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Magnetic phase transitions for body-centered tetragonal FeRh1−xPtx system
Journal of Magnetism and Magnetic Materials 226}230 (2001) 572}573
Magnetic phase transitions for body-centered tetragonal FeRh Pt system \V V Kaori Takizawa, Teruo Ono, Hideki Miyajima* Department of Physics, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku, Yokohama 223-8522, Japan
Abstract The magnetic phase transitions near the triple point of FeRh Pt system with body-centered tetragonal structure \V V were studied by means of magnetization measurement and MoK ssbauer spectroscopy. The composition (x) vs. temperature (T) phase diagram is determined. The triple point exists at x"0.715 and T"388 K. It is noted that the spin #uctuation plays an important role in the magnetic phase transition around the triple point. 2001 Elsevier Science B.V. All rights reserved. Keywords: 3d metals; Alloys; Magnetic phase diagrams; Magnetization measurements; MoK ssbauer spectroscopy; Phase transitions
Equiatomic FeRh alloy with CsCl-type structure exhibits a "rst-order transition from antiferromagnetic (AF) to ferromagnetic (FM) state at about 400 K [1]. FeRh Pt alloy with x'0.2 has the body-centered \V V tetragonal (BCT) structure of the CuAu-type, and it exhibits three di!erent magnetic phase transitions: (A) a "rst-order antiferromagnetic (AF)}paramagnetic (PM) transition (0.20(x(0.72), (B) a "rst-order AF}FM transition (0.72(x(0.81), and (C) a second-order FM}PM transition (0.81(x(1.0) [2]. The BCT FeRh Pt alloy is expected to have the triple point, at \V V which the AF, FM and PM states coexist in the region of x"0.72. However, the characteristic of the triple point has not been clari"ed. In this work, the triple point and the magnetic phase transition of the BCT FeRh Pt \V V are investigated. The sample preparation of FeRh Pt (0.70)x) \V V 0.75) and the experiments for magnetization and MoK ssbauer measurements are described in a previous paper [2]. Fig. 1 shows the temperature dependence of magnetization M for FeRh Pt and FeRh Pt in H "6 kOe. The former exhibits the "rst-order AF}PM transition at ¹ "394 K with thermal hysteresis of 5 K, * Corresponding author. Tel: #81-45-566-16-92; fax: #8145-566-16-72. E-mail address: [email protected] (H. Miyajima).
and the latter shows the "rst-order AF}FM transition at ¹ "368 K with thermal hysteresis of 7 K. The suscepti bility above ¹ or ¹ follows the Curie}Weiss law with positive asymptotic-paramagnetic Curie temperature . This means that the paramagnetic state is derived from the ferromagnetic state. The magnetization exhibits anomalous variations at ¹ , but the origin of this anomaly is not clear at present. The x}¹ phase diagram for BCT FeRh Pt \V V (0.70)x)0.75) is shown in Fig. 2. It should be noted that the lines of the AF}PM, AF}FM and FM}PM transitions intersect at the point of x"0.715 and T"388 K. As shown in Fig. 3, the hyper"ne "eld H for FeRh Pt and FeRh Pt decreases discon tinuously at ¹ or ¹ . This means that the "rst-order transition occurs at these temperatures. The result is consistent with that of the magnetization measurement. The asymptotic NeH el temperature ¹ , obtained by the , extrapolation of temperature variations of the hyper"ne "eld to zero is 430 K and 435 K for FeRh Pt and FeRh Pt , respectively. It is noted that the hyper"ne "eld shows no anomaly at ¹ for both samples. So, the temperature ¹ is not related to the transition point. According to MoK ssbauer spectra and X-ray diffraction [2], the AF, FM and PM states take a single magnetic phase. In other words, no mixing state consisting of three phases is con"ned. Consequently, the point of x"0.715 and ¹"388 K is concluded to be the triple point.
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 2 9 6 - 8
K. Takizawa et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 572}573
Fig. 1. Temperature dependence of magnetization FeRh Pt and FeRh Pt in H "6 kOe.
for
Fig. 2. x!¹ magnetic phase diagram for BCT FeRh Pt \V V (0.70)x)0.75), where AF, FM and PM represent antiferromagnetic, ferromagnetic and paramagnetic phases, respectively. The solid triangles, open triangles and open circles represent the "rst-order AF}PM transition temperature ¹ , the "rst-or der AF}FM transition temperature ¹ and the asymptotic Curie temperature , respectively.
It is known that BCC FeRh and BCC FeRhIr have the triple point in a pressure (P) vs. temperature (T) phase diagram: FeRh at about 50 kbar and 630 K [3] and
573
Fig. 3. Temperature dependence of the hyper"ne "eld H for (a) FeRh Pt and (b) FeRh Pt .
Fe Rh Ir at 20 kbar and 586 K [4]. In FeRh Pt system, the unit cell constant increases con\V V tinuously with Pt composition. In other words, as the Pt composition axis may be transposed with the inversed pressure axis, the P}T phase diagram of FeRh and Fe Rh Ir obtained by Wayne [3] and Vinokurova et al. [4], respectively, is very similar to that of the FeRh Pt system. The existence of the triple \V V point is consistent between these systems. In addition, it is found that the slope of the Arrott plots turns out to be negative in the vicinity of the transition, which may be derived from the tetragonal elongation e!ect which a!ects the band structure. This means that the longitudinal spin #uctuation is promoted and that the spin #uctuation may play an important role around the triple point.
References [1] M. Fallot, Ann. Phys. 10 (1938) 291. [2] S. Yuasa, H. Miyajima, Y. Otani, J. Phys. Soc. Japan 63 (1994) 3129. [3] R.C. Wayne, Phys. Rev. 170 (1968) 523. [4] L.I. Vinokurova, M. Pardavi-HorvaH th, Phys. Stat. Sol. B 48 (1971) K31.
