Effect of B substitution for Si on the electronic structure and magnetic properties of Heusler alloys Fe2CoSi1-xBx (x = 0–0.25)

A series of Fe ₂CoSi _1-xB _x (x = 0, 0.05, 0.1, 0.15, 0.2 and 0.25) Heusler alloys has been synthesized using melt-spinning method, which can help to reduce the precipitation of the secondary phase and form a highly ordered structure. The XRD patterns indicate that, when x = 0–0.1 the Fe ₂CoSi _1-xB _x melt-spun ribbons are single Heusler phases, but when x = 0.2 and 0.25 a small amount of secondary phase appears. The lattice constant decreases with increasing B content, suggesting that B mainly enters the substitutional sites of Si. The Curie temperatures T _C of Fe ₂CoSi _1-xB _x alloys are all higher than 1000 K, and a 21 K increase of T _C is observed as x varies from 0 to 0.25. The hysteresis loops of Fe ₂CoSi _1-xB _x alloys show soft magnetic character. Their saturation magnetizations M _s at 5 K and 300 K are similar and close to 5μ _B / f.u. The zero-gap half-metallic character reported for Fe ₂CoSi has been confirmed in present work. But with the substitution of B for Si, the Fermi level moves from the center in Fe ₂CoSi DOS to the left edge of the minority energy gap in Fe ₂CoSi _0.75B _0.25, which may disturb the half-metallic character when B content is high. When × = 0–0.1, the M _s at 5 K of Fe ₂CoSi _1-xB _x agree well with the calculated total spin moments, but when the B content is high, especially when x = 0.2 and 0.25, a deviation between the experimental and theoretical results is observed, which is related to the secondary phase in these samples.