Angeline Klemm Smith

Spin Hall switching of the magnetization in Ta/TbFeCo structures with bulk perpendicular anisotropy

Spin-orbit torques are studied in Ta/TbFeCo/MgO patterned structures, where the ferrimagnetic material TbFeCo provides a strong bulk perpendicular magnetic anisotropy (bulk-PMA) independent of the interfaces. The current-induced magnetization switching in TbFeCo is investigated in the presence of a perpendicular, longitudinal, or transverse field. An unexpected partial-switching phenomenon is observed in the presence of a transverse field unique to our bulk-PMA material. It is found that the anti-damping torque related with spin Hall effect is very strong, and a spin Hall angle is determined to be 0.12. The field-like torque related with Rashba effect is unobservable, suggesting that the interface play a significant role in Rashba-like torque.

A fast magnetoelectric device based on current-driven domain wall propagation

Several emerging spintronic devices have recently been proposed, performing computation by (a) generating spin currents based on input magnet states to switch an output magnet state using Spin-Transfer Torque (STT) [1,2], (b) using multiple nanopillars to drive a domain wall (DW) that switches an output nanopillar using STT [7], and (c) using magnetoelectric (ME) switching at the input, combined with DW automotion, to switch an output state [3]. All of these devices have delays of several nanoseconds. The energy for (a) and (b) is in the range of femtoJoules, while the ME mechanism in (c) facilitates greater energy-efficiency, in the aJ range. These numbers fall some distance away from CMOS, where gate delays and switching energies are in the range of picoseconds (ps) and attoJoules (aJ), respectively.

Planar Hall effect based characterization of spin orbital torques in Ta/CoFeB/MgO structures

The spin orbital torques in Ta/CoFeB/MgO structures are experimentally investigated utilizing the planar Hall effect and magnetoresistance measurement. By angular field characterization of the planar Hall resistance at ±current, the differential resistance which is directly related to the spin orbital torques is derived. Upon curve fitting of the analytical formulas over the experimental results, it is found that the anti-damping torque, also known as spin Hall effect, is sizable while a negligible field-like torque is observed. A spin Hall angle of about 18 ± 0.6% is obtained for the Ta layer. Temperature dependent study of the spin orbital torques is also performed. It is found that temperature does not significantly modify the spin Hall angle. By cooling down the sample down to 100 K, the obtained spin Hall angle has a maximum value of about 20.5 ± 0.43%.

Nonreciprocal behavior of the spin pumping in ultra-thin film of CoFeB

The processional magnetization induced spin current at the interface between CoFeB and Ta has been studied experimentally using spin pumping and inverse spin Hall effect for different thicknesses of CoFeB film down to 1.6 nm. It is found that upon decreasing the thickness of the CoFeB, the frequency of the peak position of the spin pumping signal reduces and dispersion relation of the ferromagnetic resonance changes from a quadratic to a linear behavior indicating the presence of an interfacial perpendicular anisotropy. Furthermore, a nonreciprocal behavior between the spin pumping signal amplitude at positive and negative fields is observed which could be as large as 100%. Our experimental results suggest reduction of the effective demagnetization field and possibly the spin waves nonreciprocal behavior mediated by the Dzyaloshinskii-Moriya interaction at the Ta/CoFeB interface are responsible for the large nonreciprocity of the spin pumping signal.

Evaluation of spin waves and ferromagnetic resonance contribution to the spin pumping in a Ta/CoFeB structure

The spin waves and ferromagnetic resonance (FMR) contribution to the spin pumping signal is studied in the Ta/CoFeB interface under different excitation bias fields. Ferromagnetic resonance is excited utilizing a coplanar waveguide and a microwave generator. Using a narrow waveguide of about 3 {\mu}m, magnetostatic surface spin waves with large wavevector (k) of about 0.81 {\mu}m^-1 are excited. A large k value results in dissociation of spin waves and FMR frequencies according to the surface spin wave dispersion relation. Spin waves and FMR contribution to the spin pumping are calculated based on the area under the Lorentzian curve fitting over experimental results. It is found that the FMR over spin waves contribution is about 1 at large bias fields in Ta/CoFeB structure. Based on our spin pumping results, we propose a method to characterize the spin wave decay constant which is found to be about 5.5 {\mu}m in the Ta/CoFeB structure at a bias field of 600 Oe.

Non-Local Lateral Spin-Valve Devices Fabricated With a Versatile Top-Down Fabrication Process

We report a method that uses top-down techniques for fabrication of non-local lateral spin-valve devices. Using this process, we demonstrate the fabrication of non-local lateral spin valves with Cu channels and Co nanopillar structures down to 75 nm × 100 nm. This will provide an appealing, cost-effective approach for industrial applications and allow for high control over material interface properties and device design. The nanopillar structures are essential to the scalability of devices, which is required for magnetic read head and logic applications to be competitive with current technologies.

Design and fabrication of nanomagnetic majority logic gate based on spin hall assisted switching

In recent years, spin-orbit torques induced by charge current in heavy metal/magnetic structures have attracted wide attention among researchers. Since the experimental demonstration of spin-orbit torques due to the spin Hall effect (SHE) being able to manipulate the magnetization direction in an in-plane MTJ structure[1], it has been heavily explored for applications in memory and logic. It provides a potential low power alternative to other techniques such as spin transfer torque or electric field control for magnetization reversal. Recently, it has been shown that the spin Hall effect can provide a clocking mechanism for logic applications where the spin Hall effect is used to change the magnetization of a perpendicularly magnetized device along the hard axis (in the plane of the film)[2]. In [2], a series of three nanomagets are spaced closely together so they interact through dipole interactions. Each nanomagnet serves as a logic bit. An additional nearby magnet is used as the input. However, due to the dipole interactions, only logic states of 101 or 010 can be obtained. For logic applications, it would be beneficial to be able to individually control the states of the individual bits to obtain a full range of logic functions.

Lateral spin valve device for magnetic reader applications fabricated by an Etch back process

A schematic design of the device with lithography defined pillars and channel is shown. The deposited film stacks have a structure of substrate/Cu(100 nm)/Co(20 nm)/Ta(4 nm). The films were deposited in vacuum with a Shamrock sputtering system. Patterning was performed using a Vistec electron beam (e-beam) lithography system. The channel was first patterned using negative resist and ion milling. After resist removal, negative resist and ion milling were used again to pattern and define the pillars. Before the e-beam resist was moved, SiO2 was deposited using e-beam evaporation to isolate the pillars and prevent shorting between top electrodes and the channel. The e-beam resist was then removed to expose the tops of the pillars for electrical contact. A final step of e-beam lithography was performed to pattern the top electrodes. E-beam evaporation was used to deposit Ti(10nm)/Au(100 nm) for the top electrodes. A schematic of the fabrication process is shown. Precise alignment between the multiple steps of e-beam lithography for the electrodes, FM pillars, and channel is essential for correct operation of the device.