Current-driven domain wall motion not only offers fascinating basic physics but offers the prospect of storage-class non-volatile solid state memories. The fruitfly sample used in experimental studies to date is the lithographically patterned permalloy nanowire. Here we describe our recent work on such structures, with patterned pinning centres, notches, halfway along their length. Our key techniques have been high-resolution magnetic imaging by XMCD PEEM and SEMPA, as well as magnetotransport measurements. We have measured the spin polarisation of the current (P~0.4) and the degree of non-adiabaticity of the spin torque (beta~0.04) in Permalloy by studying the depinning boundary of the DWs pinned in triangular notches. We have demonstrated that the depinning current at zero field may be engineered by controlling the half-angle of the triangular notch, which varies the restoring force the pinning centre applies to the wall as it attempts to leave. We have also excited pinned walls with RF currents: in a triangular notch the behaviour is highly anharmonic, whilst a well-defined eigenfrequency is found for parabolic notches, which provide an almost harmonic pinning potential.
This work was supported by the ESF (SpinCurrent consortium), the EPSRC (Spin@RT Consortium), and Diamond Light Source.
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
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