- Email: [email protected]
3Mn1−xCoxO3 compounds
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) e179–e182 www.elsevier.com/locate/jmmm
Magnetocaloric and magnetoresistance properties of La2/3Sr1/3Mn1xCoxO3 compounds R. Tetean, I.G. Deac, E. Burzo, A. Bezergheanu Faculty of Physics, Babes-Bolyai University, RO-400084 Cluj-Napoca, Romania Available online 23 February 2008
Abstract Structural, magnetic, magnetoresistance and magnetocaloric studies on La2/3Sr1/3Mn1xCoxO3 compounds were reported. The samples were prepared by the conventional ceramic method. X-ray analysis showed the presence of one phase only, in all studied samples. From electrical resistance measurements it was found that the samples show large negative magnetoresistance behavior. The magnetic measurements were performed in a large temperature range, 4.2–750 K and external magnetic fields up to 5 T. The adiabatic magnetic entropy changes, |DS|, were determined from magnetization data. Large magnetocaloric effect (MCE) has been obtained in all studied samples. r 2008 Elsevier B.V. All rights reserved. PACS: 75.30.Sg; 71.30.+h; 75.30.Kz Keywords: Magnetic oxides; Colossal magnetoresistance; Magnetocaloric effect
1. Introduction Magnetic materials showing a large magnetocaloric effect (MCE) have attracted considerable attention for their potential application in magnetic refrigeration technology [1–3]. MCE is an isothermal magnetic entropy change or an adiabatic temperature change of a magnetic material upon application of a magnetic field. The compounds that undergo temperature-driven paramagnetic to ferromagnetic transitions show relatively large ‘‘negative’’ MCE, in which the isothermal magnetic entropy change is negative [4]. Refrigeration in the temperature range 250–300 K is of particular interest due to the potential impact on energy savings and environmental concerns. The interplay between structure, magnetic and transport properties in perovskite-type manganites was the aim of many recent papers. The substitution of the trivalent element by a divalent one produces an inhomogeneous distribution of mixed valence Mn4+/Mn3+ ions to maintain charge neutrality. These systems exhibit many Corresponding author. Tel.: +40 26 459 4315; fax: +40 26 459 1906.
E-mail address: [email protected] (R. Tetean). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.02.100
significant properties like charge and orbital ordering, metal–insulator transition, ferromagnetic–paramagnetic phase change, magnetoresistance, MCE, spin-glass behavior depending on the charge density, temperature and atomic structure [5–7]. Colossal magnetoresistance phenomena were observed in the perovskite-type hole-doped manganites in which the double-exchange ferromagnetic metal phase and the charge–orbital ordered antiferromagnetic phase compete with each other. The chemical randomness or the impurity doping may cause major modifications in the electronic phase diagram as well as in the magnetoelectronic properties. At present, the perovskite manganites are the most representative materials system that can show versatile unconventional electroniclattice structural changes or insulator–metal transitions upon stimulation by external stimuli, like magnetic field, irradiation with light, X-rays or electron-beams. Many of the manganite compounds, R1–xMxMnO3, where R=rare-earth metal and M=Ca, Sr or Ba, exhibit large or unusual MCE values [8]. On the other hand, these materials display considerably small magnetic hysteresis and their Curie temperature can be tuned easily. The former is beneficial for the magnetic cooling efficiency and
ARTICLE IN PRESS R. Tetean et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e179–e182
the latter is advantageous to the wide working temperature ranges. LaCoO3 is a diamagnetic insulator at low temperatures with a low spin state (LS) of Co3+(S ¼ 0). The substitution of Sr2+ for La3+ in this compound converts a number of Co3+ to Co4+ ions. Co3+ ions can be in LS (S ¼ 0), intermediate spin (IS, S ¼ 1) or high spin (HS, S ¼ 2), while Co4+ can be in LS (S ¼ 12), IS (S ¼ 32) or HS (S ¼ 52). In this paper we analyze the influence of Co substitution at Mn sites on the physical properties and the MCE in La2/ 3Sr1/3Mn1xCoxO3 compounds. 