- Email: [email protected]
Structural phase transition in Pr0.55Ca0.45MnO3 and Nd0.5Sr0.5MnO3 associated with charge ordering transition
Materials Science and Engineering A 481–482 (2008) 555–558
Structural phase transition in Pr0.55Ca0.45MnO3 and Nd0.5Sr0.5MnO3 associated with charge ordering transition T. Murata ∗ , T. Terai, T. Fukuda, T. Kakeshita Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan Received 17 May 2006; received in revised form 8 November 2006; accepted 31 December 2006
Abstract We have investigated the microstructure formed in association with charge ordering transition in perovskite manganites Pr0.55 Ca0.45 MnO3 (PCMO) and Nd0.5 Sr0.5 MnO3 (NSMO). By optical microscope observation, surface relief is observed below the charge ordering transition temperature TCO (CO represents charge ordering phase) for both of the specimens. Two surface trace analysis and Laue back reflection technique has been revealed that microstructures are composed of twinned variants which have {1 0 1}CO and {1 1 2}CO twinning planes for PCMO and NSMO, respectively. We have also investigated the magnetic field dependence of TCO by magnetization measurements and found that the TCO decreases parabolically with increasing magnetic field. This magnetic field dependence of TCO in PCMO is in good agreement with the calculated one based on the Clausius–Clapeyron equation. © 2007 Elsevier B.V. All rights reserved. Keywords: Charge ordering transition; Twin; Perovskite manganite
1. Introduction Perovskite manganites with some trivalent rare earth metal ions R3+ replaced by divalent alkaline-earth metal ions A2+ , R1−x Ax MnO3 , have been extensively studied and found to show interesting phenomena such as metal–insulator transition, colossal magnetoresistance etc. [1–7]. In some perovskite manganites R1−x Ax MnO3 (R and A are trivalent rare earth metal ions and divalent alkaline-earth metal ions, respectively) with hole doping level corresponding to x ≈ 0.5, charge ordering transition occurs and is usually accompanied by a discontinuous change in lattice parameters [5–7]. Thus, it is expected that self-accommodated microstructures are formed to relax stress which is induced by the change in lattice parameters, like in martensitic transitions. Moreover, most of the charge ordering transitions in perovskite manganites accompanies drastic changes in magnetic and electronic properties [5–7]. Thus, it is also expected that the charge ordering transition temperature is largely changed by external field such as magnetic field; similar to martensitic transition temperatures in metallic materials. However, there are few studies on
∗
Corresponding author. E-mail address: [email protected] (T. Murata).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.205
microstructures formed by charge ordering transition [7] and on quantitative analysis of the magnetic field dependence of charge ordering transition temperature in perovskite manganites. In this study, therefore, we have analyzed the microstructure of the charge ordered phase and the magnetic field dependence of charge the ordering transition temperature of Pr0.45 Ca0.55 MnO3 (PCMO) and Nd0.5 Sr0.5 MnO3 (NSMO) by magnetization measurement, optical microscopy and the Laue back reflection technique. 2. Experimental We prepared single crystal specimens of PCMO and NSMO. Compounds of Pr2 O3 , Nd2 O3 , CaCO3 , SrCO3 and Mn3 O4 were mixed in amounts for the desired compositions, and calcined in air at 1273 K for 12 h. After intermediate grinding, these compounds were pressed into rods of about 5 mm in diameter. These rods were sintered in O2 gas flow at 1873 K for 12 h. Using the rods as starting materials, single crystals were grown by a floating zone method with a growth rate of 1.25–3.50 mm/h. The compositions of single crystal specimens were confirmed by inductively coupled plasma-atomic emission spectrometer to be nominal. Powder X-ray diffraction measurements were carried out in order to check their crystal structures at
556
T. Murata et al. / Materials Science and Engineering A 481–482 (2008) 555–558
Table 1 Lattice parameters of parent phases at 298 K and charge ordered phases at 120 K for PCMO and NSMO Parent phase ˚ (298 K) (A)
Charge ordered ˚ phase (120 K) (A)
PCMO
a = 7.647
a = 7.669 c = 7.586
NSMO
a = 5.432 b = 5.482 c = 7.640
a = 5.442 b = 5.504 c = 7.539 (β = 89.9◦ )
