C. H. Marrows

Magnetoresistance of a two-dimensional electron gas due to a single magnetic barrier and its use for nanomagnetometry

We investigate the longitudinal resistance of a semiconductor near-surface two-dimensional electron gas (2DEG) subjected to a magnetic barrier induced by the stray field from a single sub-micron ferromagnetic line on the surface of the device. The amplitude of the magnetic barrier is controlled by the application of an external magnetic field in the plane of the 2DEG. We show that this type of magnetoresistance can be used to deduce properties of the ferromagnetic line, so that our hybrid ferromagnet-semiconductor structure acts as a nanomagnetometer.

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. Historical and recent research on these topics is reviewed. Contents PAGE 1. Introduction 586 2. Spin-polarised current 587  2.1. Tunnelling current spin polarisation 589  2.2. Ballistic current spin polarisation 592  2.3. Diffusive current spin polarisation 593 3. Magnetic Domain Walls 598  3.1. Basics of domain walls 598  3.1.1. Domain wall thickness and energy 601  3.1.2. Micromagnetic calculations 603  3.1.3. Tailoring domain structures for measurements 605  3.2. Domain walls in nanostructures 609  3.3. Domain wall dynamics 610 4. Domain Wall Resistance 613  4.1. Early results 613  4.2. Theory 615  4.3. Recent experimental results 626  4.3.1. Homogeneous materials 626  4.3.2. Heterostructures 631  4.3.3. Mesoscopic devices 635  4.4. Huge domain wall MR in nanoconstrictions? 644  4.4.1. First results 645  4.4.2. Theoretical interpretation 650  4.4.3. Experimental exploration 656 5. Current-induced Domain Wall Motion 665  5.1. Experimental results 666  5.2. Theory 678 6. Conclusion 690 Acknowledgements 692 References 692

Origin of in-plane uniaxial magnetic anisotropy in CoFeB amorphous ferromagnetic thin films

Describing the origin of uniaxial magnetic anisotropy (UMA) is generally problematic in systems other than single crystals. We demonstrate an in-plane UMA in amorphous CoFeB films on GaAs(001) which has the expected symmetry of the interface anisotropy in ferromagnetic films on GaAs(001), but strength which is independent of, rather than in inverse proportion to, the film thickness. We show that this volume UMA is consistent with a bond-orientational anisotropy, which propagates the interface-induced UMA through the thickness of the amorphous film. It is explained how, in general, this mechanism may describe the origin of in-plane UMAs in amorphous ferromagnetic films.

Temperature dependence of large positive magnetoresistance in hybrid ferromagnetic/semiconductor devices

We investigate a new type of magnetoresistance(MR) in which the resistivity of a near-surface two-dimensional electron gas is controlled by the magnetization of a submicron ferromagneticgrating defined on the surface of the device. We observe an increase in resistance of up to ∼1500% at a temperature of 4 K and ∼1% at 300 K. The magnitude and temperature dependence of the MR are well accounted for by a semiclassical theory. Optimization of device parameters is expected to increase considerably the magnitude of the room temperature MR.

Quantification of magnetic domain disorder and correlations in antiferromagnetically coupled multilayers by neutron reflectometry

The in-plane correlation lengths and magnetic disorder of magnetic domains in a transition metal multilayer have been studied using neutron scattering techniques. A new theoretical framework is presented connecting the observed scattering to the in-plane correlation length and the dispersion of the local magnetization vector about the mean macroscopic direction. The results unambiguously show the highly correlated nature of the antiferromagnetically coupled domain structure vertically throughout the multilayer. We are easily able to relate the neutron determined magnetic dispersion and domain correlations to magnetization and magnetotransport experiments.

Disorder Strength and Field-Driven Ground State Domain Formation in Artificial Spin Ice: Experiment, Simulation, and Theory

Quenched disorder affects how nonequilibrium systems respond to driving. In the context of artificial spin ice, an athermal system comprised of geometrically frustrated classical Ising spins with a twofold degenerate ground state, we give experimental and numerical evidence of how such disorder washes out edge effects and provide an estimate of disorder strength in the experimental system. We prove analytically that a sequence of applied fields with fixed amplitude is unable to drive the system to its ground state from a saturated state. These results should be relevant for other systems where disorder does not change the nature of the ground state.

