Change of the magnetization reversal mechanism of polycrystalline Ni-Co thin films

Abstract

Features of the magnetization reversal of thin-film polycrystalline magnetic media are revealed by using the magneto-optical indicator film imaging technique. It is shown by a direct visualization that the increase in the Co content in ${\mathrm{Ni}}_{100\ensuremath{-}x}{\mathrm{Co}}_{x}$ alloy films leads to a change of the microscopic magnetization reversal mechanism. The magnetization reversal proceeds either by the incoherent spin rotation (at low Co content) or by the nucleation and motion of domain walls (at high Co concentration). This behavior results in a nonmonotonic dependence of the coercivity and remanent magnetization observed in macroscopic magnetization curve measurements. A micromagnetic theory of the magnetization reversal of polycrystalline magnetic media, which takes into account the intergranular exchange coupling, is presented. This theory describes qualitatively the change of the magnetization reversal scenario, and explains the surprising magnetic behavior of binary alloys of transition metals observed in the experiment. It was also found by using atomic force microscopy that as-deposited ${\mathrm{Ni}}_{100\ensuremath{-}x}{\mathrm{Co}}_{x}$ films are characterized by a substantial surface roughness that varies with the change of the Co content. The relationship between the surface roughness and the coercivity mechanism in polycrystalline media is also discussed.