Element-specific Curie temperatures and Heisenberg criticality in ferrimagnetic Gd6(Mn1−xFex)23 via Kouvel-Fisher analysis

Many large unit-cell rare-earth transition metal ternary alloys of the type R a (M 1− x M’ x ) b exhibit non-monotonic ferrimagnetic Curie temperatures ( T C ) coupled to monotonic composition-controlled magnetization. Its origin remains an important long-standing puzzle in the absence of studies probing their temperature-dependent element-specific magnetism. Here, in order to resolve this issue and identify design principles for new R-M-M’ permanent magnets, we carry out x-ray magnetic circular dichroism (XMCD) for the series Gd 6 (Mn 1− x Fe x ) 23 , x = 0.0 − 0.75. The results show that the net Mn-moment reduces and switches from parallel to antiparallel for x ≥ 0.2, while the Fe-moment is always antiparallel to the Gd-moment. Kouvel-Fisher analyses of XMCD data reveals distinct sublattice T C ’s and 3D Heisenberg criticality. Band structure calculations show magnetic moments and density of states consistent with experiments. The magnetic phase diagram shows three regions characterized by (i) Mn-sublattice bulk- T C > Gd-sublattice T C , (ii) a reduced common- T C for all sublattices, and (iii) Fe-sublattice bulk- T C > Gd-sublattice T C . The Mn-moment switching and gradual increase of Fe-moment combine to cause non-monotonic T C ’s with monotonic magnetization. The study indicates the importance of element-specific T C ’s for tuning magnetic properties.