Dependence of Magnetic Anisotropy and Magnetoresistance of Ni81Fe19-Films on Annealing

Permalloy (Py:Ni81Fe19) exhibits an anisotropic magnetoresistance (AMR) which is very often used to read magnetic signals from storage devices. Py-films of thickness 20nm were prepared by dc-magnetron sputtering in a magnetic field onto thermally oxidized Si-wafers and annealed ex situ at temperatures up to 1000K in order to investigate the dependence of the magnetic anisotropy and the AMR on heat treatments. The films exhibit an uniaxial anisotropy after preparation which changes during annealing above 520K. The AMR along the former magnetically easy axis as well as the corresponding field sensitivity are increased by a heat treatment around 700K reaching maxima of about 8% and a maximum sensitivity of 1.5%/Oe, respectively. We discuss possible sources for the change in anisotropy, i.e. strain effects, inhomogeneities, and changes of the local atomic order.