Noncollinear Magnetic Configurations Research Articles

New Model for Magnetism in Ultrathin fcc Fe on Cu(001)

Using soft-x-ray resonant magnetic scattering in combination with first-principles calculations for noncollinear magnetic configurations we present a new model of the magnetism in ultrathin fcc Fe films on Cu(001). We find the presence of blocks with robust magnetic structure, while the relative directions of the moments of different blocks are sensitive to the detailed atomic structure and temperature. The magnetic noncollinearity is directly demonstrated, which has not been possible so far.

Current-induced dynamics in noncollinear dual spin valves

Spin-transfer torque and current induced spin dynamics in spin-valve nanopillars with the free magnetic layer located between two magnetic films of fixed magnetic moments is considered theoretically. The spin-transfer torque in the limit of diffusive spin transport is calculated as a function of magnetic configuration. It is shown that non-collinear magnetic configuration of the outermost magnetic layers has a strong influence on the spin torque and spin dynamics of the central free layer. Employing macrospin simulations we make some predictions on the free layer spin dynamics in spin valves composed of various magnetic layers. We also present a formula for critical current in non-collinear magnetic configurations, which shows that the magnitude of critical current can be several times smaller than that in typical single spin valves.

Influence of a transport current on magnetic anisotropy in gyrotropic ferromagnets

Current-induced torques are commonly used to manipulate noncollinear magnetization configurations. In this paper we discuss current-induced torques present in a certain class of collinear magnetic systems, relating them to current-induced changes in magnetic anisotropy energy. We present a quantitative estimate of their characteristics in uniform strained ferromagnetic (Ga,Mn)As.

Temperature derivative of the resistance of spin valves

Spin-dependent transport properties of nanowires containing individual Co/Cu/Co pseudo-spin valves were investigated by monitoring an AC voltage induced by a temperature oscillation under a continuous DC current. We observed sharp peaks in the field-dependent AC voltage response. These peaks only appear close to the magnetic field range where conventional magnetoresistance shows a reversible and gradual transition. The effect is interpreted as an extra dissipation process with a distinct temperature dependence that occurs in non-collinear magnetic configurations when transverse moment relaxation takes place.

Carrier-induced noncollinear magnetism in perovskite manganites by first-principlescalculations

We have performed noncollinear first-principles density-functional calculations of carrier-doped perovskitemanganites La1−xSrxMnO3 (0.0≤x≤1.0). In the calculated magnetic phase diagram(T = 0) within the collinear magnetic configurations, ferromagnetic and severalantiferromagnetic configurations successively appeared as a ground state with increasingx. The calculated total energies of the ferromagnetic and A-typeantiferromagnetic phases are almost degenerate around the phase boundary,x = 0.5. Wefound that the noncollinear magnetic configurations are stable in a wide range of carrier concentrations0.3≤x≤0.6. We discuss the effect of lattice distortions on the stability of the noncollinear magneticphase.

Correlation of the angular dependence of spin-transfer torque and giant magnetoresistance in the limit of diffusive transport in spin valves

Angular variation in giant magnetoresistance and spin-transfer torque in metallic spin-valve heterostructures is analyzed theoretically in the limit of diffusive transport. It is shown that the spin-transfer torque in asymmetric spin valves can vanish in noncollinear magnetic configurations, and such a nonstandard behavior of the torque is generally associated with a nonmonotonic angular dependence of the giant magnetoresistance, with a global minimum at a noncollinear magnetic configuration.

Challenge of Magnetism in Strongly Correlated Open-Shell2pSystems

We report on theoretical investigations of the exotic magnetism in rubidium sesquioxide Rb4O6, a model correlated system with an open 2p shell. Experimental investigations indicated that Rb4O6 is a magnetically frustrated insulator. The frustration is explained here by electronic structure calculations that incorporate the correlation between the oxygen 2p electrons and deal with the mixed-valent oxygen. This leads to a physical picture where the symmetry is reduced because one third of the oxygen in Rb4O6 is nonmagnetic while the remaining two thirds assemble in antiferromagnetic arrangements. A degenerate, insulating ground state with a large number of frustrated noncollinear magnetic configurations is confidently deduced from the theoretical point of view. These findings demonstrate in general the importance of electron-electron correlations in open-shell p-electron systems.

