Structural and magnetocrystalline anisotropy contributions to the blocking temperatures of NixCo(1−x)O exchange couples (abstract)

Abstract

The simple spin and crystal structure of the antiferromagnetic (AF) monoxide NixCo1−xO facilitates structural characterization and modeling of its magnetic properties NixCo1−xO is also receiving technological attention for biasing magnetoresistive sensors as a robust alternative to FeMn. In this work, we studied the thickness dependence of the AF layer on the exchange field created through interfacial exchange coupling between NixCo1−xO and Ni81Fe19 films. The NixCo(1−x)O films were reactively sputtered on silicon substrates and capped with a 300 Å Ni81Fe19 layer. An additional silver layer was deposited as an oxidation barrier. An exchange field was observed on all the exchange couples with the NixCo(1−x)O thickness ranging from 25 to 1000 Å. The exchange couples were characterized using transmission electron microscopy (TEM) to yield information on the orientation, structure, and size of the crystallites. The films were polycrystalline with columnar grains. For oxide thicknesses below 300 Å, the film was randomly oriented but developed a texture with a preferred orientation for thicker films. We found that the blocking temperature, at which the exchange field goes to zero, decreases with decreasing oxide thickness. The blocking temperature is relatively thickness independent for higher thicknesses but decreases rapdily below a certain thickness. The decrease in the blocking temperature does not appear to be consistent with a finite size scaling law. Instead we modeled this behavior to take into account the temperature dependence associated with the changing microstructure of the AF layer with thickness, in particular the size of the crystallites. In thick oxide films, the blocking temperature corresponds to the AF ordering temperature, the Néel temperature. The decrease of the blocking temperature below the Néel temperature suggests at the competing magnetic and thermal energies necessary for exhibiting unidirectional anisotropy. The thickness sensitivity of the blocking temperatures of the AF monoxides are found to be strongly dependent on their magnetocrystalline anisotropy.