Speaker
Description
Various interactions contribute to the existence of an easy magnetization axis or plane in a system, a phenomenon known as magnetic anisotropy. Among those contributions, the so-called magnetocrystalline anisotropy (MCA) is fundamental to understand magnetism at the nanoscale, since it is intimately related to the electronic structure. MCA is due to the spin-orbit (SO) interaction and it is enhanced by reduced dimensionality. Thus, giant MCA is commonplace for single adatoms or molecules and bidimensional magnetic systems.
Since the energies associated to MCA are small (often well below the meV), perturbative models have been used to describe it [1], such as spin effective hamiltonians for isolated magnetic moments. However, these models present limitations when the ion hybridizes strongly with its surroundings. Alternatively, DFT calculations including SO terms offer a reliable description in those cases, although they might be sometimes impractical due to the accuracy needed to account for the small MCA energies.
We will discuss free-standing transition-metal-organic coordination networks. Although SO effects may seem small a priori here, this example nicely illustrates the complexity behind an easy magnetization axis or plane. For model Mn- and Ni-TCNQ rectangular lattices [2] we have found, using DFT calculations, how to tune the MCA upon alteration of the electronic structure by charge transfer from the substrate, lattice strain, or by symmetry breaking. Due to the latter effect in Ni-TCNQ, strong azimuthal anisotropy energy differences of up to 1.5 meV (measured with the OZ easy axis reference) are obtained.
The second example is the GdAu$_2$ monolayer alloy, studied both experimentally (XMCD, MOKE, and ARPES) and theoretically (DFT). In this strong SO-coupled system we have found in-plane MCA, as opposed to pure Gd surfaces, characterised by perpendicular anisotropy. Our result is a consequence of the formation of Gd(d)-Au(s) hybrid bands [3]. Further, when GdAu$_2$ is used as a substrate it induces a magnetization direction change in deposited Co [4].
References
[1] G. van der Laan, J. Phys.: Condens. Matter 10, 3239 (1998).
[2] M.N. Faraggi et al, J. Phys. Chem. C 119, 574 (2015).
[3] M. Ormaza et al. Nano Lett. 16, 4230-4235 (2016).
[4] L. Fernández et al., Nano Lett. 14, 2977-2981 (2014).
