A MAGNETIC AND STRUCTURAL STUDY OF MN, CO, AND NI SUBSTITUTED FE3GE2 HEXAGONAL GERMANIDES

Structural (x-ray and neutron diffraction), Mossbauer, and magnetic in vestigations were performed in order to study Mn, Co, and Ni substitut ed Fe3.34Ge2 hexagonal germanides. From an x-ray diffraction analysis of Fe3.34Ge2 single crystal data Fe was found to enter exclusively 2a and 2d sites of the defective structure, while vacancies only occupy t he 2d site in the congruent composition. It was also found that Ge ato ms are slightly displaced from 2c site towards an adjacent vacancy in a 2d site. The preferential entrance of Mn, Co, and Ni was determined by neutron powder diffraction and Mossbauer data in the paramagnetic r egion. The complex Mossbauer spectra in the ferromagnetic region were interpreted in terms of a distribution of crystal field gradient direc tions which has its origins from the splitting of the coordinates of G e atoms due to the presence of vacancies at 2d site. No magnetic contr ibution was found for Ni, while a small magnetic moment was found to b e supplied by Co. In the 2a site (the only one occupied by Mn) a magne tic moment of the same order as that of Fe was found for the host atom . The studied substitutions all reduce the intrinsic magnetic characte ristics (Curie temperature, T-c, anisotropy field, H-a, and saturation magnetization, M-s) of the system. The overall anisotropy is planar f or all the investigated compounds. The observed Fe planar anisotropy i s the result of a competition between an axial anisotropy of Fe in the 2a site and a nine times higher 2d planar contribution. The origin of the observed relevant Fe anisotropy seems to be due to a residual non quenched orbital moment. Its composition behavior was well described i n the frame of a localized model. The temperature behavior of the anis otropy constant for the different compounds substantially follows the Callen and Callen power law with n=3. This suggests the possibility of a single-ion nature for the transition metal anisotropy in these meta l-metalloid based compounds. (C) 1998 American Institute of Physics.