [10]. The LaCo1xFexO3 system was investigated earlier from a viewpoint of catalytic activity [11]. There are contradicting data on the crystal structure in the works devoted to this system. It was noted that the rhombohedral phase coexists with orthorhombic in a wide concentration interval of Fe content [11]. In the present work we demonstrate that crystallographically single-phase samples can be prepared by high-temperature synthesis as well as thermal treatment in reducing medium. Crystal structure and magnetic properties of LaCo0.5Fe0.5O3 nominal composition dramatically depend on oxygen content. The cobalt ions seem to be in a low spin state.
2. Experiment The La(Co1xFex)O3 samples have been prepared from the mixture of oxides La2O3, Co3O4 and Fe2O3 using standard ceramic technology. The temperature of synthesis has been varied from
1470 to 1770 K. Some of the samples were reduced in vacuumed quartz ampoules at 1170 K. Metallic tantalum was used as an oxygen getter. X-ray diffraction measurements were performed in Cuka radiation using DRON-3 M diffractometer. Crystal structure parameters were calculated using a FullProf program. Magnetic measurements have been carried out with commercial magnetometer MPMS-5 (Quantum Design). The Mo¨ssbauer spectra of the samples have been recorded with a 57 Co(Rh) source in transmission geometry. The recorded Mo¨ssbauer data were computer fitted by the MOSMOD program. Isomeric shift is given in relation to the a-Fe. The surface topography of the obtained samples was studied with a ‘‘KARL ZEISS’’ raster electronic microscope. The X-ray spectral analysis was performed using energy dispersion Si–Li semiconductor detector.
3. Results and discussion The crystal structure parameters of the LaCo0.5 Fe0.5O3 composition obtained at 1570 K in air were refined successfully assuming that the sample consists of mixture of rhombohedral ðR 3¯ cÞ and orthorhombic (Pnma) phases. Increasing synthesis temperature up to 1670 K results in the crystal structure homogeneity. High temperature favors apparently the solid solutions formation. The structure of the sample prepared at 1770 K was refined as rhombohedral (space group R 3¯ c). However, there is another method to prepare samples with homogeneous crystal structure associated with thermal treatment in reducing medium. Fig. 1 demonstrates the results of the structure calculation using FullProf program for the sample with x ¼ 0:5 prepared at 1570 K in air, at 1770 K in air as well as that reduced at T ¼ 1170 K in vacuumed quartz ampoule in the presence of metallic tantalum. For the reduction the sample was taken prepared at 1570 K in air. The treatment of the sample in the vacuumed ampoule led to the mass loss corresponding to the oxygen content decreasing by 2%, and the structure became orthorhombic (space group Pnma). According to the data of X-rays spectral microanalysis, the distribution of cobalt and
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I.O. Troyanchuk et al. / Journal of Magnetism and Magnetic Materials 298 (2006) 19–24
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20
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2
Fig. 1. FullProf refinement patterns for La(Co0.5Fe0.5)O3 samples prepared at 1570 K in air (a), at 1770 K in air (b) and that reduced in vacuumed quartz tube (c) using X-ray powder diffraction data. The observed intensities are shown by dots and the calculated ones by solid line. The positions of the Bragg reflections are shown by the small vertical lines below the patterns. The line at the bottom indicates the intensity difference between the experimental and the refined patterns.
