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Physical properties of Heusler-type Fe2VA1 compound
Journal of
,A•
maadg neusm magnetic
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 669-670
materials
Physical properties of Heusler-type Fe2VA1 compound A. Matsushita a'*, Y. Yamada b "National Research Institutefor Metals, 1-2-1 Sengen, Tsukuba, lbaraki 305-0047, Japan bDepartment of Material Science, Shimane University, Shimane, Japan
Abstract
We have investigated the effect of heat treatment on the physical properties of Heusler-type Fe2VA1 compound. Slow cooling after homogenization at 1073 K suppresses the semiconducting behavior of electrical resistivity and the enhancement of specific heat which are observed for a quenched sample. Ferromagnetic transition is observed for the slowly cooled sample while it is not observed for a quenched one. We discuss the cause of heat treatment effects. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Electrical resistivity; Specific heat; Magnetic susceptibility; Heat treatment; Ferromagnetic transition
The intermetallic compound Fe3A1 is a ferromagnet with DO3 crystal structure. Some Fe atoms of this compound can be replaced by V atoms. The Curie temperature decreases with V concentration and finally disappears around the composition of Fe2VA1 [1]. Recently, it was found that Heusler-type FezVA1 compound exhibits a heavy-fermion-like behavior, i.e., a semiconducting behavior of electrical resistivity and an enhancement of low-temperature specific heat [1]. In this paper, we report on the effects of heat treatment on these physical properties and discuss the cause of heat treatment effects. Stoichiometric mixtures of the constituent elements were melt in an argon arc furnace. A weight loss during this process was less than 0.5%. Three specimens were cut from the ingot and each specimen was sealed in an evacuated quartz ampoule for heat treatment. At first, all samples were homogenized at 1073 K for 50 h. The first sample was quenched in water after this heat treatment (sample Q). The second one was annealed at 673 K for 4 h (sample A). The third one was slowly cooled to room temperature at the rate o f - 20 K/h (sample S). A portion of each sample was ground into powder for X-ray diffraction measurement. Broadening of diffraction line owing
*Corresponding author. Fax: +81-298-59-2701; e-mail: [email protected].
to plastic deformation was observed, but the formation of Heusler-type structure was confirmed for the three sampies. The second phase was not observed. The lattice constant was 5.763 A. A difference in lattice constant was not detected for the three samples within experimental errors. Electrical resistivity was measured with a conventional four-probe method from 4 to 300 K. Specific heat data were obtained with a quasi-adiabatic method from 1.7 to 35 K. Magnetic susceptibility data were taken with a Quantum Design SQUID magnetometer from 4 to 300 K. Fig. 1 shows the effect of heat treatment on the electrical resistivity. Sample Q exhibits a semiconducting behavior, but the temperature dependence cannot be explained as a semiconductor with a single energy gap. The semiconducting behavior is surprisingly suppressed by annealing at 673 K or by slow cooling. In sample S a cusp appears at 13 K. As mentioned later a discontinuous increase of magnetic susceptibility is observed at 13 K and an anomaly of specific heat is also observed at 13 K. From these experimental results we conclude that the cusp of the electrical resistivity corresponds to a ferromagnetic ordering. It should be noted that the electrical resistivity is metallic below the Curie temperature. This fact suggests that the semiconducting behavior of the electrical resistivity is closely related to the paramagnetic state.
0304-8853/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 9 1 3-5
670
A. Matsushita, E }ramada /Journal q[' MagnetLs'm and Magnetic Materials 196-197 (1999) 669- 670 6[ ,
g
I /
•
Fe2VAI 6i
~Xx quenched
>,
quenched
~-~'~
~c':
at 673 K ,,
I
~
slowly cooled
± ££ 2-
Tc OL_ _ 0
• 2 iTc 2
F%VAI H=100G
slowly cooled 100 200 Temperature (K)
300
Fig. 1. Effect of heat treatment on the electrical resistivity of Fe2VAI. Details of heat treament are described in the text.
