UK-Japanese Collaboration on Current-Driven Domain Wall Dynamics

The project will be a collaboration between theory (Japan) and experiment (UK). The effort of this project will be directed into study of the role of the spin-orbit interaction in current-driven spin torques in magnetic nanowires. Here we will tackle two main areas. The first is the use of spin-orbit coupling to induce spin-flip scattering and hence increase the so-called non-adiabatic (beta term) torque, which is field-like in symmetry. The second is to study the use of spin orbit coupling to introduce a current-dependent effective magnetic field through the Rashba effect. Both of these are outstanding mysteries of condensed matter physics, in that there is no predictive theory to allow materials and devices with appropriate parameters to be designed. They may also hold the key to the substantially more efficient spin-torques that will allow for lower power device operation.

The project builds on existing programmes in the UK and Japan, in particular "Current-Driven Domain Wall Motion in Multilayer Nanowires" EPSRC grant EP/I011668/1 held by Dr Marrows and "Microscopic theory of spin transports", a JSPS Grant-in-Aid for Scientific Research (B) (Grant No. 22340104) held by Prof. Tatara. There is a natural complementarity between the experimental programme in the former and the programme of analytical and first-principles theory in the latter.

For a technological point of view, an important outcome of the results we shall obtain is an understanding of how these effects can be used to increase the efficiency of these torques, which will reduce the power consumption of devices based upon them, one of the main obstacles to their adoption in industry. Most famously, such domain wall devices are proposed in the so-called 'racetrack memory' architecture being developed by IBM. This uses a shift register consisting of a series of domains in a magnetic nanowire to represent the data where the current induced torque is used to shift the data stream past a read/write element. It offers the speed of conventional MRAM with the very low cost per bit of a hard disk, and is touted as a storage class memory. Other devices include novel designs for spintronic memristors or logic gates based on domain wall physics.

Aside from applications in data storage and processing, such domain wall technologies have potential use as scientific instruments, as the DW provides a strong and highly localised magnetic field and field gradient that can be moved around along the nanowire. This can be used to move around small magnetic objects, such as magnetically tagged biomolecules, or cold atoms. High frequency currents produce resonant domain wall motion that can be used to excite localised spins to build nanoscale electron paramagnetic resonance spectrometers.

Nevertheless, a there is a significant drawback to current experimental realisations of the effect: very large current densities are required to bring about domain wall motion. This both dissipates a lot of power through Joule heating of the devices, but also leads to the risk of electromigration and device failure in extended operation. This appears reasonable on the basis of the simplest form of the effect, the adiabatic torque, where the upper limit is set by conservation of angular momentum. After extensive work, it is clear that there limited further scope for improvement in this regard. Nevertheless, it is clear that other torques may be excited by a current: the so-called non-adiabatic torque and the Rashba effective field torque. These rely on the spin-orbit interaction and the details of the microscopic mechanism, and hence any upper limit to their effectiveness, are as yet unknown. Here we will study, through a combination of theory and experiment, these torques, and assess their usefulness towards breaking the impasse that is holding back technological exploitation of this exciting effect.