manganese ions is rather homogeneous. The average size of the crystallites is about 5 mm. Unit cell parameters, symmetry and reliability factors are presented in Table 1. Both orthorhombic and rhombohedral samples have a small spontaneous magnetic moment. Fig. 2 demonstrates the magnetization of the samples x ¼ 0:5 prepared at 1770 K in air and the remanent magnetization of the sample reduced in vacuum. For the reduced sample, magnetization measurements have been performed without external magnetic field on heating, after cooling down to liquid helium temperature. At room temperature the reduced sample has a small spontaneous magnetic moment which does not disappear even on heating up to 360 K (the upper temperature limit of measurement). So one can believe the critical temperature of magnetic order destruction to be slightly above 360 K. In the temperature interval 5–100 K the reduced sample exhibits an anomalous magnetization behavior indicating the possible phase transition associated apparently with spin reorientation. Small spontaneous magnetization of the samples (Fig. 2) could be explained in terms of weak ferromagnetism as a result of Dzialoshinsky–Moriya interactions, since both the samples, containing the orthorhombic and rhombohedral phases allow the appearance of weak ferromagnetic component. The rhombohedral oxidized sample exhibits phase transition into paramagnetic state at a slightly lower temperature of 300 K (Fig. 2).
Magnetization vs. field dependences evidence a giant magnetic anisotropy in the compound x ¼ 0:5: Fig. 3 shows the hysteresis loops for the sample synthesized in air at 1770 K and that reduced in quartz ampoule. The first sample is a hard magnetic material, because the coercive field at room temperature is about 1 kOe. For the second sample the oxygen loss by reduction leads to a sharp increase of magnetic anisotropy (the value of coercive field about 10 kOe at room temperature is unique for oxide compounds). Magnetic anisotropy increases with a decrease in the temperature. At 10 K the coercive field value of the oxidized sample reaches 4 kOe, whereas for the reduced one an external magnetic field of 50 kOe is too small in order to reorient the magnetic moments (Fig. 4). In order to clarify the origin of such magnetic anisotropy the reduced sample has been oxidized by annealing at T ¼ 1170 K in air. We observed that orthorhombic phase transformed in rhombohedral and magnetic anisotropy strongly decreased. The Mo¨ssbauer spectrum of the sample x ¼ 0:5 prepared at 1670 K (rhombohedral phase) at T ¼ 293 K consists of quadrupole splitted doublet: DQ ¼ 0:440 mm=s: Isomer shift value d ¼ 0:369 mm=s evidences the Fe ions to be in tri-valent state. The spectra of the orthorhombic samples consist of two or more sextets indicating the magnetic nonequivalence of Fe ions. Magnetic properties of LaCo0.5Fe0.5O3 differ strongly from those of the end members LaCoO3
ARTICLE IN PRESS I.O. Troyanchuk et al. / Journal of Magnetism and Magnetic Materials 298 (2006) 19–24
0.20
M (emu/g)
R ¼ 0:75; R ¼ 0:81; R ¼ 1:95; R ¼ 0:43;
R-factors
RF ¼ 0:52 RF ¼ 0:63 RF ¼ 1:51 RF ¼ 0:44
22
FC ZFC reduced
0.15 0.10
1.32 0.89
0.83
w2
0.05 0.00
57.523 58.192 57.533 58.405 R 3¯ c: a ¼ 5:4908ð1Þ; c ¼ 13:2201ð6Þ; Pnma: a ¼ 5:4685ð3Þ; b ¼ 7:7548ð7Þ; c ¼ 5:5001ð8Þ; a ¼ 5:4877ð3Þ; b ¼ 13:2360ð5Þ; a ¼ 5:4627ð7Þ; b ¼ 7:7573ð8Þ; c ¼ 5:5129ð1Þ;
200 T (K)
300
400
Fig. 2. Temperature dependence of remanent magnetization for the reduced La(Co0.5Fe0.5)O3d sample (asterix mark) at field H ¼ 0 Oe; FC and ZFC magnetization for La(Co0.5 Fe0.5)O3 sample prepared at 1770 K at H ¼ 100 Oe:
1.0
0.5 M (emu/g)
