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 R_a(M_1−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 Gd₆(Mn_1−xFe_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.

Ternary alloys of rare-earths and transition metals exhibit complex ferrimagnetic behavior as a function of alloy compositions. Here, X-ray magnetic circular dichroism of the Gd₆(Mn_1−xFe_x)₂₃ series is used to explain the composition dependence of sublattice Curie temperatures in terms of element-specific magnetic moment evolution.