Correlation between microstructural and magnetic properties in Fe/KCoF3 bilayers

Abstract

Molecular beam epitaxy was used to grow a ferromagnet/antiferromagnet (Fe/KCoF3) system. A series of structural, surface, and magnetic characterization techniques were used to understand the correlation between microstructural and magnetic properties in this exchange bias system. Depending on deposition conditions, the fluoride grew either in a single crystal or a polycrystalline form, which was initially confirmed by reflection high energy electron diffraction patterns and x-ray studies. The crystalline form of the KCoF3 significantly affected the magnetic properties, in particular the exchange bias and the magnetocrystalline anisotropy of the Fe layer. Transmission electron microscopy (TEM) studies were carried out to shed more light on the microstructure of the fluoride and on the interface between Fe and KCoF3 layers. Single crystals KCoF3 layers grown at elevated temperature on (001) Fe template have a (001) orientation. On the other hand, the cross-sectional TEM images of the polycrystalline fluoride deposited at room temperature show columnar structure of the grains with a column diameter of about 10 nm. In addition, planar defects were observed in the Fe layer due to the slight mismatch between Fe and KCoF3 lattices. These defects and grain boundaries in the antiferromagnet are responsible for considerable modification of magnetic properties of the structures with polycrystalline fluoride compared to those with the single crystal KCoF3. Magnetic anisotropy and the exchange bias were measured using ferromagnetic resonance and superconducting quantum interference device magnetometry, respectively. The exchange bias and blocking temperature in the samples with polycrystalline fluoride were significantly reduced, however, the low-temperature fourfold anisotropy was enhanced by a factor of 3 for the samples with 1-nm-thick Fe and polycrystalline fluoride compared to the samples with the same thickness of Fe but single crystal fluoride.