Volume, (A˚3)
100
0.0
air prepared reduced T=300 K
-0.5
-1.0 -60
-40
-20
0 H (kOe)
20
40
60
R 3¯ c þ Pnma
R 3¯ c Pnma LaCo0.5Fe0.5O3 sintered 1770 K LaCo0.5Fe0.5O3 sintered 1570 K and reduced in vacuum
Fig. 3. Hysteresis loops of the LaCo0.5Fe0.5O3 sample prepared at 1770 K in air and the reduced one. T ¼ 300 K:
LaCo0.5Fe0.5O3 sintered 1570 K
Table 1
Space group
Cell parameters, (A˚)
0
and LaFeO3 of the solid solutions system. LaCoO3 exhibits diamagnetic properties at low temperature as a result of low spin state of Co3+ ions. LaFeO3 is known to be a weak ferromagnet with Neel temperature of 750 K [12]. The small ferromagnetic component is due to the antisymmetric interactions of Dzialoshinsky–Moriya. We have failed to evaluate precisely the spontaneous magnetization of the single-phase reduced sample x ¼ 0:5 with the orthorhombic structure. However, on the basis of obtained data one can suppose that this value is about 1 emu/g, which is
ARTICLE IN PRESS I.O. Troyanchuk et al. / Journal of Magnetism and Magnetic Materials 298 (2006) 19–24
characteristic of orthoferrites, being weak ferromagnets. It is well known that the cobalt ion in the intermediate spin state has ionic radius larger than the Co3+ in low spin state [13]. It was found that the unit cell volume of LaNi0.5Co0.5O3 is more than for LaCoO3 or LaNiO3 which suggests that
the most part of cobalt ions in LaNi0.5Co0.5O3 adopts intermediate spin state. However, we observed that the unit cell volume of LaCo1xFexO3 prepared at a high temperature monitonically increases with Fe content (Fig. 5). On the basis of this fact we cannot draw any conclusions concerning the spin state of the cobalt ions, but according to the magnetic data the most probable spin state of the Co3+ ions is the low spin one. The strong magnetic anisotropy of oxidized LaCo0.5Fe0.5O3 is associated with antisymmetric exchange of Dzialoshinsky–Moriya because this type of exchange is directly connected with spin–orbital interactions [14]. The most likely magnetic anisotropy increase observed in the reduced sample is due mainly to the contribution of cobalt ions. Tri-valent cobalt ions in the intermediate or high spin state as well as di-valent cobalt ions are known to enhance the magnetic anisotropy significantly as a result of spin–orbital interaction. The main part of cobalt ions in the composition x ¼ 0:5 is apparently in tri-valent low spin state, however, the di-valent cobalt ions and oxygen vacancies should appear by reduction admitting
2.0 1.5
M (emu/g)
1.0 0.5 0.0 -0.5
air prepared reduced T=10K
-1.0 -1.5 -2.0 -60
-40
-20
0 H (kOe)
20
40
23
60
Fig. 4. Hysteresis loops of the LaCo0.5Fe0.5O3 sample prepared at 1770 K in air and the reduced one. T ¼ 10 K:
La(Co1-xFex)O3
61.0
58
Vol 60.6
56 rhomb
orth 5.56
unit cell parameter, (A)
volume, (A3)
60.8
5.52
b
a1
5.4
c
5.48 a2
5.44
5.3 0
20
40
60
80
unit cell parameters, (A)
alfa angle, (deg)
60 alfa
100
x Fig. 5. Unit cell parameters for the samples, prepared at 1770 K. (rhomb—rhombohedral region 0pxo0.55, alfa—rhombohedral angle, a1—cell parameters for R 3¯ c space group; orth—orthorhombic region, 0.55pxo1 a2, b, c—cell parameters for Pnma space group; volume is calculated per formula unit).
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the starting composition to be nearly stoichiometric on oxygen. So, the Co2+ ions could affect significantly the magnetic anisotropy value whereas the oxygen vacancies could reduce the domain boundaries mobility. Since, the magnetic domain walls become immobile, the magnetic domain reorientation takes place by the way of magnetic moments rotation similar to the case of spin-glasses.
