Structure and magnetic properties of nanogranular composites CoFeZr-alumina

Abstract

The 5 to 15 μm thick nanocomposite films (Coo.45Fe0.45Zr0.10)x(Al2O3)1-x with 30 < x < 65 at.% sputtered in a chamber evacuated with pure Ar gas were studied using Mössbauer spectroscopy, magnetization, complex magnetic permeability, and magneto-force microscopy (MFM) measurements. In particular, the films with x < 40 at.% displayed superparamagnetic state at room temperature (the lack of sextets in Mössbauer specta, non-hysteresis character of the magnetization curves, invariability of real μ’ and imaginary μ’’ parts of magnetic permeabilities with x and high resistivity. Near the percolation threshold ( x ≈ 40 - 45 at.%) Mössbauer specta displayed features of the ferromagnetic sextet and the values of μ’ and μ’’ increased, approaching the maximum, as x increasing due to the decrease of inter-particle distances. Moreover, effects of dipole-dipole and exchange interactions of the nanoparticles resulted in the nucleation of the magneto-ordered granules (magnetoclusters) including a few metallic nanoparticles. The MFM allowed to visualizing the presence of magnetic labyrinth-like magnetic contrast that was formed by the metallic nanoparticles near the percolation threshold. At x > 45 at.% the films behave like the “bulk” ferromagnetic sample with occasional inclusions of dielectric phase. 1. INTRODUCTION At present time high interest is given to granular composite materials, which consist of metallic nanosized particles randomly distributed within a dielectric matrix, because they represent nearly ideal systems for the study of nanostructured materials with a big change of their composition. For the first time, such materials (named as cermets) were manufactured at the beginning of seventies of the last century and during that decade the main concepts concerning the electrical and magnetic properties of composites were stated [1-5]. The interest of these materials was renewed in the nineties, of the previous century, because of their possible applications in the magnetic field sensors and as magnetic recording media. The study of magnetic properties of iron, cobalt or nickel nanoparticles embedded into a dielectric matrix has shown [6] that reduction of volume fraction of the metallic phase below the percolation threshold xC resulted in metallic granules becoming a singledomain and the nanocomposite as a whole displays superparamagnetic properties at room tem