Induced anisotropy in amorphous Co-Zr-M(M=Zr,Nb,Ti) and Co-Zr-Pt thin films: A magnetic and a structural study

A magnetic and structural investigation was carried out on ${\mathrm{Co}}_{95\mathrm{-}\mathit{x}}$ ${\mathrm{Zr}}_5$ ${\mathit{M}}_{\mathit{x}}$ (M=Zr, Nb, or Ti) and ${\mathrm{Co}}_{91\mathrm{-}\mathit{x}}$ ${\mathrm{Zr}}_9$ ${\mathrm{Pt}}_{\mathit{x}}$ amorphous thin films in order to determine the physical origin of the induced in-plane uniaxial anisotropy ${\mathit{K}}_{\mathit{u}}$. The films were prepared by rf sputtering and the deposition was performed in a magnetic field. The variations of ${\mathit{K}}_{\mathit{u}}$ were studied as a function of the sputtering parameters, composition, concentration, the thermomagnetic treatment, and structural-relaxation process. As a result of these investigations a model for the origin of ${\mathit{K}}_{\mathit{u}}$ is advanced, which involves the single-ion anisotropy mechanism. ${\mathit{K}}_{\mathit{u}}$ is the sum of two contributions ${\mathit{K}}_{\mathit{u}}^{\mathit{a}}$+${\mathit{K}}_{\mathit{u}}^{\mathit{s}}$ where ${\mathit{K}}_{\mathit{u}}^{\mathit{a}}$ and ${\mathit{K}}_{\mathit{u}}^{\mathit{s}}$ are the induced anisotropies related to the local anisotropy of the octahedral-icosahedral and trigonal-type clusters, respectively. The number of trigonal clusters varies as a function of ${\mathit{P}}_{\mathrm{Ar}}$ and thus explains the variation of ${\mathit{K}}_{\mathit{u}}$ with ${\mathit{P}}_{\mathrm{Ar}}$. In Co-Zr-Pt an extra contribution to ${\mathit{K}}_{\mathit{u}}$ is observed due to the pseudodipolar ordering that results from the magnetic polarization of Pt. Co-Zr-Pt films exhibit a peculiar thermomagnetic relaxation that is explained by the reversibility of the pseudodipolar anisotropy and the thermal instability of trigonal-type clusters resulting from the substitution of Pt.