4. Conclusions Using standard ceramic technology, we have managed to obtain the LaCo0.5Fe0.5O3 samples, the structural homogeneity and magnetic properties of which may be controlled by changing the temperature conditions of synthesis and oxygen content using the reduction in vacuum. Fe and Co ions are in tri-valent state, however, an insignificant quantity of di-valent cobalt can appear by reduction. The reduction is shown to lead to a strong increase of magnetic anisotropy associated with Co2+ ions and to convert the rhombohedral phase into the orthorhombic one. The spontaneous magnetization of orthorhombic compounds is supposed to be due to antisymmetric exchange of Dzialoshinsky–Moriya, whereas lowspin Co3+ ions do not affect strongly the magnetic properties.
Acknowledgments The work was supported partly by Fund for fundamental research of Belarus (Project F03-155) and the State Committee for Scientific Research (Poland) (Grant no. KBN 1 P03B 038 27). References [1] R.R. Heikes, R.C. Miller, R. Mazelsky, Physica 30 (1964) 1600. [2] G.H. Jonker, J. Appl. Phys. 37 (1966) 1424. [3] K. Asai, O. Yokokura, M. Suzuki, et al., J. Phys. Soc. Japan 66 (1997) 967. [4] K. Asai, A. Atsuro, O. Yokokura, et al., J. Phys. Soc. Japan 67 (1998) 290. [5] P.M. Raccah, J.B. Goodenough, J. Appl. Phys. 39 (1968) 1209. [6] M. Itoh, I. Natori, S. Kubota, et al., J. Phys. Soc. Japan 63 (1994) 1486. [7] K. Asai, O. Yokokura, N. Nishimori, et al., Phys. Rev. B 50 (1994) 3025. [8] V.P. Gerthsen, K.H. Ha¨rdil, Z. Naturforsch. 17a (1962) 514. [9] Y. Kobayashi, S. Murata, K. Asai, et al., J. Phys. Soc. Japan 68 (1999) 1011. [10] T. Kyomen, R. Yamazaki, M. Itoh, Phys. Rev. B 68 (2003) 104416. [11] L. Bedel, A.C. Roger, C. Estournes, et al., Catal. Today 85 (2003) 207. [12] W.C. Koehler, E.O. Wollan, J. Phys. Chem. Solids 2 (1957) 100. [13] P.G. Radaelli, S.-W. Cheong, Phys. Rev. B 66 (2002) 094408. [14] A. Weibe, H. Fehske, Eur. Phys. J. B 30 (2002) 487.
Journal of Magnetism and Magnetic Materials 298 (2006) 19–24 www.elsevier.com/locate/jmmm
Effect of oxygen deficit on magnetic properties of LaCo0.5Fe0.5O3 I.O. Troyanchuka,, D.V. Karpinskya,, R. Szymczakb, H. Szymczakb a
Institute of Solids and Semiconductors Physics, NAS of Belarus, Minsk, Belarus b Institute of Physics PAS, Warsaw, Poland Received 12 October 2004; received in revised form 14 February 2005 Available online 25 March 2005
Abstract LaCo0.5Fe0.5O3+d system has been prepared using different thermal treatments by conventional ceramic technology. The samples prepared at 1570 K in air and below this temperature are mixtures of orthorhombic and rhombohedral phases. Increasing temperature up to 1670 K stabilizes the rhombohedral phase whereas annealing in vacuumed quartz tube with a metallic getter results in orthorhombic single-phase structure. All the samples exhibit the ferromagnetic component at temperatures up to 360 K as a result of Dzialoshinsky–Moriya interactions between the Fe3+ ions. The orthorhombic samples have a giant magnetic anisotropy, mostly associated with Co2+ ions. Co3+ ions are supposed to adopt a low spin state. r 2005 Elsevier B.V. All rights reserved. PACS: 75.30.m; 75.30.Gw; 75.60.d Keywords: Magnetically ordered materials; Magnetic anisotropy; Domain effects; Magnetization and hysteresis
1. Introduction In LaCoO3 tri-valent Co3+ ions exhibit a transition of spin state around 90 K: from the low spin (LS) non-magnetic ground state ðS ¼ 0Þ to the intermediate one (IS, S ¼ 1). A diffuse Corresponding author. Tel.: +375 017 284 03 36;
fax: +375 017 284 08 88. E-mail addresses: [email protected] (I.O. Troyanchuk), [email protected] (D.V. Karpinsky).
