Defects-driven appearance of half-metallic ferrimagnetism in Co–Mn-based Heusler alloys

Abstract

Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having the formula Co2MnZ where Z a sp element, are among the most studied Heusler alloys due to their stable ferromagnetism and the high Curie temperatures which they present. Using state-of-the-art electronic structure calculations we show that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these Mn atoms have spin moments antiparallel to the other transition metal atoms. The ferrimagnetic compounds, which result from this procedure, keep the half-metallic character of the parent compounds and the large exchange-splitting of the Mn impurities atoms only marginally affects the width of the gap in the minority-spin band. The case of [Co1−xMnx]2MnSi is of particular interest since Mn3Si is known to crystallize in the Heusler L21 lattice structure of Co2MnZ compounds. Robust half-metallic ferrimagnets are highly desirable for realistic applications since they lead to smaller energy losses due to the lower external magnetic fields created with respect to their ferromagnetic counterparts.

Ferrimagnetism and antiferro‐ magnetism in half‐metallic Heusler alloys

“…Cr(Mn) impurites occupying Co sites in Co 2 Mn(Cr)Al and Co 2 Mn(Cr)Si alloys couple antiferromagnetically to the other transition metal atoms resulting in ferrimagnetic compounds with lower total spin moments similarly to our previous studies on these alloys [23,24]. V and Cr impurities occupying Mn sites in ferrimagnetic Mn 2 VAl and Mn 2 VSi alloys present spin moments parallel to the Mn atoms contrary to the behavior of the Fe and Co impurities in these alloys and they lead to larger absolute values of the total spin moments [25].…”

“…Defects in these alloys show a very interesting behavior. When we substitute Co atoms with Cr(Mn) in the Co 2 Cr(Mn)Al and Co 2 Cr(Mn)Si compounds, the impurity atoms couple antiferromagnetically with the other transition metal atoms lowering the total spin moment [23,24]. Co and Fe impurities in Mn 2 VAl and Mn 2 VSi ferrimagnetic alloys have spin moments antiparallel to Mn and thus the total spin moment reaches closer to the zero value [25].…”

Engineering the electronic, magnetic, and gap-related properties of the quinternary half-metallic Heusler alloys

“…When 25% of the Al atoms are substituted by Si, the Co spin moments increase and reach the 0. [32] where it was attributed to the pinning of the Fermi level in a narrow band created by the Cr triple degenerated majority t 2g electrons which overlaps strongly with the doubledegenerated e g majority states, contrary to the behavior of the Mn atoms [33]. As shown in [32] a small shift of the Fermi level results in large changes of the Cr spin moments.…”

“…When we compute the DOS within CPA we have to use a smearing for the DOS. Usually in previous publications with the same method (see references [32] and [33] for example) we have used a smearing of 10…”

“…Except interface states also temperature driven excitations [25,26,27] and defects [28,29] seem to destroy halfmetallicity. Also some other important aspect of these alloys like the orbital magnetism [30], the doping [31], the structural stability [36], the appearance of ferrimagnetism [32,33,34] and antiferromagnetism [35] and the interplay of exchange interactions [37,38] have been addressed in literature.…”