Domain-wall pinning, nonadiabatic spin-transfer torque, and spin-current polarization in permalloy wires doped with vanadium

Abstract

Using pulsed-current measurements we investigate the domain-wall depinning via spin-transfer torque from pinning potentials in V-doped Ni 80 Fe 20 wires. The domain-wall depinning boundary, showing the variation of threshold current density with longitudinal magnetic field is measured and reproduced using micromagnetic simulations. This method allows us to determine the spin-current polarization and nonadiabaticity parameter in these materials. By increasing the V concentration we show that the nonadiabaticity parameter is increased while the Gilbert damping is unaffected. On the other hand the spin-current polarization is decreased, resulting in larger threshold current densities. The ability to manipulate domain walls in magnetic materials by applying electrical currents 1 is of great interest for spintronics devices, such as magnetic memory devices. 2 The mechanism responsible for the interaction between spinpolarized currents and domain walls is called the spintransfer torque effect.3 The spin torques acting on a domain wall are composed of two terms, namely, the adiabatic 4 and the nonadiabatic 5,6 spin torques, with the latter being proportional to a material-dependent constant ␤ known as the nonadiabaticity parameter. A number of recent studies have investigated the contribution of the nonadiabatic spin torque in various materials and systems. [7][8][9] The relationship between the two phenomenological constants representing dissipative effects ␤ and the Gilbert damping constant ␣ has also come under debate with theoretical arguments given for both the cases of ␤ ␣ ͑Ref. 10͒ and ␤ = ␣.11 We have recently demonstrated a method to measure the spin-current polarization and nonadiabaticity parameter in Ni 80 Fe 20 .12,13 Since the threshold current required to move or depin a domain wall depends on the value of ␤, methods of controlling the nonadiabatic spin torque are of particular importance. According to theoretical formulations ␤ is inversely proportional to the spin-flip lifetime of conduction electrons. 6 In a recent work on PtCo layers, another method of determining the contribution of the nonadiabatic spin torque has been demonstrated. 14 By introducing an AlO x layer to break the symmetry properties and increase the spin-flip rate, an enhanced spin-torque effect compared to the symmetric PtCoPt structure was shown. In this work we investigate the possibility of modifying ␤ by introducing impurities in Ni 80 Fe 20 . The impurities act as scattering centers for the conduction electrons, reducing the spin-flip lifetimes and thus increasing the value of ␤. Moreover, since the damping in Ni 80 Fe 20 is known to be largely unaffected by V doping up to 10% concentration, 15 this experiment also allows us to investigate the relationship between ␤ and ␣.Notched wires were fabricated on Si/ SiO 2 substrates using e-beam lithography. The material is deposited using sputtering of V-doped Py ͑Py= Ni 80 Fe 20 ͒ targets, Py 99 V 1 , Py 97.5 V 2.5 , and Py 90 V 10 respectively, with 20 nm thickness and a 1.5 nm thi...

Spin-transfer torque efficiency measured using a Permalloy nanobridge

“…12 Analysis of the DPC images leads to DW widths ⌬ ϳ 100 nm, yielding ␤ ϳ 0.04, the same as previous estimates by us for Py in other depinning studies, 11,12 and approximately five times the Gilbert damping constant ␣ Ϸ 0.008 in our Py films prior to patterning. 30 We note, however, that this formula for ␤ is only valid when the energy barrier to be overcome is linear in H, and we are not certain that this is the case here. 31 To summarize, we have studied the effect of high current densities on the micromagnetic state of a nanoscale bridge connecting two microscale Py pads by measuring the characteristic AMR signal that indicates switching of the magnetization direction in the bridge.…”

The contribution of Diamond Light Source to the study of strongly correlated electron systems and complex magnetic structures

“…The vanadium doping did not affect the value of α indicating that the microscopic origins of α and β are different. However, even though the non-adiabatic contribution to β was found to increase with vanadium doping, the current density required to depin a domain wall also increased due to concomitant changes in the spin-polarization and saturation magnetization of the nanowire [104]. The group of Kläui (Mainz) has studied domain wall motion in Ni 80 Fe 20 nanowires doped with Ho (figure 8) using PEEM on beamline I06 [105,106].…”

“…Since the threshold current density required to depin and move a domain wall is related to the non-adiabatic part of β there have been several strategies developed to tune this parameter. The group of Marrows (Leeds) have used PEEM combined with XMCD on the Nanoscience beamline (I06) to study domain wall motion in magnetic nanowires with notable advances made using nanowires doped with vanadium [104]. By doping V into Ni 80 Fe 20 nanowires the Marrows group managed to increase the spinflip rate by introducing scattering centres for conduction electrons which in turn increased the non-adiabatic contribution to β.…”

Influence of material defects on current-driven vortex domain wall mobility

“…One way is to measure the depinning field to pull a vortex out of a pinning potential in the presence of a spin-polarized current [9][10][11]. A similar technique consists of looking at the thermal hopping between pinning sites in the presence of a spin-polarized current [12] where different values for β for the same material are estimated depending on the considered magnetic structure: a vortex domain wall or a transverse domain wall.…”