Magnetic phase transitions for body-centered tetragonal FeRh Pt system \V V Kaori Takizawa, Teruo Ono, Hideki Miyajima* Department of Physics, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku, Yokohama 223-8522, Japan
Abstract The magnetic phase transitions near the triple point of FeRh Pt system with body-centered tetragonal structure \V V were studied by means of magnetization measurement and MoK ssbauer spectroscopy. The composition (x) vs. temperature (T) phase diagram is determined. The triple point exists at x"0.715 and T"388 K. It is noted that the spin #uctuation plays an important role in the magnetic phase transition around the triple point. 2001 Elsevier Science B.V. All rights reserved. Keywords: 3d metals; Alloys; Magnetic phase diagrams; Magnetization measurements; MoK ssbauer spectroscopy; Phase transitions
Equiatomic FeRh alloy with CsCl-type structure exhibits a "rst-order transition from antiferromagnetic (AF) to ferromagnetic (FM) state at about 400 K [1]. FeRh Pt alloy with x'0.2 has the body-centered \V V tetragonal (BCT) structure of the CuAu-type, and it exhibits three di!erent magnetic phase transitions: (A) a "rst-order antiferromagnetic (AF)}paramagnetic (PM) transition (0.20(x(0.72), (B) a "rst-order AF}FM transition (0.72(x(0.81), and (C) a second-order FM}PM transition (0.81(x(1.0) [2]. The BCT FeRh Pt alloy is expected to have the triple point, at \V V which the AF, FM and PM states coexist in the region of x"0.72. However, the characteristic of the triple point has not been clari"ed. In this work, the triple point and the magnetic phase transition of the BCT FeRh Pt \V V are investigated. The sample preparation of FeRh Pt (0.70)x) \V V 0.75) and the experiments for magnetization and MoK ssbauer measurements are described in a previous paper [2]. Fig. 1 shows the temperature dependence of magnetization M for FeRh Pt and FeRh Pt in H "6 kOe. The former exhibits the "rst-order AF}PM transition at ¹ "394 K with thermal hysteresis of 5 K, * Corresponding author. Tel: #81-45-566-16-92; fax: #8145-566-16-72. E-mail address: [email protected] (H. Miyajima).
and the latter shows the "rst-order AF}FM transition at ¹ "368 K with thermal hysteresis of 7 K. The suscepti bility above ¹ or ¹ follows the Curie}Weiss law with positive asymptotic-paramagnetic Curie temperature . This means that the paramagnetic state is derived from the ferromagnetic state. The magnetization exhibits anomalous variations at ¹ , but the origin of this anomaly is not clear at present. The x}¹ phase diagram for BCT FeRh Pt \V V (0.70)x)0.75) is shown in Fig. 2. It should be noted that the lines of the AF}PM, AF}FM and FM}PM transitions intersect at the point of x"0.715 and T"388 K. As shown in Fig. 3, the hyper"ne "eld H for FeRh Pt and FeRh Pt decreases discon tinuously at ¹ or ¹ . This means that the "rst-order transition occurs at these temperatures. The result is consistent with that of the magnetization measurement. The asymptotic NeH el temperature ¹ , obtained by the , extrapolation of temperature variations of the hyper"ne "eld to zero is 430 K and 435 K for FeRh Pt and FeRh Pt , respectively. It is noted that the hyper"ne "eld shows no anomaly at ¹ for both samples. So, the temperature ¹ is not related to the transition point. According to MoK ssbauer spectra and X-ray diffraction [2], the AF, FM and PM states take a single magnetic phase. In other words, no mixing state consisting of three phases is con"ned. Consequently, the point of x"0.715 and ¹"388 K is concluded to be the triple point.
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 2 9 6 - 8
K. Takizawa et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 572}573
Fig. 1. Temperature dependence of magnetization FeRh Pt and FeRh Pt in H "6 kOe.
for
Fig. 2. x!¹ magnetic phase diagram for BCT FeRh Pt \V V (0.70)x)0.75), where AF, FM and PM represent antiferromagnetic, ferromagnetic and paramagnetic phases, respectively. The solid triangles, open triangles and open circles represent the "rst-order AF}PM transition temperature ¹ , the "rst-or der AF}FM transition temperature ¹ and the asymptotic Curie temperature , respectively.
It is known that BCC FeRh and BCC FeRhIr have the triple point in a pressure (P) vs. temperature (T) phase diagram: FeRh at about 50 kbar and 630 K [3] and
573
Fig. 3. Temperature dependence of the hyper"ne "eld H for (a) FeRh Pt and (b) FeRh Pt .
Fe Rh Ir at 20 kbar and 586 K [4]. In FeRh Pt system, the unit cell constant increases con\V V tinuously with Pt composition. In other words, as the Pt composition axis may be transposed with the inversed pressure axis, the P}T phase diagram of FeRh and Fe Rh Ir obtained by Wayne [3] and Vinokurova et al. [4], respectively, is very similar to that of the FeRh Pt system. The existence of the triple \V V point is consistent between these systems. In addition, it is found that the slope of the Arrott plots turns out to be negative in the vicinity of the transition, which may be derived from the tetragonal elongation e!ect which a!ects the band structure. This means that the longitudinal spin #uctuation is promoted and that the spin #uctuation may play an important role around the triple point.
References [1] M. Fallot, Ann. Phys. 10 (1938) 291. [2] S. Yuasa, H. Miyajima, Y. Otani, J. Phys. Soc. Japan 63 (1994) 3129. [3] R.C. Wayne, Phys. Rev. 170 (1968) 523. [4] L.I. Vinokurova, M. Pardavi-HorvaH th, Phys. Stat. Sol. B 48 (1971) K31.