The invar property of elemental FCC Co and large spontaneous magnetostriction of BCC Fe-Co

The ferromagnetic invar effect in f.c.c. 3d-transition metal alloys spans a n (s+d) valence electron concentration range of 8.6 less than or equal to e /a less than or equal to 9.5. The maximum spontaneous magnetostriction asso ciated with the invar effect occurs at ela approximate to 8.7. For e/a less than or equal to 8.6, the anti-invar effect develops, for which the associ ated 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-x Nix alloys takes place in the range of 0 less than or equal to x less than or equal to 0.6 (8 less than or equal to e/a less than or equal to 9.2). Th e electron concentration of Fe1-xCox alloys falls within this range with 8 less than or equal to e/a less than or equal to 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 less th an or equal to T less than or equal to 1500 K, the volumetric properties of FexCo1-x alloys with 0 less than or equal to x less than or equal to 1. In deed, we find invar behavior in the f.c.c. phase of the alloys with concent rations ranging from x = 0.6 to pure Co. Aside from the invar property of p ure f.c.c. Co, we observe features associated with the order-disorder trans ition (B2 <--> b.c.c.) and a large magnetostriction in the b.c.c. phase rel ated to magnetic ordering.