Unusual magnetization characteristics of Fe-Ni films with graded composition

Thin films of Fe-Ni with graded composition have been deposited on a Si (001) substrate at room temperature by co-sputtering of Fe and Ni with variable rates of the constituting elements. The composition of the films was changing linearly across the thickness from Fe 80 Ni 20 to Fe 6 Ni 94 . Five samples were studied with the thickness of 30, 50, 100, 150, and 200 nm. The hysteresis loops measured with the field applied in the film plane had square shape and the coercivity was varying from 11 to 22 Oe. However, the loops for the field perpendicular to the film plane displayed unusual shapes consisting of a double-step hysteresis loop at low fields and unhysteretic part at higher fields. The size of the steps varied with the thickness of the film. The most likely source of the double step hysteretic curves was identified as magnetostrictive stresses at the film/substrate interface. This was evidenced by the disappearance of the second hysteresis step after annealing at 200 °C for 1 h and significant changes of the hysteresis loops when the same structure was deposited starting from Fe-rich or Ni-rich compositions at the substrate.