the temperatures from 77 K to room temperature. Magnetization measurements were carried out using a superconducting quantum interference device (SQUID) magnetometer. The microstructure on the surface of the specimen was examined by Nomarski microscopy and ordinary polarizing microscopy (OLYMPUS BM60XM). The crystallographic orientation relationship between observed domains in the charge ordered phase was determined by using a back-reflection Laue camera (RINT2100) equipped with a cooling stage. 3. Results 3.1. Microstructure of the charge ordered phase From X-ray diffraction and magnetization measurements, we have confirmed that PCMO exhibits a structural transition from a paramagnetic phase with pseudo-cubic structure to a paramagnetic charge ordered phase with pseudo-tetragonal structure at TCO = 238 K, and NSMO exhibits a structural transition from a ferromagnetic phase with orthorhombic structure to an antiferromagnetic charge ordered phase with monoclinic structure at TCO = 160 K, which are in good agreement with previous reports [5,6]. The lattice parameters are shown in Table 1, and the lattice relationships between parent and charge ordered phases of these compounds are shown in Fig. 1. In Fig. 1, the crystal structure of the charge ordered phase for NSMO is treated as a pseudoorthorhombic structure for simplicity, because the monoclinic angle β is close to 90◦ . This figure also shows the unit cell of the cubic perovskite structure, which is obtained in BaTiO3 . As seen from this figure, the [1 0 0] direction of PCMO and the [1 1 0]
Fig. 1. Schematic diagram of unit cells for cubic (solid lines), pseudo-tetragonal (dashed lines) and pseudo-orthorhombic (dotted lines). The a0 represents the lattice parameter of the cubic unit cell together with MnO6 octahedra. Subscripts c, t and o represent pseudo-cubic, pseudo-tetragonal and pseudo-orthorhombic, respectively.
Fig. 2. Optical micrographs on the (0 0 1)p plane for PCMO taken at (a) 298 K and (b) 135 K, and those on the (0 1 0)p plane for NSMO taken at (c) 298 K and (d) 82 K.
direction of NSMO are parallel to the [1 0 0] direction of the cubic structure, and the [0 0 1] directions of PCMO and NSMO are parallel to the [0 0 1] direction of cubic structure. In order to observe the microstructure associated with the charge ordering transition, optical microscopy observation has been made for both specimens. Typical micrographs on (0 0 1)p (p represents the parent phase) for PCMO and (0 1 0)p planes for NSMO are shown in Fig. 2. At room temperature, as shown in Fig. 2a and c, a flat and a smooth surface is observed for PCMO and NSMO, respectively. In the cooling process, surface relief appears at TCO and microstructure is formed for PCMO and NSMO, as shown in Fig. 2b and d, respectively. This microstructure quite resembles the twinned structures of shape memory alloys. Thus, it is considered that these microstructures are composed of crystallographic domains. In the heating process, the surface relief disappears at TCO and flat and smooth surfaces are obtained again at room temperature. In order to analyze this microstructure crystallographically, we have taken Laue photographs at 77 K from a part of the surface, which contains two domains. As a result, we find that there are mirror relationships between two domains, and the mirror planes are {1 0 1}CO (CO represents the charge ordered phase) and {1 1 2}CO for PCMO and NSMO respectively. In order to confirm that these mirror planes are the interface of two domains, we use two-surface trace analysis. For PCMO, the optical micrographs of the charge ordered phase on (0 1 0)p and (1 0 0)p planes are shown in Fig. 3a and b, respectively. As seen from the figures, the trace AA along the [1 0 0]p direction is observed on the (0 1 0)p plane, and this trace is connected to the trace BB along the [0 1 1]p direction on the (1 0 0)p plane. This result means that the interface of two domains is the {1 0 1}CO plane, which is the same as the mirror plane. Also, for NSMO, the optical micrographs of the charge ordered phase on (1 0 0)p and (1 1 0)p planes are shown in Fig. 4a and b, respectively. As seen from the figures, the trace AA along the [0 2 1]p direction is observed on the (1 0 0)p plane and this trace is connected to trace BB along the [1 1¯ 1]p direction on the (1 1 0)p plane. This
T. Murata et al. / Materials Science and Engineering A 481–482 (2008) 555–558
557
Fig. 5. Magnetization curves of (a) PCMO at 230 (solid line) and 260 K (dashed line), and (b) NSMO at 140 K with magnetic field parallel to [0 0 1]p direction. Fig. 3. Optical micrographs of the charge ordered phase on (a) (0 1 0)p and (b) (1 0 0)p planes for PCMO at 208 K with common edges. The lines AA and BB are the traces of the surface relief on (0 1 0)p and (1 0 0)p planes, respectively.
