Magnetic anisotropy in Ni-Fe-Ga-Co ferromagnetic shape memory alloys in the single-variant state

The effects of the addition of Co on the magnetic anisotropy in Ni _55-xFe₁₈Ga₂₇Co_x (x = 1-6) single-variant ferromagnetic shape memory alloys have been investigated. By the addition of Co from 1 to 6 at.%, the Curie temperature T_C is increased from 318 to 405K, keeping the martensitic transformation temperatures above room temperature. As a result, the value of the uniaxial magnetic anisotropy constant |K_u| at 300K increases with increasing x of the Co concentration and the martensite phase of Ni₄₉Fe ₁₈Ga₂₇Co₆ exhibits a relatively high value of |K_u| = 1.15 × 10⁵Jm^-3 at 300K. With increasing Co concentration, on the other hand, the c axis changes from the magnetic easy axis to the hard axis at 4.2K, that is, the sign of K_u is reversed from positive to negative between 2 and 3at.% Co. Furthermore, K_u in Ni₅₃Fe₁₈Ga₂₇Co₂ is positive below 100K and negative above 100K up to T_C, reducing the magnetic anisotropy around 200K. From the present results, it is evident that the magnetic anisotropy of Ni_55-xFe₁₈Ga ₂₇Co_x (x = 1-6) single-variant ferromagnetic shape memory alloys is very sensitive to Co concentration and also temperature.