Unified theory of near-adiabatic magnetization dynamics for collinear and noncollinear magnetization

A combination of the breathing Fermi-surface model with a variant of the ab initio density-functional electron theory given by the magnetic force theorem is used to establish a unified theory for the near-adiabatic magnetization dynamics on the atomic scale. The main achievement of the theory is that it makes possible to treat both collinear as well as noncollinear magnetization configurations on equal footing. The theory yields an equation of motion of the type of the widely used Gilbert equation, however, with the constant Gilbert damping scalar replaced by an anisotropic and nonlocal damping matrix. The range of validity of the theory is discussed.

Spin transfer torques in the nonlocal lateral spin valve

We report a theoretical study on the spin and electron transport in the nonlocal lateralspin valve with a non-collinear magnetic configuration. The nonlocal magnetoresistance,defined as the voltage difference on the detection lead over the injected current, is derivedanalytically. The spin transfer torques on the detection lead are calculated. It isfound that spin transfer torques are symmetrical for parallel and antiparallelmagnetic configurations, in contrast to that in a conventional sandwiched spin valve.

Cotunneling through quantum dots coupled to magnetic leads: Zero-bias anomaly for noncollinear magnetic configurations

Cotunneling transport through quantum dots weakly coupled to non-collinearly magnetized leads is analyzed theoretically by means of the real-time diagrammatic technique. The electric current, dot occupations, and dot spin are calculated in the Coulomb blockade regime and for arbitrary magnetic configuration of the system. It is shown that an effective exchange field exerted on the dot by ferromagnetic leads can significantly modify the transport characteristics in non-collinear magnetic configurations, in particular the zero-bias anomaly found recently for antiparallel configuration. For asymmetric Anderson model, the exchange field gives rise to precession of the dot spin, which leads to a nonmonotonic dependence of the differential conductance and tunnel magnetoresistance on the angle between magnetic moments of the leads. An enhanced differential conductance and negative TMR are found for certain non-collinear configurations.

Role of nonequilibrium conduction electrons on the magnetization dynamics of ferromagnets in thes-dmodel

In the literature on the $s$-$d$ model for the magnetization dynamics in systems without and with spin-polarized transport currents, it is generally assumed that the adiabatic part of the $s$-electron magnetization is everywhere parallel to the $d$-electron magnetization. We present model calculations within the ab initio spin-density functional theory which suggest that this assumption is not valid for systems with strongly noncollinear magnetization configurations.

Possible non-collinear magnetic configurations in BaCoO 3

The magnetic properties of BaCoO 3 are strongly influenced by the frustration induced by the hexagonal structure. Previously, collinear configurations with ferromagnetically (FM) ordered hexagonal planes stacked antiferromagnetically (AF) along the c-axis (that of the Co chains) were found to be nearly degenerate and an orbitally ordered state (along c) was encountered. Here, we present a study of possible non-collinear magnetic structures in the system. We have used the WIEN2k software, which uses a full-potential, all-electron APW+lo (augmented plane waves plus local orbitals) method in a version adapted for studying non-collinear magnetic configurations. We explore this possibility of a non-collinear arrangement of the magnetic moments within the hexagonal plane, which could lead to a vanishing net magnetization within the plane. Due to the triangular arrangement of the Co chains and resulting frustrations, a simple AF order is not possible with any collinear configuration. Our calculations show that the most stable ordering is FM within the plane.

Macroscopic description of spin transfer torque

A macroscopic description of the current-induced torque due to spin transfer has been developed for layered systems consisting of ferromagnetic films, separated by nonmagnetic layers. The description is based on the classical spin diffusion equations for the distribution functions used in the theory of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR), and the relevant boundary conditions for the longitudinal and transverse components of the spin current and spin accumulation. The torque is expressed as a function of the usual parameters derived from CPP-GMR experiments and two additional parameters involved in the transverse boundary conditions. The model describes qualitatively the normal and inverse switching phenomena studied in recent experiments. We also discuss a structure for which the spin torque disappears at a noncollinear magnetic configuration.

Magnetic Couplings in MM′X Compounds of Cu2Sb Type of Structure (M,M′: d‐Metals, X: p‐Elements as P, As, Sb,...)

AbstractFor Abstract see ChemInform Abstract in Full Text.