result means that the interface of two domains is the {1 1 2}CO plane, which is the same as the mirror plane. From these results, we conclude that the charge ordered phase is composed of the twinning domains, and the twinning planes are {1 0 1}CO and {1 1 2}CO for PCMO and NSMO, respectively. These twinning planes correspond to the {1 1 0} planes of the cubic structure, which is shown in Fig. 1 with solid lines. 3.2. Magnetic field dependence of the charge ordering transition temperature
seen from the figure, the transition fields increase with decreasing temperature for both specimens. We analyze this reverse transition using the relationship based on Clausius–Clapeyron equation written as, dT M(H) =− , dH S(H)
(1)
where M(H) is the difference of the magnetization between the charge ordered phase and the parent phase under a magnetic field H and S(H) is the change of entropy. This equation can
In order to investigate the magnetic field dependence of TCO , we have made magnetization measurements for both specimens. Typical results of magnetization curves are shown in Fig. 5. For both specimens, drastic increase in magnetization is observed just below TCO . This means that reverse transition from a charge ordered phase to a parent phase occurs by applying magnetic field. The reverse transition fields are plotted in Fig. 6. As
Fig. 4. Optical micrographs of the charge ordered phase on (a) (1 0 0)p and (b) (1 1 0)p planes for NSMO at 90 K with common edges. The lines AA and BB are the traces of the surface relief on (1 0 0)p and (1 1 0)p planes, respectively.
Fig. 6. TCO as a function of magnetic field for (a) PCMO and (b) NSMO. The solid line in (a) is the calculated from results based on Eq. (3), and dotted line in (b) is the fitted result based on Eq. (4).
558
T. Murata et al. / Materials Science and Engineering A 481–482 (2008) 555–558
be integrated over H so that T0 Hc T = − M(T0 , H)dH, Q 0
4. Summary (2)
where T represents the change of TCO , T0 represents the thermodynamic equilibrium transition temperature, Hc represents the reverse transition field, and Q represents the latent heat. As for PCMO, both the parent and the charge ordered phases are paramagnetic. Thus, it is considered that the magnetizations for both phases are proportional to the magnetic field. So, we can calculate T as, T = −
T0 [χp (T0 ) − χCO (T0 )]Hc2 ∝ Hc2 , 2Q
(3)
where χp (T0 ) and χCO (T0 ) represent the magnetic susceptibility of the parent and the charge ordered phase, respectively. In order to calculate T by this equation, the value of Q is determined from differential scanning calorimetry measurements to be 392 J mol−1 , and the values of χp and χCO are determined from the slope of the magnetization curves to be 0.0296 and 0.0154, respectively. Using these values, T is calculated and plotted as a function of magnetic field, which is shown in Fig. 6a by the solid line. As seen from the figure, the calculated values are in good agreement with the experimental ones. As for NSMO, parent and charge ordered phases are ferromagnetic and antiferromagnetic, respectively. Thus, it can be considered that the magnetization of the parent phase is almost constant with magnetic field, and that of the charge ordered phase can be neglected. So, we can calculate T as, T = −
T0 Ms (T0 )Hc ∝ Hc , 2Q
(4)
where Ms (T0 ) is the spontaneous magnetization of the parent phase at T0 . From this equation, T of NSMO is proportional to the magnetic field. Unfortunately, as for NSMO, we cannot measure the latent heat because of the present instrumental limitation. Thus, we have estimated the value of Q by using a least-square fitting shown by the dotted line in Fig. 6b, and the value of Q is obtained to be 39.2 J mol−1 .
