Magnetic and magnetostrictive properties of amorphous Tb-Fe and Tb-Fe-B thin films

Magnetic and magnetostrictive properties of Tb-Fe and Tb-Fe-B thin films are systematically investigated over a wide composition range from 40.2 at.% to 68.1 at.% Tb. The films were fabricated by rf magnetron sputtering using a composite target which consists of an Fe plate and Tb chips. The microstructure, examined by X-ray diffraction, consists mainly of an amorphous phase and, at high Tb contents, a pure Tb phase also exists. A progressive change in the direction of anisotropy from the perpendicular to in-plane occurs as the Tb content increases and the boundary at which the sign of anisotropy changes occurs shifts significantly towards higher Tb contents with the addition of B. The saturation magnetization exhibits maxima at the Tb contents of 42 at.% and 48 at.% for Tb-Fe and Tb-Fe-B thin films, respectively, and it is decreased by the addition of B. The coercive force, measured in the easy direction, decreases monotonically with the Tb content. Excellent magnetostrictive characteristics, particularly at low magnetic fields, are achieved in both Tb-Fe and Tb-Fe-B thin films; for example, a magnetostriction of 138 ppm is obtained in a Tb-Fe-B thin film at a magnetic field as low as 30 Oe. The excellent magnetostrictive properties of the present thin films are supported by the equally excellent magnetic softness, the coercivity below 10 Oe and a typical squared-loop shape with the saturation field as low as 1 kOe. Due to the excellent low field magnetostrictive characteristics, the present Tb-Fe based thin films are thought to be suitable for Si based microdevices.