Magnetic Anisotropy and Crystal Domain Variant in L1₀-FePt Polycrystalline Films

We fabricated thin L1₀-Fe_xPt_100-x (x = 46-54 at.%) polycrystalline perpendicular films, and examined the x dependences of the first-and second-order uniaxial magnetic anisotropy, K_u1 and K_u2, in relation to variant formation. The values of K_u1 and K_u2 were estimated from magnetic torque curves at an applied field of 70 kOe obtained using anomalous Hall effect. K_u1 for films with thickness d of 10 nm showed a maximum at x=50 at.%, which was consistent with the behavior of the order parameter. K_u1 decreased markedly as x increased from 50 at.%, accompanied by an increase of K_u2. In-plane X-ray diffraction analysis revealed that the variant formation increased markedly as x increased, especially beyond x = 50 at.%. Moreover, the relative lattice strain evaluated using a Williamson-Hall plot increased remarkably. The significant reduction of K_u1, accompanied by the enhancement of K_u2 , was coincident with variant formation. K_u1 for films with d = 2 nm also showed a maximum at x = 50 at.%. However, the K_u1 ∼ x relation showed a good symmetry against Fe content variations from 50 at.%. No increase of K_u2 with increasing x was observed. It is likely that a large tensile stress in the initial film growth, caused by the lattice misfit between FePt and MgO underlayers, suppressed the variant formation in these very thin films.