We have investigated the microstructure associated with the charge ordering transition in perovskite manganite PCMO and NSMO. By optical microscope observation, microstructures composed of crystallographic domains are observed below the charge ordering transition temperature, TCO , for both of the specimens. By two surface trace analysis and Laue back reflection technique, we have found that microstructures are composed of twinned variants which have {1 0 1}CO and {1 1 2}CO twinning planes for PCMO and NSMO, respectively. We have also investigated the magnetic field dependence of TCO for both specimens by magnetization measurements and confirmed that the magnetic field dependence of TCO in PCMO is in good agreement with the calculated one based on the Clausius–Clapeyron equation. Acknowledgement This work was supported by Grant-In-Aid of COE (Center of Excellence) program for Advanced Structural and Functional Materials Design from MEXT of Japan. References [1] G.H. Jonker, J.H. van Santen, Physica 16 (1950) 337–349. [2] R. von Helmot, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331–2333. [3] K. Chahara, T. Ohno, M. Kasai, Y. Kozono, Appl. Phys. Lett. 63 (1993) 1990–1992. [4] T. Terai, H. Fujita, T. Murata, T. Kakeshita, K. Kishio, Trans. Mater. Res. Soc. Jpn. 28 (2003) 243–244. [5] Z. Jirak, S. Krupicka, Z. Simsa, M. Dlouha, Z. Vratislav, J. Magn. Magn. Mater. 53 (1985) 153–166. [6] Y. Tomioka, T. Okuda, Y. Okimoto, A. Asamitsu, H. Kuwahara, Y. Tokura, J. Alloys Compd. 326 (2001) 27–35. [7] I.O. Shklyarevskiy, A.B. Chizhik, S.L. Gnatchenko, P.J.M. van Bentum, P.C.M. Christianen, J.C. Maan, K.V. Kamenov, G. Balakrishnan, D.McK. Paul, J. Magn. Magn. Mater. 238 (2002) 140–144.
Structural phase transition in Pr0.55Ca0.45MnO3 and Nd0.5Sr0.5MnO3 associated with charge ordering transition T. Murata ∗ , T. Terai, T. Fukuda, T. Kakeshita Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan Received 17 May 2006; received in revised form 8 November 2006; accepted 31 December 2006
Abstract We have investigated the microstructure formed in association with charge ordering transition in perovskite manganites Pr0.55 Ca0.45 MnO3 (PCMO) and Nd0.5 Sr0.5 MnO3 (NSMO). By optical microscope observation, surface relief is observed below the charge ordering transition temperature TCO (CO represents charge ordering phase) for both of the specimens. Two surface trace analysis and Laue back reflection technique has been revealed that microstructures are composed of twinned variants which have {1 0 1}CO and {1 1 2}CO twinning planes for PCMO and NSMO, respectively. We have also investigated the magnetic field dependence of TCO by magnetization measurements and found that the TCO decreases parabolically with increasing magnetic field. This magnetic field dependence of TCO in PCMO is in good agreement with the calculated one based on the Clausius–Clapeyron equation. © 2007 Elsevier B.V. All rights reserved. Keywords: Charge ordering transition; Twin; Perovskite manganite
1. Introduction Perovskite manganites with some trivalent rare earth metal ions R3+ replaced by divalent alkaline-earth metal ions A2+ , R1−x Ax MnO3 , have been extensively studied and found to show interesting phenomena such as metal–insulator transition, colossal magnetoresistance etc. [1–7]. In some perovskite manganites R1−x Ax MnO3 (R and A are trivalent rare earth metal ions and divalent alkaline-earth metal ions, respectively) with hole doping level corresponding to x ≈ 0.5, charge ordering transition occurs and is usually accompanied by a discontinuous change in lattice parameters [5–7]. Thus, it is expected that self-accommodated microstructures are formed to relax stress which is induced by the change in lattice parameters, like in martensitic transitions. Moreover, most of the charge ordering transitions in perovskite manganites accompanies drastic changes in magnetic and electronic properties [5–7]. Thus, it is also expected that the charge ordering transition temperature is largely changed by external field such as magnetic field; similar to martensitic transition temperatures in metallic materials. However, there are few studies on