The variation of magnetization with applied magnetic field is not linear below about 4000 G. In Fig. 2 the values of HIM measured at 100 G are plotted as a function of temperature for the sample Q and S, where H is the applied magnetic field and M is the magnetization. An anomaly corresponding to the Curie temperature is observed at 13 K for sample S. The variation of H / M with temperature cannot be fitted with Curie-Weiss law. In a previous work [2] the magnetization is reported to be proportional to an applied magnetic field and the magnetic susceptibility can be explained with Curie-Weiss law with a temperature-independent term. Our results are not consistent with this work, but the magnetic susceptibility obtained at 104G gives a good agreement with those reported in Ref. [2]. The cause of the nonlinear M - H relationship observed in our samples is not clear at present. Fig. 3 shows the effect of heat treatment on the specific heat. Above 30 K the values of specific heat are identical for the three samples. As temperature decreases the value of C / T exhibits an upturn after passing a minimum. The position of the minimum is different depending on the heat treatment. The slope of the upturn is largest in sample Q and is smallest in sample S. In sample S an anomaly corresponding to the Curie temperature is observed at 13 K. Thus, the physical properties of Fe2VAI greatly depend on the heat treatment. Unfortunately, we could not find any difference of X-ray diffraction profiles among the three samples. Recently, G u o et al. have calculated the electronic structure of FezVA1 c o m p o u n d [3]. They have investigated two different cases; one is the c o m p o u n d with a Heusler-type structure and the other is a compound with a structure in which one Fe atom is replaced by a V atom. We denote the latter structure FeVFeAI. According to their calculation Fe2VA1 is a nonmagnetic semimetal while the FeVFeA1 is a ferromagnetic metal. Their study suggests a possibility that a mixing of Fe and V may give rise to a ferromagnetic transition. Therefore,
o
t 0o
Edo
3OO
Temperature IK)
Fig. 2. Effect of heat treatment on the value H/M measured at 100 G. Details of heat treatment are described in the text.
FepVAI
-•60• E i
4ol
20 =
annealed al 673K slowly-cooled
,,,~
~ 7 5 : ; o'° • quenched
°o
i~
2U-
3'0
Tempearture (K)
Fig. 3. Effect of heat treatment on the specific heat of Fe2VAI. Details of heat treatment are described in the text.
a possible explanation for the difference between samples Q and S is that mixing of a small amount of Fe and V occurs by slow cooling and the ferromagnetic transition appears in sample S. An enhancement of low-temperature specific heat has been reported for the Sc3In c o m p o u n d and was explained taking into account the effect of spin fluctuations I-4]. Sc3In is a ferromagnetic metal with an ordering temperature at 5.5 K. Therefore, we speculate that the enhancement of low-temperature specific heat is a general behavior of a ferromagnet with a low Curie temperature or of a nearly ferromagnet.
References [1] Y. Nishino, M. Kato, S. Asano, K. Soda, M. Hayasaki, U. Mizutani, Phys. Rev. Lett. 79 (1997) 1909. [2] P.J. Webster, K.R.A. Ziebeck, Phys. Lett. 98A (1983/ 51. [3] G.Y. Guo, G.A. Botton, Y. Nishino, J. Phys.: Condens. Matter 10 (1998) Ll19. [4] J. Takeuchi, Y. Masuda, J. Phys. Soc. Jpn. 46 (1979) 468.
,A•
maadg neusm magnetic
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 669-670
materials
Physical properties of Heusler-type Fe2VA1 compound A. Matsushita a'*, Y. Yamada b "National Research Institutefor Metals, 1-2-1 Sengen, Tsukuba, lbaraki 305-0047, Japan bDepartment of Material Science, Shimane University, Shimane, Japan
Abstract
We have investigated the effect of heat treatment on the physical properties of Heusler-type Fe2VA1 compound. Slow cooling after homogenization at 1073 K suppresses the semiconducting behavior of electrical resistivity and the enhancement of specific heat which are observed for a quenched sample. Ferromagnetic transition is observed for the slowly cooled sample while it is not observed for a quenched one. We discuss the cause of heat treatment effects. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Electrical resistivity; Specific heat; Magnetic susceptibility; Heat treatment; Ferromagnetic transition
The intermetallic compound Fe3A1 is a ferromagnet with DO3 crystal structure. Some Fe atoms of this compound can be replaced by V atoms. The Curie temperature decreases with V concentration and finally disappears around the composition of Fe2VA1 [1]. Recently, it was found that Heusler-type FezVA1 compound exhibits a heavy-fermion-like behavior, i.e., a semiconducting behavior of electrical resistivity and an enhancement of low-temperature specific heat [1]. In this paper, we report on the effects of heat treatment on these physical properties and discuss the cause of heat treatment effects. Stoichiometric mixtures of the constituent elements were melt in an argon arc furnace. A weight loss during this process was less than 0.5%. Three specimens were cut from the ingot and each specimen was sealed in an evacuated quartz ampoule for heat treatment. At first, all samples were homogenized at 1073 K for 50 h. The first sample was quenched in water after this heat treatment (sample Q). The second one was annealed at 673 K for 4 h (sample A). The third one was slowly cooled to room temperature at the rate o f - 20 K/h (sample S). A portion of each sample was ground into powder for X-ray diffraction measurement. Broadening of diffraction line owing
*Corresponding author. Fax: +81-298-59-2701; e-mail: [email protected].
