Single magnetic atoms, and assemblies of such atoms, on non-magnetic surfaces have recently attracted attention owing to their potential use in high-density magnetic data storage and as a platform for quantum computing. A fundamental problem resulting from their quantum mechanical nature is that the localized magnetic moments of these atoms are easily destabilized by interactions with electrons, nuclear spins and lattice vibrations of the substrate. Even when large magnetic fields are applied to stabilize the magnetic moment, the observed lifetimes remain rather short (less than a microsecond). Several routes for stabilizing the magnetic moment against fluctuations have been suggested, such as using thin insulating layers between the magnetic atom and the substrate to suppress the interactions with the substrate's conduction electrons, or coupling several magnetic moments together to reduce their quantum mechanical fluctuations. Here we show that the magnetic moments of single holmium atoms on a highly conductive metallic substrate can reach lifetimes of the order of minutes. The necessary decoupling from the thermal bath of electrons, nuclear spins and lattice vibrations is achieved by a remarkable combination of several symmetries intrinsic to the system: time reversal symmetry, the internal symmetries of the total angular momentum and the point symmetry of the local environment of the magnetic atom.

We have investigated the magnetic properties of pure ZnO thin films grown under N 2 pressure on a-, c-, and r-plane Al 2 O 3 substrates by pulsed-laser deposition. The substrate temperature and the N 2 pressure were varied from room temperature to 570°C and from 0.007 to 1.0 mbar, respectively. The magnetic properties of bare substrates and ZnO films were investigated by SQUID magnetometry. ZnO films grown on c-and a-plane Al 2 O 3 substrates did not show significant ferromagnetism. However, ZnO films grown on r-plane Al 2 O 3 showed reproducible ferromagnetism at 300 K when grown at 300-400°C and 0.1-1.0 mbar N 2 pressure. Positron annihilation spectroscopy measurements as well as density-functional theory calculations suggest that the ferromagnetism in ZnO films is related to Zn vacancies.

The heavy rare earth elements crystallize into hexagonally close packed (h.c.p.) structures and share a common outer electronic configuration, differing only in the number of 4f electrons they have. These chemically inert 4f electrons set up localized magnetic moments, which are coupled via an indirect exchange interaction involving the conduction electrons. This leads to the formation of a wide variety of magnetic structures, the periodicities of which are often incommensurate with the underlying crystal lattice. Such incommensurate ordering is associated with a 'webbed' topology of the momentum space surface separating the occupied and unoccupied electron states (the Fermi surface). The shape of this surface-and hence the magnetic structure-for the heavy rare earth elements is known to depend on the ratio of the interplanar spacing c and the interatomic, intraplanar spacing a of the h.c.p. lattice. A theoretical understanding of this problem is, however, far from complete. Here, using gadolinium as a prototype for all the heavy rare earth elements, we generate a unified magnetic phase diagram, which unequivocally links the magnetic structures of the heavy rare earths to their lattice parameters. In addition to verifying the importance of the c/a ratio, we find that the atomic unit cell volume plays a separate, distinct role in determining the magnetic properties: we show that the trend from ferromagnetism to incommensurate ordering as atomic number increases is connected to the concomitant decrease in unit cell volume. This volume decrease occurs because of the so-called lanthanide contraction, where the addition of electrons to the poorly shielding 4f orbitals leads to an increase in effective nuclear charge and, correspondingly, a decrease in ionic radii.

We propose a simplified version of self-interaction corrected local spin-density (SIC-LSD) approximation, based on multiple scattering theory, which implements self-interaction correction locally, within the KKR method. The multiple scattering aspect of this new SIC-LSD method allows for the description of crystal potentials which vary from site to site in a random fashion and the calculation of physical quantities averaged over ensembles of such potentials using the coherent potential approximation (CPA). This facilitates applications of the SIC to alloys and pseudoalloys which could describe disordered local moment systems, as well as intermediate valences. As a demonstration of the method, we study the well-known α-γ phase transition in Ce, where we also explain how SIC operates in terms of multiple scattering theory.

An ab initio study of magnetic exchange interactions in antiferromagnetic and strongly correlated 3d transition metal monoxides is presented. Their electronic structure is calculated using the local self-interaction correction approach, implemented within the Korringa-Kohn-Rostoker band structure method, which is based on multiple scattering theory. The Heisenberg exchange constants are evaluated with the magnetic force theorem. Based on these the corresponding Néel temperatures TN and spin wave dispersions are calculated. The Néel temperatures are obtained using mean field approximation, random phase approximation and Monte Carlo simulations. The pressure dependence of TN is investigated using exchange constants calculated for different lattice constants. All the calculated results are compared to experimental data.