Magnetic properties of R6Fe13-xM1+x compounds and their hydrides

The magnetic properties of various R6Fe13-xM1+x compounds (R=La, Nd, Gd, and Dy) crystallizing in the La6Co11Ga3 structure have been investigated by magnetic measurements and x-ray diffraction. It is shown that all Nd6Fe13M compounds with M=Au, Ag, Cu, Si, and Ga order antiferromagnetically around 415 K and evidence is provided that the frequently reported, increase of the magnetization at lower temperature is due to impurities. High-field measurements made at 4.2 K on Nd6-xDyxFe12.7Ga1.3 compounds with x = 0.0, 0.2, 0.5, and 1.0 show that the R-Fe coupling is not yet broken at 35 T. A large hysteresis in the field dependence of the magnetization is present in all compounds including La6Fe11Al3, indicating the role of the Fe-sublattice anisotropy. A theoretical model for the field dependence of the magnetization is constructed, based on local minimization of the free energy. By taking into account the second-and fourth-order magnetocrystalline anisotropy terms, the magnetization behavior of the compounds, including the large hysteresis, can be explained excellently. A structure of ferromagnetically ordered Fe sheets coupling antiferromagnetically to each other is proposed to explain the experimental data. The hydrides of these compounds are all ferromagnetic (light R) or ferrimagnetic (heavy R) and have an easy magnetization direction along the c axis due to the Fe-sublattice anisotropy. Their ordering temperatures are near 450 K, just slightly above the Neel temperature of the parent compounds, which can be understood from the proposed spin structure.