Abstract
Thin films of Co/Ru(0001) are known to exhibit an unusual spin reorientation transition (SRT) coupled to the completion of Co atomic layers for Co thicknesses under four layers. By means of spin-polarized low-energy electron microscopy, we follow in real space the magnetization orientation during the growth of atomically thick capping layers on Co/Ru(0001). Capping with noble metal (Cu, Ag and Au) elements modifies the SRT depending on the Co and overlayer thickness and on the overlayer material, resulting in an expanded range of structures with high perpendicular magnetic anisotropy. The origin of the SRT can be explained in terms of ab initio calculations of the layer-resolved contributions to the magnetic anisotropy energy. Besides the changes in the SRT introduced by the capping, a quantitative enhancement of the magnetic anisotropy is identified. A detailed analysis of the interplay between strain and purely electronic effects allows us to identify the conditions that lead to a high perpendicular magnetic anisotropy in thin hcp Co films.
Highlights
The magnetism of ultra-thin films is a fascinating field with important device applications[1]
A very rough model to approach the large lattice mismatch between the Co film and the Au cap from the screened KorringaKohn-Rostoker (SKKR) results would be to take ∆Eb of the inner layers from a calculation using the lattice constant of bulk Co and the surface contribution from an expanded case. This leads to an estimate of the spin reorientation transition (SRT) for a Co film capped by 2 ML Au to occur at a Co thickness of ∼ 8 ML, in excellent agreement with the spin-polarized low-energy electron microscopy (SPLEEM) measurements
By means of SPLEEM we have observed the changes in the easy axis in real-time and with spatial resolution while growing the coinage metal layers
Summary
The magnetism of ultra-thin films is a fascinating field with important device applications[1]. In some thin film systems, the easy-axis is in-plane up to a critical thickness, it turns to a perpendicular orientation, and back again to in-plane orientation at a larger thickness, i.e. they show a double spin reorientation transition (SRT) This is attributed to a complex interplay of magnetic interactions influenced by atomic structure and electronic effects. The complicated interplay of effects leading to these results is studied by means of calculations of the MAE which allow to separate the different contributions (strain, hybridization, thickness) in a layer-resolved analysis In this way we determine the factors that lead to high PMA in thin hcp Co films
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