∗
Corresponding author. E-mail address: [email protected] (T. Murata).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.205
microstructures formed by charge ordering transition [7] and on quantitative analysis of the magnetic field dependence of charge ordering transition temperature in perovskite manganites. In this study, therefore, we have analyzed the microstructure of the charge ordered phase and the magnetic field dependence of charge the ordering transition temperature of Pr0.45 Ca0.55 MnO3 (PCMO) and Nd0.5 Sr0.5 MnO3 (NSMO) by magnetization measurement, optical microscopy and the Laue back reflection technique. 2. Experimental We prepared single crystal specimens of PCMO and NSMO. Compounds of Pr2 O3 , Nd2 O3 , CaCO3 , SrCO3 and Mn3 O4 were mixed in amounts for the desired compositions, and calcined in air at 1273 K for 12 h. After intermediate grinding, these compounds were pressed into rods of about 5 mm in diameter. These rods were sintered in O2 gas flow at 1873 K for 12 h. Using the rods as starting materials, single crystals were grown by a floating zone method with a growth rate of 1.25–3.50 mm/h. The compositions of single crystal specimens were confirmed by inductively coupled plasma-atomic emission spectrometer to be nominal. Powder X-ray diffraction measurements were carried out in order to check their crystal structures at
556
T. Murata et al. / Materials Science and Engineering A 481–482 (2008) 555–558
Table 1 Lattice parameters of parent phases at 298 K and charge ordered phases at 120 K for PCMO and NSMO Parent phase ˚ (298 K) (A)
Charge ordered ˚ phase (120 K) (A)
PCMO
a = 7.647
a = 7.669 c = 7.586
NSMO
a = 5.432 b = 5.482 c = 7.640
a = 5.442 b = 5.504 c = 7.539 (β = 89.9◦ )
the temperatures from 77 K to room temperature. Magnetization measurements were carried out using a superconducting quantum interference device (SQUID) magnetometer. The microstructure on the surface of the specimen was examined by Nomarski microscopy and ordinary polarizing microscopy (OLYMPUS BM60XM). The crystallographic orientation relationship between observed domains in the charge ordered phase was determined by using a back-reflection Laue camera (RINT2100) equipped with a cooling stage. 3. Results 3.1. Microstructure of the charge ordered phase From X-ray diffraction and magnetization measurements, we have confirmed that PCMO exhibits a structural transition from a paramagnetic phase with pseudo-cubic structure to a paramagnetic charge ordered phase with pseudo-tetragonal structure at TCO = 238 K, and NSMO exhibits a structural transition from a ferromagnetic phase with orthorhombic structure to an antiferromagnetic charge ordered phase with monoclinic structure at TCO = 160 K, which are in good agreement with previous reports [5,6]. The lattice parameters are shown in Table 1, and the lattice relationships between parent and charge ordered phases of these compounds are shown in Fig. 1. In Fig. 1, the crystal structure of the charge ordered phase for NSMO is treated as a pseudoorthorhombic structure for simplicity, because the monoclinic angle β is close to 90◦ . This figure also shows the unit cell of the cubic perovskite structure, which is obtained in BaTiO3 . As seen from this figure, the [1 0 0] direction of PCMO and the [1 1 0]
Fig. 1. Schematic diagram of unit cells for cubic (solid lines), pseudo-tetragonal (dashed lines) and pseudo-orthorhombic (dotted lines). The a0 represents the lattice parameter of the cubic unit cell together with MnO6 octahedra. Subscripts c, t and o represent pseudo-cubic, pseudo-tetragonal and pseudo-orthorhombic, respectively.
