Spontaneous and field-induced magnetic phase transitions in the intermetallic compounds (Gd1-xYx) Mn2Ge2

The magnetic properties of the intermetallic compounds Gd1-xYxMn2Ge2 (0 les s than or equal to x less than or equal to 0.5) (tetragonal crystal structu re of the type ThCr2Ge2) have been measured in fields up to 40 T in the tem perature range 4.2-120 K on free powders (whose particles are free to rotat e in a magnetic field), fixed powders, and polycrystalline bulk samples. Th e temperature dependence of the crystal structure parameters of some of the se intermetallic compounds was determined from x-ray diffraction measuremen ts. It was found that the temperatures of magnetic disordering of the gadol inium magnetic subsystem and the transition of the manganese subsystem from the ferromagnetic to the antiferromagnetic state decrease as the gadoliniu m content is decreased, so that in compositions with x < 0.3 this transitio n is a first-order phase transition, and with further decrease of the gadol inium content it becomes second-order. In intermetallic compounds of this s ystem with x = 0.3 and 0.4 another spontaneous first-order transition is ob served at lower temperatures. In a magnetic field the magnetization of all the investigated intermetallic compounds except for the composition x = 0.5 undergo a metamagnetic transition. The T-x, H-x, and H-T magnetic phase di agrams are constructed. The experimental data are interpreted with allowanc e for the fact that according to the results of previous studies, the Mn-Mn exchange interaction is antiferromagnetic, and thus the examined intermeta llic compounds are two-sublattice ferrimagnets with negative exchange inter action in one of the sublattices. Calculations performed within the framewo rk of this model in the Yafet-Kittel approximation, which assumes that the sublattice with negative exchange can be divided into two sub-sublattices, permit a quantitative description of the experimental results in most cases . (C) 1999 American Institute of Physics. [S1063-7761(99)01610-8].