The Invar Property of Elemental F.C.C. Co and Large Spontaneous Magnetostriction of B.C.C. Fe-Co

Abstract

The ferromagnetic invar effect in f.c.c. 3d-transition metal alloys spans an (s+d) valence electron concentration range of 8.6 ≤ e/a ≤ 9.5. The maximum spontaneous magnetostriction associated with the invar effect occurs at e/a ≈︂ 8.7. For e/a ≤ 8.6, the anti-invar effect develops, for which the associated volume enhancement increases as the pure f.c.c. Fe composition of e/a = 8 is approached. The progressive change from anti-invar to invar in Fe1—xNix alloys takes place in the range of 0 ≤ x≤ 0.6 (8 ≤ e/a ≤ 9.2). The electron concentration of Fe1—xCox alloys falls within this range with 8 ≤ e/a ≤ 9, implying that Co should be an element having invar properties. To verify this assessment we examine, by thermal expansion measurements in the temperature range of 4 K ≤ T≤ 1500 K, the volumetric properties of FexCo1—x alloys with 0 ≤ x ≤ 1. Indeed, we find invar behavior in the f.c.c. phase of the alloys with concentrations ranging from x = 0.6 to pure Co. Aside from the invar property of pure f.c.c. Co, we observe features associated with the order–disorder transition (B2 ↔ b.c.c.) and a large magnetostriction in the b.c.c. phase related to magnetic ordering.