2. Experimental Polycrystalline samples with nominal composition La2/ (x ¼ 0.5, 0.6, 0.7 0.8, 1) were prepared by standard ceramic reaction at high temperatures. The mixtures of the respective oxides were calcinated at 1200 1C and then were pressed and sintered in air at 1300 1C for 24 h. The powder X-ray diffraction patterns were recorded by using a Bruker D8 Advance AXS diffractometer with Cu Ka radiation. An Oxford Instruments MagLab System 2000 was used for magnetization measurements. The samples were studied in magnetic fields up to 5 T in the temperature ranges 4.2–750 K. The resistivities were measured in a cryogen-free cryostat CFM-7T (Cryogenic Ltd.) by the four-probe technique. The magnetic entropy changes were determined from magnetization isotherms, between zero field and a maximum field (H0) using the thermodynamic relation 3Sr1/3Mn1xCoxO3
Mn and Co ions in the lattice, i.e. not long-range Co/Mn order [9]. Some magnetization isotherms for the compounds with x ¼ 1 and 0.6 are plotted in Fig. 1. One can see that the saturation is not attended even in 5 T external magnetic field. Similar behaviors were obtained in all cases. In addition, low magnetic hysteretic behavior was found in M(H) curves at low temperatures. The Curie temperatures decrease from 212 K at x ¼ 1 to 147 K at x ¼ 0.5. In the higher temperature region, 300–750 K, the reciprocal static magnetic susceptibility, 1/w, has almost a linear behavior. The paramagnetic Curie temperatures for the samples having high Co contents are negative, antiferromagnetic interactions becoming dominant. When x ¼ 1 the Co3+ ions were found in all the three states LS, IS and HS. The electrical resistivity measurements indicated semiconductor behavior for all the studied samples on the whole temperature range. Negative magnetoresistance MR ¼ [r(H)r(0)]/r(0) has been found for all the
1.2
0.8 0.6 0.4
DS m ðT; H 0 Þ ¼ S m ðT; H 0 Þ S m ðT; 0Þ Z H0 1 ½MðT þ DT; HÞ MðT; HÞ dH ¼ DT 0
220K
0.2 0.0
(1) where DT is the temperature increment between measured magnetization isotherms (DT ¼ 5 K for our data). The magnetic cooling efficiency was evaluated by considering the magnitude of the magnetic entropy change, DSm and its full-width at half-maximum (dTFWHM). The product of the DSm maximum and the (dTFWHM ¼ T2T1):
0.0
0.5
1.0
1.5
2.0
2.5 3.0 μ0H (T)
3.5
4.0
4.5
5.0
100 K
0.5
La2/3Sr1/3Mn0.4Co0.6O3
0.4
(2)
is the so-called relative cooling power (RCP) based on the magnetic entropy change.
M (μΒ/f.u.)
RCPðSÞ ¼ DSm ðT; HÞ dT FWHM
100K
La2/3Sr1/3CoO3
1.0
M (μΒ/f.u.)
e180
0.3
0.2
3. Results and discussions The X-ray diffraction patterns of La2/3Sr1/3Mn1xCoxO3 showed that the compounds are single phases, within the limit of experimental errors. All the compounds crystallize in a rhombohedral structure. The lattice parameters decrease slightly when the Co content increases. The monotonic decrease of the unit cell volume, with increasing cobalt content, indicate a random distribution of the
215 K 0.1
0.0
0
1
2
3 μ0H (T)
4
5
Fig. 1. Magnetization isotherms for the compounds with x ¼ 1 and 0.6.
ARTICLE IN PRESS R. Tetean et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e179–e182 0
La2/3Sr1/3CoxMn1-xO3 X=1
MR (%)
-5
-10 25 K 50 K
-15
80 K -20
0
1
2
3
4 5 μ0H (T)
7
6
8
Fig. 2. The magnetic field dependences of the MR at different temperatures for the sample with x ¼ 1.
1.4 1.2
−ΔSm (J/kgK) −Δ
1 0.8 0.6 0.4 0.2 0 100
150
200 T (K)
250
values of entropy change occur almost around the transition temperatures for all the compounds. In our case the maximum value is around 1.37 J/kg K in a 5 T magnetic field for the compound with x ¼ 1. The magnetic entropy change decreases with about 0.6 J/kg K in a field of 3 T. These values are somewhat smaller than those evidenced in other perovskites but high enough for technical interest. As the most ferromagnetic materials, La2/3Sr1/ 3Mn1xCoxO3 show a second-order magnetic phase transition. It should be noted that a first-order transition is able to concentrate the MCE in a narrow temperature range, whereas second-order transitions are usually spread over a broad temperature range, which is beneficial for active magnetic refrigeration [11,12]. In our samples the MCE broadens on a large temperature range as one could see in Fig. 3. The relative cooling power is maximum for the sample with x ¼ 1 being 128.5 J/kg for DB ¼ 5 T, 74.4 J/kg for DB ¼ 3 T and 23 J/kg for DB ¼ 1 T. 4. Conclusions
ΔB=0.5T ΔB=1.0T ΔB=3.0T ΔB=5.0T
La2/3Sr1/3CoO3
e181
300
Fig. 3. The temperature dependences of the magnetic entropy change in DB ¼ 0.5, 1, 3 and 5 T.
