Spin-polarised currents and magnetic domain walls

Electrical currents flowing in ferromagnetic materials are spin-polarised as a result of the spin-dependent band structure. When the spatial direction of the polarisation changes, in a domain structure, the electrons must somehow accommodate the necessary change in direction of their spin angular momentum as they pass through the wall. Reflection, scattering, or a transfer of angular momentum onto the lattice are all possible outcomes, depending on the circumstances. This gives rise to a variety of different physical effects, most importantly a contribution to the electrical resistance caused by the wall, and a motion of the wall driven by the spin-polarised current

Effects of impurity atoms on sputtered GMR multilayers

We have investigated the effects of residual gas impurity atoms on interlayer exchange coupling and giant magnetoresistance (GMR) in Co(9Ä)/Cu(9Ä) multilayers. Structural analysis was performed by Co(59) NMR. We deposited sub-monolayer quantities of residual gases at different points in the Co/Cu bilayer; the interfaces, or the middle of the Cu spacers or CO magnetic layers. Impurities at the interface lower the GMR and increase remenant fraction and saturation field. We are able to model these results phenomenologically by adding biquadratic coupling. Impurities in the bulk of the Cu layers lower GMR still further, and such samples are well described by models containing almost 100% biquadratic coupling. We have demonstrated that the ttansport parameters in our samples are largely unaffected by small quantities of impurities, but that the interlayer coupling is extremely sensitive to them, particularly in the bulk of the Cu spacer layers

Long-ranged magnetic proximity effects in noble metal-doped cobalt probed with spin-dependent tunnelling

We inserted non-magnetic layers of Au and Cu into sputtered AlO$_{x}$-based magnetic tunnel junctions and Meservey–Tedrow junctions in order to study their effect on tunnelling magnetoresistance (TMR) and spin polarization (TSP). When either Au or Cu are inserted into a Co/AlO$_{x}$ interface, we find that TMR and TSP remain finite and measurable for thicknesses up to several nanometres. High-resolution transmission electron microscopy shows that the Cu and Au interface layers are fully continuous when their thickness exceeds ~3nm, implying that spin-polarized carriers penetrate the interface noble metal to distances exceeding this value. A power law model based on exchange scattering is found to fit the data better than a phenomenological exponential decay. The discrepancy between these length scales and the much shorter ones reported from x-ray magnetic circular dichroism studies of magnetic proximitization is ascribed to the fact that our tunnelling transport measurements selectively probe s-like electrons close to the Fermi level. When a 0.1 nm thick Cu or Au layer is inserted within the Co, we find that the suppression of TMR and TSP is restored on a length scale of ≤ 1nm, indicating that this is a sufficient quantity of Co to form a fully spin-polarized band structure at the interface with the tunnel barrier

Determination of the copper layer thickness in spin valves by grazing incidence x-ray fluorescence

We show that at the standard laboratory wavelength of CuKα the scattering factors of Cu and Ni(-0.8)Fe(-0.2) are identical, thereby making it impossible to distinguish the boundary of the Cu spacer layer in a Cdpermalloy spin valve structure from grazing incidence x-ray reflectivity curves. Use of grazing incidence fluorescence, in conjunction with x-ray reflectivity provides sufficient information to control the Cu layer thickness. We demonstrate the technique on two spin valves with Cu spacer layers differing in thickness by a factor of 2.5

Time dependence studies on giant magnetoresistive Co/Cu multilayers

Time dependence studies consisting of applying current steps at fixed applied fields have been carried out on bilinear and biquadratic giant magnetoresistive (GMR) Co/Cu multilayers in a temperature controlled environment. It has been shown that the voltage responses to current steps of these aged multilayers are greater in magnitude before field cycling compared to those made after field cycling. Normalized voltage measurements for some samples suggest a magnetic viscosity effect due to a current step at zero-field is present and before field cycling. The effect is reduced after field cycling. This behavior suggests that the effect being seen is purely magnetic in origin, as only the field is being varied. A ln( ) type function has been curve fitted to the zero field voltage response to a current step before field cycling. Voltage measurements made on the Co/Cu films at different field values show that as the applied fields are increased the voltage response has a reduced ln(t) character

