Spin-polarised currents and magnetic domain walls

Abstract

Electrical currents flowing in ferromagnetic materials are spin-polarised as a result of the spin-dependent band structure. When the spatial direction of the polarisation changes, in a domain structure, the electrons must somehow accommodate the necessary change in direction of their spin angular momentum as they pass through the wall. Reflection, scattering, or a transfer of angular momentum onto the lattice are all possible outcomes, depending on the circumstances. This gives rise to a variety of different physical effects, most importantly a contribution to the electrical resistance caused by the wall, and a motion of the wall driven by the spin-polarised current. Historical and recent research on these topics is reviewed. Contents PAGE 1. Introduction 586 2. Spin-polarised current 587  2.1. Tunnelling current spin polarisation 589  2.2. Ballistic current spin polarisation 592  2.3. Diffusive current spin polarisation 593 3. Magnetic Domain Walls 598  3.1. Basics of domain walls 598  3.1.1. Domain wall thickness and energy 601  3.1.2. Micromagnetic calculations 603  3.1.3. Tailoring domain structures for measurements 605  3.2. Domain walls in nanostructures 609  3.3. Domain wall dynamics 610 4. Domain Wall Resistance 613  4.1. Early results 613  4.2. Theory 615  4.3. Recent experimental results 626  4.3.1. Homogeneous materials 626  4.3.2. Heterostructures 631  4.3.3. Mesoscopic devices 635  4.4. Huge domain wall MR in nanoconstrictions? 644  4.4.1. First results 645  4.4.2. Theoretical interpretation 650  4.4.3. Experimental exploration 656 5. Current-induced Domain Wall Motion 665  5.1. Experimental results 666  5.2. Theory 678 6. Conclusion 690 Acknowledgements 692 References 692