Four-fold magnetic anisotropy in a Co film on MgO(001)

The development of devices based on magnetic tunnel junctions has raised new interests on the structural and magnetic properties of the interface Co/MgO. In this context, we have grown ultrathin Co films (≤30Å) by molecular-beam epitaxy on MgO(001) substrates kept at different temperatures (TS). Their structural and magnetic properties were correlated and discussed in the context of distinct magnetic anisotropies for Co phases reported in the literature. The sample characterization has been done by reflection high energy electron diffraction, magneto-optical Kerr effect and ferromagnetic resonance. The main focus of the work is on a sample deposited at TS=25°C, as its particular way of growth has enabled a bct Co structure to settle on the substrate, where it is not normally obtained without specific seed layers. This sample presented the best crystallinity, softer magnetic properties and a four-fold in-plane magnetic anisotropy with Co〈110〉 easy directions. Concerning the samples prepared at TS=200 and 500°C, they show fcc and polycrystalline structures, respectively and more intricate magnetic anisotropy patterns. ►Results suggest the lattice is already after the Bain transformation for TS=25°C, and the Co film has a bct structure instead of an fct one. ►For deposition temperature of TS=25°C, a four-fold in-plane magnetic anisotropy with Co〈110〉 easy directions has been obtained. ►The growth mode of Co on MgO single crystals at different temperatures resulted in bct Co at TS=25°C, fcc Co at TS=200°C and polycrystalline Co at TS=500°C.