compounds at low temperatures. MR does not exceed 18% as obtained for the sample with x ¼ 0.7 in 7 T, at 195 K. As an example in Fig. 2. is presented the magnetic field dependence of MR for the sample with x ¼ 1. The lowest value of resistivity was found when x ¼ 1. No feature at the transition of the system from the paramagnetic phase in the cluster glass phase was observed. The resistivity decreases with increasing Co content, reaching the lowest value for x ¼ 1 suggesting the important role of the Co3+ subsystem in electrical conduction. The low value of resistivity in the high-temperature region can be the result of a carrier hopping mechanism together with a charge disproportionation effect of Co3+ ions in Co2+ and Co4+ [10]. The temperature dependences of magnetic entropy change in 0.5, 1, 3 and 5 T external applied fields for the compound with x ¼ 1 are plotted in Fig. 3. The maximum
The single-phase La2/3Sr1/3Mn1xCoxO3 compounds with 0.5pxp1.0 were prepared. All the compounds crystallize in a rhombohedral structure. The lattice parameters decrease slightly when the Co content increases. The Curie temperatures decrease from 212 K at x ¼ 1 to 147 K for the sample with x ¼ 0.5. At low temperatures low hysteretic behavior was observed. The paramagnetic Curie temperatures for high Co content are negative, suggesting that antiferromagnetic interactions become dominant. Negative magnetoresistance was evidenced in all cases. The resistivity decreases with increasing Co content, reaching the lowest value for x ¼ 1 suggesting the important role of the Co3+ subsystem in electrical conduction. The low value of resistivity, in the hightemperature region can be the result of a carrier hopping mechanism together with a charge disproportionation effect of Co3+ ions in Co2+ and Co4+. The maximum magnetic entropy change was obtained for the sample with x ¼ 1 in DB ¼ 5 T, being 1.37 J/(kg K). The RCP have values comparable with the values obtained in other perovskite-type compounds. The studied compounds may be considered as magnetic materials operated in the intermediate temperature range. Acknowledgment This work was supported by the Ministry of Education and Research of Romania, Grant 2-Cex-06-11-102/ 25.10.2006. References [1] K.A. Gschneider Jr., V.K. Pecharsky, Annu. Rev. Mater. Sci. 30 (2000) 387. [2] V.K. Pecharsky, K.A. Gschneider Jr., Phys. Rev. Lett. 78 (1997) 4494.
ARTICLE IN PRESS e182
R. Tetean et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e179–e182
[3] V.K. Pecharsky, K.A. Gschneider, Appl. Phys. Lett. 70 (1997) 3299. [4] H. Wada, Y. Tanabe, Appl. Phys. Lett. 79 (2001) 3302. [5] H.Y. Hwang, S.W. Cheong, P.G. Radaelli, M. Marenzio, B. Batlogg, Phys. Rev. Lett. 75 (1995) 914. [6] P.G. Radaelli, D.E. Cox, M. Marenzio, S.W. Cheong, P.E. Schiffer, A.P. Ramirez, Phys. Rev. Lett. 75 (1995) 4488. [7] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri, C.N.R. Rao, J. Solid State Chem. 120 (1995) 204.
[8] M.-H. Phan, S.-C. Yu, J. Magn. Magn. Mater. 308 (2007) 325. [9] C. Autret, J. Hejtmanek, K. Knizek, M. Marysko, Z. Jirak, Mdlouba, S. Vratislav, J. Phys.: Condens. Matter 17 (2005) 160 and references therein. [10] I.O. Troyanchuk, et al., J. Magn. Magn. Mater. 210 (2000) 63. [11] E. Bruck, J. Phys. D: Appl. Phys. (2005) R381. [12] M.-H. Phan, S.-C. Yu, N.H. Hur, Appl. Phys. Lett. 86 (2005) 072504.
