Current-driven magnetic rearrangements in spin-polarized point contacts

Abstract

A new method for investigating the dynamics of atomic magnetic moments in current-carrying magnetic point contacts under bias is presented. This combines the non-equilibrium Green's function (NEGF) method for evaluating the current and the charge density with a description of the dynamics of the magnetization in terms of quasistatic thermally-activated transitions between stationary configurations. This method is then implemented in a tight-binding (TB) model with parameters chosen to simulate the main features of the electronic structures of magnetic transition metals. We investigate the domain wall (DW) migration in magnetic monoatomic chains sandwiched between magnetic leads, and for realistic parameters find that collinear arrangement of the magnetic moments of the chain is always favorable. Several stationary magnetic configurations are identified, corresponding to a different number of Bloch walls in the chain and to a different current. The relative stability of these configurations depends on the geometrical details of the junction and on the bias, however we predict transitions between different configurations with activation barriers of the order of a few tens of meV. Since different magnetic configurations are associated to different resistances, this suggests an intrinsic random telegraph noise at microwave frequencies in the I-V characteristics of magnetic atomic point contacts at room temperature. Finally, we investigate whether or not current induced torques are conservative

Nonconservative generalized current-induced forces

“…An example of such generalized coordinates are magnetic degrees of freedom in spinpolarized nanowires. 4 The key finding ͓Eq. ͑24͔͒ is that the generalized self-consistent steady-state forces remain, in general, nonconservative.…”

“…1 Nonetheless, a fundamental question about these forces was, until recently, not entirely clear: are they conservative or not, with arguments both ways. [1][2][3][4] The interest in these forces is partly physical and partly practical: in nanoscale conductors, where the current densities can be huge, these forces can, correspondingly, be large, opening up the question of how these corrections to ordinary interatomic forces affect the structure and functionality of atomic-scale devices. [5][6][7] Recently it was shown that current-induced forces are, indeed, not conservative, by evaluating explicitly their curl under steady-state conditions, 8 and that, therefore, these forces can do net work on the atoms around closed paths.…”

Spin-transfer torque in magnetic tunnel junctions

“…The choice of the parameters including the hopping matrix element provides a realistic choice for systems based on magnetic transition metals and their alloys. 50,51 In order to increase the band filling, we believe that different class of magnetic materials should be suggested, for example, alloying Fe͑Co͒ with impurities which can fill up the majority ⌬ 1 band.…”

Nonconservative dynamics in long atomic wires

“…The average force on the other hand contains, among other contributions, the familiar electron wind force [4][5][6][7][8][9]. In recent years the wind force has become the focus of renewed attention due to the realization that it is nonconservative [3,5,[10][11][12][13][14][15][16]. The importance of these nonconservative forces cannot be overestimated.…”