Spin-polarized tunneling with Au impurity layers

We have inserted nonmagnetic impurity layers of Au into sputtered AlOx-based magnetic tunnel junctions (F/I/F) and Meservey–Tedrow junctions (S/I/F) in order to study their effect on the tunneling magnetoresistance (TMR) and spin polarization (TSP). Both room temperature TMR and the TSP at 250 mK decay exponentially as an interfacial Au layer is introduced between the barrier and one Co electrode, with 1/e decay lengths λTMR=11±3 Å and λTSP=14±2 Å. We also inserted a 1 Å thick Au layer at a variable distance from the barrier/Co interface and find that both the TMR and TSP recover to the undoped value with the shorter exponential length scales of λTMR=7±4 Å and λTSP=6±2 Å

Room temperature magnetic stabilization of buried cobalt nanoclusters within a ferromagnetic matrix studied by soft x-ray magnetic circular dichroism

Single dusting layers of size-selected Co nanoclusters (NCs) of sizes ranging from 1.5–5.5 nm have been deposited by a gas-phase aggregation method in ultrahigh vacuum, and embedded within a NiFe matrix. Magnetic hysteresis loops have been obtained using soft x-ray magnetic circular dichroism, which shows that these Co NCs embedded in NiFe exhibit room temperature ferromagnetism with identical coercivity to the surrounding NiFe film. The strong local exchange field at the interface between NiFe and Co NCs, combined with the magnetic anisotropy of the NiFe film, allows stabilization of NC ferromagnetism which persists to room temperature

The spin polarization of Mn atoms in paramagnetic CuMn alloys induced by a Co layer

Copyright © 2009 American Institute of PhysicsUsing the surface, interface, and element specificity of x-ray resonant magnetic scattering in combination with x-ray magnetic circular dichroism, we have spatially resolved the polarization, and hence the spin accumulation in Mn high susceptibility material in close proximity to a ferromagnetic layer. The magnetic polarization of Mn and Cu 3d electrons in paramagnetic CuMn layers is detected in a Co/Cu x /CuMn structure for varying copper layer thicknesses x . The size of the Mn and Cu L2–3-edge dichroism shows a decrease in the polarization for increasing copper thickness indicating the dominant interfacial nature of the Cu and Mn spin polarization. The Mn polarization appears to be much higher than that of Cu

Spin-orbit interaction in InAs/GaSb heterostructures quantified by weak antilocalization

We study the spin-orbit interaction (SOI) in InAs/GaSb and InAs quantum wells. We show through temperature- and gate-dependent magnetotransport measurements of weak antilocalization that the dominant spin-orbit relaxation mechanism in our low-mobility heterostructures is Elliott-Yafet and not Dyakonov-Perel in the form of the Rashba or Dresselhaus SOI as previously suggested. We compare our findings with recent work on this material system and show that the SOI length lies within the same range. The SOI length may be controlled using an electrostatic gate, opening up prospects for developing spintronic applications

Phase coexistence and transitions between antiferromagnetic and ferromagnetic states in a synthetic antiferromagnet

In synthetic antiferromagnets (SAFs), antiferromagnetic (AFM) order and synthesis using conventional sputtering techniques is combined to produce systems that are advantageous for spintronics applications. Here we present the preparation and study of SAF multilayers possessing both perpendicular magnetic anisotropy and the Dzyaloshinskii-Moriya interaction. The multilayers have an antiferromagnetically aligned ground state but can be forced into a full ferromagnetic (FM) alignment by applying an out-of-plane field ∼100mT. We study the spin textures in these multilayers in their ground state as well as around the transition point between the AFM and FM states at fields ∼40 mT by imaging the spin textures using complementary methods: photoemission electron, magnetic force, and Lorentz transmission electron microscopies. The transformation into a FM state by field proceeds by a nucleation and growth process, where skyrmionic nuclei form and then broaden into regions containing a ferromagnetically aligned labyrinth pattern that eventually occupies the whole film. Remarkably, this process occurs without any significant change in the net magnetic moment of the multilayer. The mix of antiferromagnetically and ferromagnetically aligned regions on the micron scale in the middle of this transition is reminiscent of a first-order phase transition that exhibits phase coexistence. These results are important for guiding the design of spintronic devices whose operation is based on spin textures in perpendicularly magnetized SAFs