A magnetic and structural study of Mn, Co, and Ni substituted Fe3Ge2 hexagonal germanides

Abstract

Structural (x-ray and neutron diffraction), Mössbauer, and magnetic investigations were performed in order to study Mn, Co, and Ni substituted 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 the 2d site in the congruent composition. It was also found that Ge atoms 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 Mössbauer data in the paramagnetic region. The complex Mössbauer spectra in the ferromagnetic region were interpreted in terms of a distribution of crystal field gradient directions which has its origins from the splitting of the coordinates of Ge atoms due to the presence of vacancies at 2d site. No magnetic contribution was found for Ni, while a small magnetic moment was found to be supplied by Co. In the 2a site (the only one occupied by Mn) a magnetic moment of the same order as that of Fe was found for the host atom. The studied substitutions all reduce the intrinsic magnetic characteristics (Curie temperature, Tc, anisotropy field, Ha, and saturation magnetization, Ms) of the system. The overall anisotropy is planar for all the investigated compounds. The observed Fe planar anisotropy is 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 nonquenched orbital moment. Its composition behavior was well described in the frame of a localized model. The temperature behavior of the anisotropy 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 metal-metalloid based compounds.