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We have investigated the origin of thickness dependent spin reorientation transition (SRT) of B2 type FeCo alloy using the full potential linearized augmented plane wave method. It has been reported that FeCo alloy films on various substrates show a SRT from perpendicular to in-plane magnetization at an approximate thickness of 15 monolayers (MLs). The enhanced perpendicular magnetic anisotropy in bulk FeCo is attributed to a tetragonal distortion. However, we have found that the tetragonal distortion tends to suppress the magnetocrystalline anisotropy (MCA) energy at increasing film thickness in two-dimensional structure. In contrast, the magnitude of the shape anisotropy energy increases at increasing FeCo film thickness. Interestingly, the shape anisotropy overcomes the MCA and the SRT, from perpendicular anisotropy to in-plane magnetization, which occurs at a thickness of 15 ML. Consequently, we are able to clearly understand the physical mechanism of the thickness dependent SRT in terms of the competing reactions of these two counteracting contributions. [ABSTRACT FROM AUTHOR]

Abstract: Using the full potential linearized augmented plane wave method, we have investigated the effect of Fe adlayer and Co underlayer on the magnetic properties of Fe/FeCo/Cu(001) and Fe/FeCo/Co/Cu(001) systems. It is found that the magnetic layers display close to half metallic state and this is independent on Fe adlayer thickness or Co underlayer. The pure FeCo/Cu(001) has an in-plane magnetic anisotropy and the Co underlayer induces a perpendicular magnetic anisotropy. Besides, the spin reorientation transition is realized with increasing Fe adlayer thickness. Both Fe/FeCo/Cu(001) and Fe/FeCo/Co/Cu(001) systems manifest similar behavior although the strength of anisotropy energy is different. This result indicates that the Fe adlayer plays an important role for the magnetic anisotropy because the direction of magnetization is almost independent on the presence of Co underlayer, while the Co underlayer affects the magnitude of magnetic anisotropy energy. [Copyright &y& Elsevier]