Enhancement of local magnetic ordering temperature due to non-magnetic disorder in anti-Heusler alloy Al$_2$MnCu

Abstract

Structural disorder is generally regarded as detrimental to magnetism. Since magnetic properties are governed by both the arrangement of magnetic atoms and the crystal environment, including non-magnetic atoms that mediate exchange interactions, the distinct role of magnetic and non-magnetic disorder remains less understood. The recently reported anti-Heusler alloy, Al$_2$MnCu, forming with $\sim$50\% intra-symmetry site disorder (B2) involving the magnetic Mn atom and $\sim$5\% inter-symmetry site disorder associated with non-magnetic Al and Cu atoms, offers a good platform to disentangle the individual roles of magnetic and non-magnetic atomic disorder. Our magnetic critical analysis, in the vicinity of the Curie temperature ($T_{\rm C}$), does not lead to any conventional single universality class expected for a standard ferromagnet; instead, the non-monotonic temperature dependence of effective critical exponents points towards more complex magnetic interactions, akin to inhomogeneous ferromagnets. Dc magnetometry complemented by higher-harmonic ac magnetic susceptibility data, further confirms the persistence of locally ordered magnetic clusters above $T_{\rm C}$. Theoretical analysis establishes that the enhancement of the local magnetic ordering temperature arises directly from the strengthening of exchange interactions due to the small degree of non-magnetic disorder, whereas $T_{\rm C}$ is hardly influenced by the much larger level of magnetic-atom disorder. The combined experimental and theoretical investigations on the anti-Heusler alloy Al$_2$MnCu reveal and explain the unusual observation of nonmagnetic disorder-induced elevation of $T_{\rm C}$, providing a paradigm for the counterintuitive role of disorder in magnetic materials.