Unexpected unidirectional anisotropy in amorphous Tb-Fe/Ni-Fe-Mo bilayer films

The exchange interaction between amorphous Tb-Fe (a-Tb-Fe) and crystalline Ni-Fe-Mo produces a unidirectional anisotropy in the magnetically soft Ni-Fe-Mo layer. This anisotropy appeared anomalous because the magnetizations of the layers were previously believed to be orthogonal to each other. The magnitude of the unidirectional anisotropy is quite large by comparison to values quoted in the literature for Permalloy of a comparable thickness exchange coupled to an in-plane antiferromagnet. Precise measurement of the direction of the easy axis in a-Tb-Fe shows that it is in fact tilted at a small angle with respect to the normal to the Ni-Fe-Mo moment which lies in the film plane. An expression relating the unidirectional anisotropy to measured properties of the a-Tb-Fe layer fits the experimental results. The tilt appears to be the consequence of a nonperpendicular incident angle of deposition and depends critically on deposition parameters such as substrate temperature, Tb-Fe layer thickness and composition, and the incident angles of the Tb and Fe vapor beams.