metal–insulator (MI) transition is observed in this compound at 500 K [1–4]. The resistivity is characterized with an activated character at low temperatures, decreases by some orders of the value at 500 K and increases with temperature at higher temperatures [1]. For Sr-doped system La1xSrxCoO3 the transition of the spin state at 90 K disappears, and cobalt ions remain magnetic down to the lowest temperature. Samples with Sr content x40.2 exhibit a spontaneous magnetization at low temperatures [5]. Some researchers
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.03.009
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have supposed a coexistence of both cluster spinglass state and ferromagnetism in the region 0.3oxo0.5 on the base of the data on magnetization and neutron diffraction [6]. For less Sr content value (xo0.2) the magnetization measurements indicate the spin-glass ground state [6,7] where strong ferromagnetic short-order correlations are observed according to experimental data on neutron diffuse scattering [7]. The crystal lattice expansion as a result of Sr-substitution was supposed to stabilize the intermediate spin state of Co3+ [7], whereas the ferromagnetic short order could be caused by charge carrier’s introduction also changing the electrotransport properties. For L1xSrxCoO3 at xo0.2 the r value is less by some orders than for LaCoO3, though it retains the semiconductor behavior [8]. Above 500 K, r increases with temperature, indicating insulator– metal (IM) transition at 500 K, as in the case of LaCoO3. In [9] the possibility was shown of changing the magnetic state of Co3+ ions in LaCoO3 by substitution not only for Sr ions but also for isovalent Ni3+ ions. The La(Co1xNix)O3 samples with x ¼ 0:5 exhibit ferromagnetic component at low temperatures [10]. The LaCo1xFexO3 system was investigated earlier from a viewpoint of catalytic activity [11]. There are contradicting data on the crystal structure in the works devoted to this system. It was noted that the rhombohedral phase coexists with orthorhombic in a wide concentration interval of Fe content [11]. In the present work we demonstrate that crystallographically single-phase samples can be prepared by high-temperature synthesis as well as thermal treatment in reducing medium. Crystal structure and magnetic properties of LaCo0.5Fe0.5O3 nominal composition dramatically depend on oxygen content. The cobalt ions seem to be in a low spin state.
2. Experiment The La(Co1xFex)O3 samples have been prepared from the mixture of oxides La2O3, Co3O4 and Fe2O3 using standard ceramic technology. The temperature of synthesis has been varied from
1470 to 1770 K. Some of the samples were reduced in vacuumed quartz ampoules at 1170 K. Metallic tantalum was used as an oxygen getter. X-ray diffraction measurements were performed in Cuka radiation using DRON-3 M diffractometer. Crystal structure parameters were calculated using a FullProf program. Magnetic measurements have been carried out with commercial magnetometer MPMS-5 (Quantum Design). The Mo¨ssbauer spectra of the samples have been recorded with a 57 Co(Rh) source in transmission geometry. The recorded Mo¨ssbauer data were computer fitted by the MOSMOD program. Isomeric shift is given in relation to the a-Fe. The surface topography of the obtained samples was studied with a ‘‘KARL ZEISS’’ raster electronic microscope. The X-ray spectral analysis was performed using energy dispersion Si–Li semiconductor detector.