Journal of Magnetism and Magnetic Materials 320 (2008) e179–e182 www.elsevier.com/locate/jmmm
Magnetocaloric and magnetoresistance properties of La2/3Sr1/3Mn1xCoxO3 compounds R. Tetean, I.G. Deac, E. Burzo, A. Bezergheanu Faculty of Physics, Babes-Bolyai University, RO-400084 Cluj-Napoca, Romania Available online 23 February 2008
Abstract Structural, magnetic, magnetoresistance and magnetocaloric studies on La2/3Sr1/3Mn1xCoxO3 compounds were reported. The samples were prepared by the conventional ceramic method. X-ray analysis showed the presence of one phase only, in all studied samples. From electrical resistance measurements it was found that the samples show large negative magnetoresistance behavior. The magnetic measurements were performed in a large temperature range, 4.2–750 K and external magnetic fields up to 5 T. The adiabatic magnetic entropy changes, |DS|, were determined from magnetization data. Large magnetocaloric effect (MCE) has been obtained in all studied samples. r 2008 Elsevier B.V. All rights reserved. PACS: 75.30.Sg; 71.30.+h; 75.30.Kz Keywords: Magnetic oxides; Colossal magnetoresistance; Magnetocaloric effect
1. Introduction Magnetic materials showing a large magnetocaloric effect (MCE) have attracted considerable attention for their potential application in magnetic refrigeration technology [1–3]. MCE is an isothermal magnetic entropy change or an adiabatic temperature change of a magnetic material upon application of a magnetic field. The compounds that undergo temperature-driven paramagnetic to ferromagnetic transitions show relatively large ‘‘negative’’ MCE, in which the isothermal magnetic entropy change is negative [4]. Refrigeration in the temperature range 250–300 K is of particular interest due to the potential impact on energy savings and environmental concerns. The interplay between structure, magnetic and transport properties in perovskite-type manganites was the aim of many recent papers. The substitution of the trivalent element by a divalent one produces an inhomogeneous distribution of mixed valence Mn4+/Mn3+ ions to maintain charge neutrality. These systems exhibit many Corresponding author. Tel.: +40 26 459 4315; fax: +40 26 459 1906.
E-mail address: [email protected] (R. Tetean). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.02.100
significant properties like charge and orbital ordering, metal–insulator transition, ferromagnetic–paramagnetic phase change, magnetoresistance, MCE, spin-glass behavior depending on the charge density, temperature and atomic structure [5–7]. Colossal magnetoresistance phenomena were observed in the perovskite-type hole-doped manganites in which the double-exchange ferromagnetic metal phase and the charge–orbital ordered antiferromagnetic phase compete with each other. The chemical randomness or the impurity doping may cause major modifications in the electronic phase diagram as well as in the magnetoelectronic properties. At present, the perovskite manganites are the most representative materials system that can show versatile unconventional electroniclattice structural changes or insulator–metal transitions upon stimulation by external stimuli, like magnetic field, irradiation with light, X-rays or electron-beams. Many of the manganite compounds, R1–xMxMnO3, where R=rare-earth metal and M=Ca, Sr or Ba, exhibit large or unusual MCE values [8]. On the other hand, these materials display considerably small magnetic hysteresis and their Curie temperature can be tuned easily. The former is beneficial for the magnetic cooling efficiency and
ARTICLE IN PRESS R. Tetean et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e179–e182
the latter is advantageous to the wide working temperature ranges. LaCoO3 is a diamagnetic insulator at low temperatures with a low spin state (LS) of Co3+(S ¼ 0). The substitution of Sr2+ for La3+ in this compound converts a number of Co3+ to Co4+ ions. Co3+ ions can be in LS (S ¼ 0), intermediate spin (IS, S ¼ 1) or high spin (HS, S ¼ 2), while Co4+ can be in LS (S ¼ 12), IS (S ¼ 32) or HS (S ¼ 52). In this paper we analyze the influence of Co substitution at Mn sites on the physical properties and the MCE in La2/ 3Sr1/3Mn1xCoxO3 compounds. 