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THERMAL EXPANSION ANOMALIES AT THE
MAGNETIC TRANSITION OF Mn3Ga1-xAlxC (x
≤
0.05)
T. Suzuki, T. Kanomata, T. Kaneko
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, dCembre 1988
THERMAL EXPANSION ANOMALIES AT
THE MAGNETIC TRANSITION OF
Mn3Gal-zAlxC (a:
50.05)T. Suzuki (I), T. Kanomata (I) and T. Kaneko (2)
(I) Faculty of Engineering, Tohoku Gakuin University, Tagajo 985, Japan
(2) Institute for Materials Research, Tohoku University, Semdai 980, Japan
Abstract. - The temperature variations of the lattice parameter of the pseudoternary compounds Mn3Gal-,Al,C (x j 0.05) were measured by using of powder X-ray diffraction. The anomalies which correspond to the magnetic phase transitions were observed in the thermal expansion curves.
The intermetallic compound MnaGaC is an or- dered cubic crystal of a perovskite type structure. The magnetic properties are characterized by anti- ferromagnetic (AF)-ferromagnetic(F) phase transition which is accompanied with a discontinuous change of the lattice parameter [I]. Kaneko et al. [2] mea- sured the pressure effect of this material and found that a new intermediate (I) magnetic phase appears between AF and F state at pressures above 3 kbar. Kanomata et al. [3] showed that the intermediate phase exists in Mn3Ga0.9sAl0.02C even under normal pressure, and suggested that this I phase corresponds to the pressure induced I phase for MnsGaC. They pointed also out that the magnetic exchange interac- tion is strongly dependent on the lattice parameter. It is known that the measurement of the exchange strictions is useful for the study of the lattice pa- rameter dependence on the exchange interaction. In this paper, we studied the temperature dependence on the lattice parameter of pseudoternary compounds
Fig. 1.
-
Magnetization us. temperature curves for Mn3Gal-,Al,C.MnsGal-,Al,C (x
5
0.05).
The temperature depen- dence of the magnetization is dso reported for these compounds.The samples were prepared by the procedure de- scribed in the previous work [3]. X-ray diffraction measurements showed that all diffraction lines were indexed with a cubic perovskite structure, and their intensities agreed with the calculated ones. The lat- tice parameters a t room temperature were found to decrease with increase of z a = 3.8955
a,
3.8935 and 3.8896A
for x = 0.015,0.03 and 0.05, respectively.We measured the temperature dependence of the magnetization by using of magnetic balance under 640 Oe for these samples. The results axe shown in fig-
ure 1. As seen in the figure, the compound of x = 0.015
has three magnetic transition temperatures (Ttl, Tt2 and
T,)
which correspond to the ones from A F phase to I phase, from I phase to F phase and F phase to para- magnetic phase, respectively. The curve for x = 0.03 and 0.05 have also three magnetic transition points. However, the magnetizations for these samples below Ttl are large compared with those ofx
= 0.0 and 0.015 at antiferromagnetic state. It is not clear a t present why they have such a large value.The temperature variations of the lattice parameter for these samples were measured in the temperature range from 90 t o 320 K. The results are shown in fig- ure 2. The heavy line of MnaGaC is the quoted one from literature [2, 41. Each curve shows an abrupt change and changes of its slope at the temperatures which correspond to Ttl, Tt2 and T, observed in fig- ure 1 (the abrupt change was not observed for x = 0.05 in the temperature range investigated). Ttl, Ttz, Tcl the relative change of a at Ttl and the difference of the linear thermal expansion coefficients Aa2 at Tt2 and Aa, at
T,
were determined from figure 2 and the values are given in table I. The entropy change A S at Ttl is estimated from the following expression,where AV and dTtl
/
dp are the volume change at'28 - 160 JOURNAL DE PHYSIQUE
TEMPERATURE (K)
Fig. 3. - Composition dependence of the magnetic phase transition temperatures for Mn3Gal-,A1,C.
Fig.2. - Lattice parameter a us. temperature curves for
MnsGal-,Ai,C. sistent with the pressure dependence of the transition temperatures in Mn3GaC obtained in reference [2].
Ttl and the pressure derivatuve of Ttl. The values of AV obtained in this experiment and dTtl/ dp [5]
for the sample of z = 0.03 are -0.199 cm3/mol and
-
1.06 K/kbar. By using of these values, A S is calcu- lated t o be 2.26 R from above equation. This value is larger than the value 0.5 R obtained in the phase tran- sition from AF to F phase of MnaGaC [2]. It should be noticed that this entropy change A S is that of the transition from AF to I phase.Table I.
-
T h e values of Ttl, Tt2, Tc, Aala, Aa2 and A&.Figure 3 shows the composition dependence of each transition points obtained from the magnetization and the thermal expansion curves. The transition tempera- tures from both measurements show a good agreement. With increase of composition x, Tc increases slightly and both Tt; and Ttz decrease. This tendency con-
We estimated the pressure dependence of the lat- tice parameter by using of the value of compressibility K = 0.94 x 10-~/kbar for MnsGaC given by [5]. The result shows that the lattice parameter of x = 0.03 cor- responds t o that of MnaGaC at p = 2 kbar. The inter- mediate phase, however, does not appear in Mn3GaC at p = 2 kbar, while the I phase of x = 0.03 a p
pears even at normal pressure. Therefore, it is con- cluded that the magnetic phase transitions in these compounds are due t o the change of electronic state rather than the effect of lattice ;mameter reduction.
[I] Bouchaud, J. P., Fruchart, R., Guillot, M.,
Bartholin, H. and Chaisse, F., C.R. Hebd. Sian. Acad. Sci. Paris 261 (1965) 655.
121 Kaneko, T., Kanomata, T. and Shirakawa, K., J.
Phys. Soc. Jpn 56 (1987) 4047.
[3] Kanomata, T., Yasui, H., Yoshida, H. and Kaneko, T., J. Magn. Magn. Mater. 70 (1987) 263.
[4] Fruchart, E., Lorthioir, G. and Fruchart, R.,
Mater. Res. Bull. 8 (1973) 21.
[5] Kaneko, T. and Kanomata, T., J . Phys. France