Direct observation of Oersted-field-induced magnetization dynamics in magnetic nanostripes

Abstract

We have used time-resolved x-ray photoemission electron microscopy to investigate the magnetization dynamics induced by nanosecond current pulses in NiFe/Cu/Co nanostripes. A large tilt of the NiFe magnetization in the direction transverse to the stripe is observed during the pulses. We show that this effect cannot be quantitatively understood from the amplitude of the Oersted field and the shape anisotropy. High-frequency oscillations observed at the onset of the pulses are attributed to precessional motion of the NiFe magnetization about the effective field. We discuss the possible origins of the large magnetization tilt and the potential implications of the static and dynamic effects of the Oersted field on current-induced domain-wall motion in such stripes. The possibility of manipulating the magnetic configuration of nanostructures by using electrical currents is a recent, exciting development in spintronics. Electrical currents can affect the magnetization of magnetic nanostructures through both the charge and the spin of the conduction electrons. In recent years it has been shown that spin-transfer torque (STT) 1,2 and Rashba spin-orbit torque effects 3 act on the magnetization, in addition to the classical Oersted magnetic field H Oe . In general, the combination of these effects should be taken into account in the description of the magnetization dynamics during the application of a current pulse. For instance, it was shown that the contribution of the Oersted field and not only STT is needed to explain the magnetization reversal in trilayered pillars induced by a current flowing perpendicular to the plane of the layers.4,5 For in-plane currents, H Oe has been invoked to explain magnetization reversal in mesoscopic NiFe/Cu/Co/Au bars 6 and the resonant depinning of constricted domain walls (DWs) in NiFe/Cu/Co trilayers. 7Several studies on the effects of current pulses on the magnetization of nanostripes, mainly concerning currentinduced domain-wall motion (CIDM), have been based on the observation of the domain structure before and after the application of a current pulse. 8,9 However, the effect of the Oersted field on the magnetization can only be investigated by direct, dynamic observations during the current pulses. This has been achieved in this work, using time-resolved x-ray magnetic circular dichroism combined with photoemission electron microscopy (XMCD-PEEM). Our results show that the current-induced field during nanosecond pulses causes both quasistatic and precessional effects on the NiFe magnetization. These effects may contribute to the increased efficiency of current-induced domain-wall motion observed in such trilayers. 10-12Stacks of Cu(2 nm)/Ni 80 Fe 20 (5 nm)/Cu(5 nm)/Co(5 nm)/ CoO(6 nm) deposited on highly resistive Si(100) (ρ > 300 cm) were patterned in 400-nm-wide zigzag stripes, with angles of 90• and 13-μm-long straight sections, combining electron-beam lithography and ion-beam etching. Contact electrodes made of Ti/Au were subsequently deposited using evaporation and a lift-off...

Current-induced motion and pinning of domain walls in spin-valve nanowires studied by XMCD-PEEM

“…Besides its influence on the quasistatic magnetic configuration in SV nanowires, we also clearly observed that the Oersted field torque affects the magnetization dynamics, by inducing a precession of the magnetization around the effective transverse field. 51 This phenomenon might be responsible for domain nucleation above a threshold current density. This nucleation is most likely to occur at locations where a magnetization tilt in the transverse direction is already present before the current pulse, like at position B in Fig.…”

Current-induced spin-orbit torques in ferromagnetic and antiferromagnetic systems

“…Moreover, typical thicknesses of the FM film were 10 nm, so that the effect of the interfacial torques is strongly reduced. Note that the Oersted field effect was directly observed in the case of a bilayer sample (Uhlír et al, 2011) by time-resolved photoelectron emission microscopy using xray magnetic circular dichroism (XMCD-PEEM), a technique that could be used to measure the FL SOT in situ. Simulations have shown that FL SOT modifies the STT driven dynamics (Seo et al, 2012).…”

Direct Observation of Massless Domain Wall Dynamics in Nanostripes with Perpendicular Magnetic Anisotropy

“…Temporal resolution was obtained by synchronizing the nanosecond current pulses applied to the sample with the x-ray pulses of the synchrotron single-bunch mode, where photon bunches reach the sample at a repetition rate of 846 kHz. The temporal evolution of the domain wall position in the nanostripes was obtained by recording images for different delays between the current and photon pulses[17,23]. If events are reproducible for each current pulse, the temporal resolution of this pump-probe technique is limited only by the duration of the photon pulses (about 50 ps) and the jitter between pump and probe (about 100 ps).…”