Time-resolved domain wall motion in novel spin transfer torque devices

Abstract

In the past decade, the interaction of spin polarised electrons with ferromagnetic domains and domain walls has stimulated an immense volume of research work in the field of spintronics worldwide. Perpendicular magnetic anisotropy (PMA) in thin film magnetic materials has been widely exploited as a building block for realising spintronic devices. Co/Pt multilayer thin films with PMA have been extensively investigated for their potential applications in magnetic recording media and MRAM-like devices. Much work has been done to develop ways to control the magnetic properties of these films. Ga+ focused ion beam (FIB) irradiation is a well-established technique for controlling the magnetic properties of systems with perpendicular magnetic anisotropy (PMA). Here we have systematically investigated the coercivity, magnetic anisotropy and surface roughness of Ta(4 nm)/Pt(3 nm)/Co(x nm)/Pt(1.8 nm) multilayer films as a function of Ga+ FIB irradiation dose. The influence of the thickness of Co and Pt layers on the coercivity and switching behaviour was systematically investigated and the conditions established for realising structures with medium coercivity (~100 Oe) and sharp switching that are well-suited for current-driven domain wall motion studies. An unexpected increase in the coercivity at very low Ga+ ion doses followed by a reduction at higher irradiation levels is observed. This correlates with an increase in surface roughness which we tentatively attribute to the ion induced formation of highly strained nanoscale regions of ordered CoPt alloy at the Co/Pt interface. The possibility to both increase and decrease coercivity with very low dose Ga+ ion irradiation could have important applications in the design of novel spintronic devices. In addition, annealing at relatively low temperatures (< 200 C) is shown to drastically increase the magnetic anisotropy and switching field for all Co thicknesses whereby the ratio of increase in coercivity and magnetic anisotropy is higher in thicker Co films. Optimised Co/Pt multilayer films were lithographically patterned into nanowire devices for time-resolved Extraordinary Hall Effect (EHE) measurements. The devices were based on 50 ohms coplanar waveguides incorporating single Hall cross structures. The coercivity of the region surrounding the Co/Pt Hall crosses was reduced by local Ga+ FIB irradiation, allowing the controlled nucleation of domain walls (DWs) at the edges of these regions by application of an appropriate field sequence. Domain imaging was carried out to study the evolution of artificial domains and DW dynamics under the application of magnetic fields and current pulses. DW creep measurements were carried out to study the effect of bias field on DW motion. Using pulsed currents, spin transfer torque-driven domain wall motion was demonstrated in these artificial domain structures and the DW position and velocity tracked using time-resolved EHE measurements.

Date of Award	20 Apr 2016
Original language	English
Awarding Institution	University of Bath
Supervisor	Simon Bending (Supervisor)