to plastic deformation was observed, but the formation of Heusler-type structure was confirmed for the three sampies. The second phase was not observed. The lattice constant was 5.763 A. A difference in lattice constant was not detected for the three samples within experimental errors. Electrical resistivity was measured with a conventional four-probe method from 4 to 300 K. Specific heat data were obtained with a quasi-adiabatic method from 1.7 to 35 K. Magnetic susceptibility data were taken with a Quantum Design SQUID magnetometer from 4 to 300 K. Fig. 1 shows the effect of heat treatment on the electrical resistivity. Sample Q exhibits a semiconducting behavior, but the temperature dependence cannot be explained as a semiconductor with a single energy gap. The semiconducting behavior is surprisingly suppressed by annealing at 673 K or by slow cooling. In sample S a cusp appears at 13 K. As mentioned later a discontinuous increase of magnetic susceptibility is observed at 13 K and an anomaly of specific heat is also observed at 13 K. From these experimental results we conclude that the cusp of the electrical resistivity corresponds to a ferromagnetic ordering. It should be noted that the electrical resistivity is metallic below the Curie temperature. This fact suggests that the semiconducting behavior of the electrical resistivity is closely related to the paramagnetic state.
0304-8853/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 9 1 3-5
670
A. Matsushita, E }ramada /Journal q[' MagnetLs'm and Magnetic Materials 196-197 (1999) 669- 670 6[ ,
g
I /
•
Fe2VAI 6i
~Xx quenched
>,
quenched
~-~'~
~c':
at 673 K ,,
I
~
slowly cooled
± ££ 2-
Tc OL_ _ 0
• 2 iTc 2
F%VAI H=100G
slowly cooled 100 200 Temperature (K)
300
Fig. 1. Effect of heat treatment on the electrical resistivity of Fe2VAI. Details of heat treament are described in the text.
The variation of magnetization with applied magnetic field is not linear below about 4000 G. In Fig. 2 the values of HIM measured at 100 G are plotted as a function of temperature for the sample Q and S, where H is the applied magnetic field and M is the magnetization. An anomaly corresponding to the Curie temperature is observed at 13 K for sample S. The variation of H / M with temperature cannot be fitted with Curie-Weiss law. In a previous work [2] the magnetization is reported to be proportional to an applied magnetic field and the magnetic susceptibility can be explained with Curie-Weiss law with a temperature-independent term. Our results are not consistent with this work, but the magnetic susceptibility obtained at 104G gives a good agreement with those reported in Ref. [2]. The cause of the nonlinear M - H relationship observed in our samples is not clear at present. Fig. 3 shows the effect of heat treatment on the specific heat. Above 30 K the values of specific heat are identical for the three samples. As temperature decreases the value of C / T exhibits an upturn after passing a minimum. The position of the minimum is different depending on the heat treatment. The slope of the upturn is largest in sample Q and is smallest in sample S. In sample S an anomaly corresponding to the Curie temperature is observed at 13 K. Thus, the physical properties of Fe2VAI greatly depend on the heat treatment. Unfortunately, we could not find any difference of X-ray diffraction profiles among the three samples. Recently, G u o et al. have calculated the electronic structure of FezVA1 c o m p o u n d [3]. They have investigated two different cases; one is the c o m p o u n d with a Heusler-type structure and the other is a compound with a structure in which one Fe atom is replaced by a V atom. We denote the latter structure FeVFeAI. According to their calculation Fe2VA1 is a nonmagnetic semimetal while the FeVFeA1 is a ferromagnetic metal. Their study suggests a possibility that a mixing of Fe and V may give rise to a ferromagnetic transition. Therefore,
o
t 0o
Edo
3OO
Temperature IK)
Fig. 2. Effect of heat treatment on the value H/M measured at 100 G. Details of heat treatment are described in the text.
FepVAI
-•60• E i
4ol
20 =
annealed al 673K slowly-cooled
,,,~
~ 7 5 : ; o'° • quenched
°o
i~
2U-
3'0
Tempearture (K)
Fig. 3. Effect of heat treatment on the specific heat of Fe2VAI. Details of heat treatment are described in the text.
a possible explanation for the difference between samples Q and S is that mixing of a small amount of Fe and V occurs by slow cooling and the ferromagnetic transition appears in sample S. An enhancement of low-temperature specific heat has been reported for the Sc3In c o m p o u n d and was explained taking into account the effect of spin fluctuations I-4]. Sc3In is a ferromagnetic metal with an ordering temperature at 5.5 K. Therefore, we speculate that the enhancement of low-temperature specific heat is a general behavior of a ferromagnet with a low Curie temperature or of a nearly ferromagnet.
References [1] Y. Nishino, M. Kato, S. Asano, K. Soda, M. Hayasaki, U. Mizutani, Phys. Rev. Lett. 79 (1997) 1909. [2] P.J. Webster, K.R.A. Ziebeck, Phys. Lett. 98A (1983/ 51. [3] G.Y. Guo, G.A. Botton, Y. Nishino, J. Phys.: Condens. Matter 10 (1998) Ll19. [4] J. Takeuchi, Y. Masuda, J. Phys. Soc. Jpn. 46 (1979) 468.