The field of spin electronics (spintronics) was initiated by the discovery of giant magnetoresistance (GMR) for which Fert[1] and Grunberg[2] were awarded the 2007 Nobel Prize for Physics. GMR arises from differential scattering of the majority and m
inority spin electrons by a ferromagnet (FM) so that the resistance when the FM layers separated by non-magnetic (NM) spacers are aligned by an applied field is different to when they are antiparallel. In 1996 Slonczewski[3] and Berger[4] predicted that a large spin-polarised current could transfer spin-angular momentum and so exert a spin transfer torque (STT) sufficient to switch thin FM layers between stable magnetisation states[5] and, for even higher current densities, drive continuous precession which emits microwaves[6]. Thus, while GMR is a purely passive phenomenon which ultimately depends on the intrinsic band structure of the FM, STT adds an active element to spintronics by which the direction of the magnetisation may be manipulated. Here we show that highly non-equilibrium spin injection can modify the scattering asymmetry and, by extension, the intrinsic magnetism of a FM. This phenomenon is completely different to STT and provides a third ingredient which should further expand the range of opportunities for the application of spintronics.
Using a three-dimensional focused-ion beam lithography process we have fabricated nanopillar devices which show spin transfer torque switching at zero external magnetic fields. Under a small in-plane external bias field, a field-dependent peak in the
differential resistance versus current is observed similar to that reported in asymmetrical nanopillar devices. This is interpreted as evidence for the low-field excitation of spin waves which in our case is attributed to a spin-scattering asymmetry enhanced by the IrMn exchange bias layer coupled to a relatively thin CoFe fixed layer.
We present the results of experimental investigations of magnetic switching and magnetotransport in a new generation of magnetic devices containing artificially patterned domains. Our devices are realised by locally reducing the coercive field of a p
erpendicularly magnetised Pt (3.5 nm)/Co (0.5 nm)/Pt (1.6 nm) trilayer structure using a gallium focused ion beam (FIB). Artificial domain walls are created at the interfaces between dosed and undosed regions when an external magnetic field switches the former but not the latter. We have exploited this property to create stripe-like domains with widths down to sub-micron lengthscales, separated by undosed regions. Using the extraordinary Hall effect to monitor the local magnetisation we have investigated the reversal dynamics of these artificial domains by measuring major and minor hysteresis loops. The coercive field of regions irradiated with identical doses systematically increases as their size decreases. In the lower branch of minor loops, reversal is seen to occur via a few large Barkhausen events. Preliminary measurements of transport across domain walls reveal a positive domain wall resistance, that does not change sign from 4.2 K to 300 K.
