Gordon Stecklein

Dynamic detection of electron spin accumulation in ferromagnet-semiconductor devices by ferromagnetic resonance

A distinguishing feature of spin accumulation in ferromagnet–semiconductor devices is its precession in a magnetic field. This is the basis for detection techniques such as the Hanle effect, but these approaches become ineffective as the spin lifetime in the semiconductor decreases. For this reason, no electrical Hanle measurement has been demonstrated in GaAs at room temperature. We show here that by forcing the magnetization in the ferromagnet to precess at resonance instead of relying only on the Larmor precession of the spin accumulation in the semiconductor, an electrically generated spin accumulation can be detected up to 300 K. The injection bias and temperature dependence of the measured spin signal agree with those obtained using traditional methods. We further show that this approach enables a measurement of short spin lifetimes (<100 ps), a regime that is not accessible in semiconductors using traditional Hanle techniques.

Band structure characterization of WS2 grown by chemical vapor deposition

Growth by chemical vapor deposition (CVD) leads to multilayer WS2 of very high quality, based on high-resolution angle-resolved photoemission spectroscopy. The experimental valence band electronic structure is considered to be in good agreement with that obtained from density functional theory calculations. We find the spin-orbit splitting at the K¯ point to be 420  ± 20 meV with a hole effective mass of −0.35  ± 0.02 me for the upper spin-orbit component (the branch closer to the Fermi level) and −0.43  ± 0.07 me for the lower spin-orbit component. As predicted by theory, a thickness-dependent increase of bandwidth is observed at the top of the valence band, in the region of the Brillouin zone center. The top of the valence band of the CVD-prepared films exhibits a substantial binding energy, consistent with n-type behavior, and in agreement with transistor characteristics acquired using devices incorporating the same WS2 material.

Electrical detection of ferromagnetic resonance in ferromagnet/n-GaAs heterostructures by tunneling anisotropic magnetoresistance

We observe a dc voltage peak at ferromagnetic resonance (FMR) in samples consisting of a single ferromagnetic (FM) layer grown epitaxially on the n-GaAs (001) surface. The FMR peak is detected as an interfacial voltage with a symmetric line shape and is present in samples based on various FM/n-GaAs heterostructures, including Co2MnSi/n-GaAs, Co2FeSi/n-GaAs, and Fe/n-GaAs. We show that the interface bias voltage dependence of the FMR signal is identical to that of the tunneling anisotropic magnetoresistance (TAMR) over most of the bias range. Furthermore, we show how the precessing magnetization yields a dc FMR signal through the TAMR effect and how the TAMR phenomenon can be used to predict the angular dependence of the FMR signal. This TAMR-induced FMR peak can be observed under conditions where no spin accumulation is present and no spin-polarized current flows in the semiconductor.

Independent gate control of injected and detected spin currents in CVD graphene nonlocal spin valves

Graphene is an ideal material for spintronic devices due to its low spin-orbit coupling and high mobility. One of the most important potential applications of graphene spintronics is for use in neuromorphic computing systems, where the tunable spin resistance of graphene can be used to apply analog weighting factors. A key capability needed to achieve spin-based neuromorphic computing systems is to achieve distinct regions of control, where injected and detected spin currents can be tuned independently. Here, we demonstrate the ability to achieve such independent control using a graphene spin valve geometry where the injector and detector regions are modulated by two separate bottom gate electrodes. The spin transport parameters and their dependence on each gate voltage are extracted from Hanle precession measurements. From this analysis, local spin transport parameters and their dependence on the local gate voltage are found, which provide a basis for a spatially-resolved spin resistance network that sim...

Dynamic detection of spin accumulation in ferromagnet-semiconductor devices by ferromagnetic resonance (Conference Presentation)

A distinguishing feature of spin accumulation in ferromagnet-semiconductor devices is its precession in a magnetic field. This is the basis for detection techniques such as the Hanle effect, but these approaches become ineffective as the spin lifetime in the semiconductor decreases. For this reason, no electrical Hanle measurement has been demonstrated in GaAs at room temperature. We show here that by forcing the magnetization in the ferromagnet to precess at resonance instead of relying only on the Larmor precession of the spin accumulation in the semiconductor, an electrically generated spin accumulation can be detected up to 300~K. The injection bias and temperature dependence of the measured spin signal agree with those obtained using traditional methods. We further show that this new approach enables a measurement of short spin lifetimes (< 100~psec), a regime that is not accessible in semiconductors using traditional Hanle techniques. The measurements were carried out on epitaxial Heusler alloy (Co2FeSi or Co2MnSi)/n-GaAs heterostructures. Lateral spin valve devices were fabricated by electron beam and photolithography. We compare measurements carried out by the new FMR-based technique with traditional non-local and three-terminal Hanle measurements. A full model appropriate for the measurements will be introduced, and a broader discussion in the context of spin pumping experimenments will be included in the talk. The new technique provides a simple and powerful means for detecting spin accumulation at high temperatures. Reference: C. Liu, S. J. Patel, T. A. Peterson, C. C. Geppert, K. D. Christie, C. J. Palmstrøm, and P. A. Crowell, “Dynamic detection of electron spin accumulation in ferromagnet-semiconductor devices by ferromagnetic resonance,” Nature Communications 7, 10296 (2016). http://dx.doi.org/10.1038/ncomms10296

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.

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.