3. Results and discussion The crystal structure parameters of the LaCo0.5 Fe0.5O3 composition obtained at 1570 K in air were refined successfully assuming that the sample consists of mixture of rhombohedral ðR 3¯ cÞ and orthorhombic (Pnma) phases. Increasing synthesis temperature up to 1670 K results in the crystal structure homogeneity. High temperature favors apparently the solid solutions formation. The structure of the sample prepared at 1770 K was refined as rhombohedral (space group R 3¯ c). However, there is another method to prepare samples with homogeneous crystal structure associated with thermal treatment in reducing medium. Fig. 1 demonstrates the results of the structure calculation using FullProf program for the sample with x ¼ 0:5 prepared at 1570 K in air, at 1770 K in air as well as that reduced at T ¼ 1170 K in vacuumed quartz ampoule in the presence of metallic tantalum. For the reduction the sample was taken prepared at 1570 K in air. The treatment of the sample in the vacuumed ampoule led to the mass loss corresponding to the oxygen content decreasing by 2%, and the structure became orthorhombic (space group Pnma). According to the data of X-rays spectral microanalysis, the distribution of cobalt and
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80
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Fig. 1. FullProf refinement patterns for La(Co0.5Fe0.5)O3 samples prepared at 1570 K in air (a), at 1770 K in air (b) and that reduced in vacuumed quartz tube (c) using X-ray powder diffraction data. The observed intensities are shown by dots and the calculated ones by solid line. The positions of the Bragg reflections are shown by the small vertical lines below the patterns. The line at the bottom indicates the intensity difference between the experimental and the refined patterns.
manganese ions is rather homogeneous. The average size of the crystallites is about 5 mm. Unit cell parameters, symmetry and reliability factors are presented in Table 1. Both orthorhombic and rhombohedral samples have a small spontaneous magnetic moment. Fig. 2 demonstrates the magnetization of the samples x ¼ 0:5 prepared at 1770 K in air and the remanent magnetization of the sample reduced in vacuum. For the reduced sample, magnetization measurements have been performed without external magnetic field on heating, after cooling down to liquid helium temperature. At room temperature the reduced sample has a small spontaneous magnetic moment which does not disappear even on heating up to 360 K (the upper temperature limit of measurement). So one can believe the critical temperature of magnetic order destruction to be slightly above 360 K. In the temperature interval 5–100 K the reduced sample exhibits an anomalous magnetization behavior indicating the possible phase transition associated apparently with spin reorientation. Small spontaneous magnetization of the samples (Fig. 2) could be explained in terms of weak ferromagnetism as a result of Dzialoshinsky–Moriya interactions, since both the samples, containing the orthorhombic and rhombohedral phases allow the appearance of weak ferromagnetic component. The rhombohedral oxidized sample exhibits phase transition into paramagnetic state at a slightly lower temperature of 300 K (Fig. 2).
Magnetization vs. field dependences evidence a giant magnetic anisotropy in the compound x ¼ 0:5: Fig. 3 shows the hysteresis loops for the sample synthesized in air at 1770 K and that reduced in quartz ampoule. The first sample is a hard magnetic material, because the coercive field at room temperature is about 1 kOe. For the second sample the oxygen loss by reduction leads to a sharp increase of magnetic anisotropy (the value of coercive field about 10 kOe at room temperature is unique for oxide compounds). Magnetic anisotropy increases with a decrease in the temperature. At 10 K the coercive field value of the oxidized sample reaches 4 kOe, whereas for the reduced one an external magnetic field of 50 kOe is too small in order to reorient the magnetic moments (Fig. 4). In order to clarify the origin of such magnetic anisotropy the reduced sample has been oxidized by annealing at T ¼ 1170 K in air. We observed that orthorhombic phase transformed in rhombohedral and magnetic anisotropy strongly decreased. The Mo¨ssbauer spectrum of the sample x ¼ 0:5 prepared at 1670 K (rhombohedral phase) at T ¼ 293 K consists of quadrupole splitted doublet: DQ ¼ 0:440 mm=s: Isomer shift value d ¼ 0:369 mm=s evidences the Fe ions to be in tri-valent state. The spectra of the orthorhombic samples consist of two or more sextets indicating the magnetic nonequivalence of Fe ions. Magnetic properties of LaCo0.5Fe0.5O3 differ strongly from those of the end members LaCoO3