Spin-orbit strength driven crossover between intrinsic and extrinsic mechanisms of the anomalous hall effect in the epitaxial L1{0}-ordered ferromagnets FePd and FePt.

We determine the composition of intrinsic as well as extrinsic contributions to the anomalous Hall effect (AHE) in the isoelectronic L1_{0} FePd and FePt alloys. We show that the AHE signal in our 30 nm thick epitaxially deposited films of FePd is mainly due to an extrinsic side jump, while in the epitaxial FePt films of the same thickness and degree of order the intrinsic contribution is dominating over the extrinsic mechanisms of the AHE. We relate this crossover to the difference in spin-orbit strength of Pt and Pd atoms and suggest that this phenomenon can be used for tuning the origins of the AHE in complex alloys.

Magnetic microscopy and topological stability of homochiral Néel domain walls in a Pt/Co/AlOx trilayer.

The microscopic magnetization variation in magnetic domain walls in thin films is a crucial property when considering the torques driving their dynamic behaviour. For films possessing out-of-plane anisotropy normally the presence of Néel walls is not favoured due to magnetostatic considerations. However, they have the right structure to respond to the torques exerted by the spin Hall effect. Their existence is an indicator of the interfacial Dzyaloshinskii–Moriya interaction (DMI). Here we present direct imaging of Néel domain walls with a fixed chirality in device-ready Pt/Co/AlOx films using Lorentz transmission electron and Kerr microscopies. It is shown that any independently nucleated pair of walls in our films form winding pairs when they meet that are difficult to annihilate with field, confirming that they all possess the same topological winding number. The latter is enforced by the DMI. The field required to annihilate these winding wall pairs is used to give a measure of the DMI strength. Such domain walls, which are robust against collisions with each other, are good candidates for dense data storage.Next-generation concepts for solid-state memory devices are based on current-driven domain wall propagation, where the wall velocity governs the device performance. It has been shown that the domain wall velocity and the direction of travel is controlled by the nature of the wall and its chirality. This chirality is attributed to effects emerging from the lack of inversion symmetry at the interface between a ferromagnet and a heavy metal, leading to an interfacial DzyaloshinskiiMoriya interaction that can control the shape and chirality of the magnetic domain wall. Here we present direct imaging of domain walls in Pt/Co/AlOx films using Lorentz transmission electron microscopy, demonstrating the presence of homochiral, and thus topologically protected, Néel walls. Such domain walls are good candidates for dense data storage, bringing the bit size down close to the limit of the domain wall width.

Hall-effect characterization of the metamagnetic transition in FeRh

The antiferromagnetic ground state and the metamagnetic transition to the ferromagnetic state of CsCl-ordered FeRh epilayers have been characterized using Hall and magnetoresistance measurements. On cooling into the ground state, the metamagnetic transition is found to coincide with a suppression in carrier density of at least an order of magnitude below the typical metallic level that is shown by the ferromagnetic state. The carrier density in the antiferromagnetic state is limited by intrinsic doping from Fe/Rh substitution defects, with approximately two electrons per pair of atoms swapped, showing that the decrease in carrier density could be even larger in more perfect specimens. The surprisingly large change in carrier density is a clear quantitative indication of the extent of change at the Fermi surface at the metamagnetic transition, confirming that entropy release at the transition is of electronic origin, and hence that an electronic transition underlies the metamagnetic transition. Regarding the nature of this electronic transition, it is suggested that an orbital selective Mott transition, selective to strongly-correlated Fe 3d electrons, could cause the reduction in the Fermi surface and change in sign of the magnetic exchange from FM to AF on cooling.

Role of B diffusion in the interfacial Dzyaloshinskii-Moriya interaction in Ta/Co20 F e60 B20/MgO nanowires

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