Fig. 2. Optical micrographs on the (0 0 1)p plane for PCMO taken at (a) 298 K and (b) 135 K, and those on the (0 1 0)p plane for NSMO taken at (c) 298 K and (d) 82 K.
direction of NSMO are parallel to the [1 0 0] direction of the cubic structure, and the [0 0 1] directions of PCMO and NSMO are parallel to the [0 0 1] direction of cubic structure. In order to observe the microstructure associated with the charge ordering transition, optical microscopy observation has been made for both specimens. Typical micrographs on (0 0 1)p (p represents the parent phase) for PCMO and (0 1 0)p planes for NSMO are shown in Fig. 2. At room temperature, as shown in Fig. 2a and c, a flat and a smooth surface is observed for PCMO and NSMO, respectively. In the cooling process, surface relief appears at TCO and microstructure is formed for PCMO and NSMO, as shown in Fig. 2b and d, respectively. This microstructure quite resembles the twinned structures of shape memory alloys. Thus, it is considered that these microstructures are composed of crystallographic domains. In the heating process, the surface relief disappears at TCO and flat and smooth surfaces are obtained again at room temperature. In order to analyze this microstructure crystallographically, we have taken Laue photographs at 77 K from a part of the surface, which contains two domains. As a result, we find that there are mirror relationships between two domains, and the mirror planes are {1 0 1}CO (CO represents the charge ordered phase) and {1 1 2}CO for PCMO and NSMO respectively. In order to confirm that these mirror planes are the interface of two domains, we use two-surface trace analysis. For PCMO, the optical micrographs of the charge ordered phase on (0 1 0)p and (1 0 0)p planes are shown in Fig. 3a and b, respectively. As seen from the figures, the trace AA along the [1 0 0]p direction is observed on the (0 1 0)p plane, and this trace is connected to the trace BB along the [0 1 1]p direction on the (1 0 0)p plane. This result means that the interface of two domains is the {1 0 1}CO plane, which is the same as the mirror plane. Also, for NSMO, the optical micrographs of the charge ordered phase on (1 0 0)p and (1 1 0)p planes are shown in Fig. 4a and b, respectively. As seen from the figures, the trace AA along the [0 2 1]p direction is observed on the (1 0 0)p plane and this trace is connected to trace BB along the [1 1¯ 1]p direction on the (1 1 0)p plane. This
T. Murata et al. / Materials Science and Engineering A 481–482 (2008) 555–558
557
Fig. 5. Magnetization curves of (a) PCMO at 230 (solid line) and 260 K (dashed line), and (b) NSMO at 140 K with magnetic field parallel to [0 0 1]p direction. Fig. 3. Optical micrographs of the charge ordered phase on (a) (0 1 0)p and (b) (1 0 0)p planes for PCMO at 208 K with common edges. The lines AA and BB are the traces of the surface relief on (0 1 0)p and (1 0 0)p planes, respectively.
result means that the interface of two domains is the {1 1 2}CO plane, which is the same as the mirror plane. From these results, we conclude that the charge ordered phase is composed of the twinning domains, and the twinning planes are {1 0 1}CO and {1 1 2}CO for PCMO and NSMO, respectively. These twinning planes correspond to the {1 1 0} planes of the cubic structure, which is shown in Fig. 1 with solid lines. 3.2. Magnetic field dependence of the charge ordering transition temperature
seen from the figure, the transition fields increase with decreasing temperature for both specimens. We analyze this reverse transition using the relationship based on Clausius–Clapeyron equation written as, dT M(H) =− , dH S(H)
(1)
where M(H) is the difference of the magnetization between the charge ordered phase and the parent phase under a magnetic field H and S(H) is the change of entropy. This equation can
In order to investigate the magnetic field dependence of TCO , we have made magnetization measurements for both specimens. Typical results of magnetization curves are shown in Fig. 5. For both specimens, drastic increase in magnetization is observed just below TCO . This means that reverse transition from a charge ordered phase to a parent phase occurs by applying magnetic field. The reverse transition fields are plotted in Fig. 6. As
Fig. 4. Optical micrographs of the charge ordered phase on (a) (1 0 0)p and (b) (1 1 0)p planes for NSMO at 90 K with common edges. The lines AA and BB are the traces of the surface relief on (1 0 0)p and (1 1 0)p planes, respectively.
