Field-induced magnetic phase transitions and correlated electronic states in the hexagonal RAgGE and RPtIn series

Abstract

The present work was initially motivated by the desire to continue the study of complex metamagnetism in relation to the crystal structure of various compounds; this study already included tetragonal compounds like HoNi2B2C (Canfield 1997b; Kalatsky 1998) and DyAgSb2 (Myers 1999), in which the rare earths occupy unique tetragonal positions. We intended to find hexagonal systems suited for such a study, with complex metamagnetic properties, and the search for extremely anisotropic hexagonal compounds turned into a rewarding exploration. We identified and grew most of the heavy rare earth members of two isostructural series, RAgGe and RPtIn, both belonging to the hexagonal Fe2P family of materials. In each of these series we found one compound, TmAgGe, and TbPtIn respectively, that was suitable for a simple study of angular dependent metamagnetism: they had three rare earth ions in the unit cell, positioned at a unique crystallographic site with orthorhombic point symmetry. The magnetization of both TmAgGe and TbPtIn was extremely anisotropic, with larger values for the in-plane orientation of the applied field than in the axial direction. Complex metamagnetic transitions existed for field within the ab-plane, and, similar to the case of the tetragonal compounds RNi2B2C and DyAgSb2, they depended on the field orientation within the basal plane. We were thus able to develop a two-dimensional model, the three co-planar Ising-like systems model, which described well the angular dependence of the metamagnetic transitions in the TmAgGe and TbPtIn hexagonal compounds. Having three magnetic moments in the hexagonal unit cell, in orthorhombic point symmetry positions, added to the complexity of the analysis compared to the case of tetragonal compounds having one rare earth atom per unit cell, in tetragonal point symmetry. However, the three co-planar Ising-like systems model yielded complex, but intelligible angular dependencies of the critical fields and locally saturated magnetizations for the various metamagnetic transitions observed experimentally. Having found two systems with different.rare earth ions (Tm and Tb) and different ligands (Ag, Ge and Pt, In) gives us some confidence that this behavior may be generic to the Fe2P-based compounds, and potentially even more widely applicable. Furthermore, we generalized this model to a three non-planar Issing-like systems model, in an attempt to understand the nature of the magnetic order in the non-planar magnetic RPtIn compounds (R = Dy-Tm); even though a more detailed analysis is needed to optimize it, this three-dimensional model could also be developed into a useful tool for characterizing hexagonal compounds with orthorhombic point symmetry of the rare earth site.