NON-COLLINEAR MAGNETISM IN THE Fe3 MICROCLUSTER: FREE-STANDING VS SUPPORTED ENVIRONMENTS

The possibility of a non-collinear magnetic configuration of the Fe 3 microcluster supported on the Ni (001) surface has been investigated. The morphology of the supported cluster has been calculated by means of the modified embedded atom model with quenched molecular dynamics simulations, and the electronic structure for the most stable geometrical configuration has been studied using a self-consistent non-collinear spd tight-binding method parameterised to ab-initio tight-binding linear muffin tin orbital results. Our predictions are compared with previous results for the free-standing Fe 3 microcluster. The influence of the substrate in both the structure and the magnetic properties, particularly the onset of non-collinear magnetism, is discussed in detail.

Noncollinear magnetic order in the six-atom Mn cluster

We report ab initio calculations of the structures, binding energies, and magnetic moments of the lowest-energy isomers of the cluster Mn6 that were performed using SIESTA, a density-functional method that employs linear combinations of pseudoatomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the local spin-density approximation for exchange and correlation. Our results predict that ground-state Mn6 has a noncollinear magnetic configuration.

Magnetic couplings in [formula omitted] compounds of Cu 2Sb type of structure ( [formula omitted], [formula omitted] as P, As, Sb, …)

Most of the M M ′ X tetragonal compounds of Cu 2Sb type structure, where M , M ′ are d-metals and X = P , As, Sb, Ge, etc. exhibit interesting magnetic properties. These compounds have been characterised by magnetisation measurements as well as by neutron diffraction for establishing the magnetic configurations. A first review is made of the different types of magnetic arrangements. Based on the molecular field model for magnetic couplings—thus applying a 2nd order interaction model—the different collinear arrangements derived from experiments are described using the Bertaut's Irreducible Representation Analysis. An exchange matrix based on the Heisenberg type Hamiltonian has been built up on the framework of the Bertaut's Phase Comparison Analysis between the magnetic atoms. Thus, simple arrangements of the magnetic moments have been derived. Long range non-collinear magnetic configurations appear to be highly unlikely, due to very peculiar conditions. This is confirmed experimentally with the many different, but rather simple ferromagnetic, ferrimagnetic and antiferromagnetic configurations that were established.

Magnetic interactions and phase transformations in FeM, M = (Pt, Rh) ordered alloys

We present results of the first-principles calculations for selected non-collinear magnetic configurations in L10 FePt and B2 FeRh. These results have been interpreted within the unified model of magnetic interactions which treats the Fe atomic moments as sufficiently localized degrees of freedom. The exchange fields of Fe atomic moment determine both the orientation and the value of the induced Rh/Pt atomic moments. The Pt moment fluctuations in FePt follow a relatively simple form while for the FeRh significant contributions of the non-linear susceptibility have to be taken into account. The latter, along with the remarkably strong Rh mediated exchange lead to a dominating contribution from the bi-quadratic non-Heisenberg terms. The effect of magnetic interactions mediated by the induced Rh/Pt moment on the order-order (FeRh) and order disorder (FePt) magnetic phase transition is discussed.

About noncollinear magnetic structures in FCC-Fe on Cu(1 0 0)

We present first-principle total energy studies of Fe films on a Cu(001) substrate with thicknesses of 5–7 monolayer. The films are modeled by symmetric nFe/8Cu/n Fe slabs. Both collinear and noncollinear magnetic configurations are considered. It was found that the energy difference between the noncollinear configurations and the favored collinear double-layered antiferromagnetic state is positive for all investigated systems.

Magnetic structure and transport properties of noncollinearLaMn2X2(X=Ge,Si)systems

Electronic, magnetic, and transport properties of the noncollinear naturally multilayered compounds ${\mathrm{LaMn}}_{2}{\mathrm{Ge}}_{2}$ and ${\mathrm{LaMn}}_{2}{\mathrm{Si}}_{2}$ are addressed by first-principles calculations based on the density-functional theory. At low temperatures, these systems show a magnetic state with the $\mathrm{Mn}$ moments ordered in a conical arrangement (spin spiral) with a ferromagnetic coupling along the $c$ axis and an in-plane antiferromagnetic coupling. The magnetic structures are studied by means of the full-potential linearized augmented-plane-wave method within both the generalized-gradient approximation and the local-density approximation. In both compounds, a conical magnetic state is obtained with energies lower than canted and collinear structures. The trends in the experimentally observed magnetic configuration when replacing $\mathrm{Ge}$ by $\mathrm{Si}$ are discussed. The origin of the experimentally observed inverse giant magnetoresistance in ${\mathrm{LaMn}}_{2}{\mathrm{Ge}}_{2}$ is traced back to the presence of many noncollinear low-energy magnetic configurations.