THERMAL EXPANSION ANOMALIES AT THE MAGNETIC TRANSITION OF Mn3Ga1-xAlxC (x 0.05)

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 ordered cubic crystal of a perovskite type structure. The magnetic properties are characterized by antiferromagnetic (AF)-ferromagnetic(F) phase transition which is accompanied with a discontinuous change of the lattice parameter [I]. Kaneko et al. [2] measured 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 interaction 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 parameter 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 dependence of the magnetization is dso reported for these compounds. The samples were prepared by the procedure described 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 lattice parameters at room temperature were found to decrease with increase of z a = 3.8955 a, 3.8935 and 3.8896 A 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 figure 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 paramagnetic 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 of x = 0.0 and 0.015 at antiferromagnetic state. It is not clear at 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 to 320 K. The results are shown in figure 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 figure 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 AS at Ttl is estimated from the following expression, where AV and dTtl / dp are the volume change at Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988866 '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, AS is calculated to be 2.26 R from above equation. This value is larger than the value 0.5 R obtained in the phase transition from AF to F phase of MnaGaC [2]. It should be noticed that this entropy change AS is that of the transition from AF to I phase. Table I. The 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 temperatures from both measurements show a good agreement. With increase of composition x, Tc increases slightly and both Tt; and Ttz decrease. This tendency conWe estimated the pressure dependence of the lattice 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 corresponds to that of MnaGaC at p = 2 kbar. The intermediate 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 concluded that the magnetic phase transitions in these compounds are due to 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 (1988).