First-principles analysis of spin-disorder resistivity of Fe and Ni

Spin-disorder resistivity of Fe and Ni and its temperature dependence are analyzed using noncollinear density functional calculations within the supercell method. Different models of thermal spin disorder are considered, including the mean-field approximation and the nearest-neighbor Heisenberg model. Spin-disorder resistivity is found to depend weakly on magnetic short-range order. If the local moments are kept frozen at their zero-temperature values, very good agreement with experiment is obtained for Fe, but for Ni the resistivity at elevated temperatures is significantly overestimated. Agreement with experiment for Fe is improved if the local moments are iterated to self-consistency. The overestimation of the resistivity for paramagnetic Ni is attributed to the reduction of the local moments down to 0.35 Bohr magnetons. Overall, the results suggest that low-energy spin fluctuations in Fe and Ni are better viewed as classical rotations of local moments rather than quantized spin fluctuations that would require an (S+1)/S correction.Comment: 10 pages (RevTeX), 6 eps figure

Giant Electroresistance in Ferroelectric Tunnel Junctions

The interplay between the electron transport in metal/ferroelectric/metal junctions with ultrathin ferroelectric barriers and the polarization state of a barrier is investigated. Using a model which takes into account screening of polarization charges in metallic electrodes and direct quantum tunneling across a ferroelectric barrier we calculate the change in the tunneling conductance associated with the polarization switching. We find the conductance change of a few orders of magnitude for metallic electrodes with significantly different screening lengths. This giant electroresistance effect is the consequence of a different potential profile seen by transport electrons for the two opposite polarization orientations.Comment: 4 page

The Origin of Tunneling Anisotropic Magnetoresistance in Break Junctions

First-principles calculations of electron tunneling transport in Ni and Co break junctions reveal strong dependence of the conductance on the magnetization direction, an effect known as tunneling anisotropic magnetoresistance (TAMR). The origin of this phenomenon stems from resonant states localized in the electrodes near the junction break. The energy and broadening of these states is strongly affected by the magnetization orientation due to spin-orbit coupling, causing TAMR to be sensitive to bias voltage on a scale of a few mV. Our results bear a resemblance to recent experimental data and suggest that TAMR driven by resonant states is a general phenomenon typical for magnetic broken contacts and other experimental geometries where a magnetic tip is used to probe electron transport.Comment: 4 pages, 3 figure

Electronic and Magnetic Properties of Endohedrally Doped Fullerene Mn@C60: A Total Energy Study

We perform total energy calculations on a manganese atom encapsulated inside a C60 cage using density functional theory with the generalized gradient approximation through three optimization schemes and along four paths inside the cage. We find that when Mn is located in the central region, its electronic and magnetic properties are not exactly the same as those of a free Mn atom due to weak coupling between Mn and the cage. As Mn is shifted toward to the edge, the total energy and spin start to change significantly when Mn is situated about one-third of the way between the cage center and edge, and the total energy reaches a local minimum. Finally the interaction between Mn and the cage turns repulsive as Mn approaches the edge. We also find that, along the lowest energy path, there exist three consecutive local energy minima and each of these has a different spin M. The ground state has the lowest M=3, Mn is located about 1.6 Å away from the cage center, and the binding energy is 0.08 eV. We attribute the decrease in total energy and spin to Mn and C hybridization

Theoretical Study of the Magnetic Ordering in Rare-Earth Compounds with Face-Centered-Cubic Structure

We present a detailed theoretical study of the magnetic ordering in heavy rare-earth compounds with a face-centered-cubic structure. In addition to the exchange interactions which are counted up to the third nearest neighbors, the effect of the dipolar interactions and magnetic anisotropic effect are also included in our model Hamiltonian. The interactions parameters are obtained from first-principles band-structure calculations by fitting the total energies of different magnetic configurations to the Heisenberg Model. Thus from utilizing the Monte Carlo simulations, we explained the formation of different magnetic structures in the rare-earth compounds

Ballistic anisotropic magnetoresistance

Electronic transport in ferromagnetic ballistic conductors is predicted to exhibit ballistic anisotropic magnetoresistance (BAMR) - a change in the ballistic conductance with the direction of magnetization. This phenomenon originates from the effect of the spin-orbit interaction on the electronic band structure which leads to a change in the number of bands crossing the Fermi energy when the magnetization direction changes. We illustrate the significance of this phenomenon by performing ab-initio calculations of the ballistic conductance in ferromagnetic Ni and Fe nanowires which display a sizable BAMR when the magnetization changes direction from parallel to perpendicular to the wire axis

Magnetic Moment Softening and Domain Wall Resistance in Ni Nanowires

Magnetic moments in atomic scale domain walls formed in nanoconstrictions and nanowires are softened which affects dramatically the domain wall resistance. We perform ab initio calculations of the electronic structure and conductance of atomic-size Ni nanowires with domain walls only a few atomic lattice constants wide. We show that the hybridization between noncollinear spin states leads to a reduction of the magnetic moments in the domain wall. This magnetic moment softening strongly enhances the domain wall resistance due to scattering produced by the local perturbation of the electronic potential.Comment: 4 pages, 5 figure

Surface Magnetoelectric Effect in Ferromagnetic Metal Films

A surface magnetoelectric effect is revealed by density-functional calculations that are applied to ferromagnetic Fe(001), Ni(001) and Co(0001) films in the presence of external electric field. The effect originates from spin-dependent screening of the electric field which leads to notable changes in the surface magnetization and the surface magnetocrystalline anisotropy. These results are of considerable interest in the area of electrically-controlled magnetism and magnetoelectric phenomena

Electronic and magnetic properties of endohedrally doped fullerene Mn@C60: A total energy study

We perform total energy calculations on a manganese atom encapsulated inside a C(60) cage using density functional theory with the generalized gradient approximation through three optimization schemes and along four paths inside the cage. We find that when Mn is located in the central region, its electronic and magnetic properties are not exactly the same as those of a free Mn atom due to weak coupling between Mn and the cage. As Mn is shifted toward to the edge, the total energy and spin start to change significantly when Mn is situated about one-third of the way between the cage center and edge, and the total energy reaches a local minimum. Finally the interaction between Mn and the cage turns repulsive as Mn approaches the edge. We also find that, along the lowest energy path, there exist three consecutive local energy minima and each of these has a different spin M. The ground state has the lowest M=3, Mn is located about 1.6 angstrom away from the cage center, and the binding energy is 0.08 eV. We attribute the decrease in total energy and spin to Mn and C hybridization. (C) 2008 American Institute of Physics.http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000253336800015&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=8e1609b174ce4e31116a60747a720701Physics, Atomic, Molecular & ChemicalSCI(E)EI13ARTICLE7null12

Correlation effects and electronic structure of Gd@C\u3csub\u3e60\u3c/sub\u3e

We have investigated the electronic structure of Gd@C60 using ab initio calculations, photoemission and resonant photoemission (constant initial state spectroscopy). In comparing our calculations based on the local spin density approximation and the Hubbard model description with the observed photoemission spectra, we conclude that Gd 4f states exhibit enhanced correlation energies. These correlation energies have values larger than those normally observed in metallic gadolinium and gadolinium compounds. We attributed the enhanced correlation to the diminished screening of the encapsulated Gd. Both calculation and experiment confirm a strong hybridization between the valence states of Gd and the C 2p states of the fullerene cage