SPIN

We report a systematic study of the structural and quasi-static magnetic properties, as well as of the dynamic magnetic response through MI effect, in Co₂FeAl and MgO//Co₂FeAl single layers and a MgO//Co₂FeAl/Ag/Co₂FeAl trilayered film, all grown onto an amorphous substrate. We present a new route to induce the crystalline structure in the Co₂FeAl alloy and verify that changes in the structural phase of this material leads to remarkable modifications of the magnetic anisotropy and, consequently, dynamic magnetic behavior. Considering the electrical and magnetic properties of the Co₂FeAl, our results open new possibilities for technological applications of this full-Heusler alloy in rigid and flexible spintronic devices.

The temperature-dependent magnetic properties are investigated in amorphous (Fe₂Co)${}_{(1-x)}$Yb_x ($x=0.54$, 0.64) thin films with in-plane uniaxial anisotropy. The decreases of saturation magnetization and easy axis coercivity with increasing temperature were observed and quite well explained by the Bloch’s law of $T^{3∕2}$ dependence based on three-dimensional (3D) spin wave excitations and the thermally activated domain wall motion model of $T^{1∕2}$ dependence, respectively. The decrease of in-plane uniaxial anisotropy constant is also observed and can be quite well fitted at temperature below 300K. The magnetic damping constant, which was deduced from the angular dependent ferromagnetic resonance spectra, shows a minima over the temperature range 100–435K, just like the previous results from 3D-transition metals based on Kamberský’s torque-correlation model. However, a positive correlation between damping and the in-plane uniaxial anisotropy constant was obtained with a clear deviation from the linear relationship. This deviation indicates that temperature-dependent damping and anisotropy may have different origins instead of the common source of the temperature-dependent spin-orbital coupling strength.

The ultrafast demagnetization processes (UDP) in Ta ($t$ nm)/TbFeCo (20 nm) films have been studied using the time-resolved magneto-optical Kerr effect (TRMOKE). With a fixed pump fluence of 2 mJ/cm², for the sample without a Ta underlayer ($t=0$nm), we observed the UDP showing a two-step decay behavior, with a relatively longer decay time (${\tau }_2)$ around 3.0 ps in the second step due to the equilibrium of spin-lattice relaxation following the 4$f$ occupation. As a 10nm Ta layer is deposited, the two-step demagnetization still exists while ${\tau }_2$ decreases to $\sim$1.9ps. Nevertheless, the second-step decay (${\tau }_2=0ps$) disappears as the Ta layer thickness is increased up to 20 nm, only the first-step UDP occurs within 500 fs, followed by a fast recovery process. The rapid magnetization recovery rate strongly depends on the pump fluence. We infer that the Ta layer provides conduction electrons involving the thermal equilibrium of spin-lattice interaction and serves as heat bath taking away energy from spins of TbFeCo alloy film in UDP.

Current-induced domain wall motion (CIDWM) in perpendicularly magnetized materials exhibits large potential in spintronic device applications. The Dzyaloshinskii domain walls (DWs) are nucleated in ultrathin ferromagnetic/heavy-metal bilayers with high perpendicular magnetocrystalline anisotropy (PMA) in the presence of interfacial Dzyaloshinskii–Moriya interaction (DMI). Here, we investigate the effect of magnetic fields on Dzyaloshinskii DWs driven by spin Hall effect (SHE) by means of micromagnetic simulations. We find that magnetic fields can modify the dynamics of Dzyaloshinskii DW. When applying out-of-plane magnetic fields, the velocity of Dzyaloshinskii DWs increases when the field-driven and current-driven DW motion are in same direction, while it decreases with opposite direction. In the case of in-plane longitudinal magnetic fields, Dzyaloshinskii DW velocity increases when the direction of the magnetic field and Dzyaloshinskii DW propagation direction are same, and it decreases when applying opposite in-plane magnetic fields. These manifestations may offer a new method for manipulating Dzyaloshinskii DWs and promise applications in DW-based nanodevices.

Magnetic Proximity Effect (MPE) at the interface of Y₃Fe₅O${}_{12}$ (YIG)$\setminus$Pd is investigated using X-ray Magnetic Circular Dichroism (XMCD) at the Pd-edges. No evidence of MPE is found even for Pd film thicknesses down to 0.5nm. The same bilayers have been thereafter used for inverse spin Hall Effect (ISHE) measurements using the spin-pumping technique. A large signal can be observed at the ferromagnetic resonance. The angular dependence of the Pd dc resistance has been measured as a function of the YIG magnetization direction. It shows the expected dependences for the spin Hall magnetoresistance (SMR). Our methodology allows to state that ISHE and SMR are intrinsic phenomena whose underlying physics is uncorrelated to MPE.

Magnetic skyrmions are topological structures which can be used to store information as data bits in metallic racetrack memories. Their good properties, such as their stability, small size and low currents needed to drive them make them better candidates than traditional magnetic domain walls for the building of the next generation data storage. A skyrmion racetrack memory has been suggested, with the binary data encoded in the distance between skyrmions when the racetrack is a single lane. Here, we propose a new skyrmion-based two-lane racetrack structure separated by a high-$K$ (high magnetocrystalline anisotropy) middle lane, which confines the skyrmions in their respective lanes. This design gives a new data presentation for the skyrmions on the racetrack. Phase diagrams for the skyrmion motion on the proposed racetrack as functions of the current density, middle lane anisotropy, middle lane width and DMI constant have been calculated and given, demonstrating that skyrmions can be driven in different lanes of the racetrack. This design offers the possibility of building an ultrafast and energy-efficient skyrmion transport device.

A bottom-up approach for the fabrication of an assembly of electrodeposited nanowires has been combined to single nanowire electrical connection techniques to investigate the spin-transfer-torque and microwave emission of specially designed nanowires containing Co/Cu/Co pseudo spin-valves (SVs). Porous alumina templates are used for the growth by electrodeposition of metallic in-series connected SVs. Under specific magnetic field and injected current conditions, emission of microwave current is detected with frequency in the GHz range and linewidth as low as 1.8MHz. Microwave signals have been obtained even at zero magnetic field and high frequency versus magnetic field tunability was demonstrated. Our findings are in good agreement with micromagnetic simulations. In addition, it appears that in our particular geometry, the microwave emission is generated by the vortex gyrotropic motion which occurs in, at least, one of the two magnetic layers of our SV structures.

In a conventional field-driven domain wall (DM) motion, the external field is applied along the magnetization of the magnetic domain which is expanded during the process. Recently, a novel approach is proposed to utilize a perpendicular field pulse rather than an in-plane one to drive DWs in magnetic nanostripes. In this paper, we apply this idea to transverse DWs in thin cylindrical nanowires. In the case of driving a single DW, this approach displays apparent advantages. Owing to the zero DW mass and inertia, a net DW displacement can be attained by applying a field pulse which is initially vertical to the DW magnetization. Therefore, no artificial pinning sites are required as in flat strips. Furthermore, we propose to rotate the external field with a particular frequency to maintain a right angle between the field and the DW magnetization. In this way, a continuous DW motion with maximum speed can be realized. The feasibility of this proposed approach is demonstrated by micromagnetic simulations and an analytical calculation.