A new twist to an old problem

A magnet can point up or down in a magnetic field. In magnetic materials, information is stored digitally by using two such "states" to represent ones and zeros. A proposal to store information in high-density "racetrack" memories has focussed attention on how electric currents in a magnetic material are affected by twisting of the magnetism in between regions - "domains" - where the magnetism is either all "up" or all "down". By performing extensive quantum mechanical calculations on a supercomputer, we find that a twist in the magnetism - a "domain wall" (Fig.1) - makes a finite contribution to the resistance of a material no matter how slowly the twisting occurs, when a relativistic effect, the spin-orbit coupling, is taken into account. Our finding for domain walls in the technologically important Ni80Fe20 magnetic alloy, Permalloy, contradicts received wisdom for disordered materials and suggests that it should be possible to detect the number of domain walls in a nanowire with just electrical transport measurements.

In this work, we investigated diffusive transport through a number of domain wall (DW) profiles (Fig.1) of Permalloy taking into account simultaneously noncollinearity, alloy disorder, and spin-orbit-coupling fully quantum mechanically, from first principles. In addition to observing the known effects of magnetization mistracking and anisotropic magnetoresistance, we discovered a not-previously identified contribution to the resistance of a DW that comes from spin-orbit-coupling-mediated spin-flip scattering in a textured diffusive ferromagnet. This adiabatic DW resistance, which should exist in all diffusive DWs, can be observed by varying the DW width in a systematic fashion in suitably designed nanowires.


Figure 1. Schematic illustration of the magnetic configurations of (a) Bloch, (b) rotated Néel, and (c) Néel DWs. (d) Sketch of the scattering geometry used in the calculations in which a finite thickness of Ni80Fe20 substitutional alloy is sandwiched between semiinfinite copper leads and alloy disorder is modelled using a lateral supercell periodically repeated in the x-y plane. Transport is in the z direction.