ARTICLE IN PRESS I.O. Troyanchuk et al. / Journal of Magnetism and Magnetic Materials 298 (2006) 19–24
0.20
M (emu/g)
R ¼ 0:75; R ¼ 0:81; R ¼ 1:95; R ¼ 0:43;
R-factors
RF ¼ 0:52 RF ¼ 0:63 RF ¼ 1:51 RF ¼ 0:44
22
FC ZFC reduced
0.15 0.10
1.32 0.89
0.83
w2
0.05 0.00
57.523 58.192 57.533 58.405 R 3¯ c: a ¼ 5:4908ð1Þ; c ¼ 13:2201ð6Þ; Pnma: a ¼ 5:4685ð3Þ; b ¼ 7:7548ð7Þ; c ¼ 5:5001ð8Þ; a ¼ 5:4877ð3Þ; b ¼ 13:2360ð5Þ; a ¼ 5:4627ð7Þ; b ¼ 7:7573ð8Þ; c ¼ 5:5129ð1Þ;
200 T (K)
300
400
Fig. 2. Temperature dependence of remanent magnetization for the reduced La(Co0.5Fe0.5)O3d sample (asterix mark) at field H ¼ 0 Oe; FC and ZFC magnetization for La(Co0.5 Fe0.5)O3 sample prepared at 1770 K at H ¼ 100 Oe:
1.0
0.5 M (emu/g)
Volume, (A˚3)
100
0.0
air prepared reduced T=300 K
-0.5
-1.0 -60
-40
-20
0 H (kOe)
20
40
60
R 3¯ c þ Pnma
R 3¯ c Pnma LaCo0.5Fe0.5O3 sintered 1770 K LaCo0.5Fe0.5O3 sintered 1570 K and reduced in vacuum
Fig. 3. Hysteresis loops of the LaCo0.5Fe0.5O3 sample prepared at 1770 K in air and the reduced one. T ¼ 300 K:
LaCo0.5Fe0.5O3 sintered 1570 K
Table 1
Space group
Cell parameters, (A˚)
0
and LaFeO3 of the solid solutions system. LaCoO3 exhibits diamagnetic properties at low temperature as a result of low spin state of Co3+ ions. LaFeO3 is known to be a weak ferromagnet with Neel temperature of 750 K [12]. The small ferromagnetic component is due to the antisymmetric interactions of Dzialoshinsky–Moriya. We have failed to evaluate precisely the spontaneous magnetization of the single-phase reduced sample x ¼ 0:5 with the orthorhombic structure. However, on the basis of obtained data one can suppose that this value is about 1 emu/g, which is
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characteristic of orthoferrites, being weak ferromagnets. It is well known that the cobalt ion in the intermediate spin state has ionic radius larger than the Co3+ in low spin state [13]. It was found that the unit cell volume of LaNi0.5Co0.5O3 is more than for LaCoO3 or LaNiO3 which suggests that
the most part of cobalt ions in LaNi0.5Co0.5O3 adopts intermediate spin state. However, we observed that the unit cell volume of LaCo1xFexO3 prepared at a high temperature monitonically increases with Fe content (Fig. 5). On the basis of this fact we cannot draw any conclusions concerning the spin state of the cobalt ions, but according to the magnetic data the most probable spin state of the Co3+ ions is the low spin one. The strong magnetic anisotropy of oxidized LaCo0.5Fe0.5O3 is associated with antisymmetric exchange of Dzialoshinsky–Moriya because this type of exchange is directly connected with spin–orbital interactions [14]. The most likely magnetic anisotropy increase observed in the reduced sample is due mainly to the contribution of cobalt ions. Tri-valent cobalt ions in the intermediate or high spin state as well as di-valent cobalt ions are known to enhance the magnetic anisotropy significantly as a result of spin–orbital interaction. The main part of cobalt ions in the composition x ¼ 0:5 is apparently in tri-valent low spin state, however, the di-valent cobalt ions and oxygen vacancies should appear by reduction admitting
2.0 1.5
M (emu/g)
1.0 0.5 0.0 -0.5
air prepared reduced T=10K
-1.0 -1.5 -2.0 -60
-40
-20
0 H (kOe)
20
40
23
60
Fig. 4. Hysteresis loops of the LaCo0.5Fe0.5O3 sample prepared at 1770 K in air and the reduced one. T ¼ 10 K:
La(Co1-xFex)O3
61.0
58
Vol 60.6
56 rhomb
orth 5.56
unit cell parameter, (A)
volume, (A3)
60.8
5.52
b
a1
5.4
c
5.48 a2
5.44
5.3 0
20
40
60
80
unit cell parameters, (A)
alfa angle, (deg)
60 alfa
100
x Fig. 5. Unit cell parameters for the samples, prepared at 1770 K. (rhomb—rhombohedral region 0pxo0.55, alfa—rhombohedral angle, a1—cell parameters for R 3¯ c space group; orth—orthorhombic region, 0.55pxo1 a2, b, c—cell parameters for Pnma space group; volume is calculated per formula unit).