2. Experimental Polycrystalline samples with nominal composition La2/ (x ¼ 0.5, 0.6, 0.7 0.8, 1) were prepared by standard ceramic reaction at high temperatures. The mixtures of the respective oxides were calcinated at 1200 1C and then were pressed and sintered in air at 1300 1C for 24 h. The powder X-ray diffraction patterns were recorded by using a Bruker D8 Advance AXS diffractometer with Cu Ka radiation. An Oxford Instruments MagLab System 2000 was used for magnetization measurements. The samples were studied in magnetic fields up to 5 T in the temperature ranges 4.2–750 K. The resistivities were measured in a cryogen-free cryostat CFM-7T (Cryogenic Ltd.) by the four-probe technique. The magnetic entropy changes were determined from magnetization isotherms, between zero field and a maximum field (H0) using the thermodynamic relation 3Sr1/3Mn1xCoxO3
Mn and Co ions in the lattice, i.e. not long-range Co/Mn order [9]. Some magnetization isotherms for the compounds with x ¼ 1 and 0.6 are plotted in Fig. 1. One can see that the saturation is not attended even in 5 T external magnetic field. Similar behaviors were obtained in all cases. In addition, low magnetic hysteretic behavior was found in M(H) curves at low temperatures. The Curie temperatures decrease from 212 K at x ¼ 1 to 147 K at x ¼ 0.5. In the higher temperature region, 300–750 K, the reciprocal static magnetic susceptibility, 1/w, has almost a linear behavior. The paramagnetic Curie temperatures for the samples having high Co contents are negative, antiferromagnetic interactions becoming dominant. When x ¼ 1 the Co3+ ions were found in all the three states LS, IS and HS. The electrical resistivity measurements indicated semiconductor behavior for all the studied samples on the whole temperature range. Negative magnetoresistance MR ¼ [r(H)r(0)]/r(0) has been found for all the
1.2
0.8 0.6 0.4
DS m ðT; H 0 Þ ¼ S m ðT; H 0 Þ S m ðT; 0Þ Z H0 1 ½MðT þ DT; HÞ MðT; HÞ dH ¼ DT 0
220K
0.2 0.0
(1) where DT is the temperature increment between measured magnetization isotherms (DT ¼ 5 K for our data). The magnetic cooling efficiency was evaluated by considering the magnitude of the magnetic entropy change, DSm and its full-width at half-maximum (dTFWHM). The product of the DSm maximum and the (dTFWHM ¼ T2T1):
0.0
0.5
1.0
1.5
2.0
2.5 3.0 μ0H (T)
3.5
4.0
4.5
5.0
100 K
0.5
La2/3Sr1/3Mn0.4Co0.6O3
0.4
(2)
is the so-called relative cooling power (RCP) based on the magnetic entropy change.
M (μΒ/f.u.)
RCPðSÞ ¼ DSm ðT; HÞ dT FWHM
100K
La2/3Sr1/3CoO3
1.0
M (μΒ/f.u.)
e180
0.3
0.2
3. Results and discussions The X-ray diffraction patterns of La2/3Sr1/3Mn1xCoxO3 showed that the compounds are single phases, within the limit of experimental errors. All the compounds crystallize in a rhombohedral structure. The lattice parameters decrease slightly when the Co content increases. The monotonic decrease of the unit cell volume, with increasing cobalt content, indicate a random distribution of the
215 K 0.1
0.0
0
1
2
3 μ0H (T)
4
5
Fig. 1. Magnetization isotherms for the compounds with x ¼ 1 and 0.6.
ARTICLE IN PRESS R. Tetean et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e179–e182 0
La2/3Sr1/3CoxMn1-xO3 X=1
MR (%)
-5
-10 25 K 50 K
-15
80 K -20
0
1
2
3
4 5 μ0H (T)
7
6
8
Fig. 2. The magnetic field dependences of the MR at different temperatures for the sample with x ¼ 1.
1.4 1.2
−ΔSm (J/kgK) −Δ
1 0.8 0.6 0.4 0.2 0 100
150
200 T (K)
250
values of entropy change occur almost around the transition temperatures for all the compounds. In our case the maximum value is around 1.37 J/kg K in a 5 T magnetic field for the compound with x ¼ 1. The magnetic entropy change decreases with about 0.6 J/kg K in a field of 3 T. These values are somewhat smaller than those evidenced in other perovskites but high enough for technical interest. As the most ferromagnetic materials, La2/3Sr1/ 3Mn1xCoxO3 show a second-order magnetic phase transition. It should be noted that a first-order transition is able to concentrate the MCE in a narrow temperature range, whereas second-order transitions are usually spread over a broad temperature range, which is beneficial for active magnetic refrigeration [11,12]. In our samples the MCE broadens on a large temperature range as one could see in Fig. 3. The relative cooling power is maximum for the sample with x ¼ 1 being 128.5 J/kg for DB ¼ 5 T, 74.4 J/kg for DB ¼ 3 T and 23 J/kg for DB ¼ 1 T. 4. Conclusions
ΔB=0.5T ΔB=1.0T ΔB=3.0T ΔB=5.0T
La2/3Sr1/3CoO3
e181
300
Fig. 3. The temperature dependences of the magnetic entropy change in DB ¼ 0.5, 1, 3 and 5 T.
