Al$_2$MnCu: A magnetically ordered member of the Heusler alloy family despite having a valence electron count of 24

TL;DR

This paper reports the discovery of Al$_2$MnCu, a Heusler alloy with a valence electron count of 24 that exhibits ferromagnetic order, challenging the traditional Slater-Pauling rule predictions.

Contribution

The study introduces a new anti-Heusler alloy, Al$_2$MnCu, that magnetically orders despite having a VEC of 24, and proposes a molecular orbital hybridization model to explain this anomaly.

Findings

Al$_2$MnCu exhibits ferromagnetic order with T_C ~ 315 K.

The alloy has a magnetic moment of approximately 1.8 μ_B per formula unit.

It deviates from the Slater-Pauling rule predictions for VEC=24.

Abstract

The magnetic property of the Heusler alloys can be predicted by the famous Slater-Pauling (S-P) rule, which states the total magnetic moment ($m_t$) of such materials can be expressed as $m _t\,=\,(N_V-24)\,\mu_B/f.u.$, 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 of such reported compounds belong to the VEC 24 category. Here, we report a new anti-Heusler alloy, Al$_2$MnCu, that undergoes long-range ferromagnetic (FM) ordering with $T_{\rm C}\sim$315 K and a large magnetic moment of $\sim$1.8 $\mu_B$/f.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.