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the starting composition to be nearly stoichiometric on oxygen. So, the Co2+ ions could affect significantly the magnetic anisotropy value whereas the oxygen vacancies could reduce the domain boundaries mobility. Since, the magnetic domain walls become immobile, the magnetic domain reorientation takes place by the way of magnetic moments rotation similar to the case of spin-glasses.
4. Conclusions Using standard ceramic technology, we have managed to obtain the LaCo0.5Fe0.5O3 samples, the structural homogeneity and magnetic properties of which may be controlled by changing the temperature conditions of synthesis and oxygen content using the reduction in vacuum. Fe and Co ions are in tri-valent state, however, an insignificant quantity of di-valent cobalt can appear by reduction. The reduction is shown to lead to a strong increase of magnetic anisotropy associated with Co2+ ions and to convert the rhombohedral phase into the orthorhombic one. The spontaneous magnetization of orthorhombic compounds is supposed to be due to antisymmetric exchange of Dzialoshinsky–Moriya, whereas lowspin Co3+ ions do not affect strongly the magnetic properties.
Acknowledgments The work was supported partly by Fund for fundamental research of Belarus (Project F03-155) and the State Committee for Scientific Research (Poland) (Grant no. KBN 1 P03B 038 27). References [1] R.R. Heikes, R.C. Miller, R. Mazelsky, Physica 30 (1964) 1600. [2] G.H. Jonker, J. Appl. Phys. 37 (1966) 1424. [3] K. Asai, O. Yokokura, M. Suzuki, et al., J. Phys. Soc. Japan 66 (1997) 967. [4] K. Asai, A. Atsuro, O. Yokokura, et al., J. Phys. Soc. Japan 67 (1998) 290. [5] P.M. Raccah, J.B. Goodenough, J. Appl. Phys. 39 (1968) 1209. [6] M. Itoh, I. Natori, S. Kubota, et al., J. Phys. Soc. Japan 63 (1994) 1486. [7] K. Asai, O. Yokokura, N. Nishimori, et al., Phys. Rev. B 50 (1994) 3025. [8] V.P. Gerthsen, K.H. Ha¨rdil, Z. Naturforsch. 17a (1962) 514. [9] Y. Kobayashi, S. Murata, K. Asai, et al., J. Phys. Soc. Japan 68 (1999) 1011. [10] T. Kyomen, R. Yamazaki, M. Itoh, Phys. Rev. B 68 (2003) 104416. [11] L. Bedel, A.C. Roger, C. Estournes, et al., Catal. Today 85 (2003) 207. [12] W.C. Koehler, E.O. Wollan, J. Phys. Chem. Solids 2 (1957) 100. [13] P.G. Radaelli, S.-W. Cheong, Phys. Rev. B 66 (2002) 094408. [14] A. Weibe, H. Fehske, Eur. Phys. J. B 30 (2002) 487.