Magnetic Coupling Between Non-Magnetic Ions: Eu3+ in EuN and EuP

We consider the electronic structure of, and magnetic exchange (spin) interactions between, nominally nonmagnetic Eu^3+ ions (4f^6, S=3, L=3, J=0) within the context of the rocksalt structure compounds EuN and EuP. Both compounds are ionic [Eu^3+; N^3- and P^3-] semimetals similar to isovalent GdN. Treating the spin polarization within the 4f shell, and then averaging consistent with the J=0 configuration, we estimate semimetallic band overlaps (Eu 5d with pnictide 2p or 3p) of ~0.1 eV (EuN) and ~1.0 eV (EuP) that increase (become more metallic) with pressure. The calculated bulk modulus is 130 (86) GPa for EuN (EuP). Exchange (spin-spin) coupling calculated from correlated band theory is small and ferromagnetic in sign for EuN, increasing in magnitude with pressure. Conversely, the exchange coupling is antiferromagnetic in sign for EuP and is larger in magnitude, but decreases with compression. Study of a two-site model with S_1*S_2 coupling within the J=0,1 spaces of each ion illustrates the dependence of the magnetic correlation functions on the model parameters, and indicates that the spin coupling is sufficient to alter the Van Vleck susceptibility. We outline a scenario of a spin-correlation transition in a lattice of S=3, L=3, J=0 nonmagnetic ions

X-ray absorption branching ratio in actinides: LDA+DMFT approach

To investigate the x-ray absorption (XAS) branching ratio from the core 4d to valence 5f states, we set up a theoretical framework by using a combination of density functional theory in the local density approximation and Dynamical Mean Field Theory (LDA+DMFT), and apply it to several actinides. The results of the LDA+DMFT reduces to the band limit for itinerant systems and to the atomic limit for localized f electrons, meaning a spectrum of 5f itinerancy can be investigated. Our results provides a consistent and unified view of the XAS branching ratio for all elemental actinides, and is in good overall agreement with experiments.Comment: 6 pages, 4 figure

Magnetoelastic mechanism of spin-reorientation transitions at step-edges

The symmetry-induced magnetic anisotropy due to monoatomic steps at strained Ni films is determined using results of first - principles relativistic full-potential linearized augmented plane wave (FLAPW) calculations and an analogy with the N\'eel model. We show that there is a magnetoelastic anisotropy contribution to the uniaxial magnetic anisotropy energy in the vicinal plane of a stepped surface. In addition to the known spin-direction reorientation transition at a flat Ni/Cu(001) surface, we propose a spin-direction reorientation transition in the vicinal plane for a stepped Ni/Cu surface due to the magnetoelastic anisotropy. We show that with an increase of Ni film thickness, the magnetization in the vicinal plane turns perpendicular to the step edge at a critical thickness calculated to be in the range of 16-24 Ni layers for the Ni/Cu(1,1,13) stepped surface.Comment: Accepted for publication in Phys. Rev.

Implementation of the LDA+U method using the full potential linearized augmented plane wave basis

We provide a straightforward and efficient procedure to combine LDA+U total energy functional with the full potential linearized augmented plane wave method. A detailed derivation of the LDA+U Kohn-Sham type equations is presented for the augmented plane wave basis set, and a simple ``second-variation'' based procedure for self-consistent LDA+U calculations is given. The method is applied to calculate electronic structure and magnetic properties of NiO and Gd. The magnetic moments and band eigenvalues obtained are in very good quantitative agreement with previous full potential LMTO calculations. We point out that LDA+U reduces the total d charge on Ni by 0.1 in NiO

Electronic structure and magnetic properties of cobalt intercalated in graphene on Ir(111)

Using a combination of photoemission and x-ray magnetic circular dichroism (XMCD), we characterize the growth and the electronic as well as magnetic structure of cobalt layers intercalated in between graphene and Ir(111). We demonstrate that magnetic ordering exists beyond one monolayer intercalation, and determine the Co orbital and spin magnetic moments. XMCD from the carbon edge shows an induced magnetic moment in the graphene layer, oriented antiparallel to that of cobalt. The XMCD experimental data are discussed in comparison to our results of first-principles electronic structure calculations. It is shown that good agreement between theory and experiment for the Co magnetic moments can be achieved when the local-spin-density approximation plus the Hubbard U (LSDA+U) is used

Electron correlation effects and magnetic ordering at the Gd(0001) surface

Effects of electron correlation on the electronic structure and magnetic properties of the Gd(0001) surface are investigated using of the full-potential linearized augmented plane wave implementation of correlated band theory ("LDA+U"). The use of LDA+U instead of LDA (local density approximation) total energy calculations produces the correct ferromagnetic ground state for both bulk Gd and the Gd surface. Surface strain relaxation leads to an 90 % enhancement of the interlayer surface-to-bulk effective exchange coupling. Application of a Landau-Ginzburg type theory yields a 30 % enhancement of the Curie temperature at the surface, in very good agreement with the experiment.Comment: revised version: minor typos correcte

Magnetism, Spin-Orbit Coupling, and Superconducting Pairing in UGe2_2

A consistent picture on the mean-field level of the magnetic properties and electronic structure of the superconducting itinerant ferromagnet UGe$_2$ is shown to require inclusion of correlation effects beyond the local density approximation (LDA). The "LDA+U" approach reproduces both the magnitude of the observed moment, composed of strongly opposing spin and orbital parts, and the magnetocrystalline anisotropy. The largest Fermi surface sheet is comprised primarily of spin majority states with orbital projection $m_{\ell}$=0, suggesting a much simpler picture of the pairing than is possible for general strong spin-orbit coupled materials. This occurrence, and the quasi-two-dimensional geometry of the Fermi surface, support the likelihood of magnetically mediated p-wave triplet pairing.Comment: accepted for publication in Phys. Rev. Lett; URL for better quality image of Fig.3 (2MB) at http://yammer.ucdavis.edu/public/UGe2/fig3.ep