Fig. 6. TCO as a function of magnetic field for (a) PCMO and (b) NSMO. The solid line in (a) is the calculated from results based on Eq. (3), and dotted line in (b) is the fitted result based on Eq. (4).
558
T. Murata et al. / Materials Science and Engineering A 481–482 (2008) 555–558
be integrated over H so that T0 Hc T = − M(T0 , H)dH, Q 0
4. Summary (2)
where T represents the change of TCO , T0 represents the thermodynamic equilibrium transition temperature, Hc represents the reverse transition field, and Q represents the latent heat. As for PCMO, both the parent and the charge ordered phases are paramagnetic. Thus, it is considered that the magnetizations for both phases are proportional to the magnetic field. So, we can calculate T as, T = −
T0 [χp (T0 ) − χCO (T0 )]Hc2 ∝ Hc2 , 2Q
(3)
where χp (T0 ) and χCO (T0 ) represent the magnetic susceptibility of the parent and the charge ordered phase, respectively. In order to calculate T by this equation, the value of Q is determined from differential scanning calorimetry measurements to be 392 J mol−1 , and the values of χp and χCO are determined from the slope of the magnetization curves to be 0.0296 and 0.0154, respectively. Using these values, T is calculated and plotted as a function of magnetic field, which is shown in Fig. 6a by the solid line. As seen from the figure, the calculated values are in good agreement with the experimental ones. As for NSMO, parent and charge ordered phases are ferromagnetic and antiferromagnetic, respectively. Thus, it can be considered that the magnetization of the parent phase is almost constant with magnetic field, and that of the charge ordered phase can be neglected. So, we can calculate T as, T = −
T0 Ms (T0 )Hc ∝ Hc , 2Q
(4)
where Ms (T0 ) is the spontaneous magnetization of the parent phase at T0 . From this equation, T of NSMO is proportional to the magnetic field. Unfortunately, as for NSMO, we cannot measure the latent heat because of the present instrumental limitation. Thus, we have estimated the value of Q by using a least-square fitting shown by the dotted line in Fig. 6b, and the value of Q is obtained to be 39.2 J mol−1 .
We have investigated the microstructure associated with the charge ordering transition in perovskite manganite PCMO and NSMO. By optical microscope observation, microstructures composed of crystallographic domains are observed below the charge ordering transition temperature, TCO , for both of the specimens. By two surface trace analysis and Laue back reflection technique, we have found that microstructures are composed of twinned variants which have {1 0 1}CO and {1 1 2}CO twinning planes for PCMO and NSMO, respectively. We have also investigated the magnetic field dependence of TCO for both specimens by magnetization measurements and confirmed that the magnetic field dependence of TCO in PCMO is in good agreement with the calculated one based on the Clausius–Clapeyron equation. Acknowledgement This work was supported by Grant-In-Aid of COE (Center of Excellence) program for Advanced Structural and Functional Materials Design from MEXT of Japan. References [1] G.H. Jonker, J.H. van Santen, Physica 16 (1950) 337–349. [2] R. von Helmot, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331–2333. [3] K. Chahara, T. Ohno, M. Kasai, Y. Kozono, Appl. Phys. Lett. 63 (1993) 1990–1992. [4] T. Terai, H. Fujita, T. Murata, T. Kakeshita, K. Kishio, Trans. Mater. Res. Soc. Jpn. 28 (2003) 243–244. [5] Z. Jirak, S. Krupicka, Z. Simsa, M. Dlouha, Z. Vratislav, J. Magn. Magn. Mater. 53 (1985) 153–166. [6] Y. Tomioka, T. Okuda, Y. Okimoto, A. Asamitsu, H. Kuwahara, Y. Tokura, J. Alloys Compd. 326 (2001) 27–35. [7] I.O. Shklyarevskiy, A.B. Chizhik, S.L. Gnatchenko, P.J.M. van Bentum, P.C.M. Christianen, J.C. Maan, K.V. Kamenov, G. Balakrishnan, D.McK. Paul, J. Magn. Magn. Mater. 238 (2002) 140–144.