Jun Wu

Observation of the nonlocal spin-orbital effective field

The spin-orbital interaction in heavy nonmagnetic metal/ferromagnetic metal bilayer systems has attracted great attention and exhibited promising potentials in magnetic logic devices, where the magnetization direction is controlled by passing an electric current. It is found that the spin-orbital interaction induces both an effective field and torque on the magnetization, which have been attributed to two different origins: the Rashba effect and the spin Hall effect. It requires quantitative analysis to distinguish the two mechanisms. Here we show sensitive spin-orbital effective field measurements up to 10 nm thick ferromagnetic layer and find the effective field rapidly diminishes with the increase of the ferromagnetic layer thickness. We further show that this effective field persists even with the insertion of a copper spacer. The nonlocal measurement suggests that the spin-orbital effective field does not rely on the heavy normal metal/ferromagnetic metal interface.

Quantifying interface and bulk contributions to spin–orbit torque in magnetic bilayers

Spin-orbit interaction-driven phenomena such as the spin Hall and Rashba effect in ferromagnetic/heavy metal bilayers enables efficient manipulation of the magnetization via electric current. However, the underlying mechanism for the spin-orbit interaction-driven phenomena remains unsettled. Here we develop a sensitive spin-orbit torque magnetometer based on the magneto-optic Kerr effect that measures the spin-orbit torque vectors for cobalt iron boron/platinum bilayers over a wide thickness range. We observe that the Slonczewski-like torque inversely scales with the ferromagnet thickness, and the field-like torque has a threshold effect that appears only when the ferromagnetic layer is thinner than 1u2009nm. Through a thickness-dependence study with an additional copper insertion layer at the interface, we conclude that the dominant mechanism for the spin-orbit interaction-driven phenomena in this system is the spin Hall effect. However, there is also a distinct interface contribution, which may be because of the Rashba effect.

Designing and Tuning Magnetic Resonance with Exchange Interaction

Exchange interaction at the interface between magnetic layers exhibits significant contribution to the magnetic resonance frequency. The in situ tuning of the resonance frequency, as large as 10 GHz, is demonstrated in a spintronics microwave device through manipulating the interface exchange interaction.

Spin wave resonance detection using magnetic tunnel junction structure

We have demonstrated that spin wave resonance in a permalloy microstrip can be detected by an electrical method based on magnetic tunnel junction structures. The detection method promises high spatial resolution and sensitivity. Both even and odd spin wave resonance modes can be clearly observed in a permalloy microstrip. The spin wave induced voltage is proportional to the input microwave power at each resonance mode. Data analysis using the model of quantized dipole-exchange spin wave resonance suggests the edge pinning of spin wave sensitively depends on the order of the spin wave mode, as well as on the excitation frequency for modes of the higher order.

Electrical detection of nonlinear ferromagnetic resonance in single elliptical permalloy thin film using a magnetic tunnel junction

A quantitative method to detect ferromagnetic resonance using magnetic tunnel junction structure has been developed. Experimental results reveal three distinct regions for single elliptical permalloy film of micrometer lateral size. Above the spin wave instability threshold, the experimental results show a linear response of the longitudinal magnetization component to the microwave field amplitude over a large range rather than a lock-up phenomenon appeared in macroscopic permalloy films and then a phase limiting behavior. The linear behavior can be described by the theoretical model describing subsidiary resonance.

Large spin Hall angle in vanadium film

We report a large spin Hall angle observed in vanadium films sputter-grown at room temperature, which have small grain size and consist of a mixture of body centered tetragonal (bct) and body centered cubic (bcc) structures. The spin Hall angle is as large as θVu2009=u2009−0.071u2009±u20090.003, comparable to that of platinum, θPtu2009=u20090.076u2009±u20090.007, and is much larger than that of bcc V film grown at 400u2009°C, θV_bccu2009=u2009−0.012u2009±u20090.002. Similar to β-tantalum and β-tungsten, the sputter-grown V films also have a high resistivity of more than 200u2009μΩ∙cm. Surprisingly, the spin diffusion length is still long at 16.3u2009nm. This finding not only indicates that specific crystalline structure can lead to a large spin Hall effect but also suggests 3d light metals should not be ruled out in the search for materials with large spin Hall angle.

Spin Hall Angle and Spin Diffusion Length in Au–Cu Alloy

We experimentally measured the fieldlike spin-orbit torque (SOT) by the second-order planar Hall effect, as well as the dampinglike SOT by the polar magnetooptic Kerr effect in NiFe(1.5 nm)/AuCu(x nm) bilayers with 2 nm ≤ x ≤ 8 nm. We found that the spin diffusion length in the Au-Cu alloy is ~5 nm, which is shorter than that of pure Au and Cu. When the thickness of the AuCu layer is above the spin diffusion length, the spin Hall angle of the Au-Cu alloy is ~1.3 ± 0.2%, which is much larger than that of pure Cu and Au. The results imply that large spin Hall angle materials can be explored in the nonmagnetic alloys of constituent elements having weak or negligible spin-orbit coupling.