Ab initio study of FeRh multilayers supported on MgO(0 0 1)

Abstract

Low dimensional films are materials of interest for the changes of the structural, electronic and magnetic properties they undergo, especially when they form interfaces with a substrate. The iron-rhodium (FeRh) alloy is an excellent example. Experimentally, FeRh films are deposited on single crystal substrates like MgO, sapphire or silicon. Particularly, MgO (0 0 1) is an excellent support for FeRh because it is possible to deposit high quality epitaxial films on it. MgO is also highly stable at high temperatures and tensile in-plane strain favors the FM over the AFM state. In order to improve the knowledge of the supported bimetallic systems properties, theoretical calculations using density functional theory (DFT) have been carried out. The different thicknesses considered for the multilayers are 0.6 nm, 0.9 nm and 1.2 nm for both, terminated in Fe or Rh. To complete the study, we present the results obtained on the influence of the termination of the surfaces, the number of alloy layers, the different magnetic configurations (FM – AFM), the charge transfer and the adhesion of the films to the MgO substrate. The analysis of the results shows that as the thickness of the film grows, the adhesion energy tends to an average value of the order of 1.5 J/m2 ; on the other hand, the AFM coupling facilitates the takeoff compared to the FM coupling. Moreover, comparing the difference between FM and AFM results for different thicknesses, the percentages are slightly lower for. –Rh terminated films. Regarding the out-of-plane relaxation percentages, they depend on the thickness of the film and the magnetic coupling. For the AFM coupling of films -Rh terminated, lower values are observed compared to those obtained for the FM coupling and this behavior is maintained for all three thicknesses studied. However, for the -Fe terminated films as the film grows the relaxations are practically twice those corresponding to the cases of films terminated in Rh tending to tetragonal structures. With respect to magnetic properties, in the three presented systems for -Rh terminated multilayers, the tendency of magnetic moments values for the FM coupling is maintained in around 3.2 μB/at for the case of Fe and 1.0 μB/at for the case of Rh. For AFM the most noticeable difference is the cancellation of the magnetic moments of Rh. However, the situation is different for the Fe atoms in the -Fe terminated multilayers. For 0.9 nm the Fe atoms in contact with the substrate undergo the least relaxation and their magnetic moments are parallel and equal to −1 μB/at. The Fe atoms of the other layers recovered their values of ± 3 μB/at. But when the film grows the Rh atoms acquire a small magnetization in increasing order as it approaches to the free surface. The presence of residual ferromagnetism in the interfaces of the FeRh films deposited on MgO (0 0 1) has also been observed experimentally by other groups. In summary, we can conclude that the obtained results are mostly influenced by the surface termination. (-Rh or -Fe).