Stuart Stephen Papworth Parkin

Memory on the racetrack.

Racetrack memory stores digital data in the magnetic domain walls of nanowires. This technology promises to yield information storage devices with high reliability, performance and capacity.

Role of transparency of platinum-ferromagnet interfaces in determining the intrinsic magnitude of the spin Hall effect

The spin Hall effect induces spin currents in nonmagnetic layers, which can control the magnetization of neighbouring ferromagnets. The transparency of the interface is shown to strongly influence the efficiency of such manipulation.

Giant reversible, facet-dependent, structural changes in a correlated-electron insulator induced by ionic liquid gating

Significance We report a remarkable reversible change in structure of vanadium dioxide films when gated with an ionic liquid. We show that the film expands by more than 3% in the out-of-plane direction when gated to the metallic state. This giant structural change is not only more than 10 times larger than the one at the thermally controlled insulator-to-metal transition measured in the same films, but is in the opposite direction—an expansion rather than a contraction. These results are very important to the field of ionic liquid gating, which has largely ignored the possibility that the high electric fields created on gating at the liquid–oxide interface can result in significant structural changes rather than a purely electrostatic phenomenon. The use of electric fields to alter the conductivity of correlated electron oxides is a powerful tool to probe their fundamental nature as well as for the possibility of developing novel electronic devices. Vanadium dioxide (VO2) is an archetypical correlated electron system that displays a temperature-controlled insulating to metal phase transition near room temperature. Recently, ionic liquid gating, which allows for very high electric fields, has been shown to induce a metallic state to low temperatures in the insulating phase of epitaxially grown thin films of VO2. Surprisingly, the entire film becomes electrically conducting. Here, we show, from in situ synchrotron X-ray diffraction and absorption experiments, that the whole film undergoes giant, structural changes on gating in which the lattice expands by up to ∼3% near room temperature, in contrast to the 10 times smaller (∼0.3%) contraction when the system is thermally metallized. Remarkably, these structural changes are fully reversible on reverse gating. Moreover, we find these structural changes and the concomitant metallization are highly dependent on the VO2 crystal facet, which we relate to the ease of electric-field–induced motion of oxygen ions along chains of edge-sharing VO6 octahedra that exist along the (rutile) c axis.

Basics and prospective of magnetic Heusler compounds

Heusler compounds are a remarkable class of materials with more than 1000 members and a wide range of extraordinary multi-functionalities including halfmetallic high-temperature ferri- and ferromagnets, multi-ferroics, shape memory alloys, and tunable topological insulators with a high potential for spintronics, energy technologies, and magneto-caloric applications. The tunability of this class of materials is exceptional and nearly every functionality can be designed. Co2-Heusler compounds show high spin polarization in tunnel junction devices and spin-resolved photoemission. Manganese-rich Heusler compounds attract much interest in the context of spin transfer torque, spin Hall effect, and rare earth free hard magnets. Most Mn2-Heusler compounds crystallize in the inverse structure and are characterized by antiparallel coupling of magnetic moments on Mn atoms; the ferrimagnetic order and the lack of inversion symmetry lead to the emergence of new properties that are absent in ferromagnetic centrosymmetric Heusler structures, such as non-collinear magnetism, topological Hall effect, and skyrmions. Tetragonal Heusler compounds with large magneto crystalline anisotropy can be easily designed by positioning the Fermi energy at the van Hove singularity in one of the spin channels. Here, we give a comprehensive overview and a prospective on the magnetic properties of Heusler materials.

Magnetic Heusler Compounds

Abstract Heusler compounds are a remarkable class of intermetallic materials with 1:1:1 (often called Half-Heusler) or 2:1:1 composition comprising more than 1500 members. New properties and potential fields of applications emerge constantly; the prediction of topological insulators is the most recent example. Surprisingly, the properties of many Heusler compounds can easily be predicted by the valence electron count or within a rigid band approach. The wide range of the multifunctional properties of Heusler compounds is reflected in extraordinary magnetooptical, magnetoelectronic, and magnetocaloric properties. Co 2 -Heusler compounds are predicted and proven half-metallic ferromagnets showing Slater–Pauling type behavior. The recently discovered Mn 2 -Heusler compounds are another class of half metallic Heusler compounds which even can be designed to be compensated ferrimagnets. Tetragonal Heusler compounds Mn 2 YZ as potential materials for STT applications can be easily designed by positioning the Fermi energy at the van Hove singularity in one of the spin channels.

Interface Fe magnetic moment enhancement in MgO/Fe/MgO trilayers

We model room temperature soft x-ray resonant magnetic reflectivity to determine a 24% increase of the Fe magnetic moment of the 2–3 monolayers next to both MgO interfaces in a MgO(3u2009nm)/Fe(12u2009nm)/MgO(001) heterostructure. This direct measurement of such enhanced interface magnetic moments for buried interfaces confirms theoretical predictions and highlights the importance of considering inhomogeneous in-depth magnetic profile in Fe/MgO based magnetic tunnel junctions.

Ferromagnetic Resonance Study of

Ferromagnetic resonance (FMR) was used to study the magnetic properties of Fe3O4 films to characterize the Verwey transition. Epitaxial, smooth films were deposited by molecular beam epitaxy. FMR spectra were recorded using a 9.49 GHz Brucker EMX system from room temperature to 80 K with a magnetic field applied perpendicular to the film surface. A single resonance feature was observed above the Verwey transition temperature (Tv~ 105 K). However below Tv additional features were observed indicating the presence of complex magnetic interactions below Tv.

{\hbox {Fe}}_{3}{\hbox {O}}_{4}

The authors discuss the dynamics of domain walls in thin permalloy magnetic nanowires through the anisotropic magnetoresistance effect. Upon the introduction of a single DW into the magnetic nanowire, only one resonant mode was observed. Two resonant modes were found in coupled DWs.

Thin Film

Ferromagnetic resonance (FMR) is used to investigate the magnetic anisotropy of a 25.5 nm thin Fe₃O₄ film above and below its Verwey transition temperature (TV). Epitaxial smooth film of Fe₃O₄ is deposited by oxygen-assisted molecular beam epitaxy. FMR spectra are recorded using a 9.49 GHz Brucker EMX system from 80 K to room temperature with a magnetic field applied along [100] direction. The resonance field (Hr) and linewidth (AH) are extracted by fitting the FMR line with a Lorentzian function. Temperature dependence of H_r and ΔH shows a transition at T_V. Furthermore, the data of in-plane angular dependence of H_r at 300 and 83 K indicated a change in symmetry from fourfold to twofold below TV, consistent with a transformation of Fe₃O₄ from cubic (fourfold) to monoclinic (twofold) structure with its easy axis switching from [110] to [100]. Complex symmetry pattern is found in angular dependence of ΔH below TV, which is attributed to the in-plane twinning of Fe₃O₄ lattice.