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

Abstract

Many large unit-cell rare-earth transition metal ternary alloys of the type Ra(M1−xM’x)b exhibit non-monotonic ferrimagnetic Curie temperatures (TC) 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 Gd6(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 TC’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-TC > Gd-sublattice TC, (ii) a reduced common-TC for all sublattices, and (iii) Fe-sublattice bulk-TC > Gd-sublattice TC. The Mn-moment switching and gradual increase of Fe-moment combine to cause non-monotonic TC’s with monotonic magnetization. The study indicates the importance of element-specific TC’s for tuning magnetic properties.