M. Carmen Muñoz

Noble metal capping effects on the spin-reorientation transitions of Co/Ru(0001)

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.

Ab initio study of the p-hole magnetism at polar surfaces of ZnO: the role of correlations

A standard local density approximation and its self-interaction corrected version are applied to study spontaneous magnetization, promoted by localized p electron holes, of polar oxygen-terminated ZnO surfaces. The electronic properties and magnetic exchange interactions of three different facets are calculated. It is demonstrated that partially filled oxygen p orbitals of the polar surfaces exhibit magnetic moment formation and long range magnetic order leading to the occurrence of a ferromagnetic ground state. Monte Carlo simulations predict Curie temperatures above room temperature. In contrast to isolated defects in bulk materials, applying correlation corrections to the localized p-like surface states does not lead to a collapse of magnetic interaction: as the weakening of the magnetic interaction, caused by the reduced electronic overlap, is compensated by a strengthening due to an increase of the magnetic moments, the ferromagnetism can principally persist above room temperature, provided a large hole concentration exists.

Surface ferromagnetism in non-magnetic and dilute magnetic oxides

We explore hole-doping as an efficient route to develop magnetism in simple nonmagnetic oxides. Based on ab-initio calculations of the prototypical material ZnO, we show that local spin polarization can be obtained even in the absence of magnetic dopants both in the bulk or at surfaces. However, the onset of long-range magnetic order additionally requires extended states, that can be achieved in surfaces without distorting the host lattice. We propose different possibilities which either profit the spontaneous polarization of Oxygen p states under defective charge conditions, or require the presence of adsorbates.

A realistic topological p–n junction at the Bi2Se3 (0001) surface based on planar twin boundary defects

We propose a realistic topological p−n junction (TPNJ) by matching two Bi2Se3 (0001) slabs with opposite arrangements of planar twin boundary defects. The atomistic modeling of such a device leads to dislocation defects in the hexagonal lattice in several quintuple layers. Nevertheless, total energy calculations reveal that the interface relaxes, yielding a smooth geometrical transition that preserves the nearest-neighbors fcc-type geometry throughout these defect layers. The electronic, magnetic, and transport properties of the junction have then been calculated at the ab initio level under open boundary conditions, i.e., employing a thin-film geometry that is infinite along the electron transport direction. Indeed, a p−n junction is obtained with a built-in potential as large as 350 meV. The calculations further reveal the spin texture across the interface with unprecedented detail. As the main result, we obtain non-negligible transmission probabilities around the Γ point, which involve an electron spin-flip process while crossing the interface.