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(M1-xM'(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(Mn1-xFex)(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.