Atomic disorder, magnetic and electronic properties of equiatomic quaternary Heusler alloy FeRuCrSi: Experimental and theoretical investigations

The equiatomic quaternary Heusler alloy FeRuCrSi was synthesized and its atomic ordering, magnetic properties and electronic structure were investigated experimentally and theoretically. The FeRuCrSi formed a single Heusler phase and had a lattice constant of 5.811 & ANGS; at room temperature. An L21B-type disorder related to the random occupation of Fe and Ru at 4a and 4b sites was identified in the X-ray diffraction pattern. The Curie temperature of FeRuCrSi was 346 K and the saturation magnetization at 5 K was 1.84 & mu;B/f.u., which is smaller than the integral value of 2.00 & mu;B required for ideal spin gapless semiconductor character and is likely related to the L21B-type disorder. First-principles calculations revealed that ordered FeRuCrSi with YII-type structure had the lowest total energy at 0 K. However, the energy difference between it and the L21B structure was only 0.09 eV/f.u., thus the L21B disorder dominated in the experimental sample because of the enthalpy of mixing and the "freezing" of high-temperature disorder at room temperature. The YII-type FeRuCrSi was a SGS with a half -metallic gap in the minority density of states (DOS) and a zero-width gap in the majority DOS. However, the L21B-type disorder disturbed the SGS character by narrowing the two gaps. The total spin moment of ordered FeRuCrSi was 2.00 & mu;B/f.u., mainly determined by the antiparallel coupled Cr and Fe/Ru spin moments. However, the total moment was slightly reduced in L21B-type FeRuCrSi. All this suggests that the possible L21B disorder should be carefully considered when designing and synthesizing half-metals/SGS in Heusler alloys for spintronic applications.