compounds at low temperatures. MR does not exceed 18% as obtained for the sample with x ¼ 0.7 in 7 T, at 195 K. As an example in Fig. 2. is presented the magnetic field dependence of MR for the sample with x ¼ 1. The lowest value of resistivity was found when x ¼ 1. No feature at the transition of the system from the paramagnetic phase in the cluster glass phase was observed. The resistivity decreases with increasing Co content, reaching the lowest value for x ¼ 1 suggesting the important role of the Co3+ subsystem in electrical conduction. The low value of resistivity in the high-temperature region can be the result of a carrier hopping mechanism together with a charge disproportionation effect of Co3+ ions in Co2+ and Co4+ [10]. The temperature dependences of magnetic entropy change in 0.5, 1, 3 and 5 T external applied fields for the compound with x ¼ 1 are plotted in Fig. 3. The maximum
The single-phase La2/3Sr1/3Mn1xCoxO3 compounds with 0.5pxp1.0 were prepared. All the compounds crystallize in a rhombohedral structure. The lattice parameters decrease slightly when the Co content increases. The Curie temperatures decrease from 212 K at x ¼ 1 to 147 K for the sample with x ¼ 0.5. At low temperatures low hysteretic behavior was observed. The paramagnetic Curie temperatures for high Co content are negative, suggesting that antiferromagnetic interactions become dominant. Negative magnetoresistance was evidenced in all cases. The resistivity decreases with increasing Co content, reaching the lowest value for x ¼ 1 suggesting the important role of the Co3+ subsystem in electrical conduction. The low value of resistivity, in the hightemperature region can be the result of a carrier hopping mechanism together with a charge disproportionation effect of Co3+ ions in Co2+ and Co4+. The maximum magnetic entropy change was obtained for the sample with x ¼ 1 in DB ¼ 5 T, being 1.37 J/(kg K). The RCP have values comparable with the values obtained in other perovskite-type compounds. The studied compounds may be considered as magnetic materials operated in the intermediate temperature range. Acknowledgment This work was supported by the Ministry of Education and Research of Romania, Grant 2-Cex-06-11-102/ 25.10.2006. References [1] K.A. Gschneider Jr., V.K. Pecharsky, Annu. Rev. Mater. Sci. 30 (2000) 387. [2] V.K. Pecharsky, K.A. Gschneider Jr., Phys. Rev. Lett. 78 (1997) 4494.
ARTICLE IN PRESS e182
R. Tetean et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e179–e182
[3] V.K. Pecharsky, K.A. Gschneider, Appl. Phys. Lett. 70 (1997) 3299. [4] H. Wada, Y. Tanabe, Appl. Phys. Lett. 79 (2001) 3302. [5] H.Y. Hwang, S.W. Cheong, P.G. Radaelli, M. Marenzio, B. Batlogg, Phys. Rev. Lett. 75 (1995) 914. [6] P.G. Radaelli, D.E. Cox, M. Marenzio, S.W. Cheong, P.E. Schiffer, A.P. Ramirez, Phys. Rev. Lett. 75 (1995) 4488. [7] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri, C.N.R. Rao, J. Solid State Chem. 120 (1995) 204.
[8] M.-H. Phan, S.-C. Yu, J. Magn. Magn. Mater. 308 (2007) 325. [9] C. Autret, J. Hejtmanek, K. Knizek, M. Marysko, Z. Jirak, Mdlouba, S. Vratislav, J. Phys.: Condens. Matter 17 (2005) 160 and references therein. [10] I.O. Troyanchuk, et al., J. Magn. Magn. Mater. 210 (2000) 63. [11] E. Bruck, J. Phys. D: Appl. Phys. (2005) R381. [12] M.-H. Phan, S.-C. Yu, N.H. Hur, Appl. Phys. Lett. 86 (2005) 072504.