Al 2 MnCu : A magnetically ordered member of the Heusler alloy family despite having a valence electron count of 24

The magnetic property of Heusler alloys can be predicted by the famous Slater-Pauling (S-P) rule, which states that the total magnetic moment $({m}_t)$ of such materials can be expressed as ${m}_t\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}({N}_V\ensuremath{-}24)\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_B/\text{f.u.}$, where ${N}_V$ is the total valence electron count (VEC). Consequently, no Heusler alloys having VEC = 24 are theoretically expected, as well as experimentally reported, to have any magnetic ordering. Recently, a special class of Heusler alloys with 50% concentration of $p$-block elements (anti-Heusler) have been identified, although none such reported compounds belong to the VEC 24 category. Here, we report an anti-Heusler alloy, ${\mathrm{Al}}₂\mathrm{MnCu}$, that undergoes long-range ferromagnetic ordering with ${T}_{\mathrm{C}}\ensuremath{\sim}315$ K and a large magnetic moment of $\ensuremath{\sim}1.8\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_B/\mathrm{f}.\mathrm{u}.$ despite having VEC 24. A phenomenological model based on molecular orbital hybridization is also proposed to understand the magnetism and unusual deviation from the standard S-P rule.