Direct Electrical Detection of Spin Hall Effect Using “Hall-Effect Like” Measurement Configuration on a Series of β-Tungsten Channels Spanning the Metal-Insulator Transition

Mandal, Arpita (2021) Direct Electrical Detection of Spin Hall Effect Using “Hall-Effect Like” Measurement Configuration on a Series of β-Tungsten Channels Spanning the Metal-Insulator Transition. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Arpita Mandal (10RS008)) 10RS008.pdf - Submitted Version Restricted to Repository staff only Download (12MB)

Abstract

A pure spin current is said to be generated in a non-magnetic conductor when upand down- spin carriers move in opposite directions, so that there is no net charge transport. Spintronics researchers are fascinated by this novel concept, that can potentially allow dissipation less information transfer by transport of spin angular momentum. Spin Hall effect refers to the phenomenon where a (spin unpolarized) charge current flowing through a normal (non-ferromagnetic) conductor generates a pure spin current in the transverse direction. Spin Hall effect is a result of spin dependent scattering of carriers due to spin-orbit interaction and along with anomalous Hall effect in ferromagnetic conductors, are among the rare exhibits of relativistic effects in condensed matter physics. For an open circuit condition in the transverse direction, the pure spin current generated due to spin Hall effect results in spin accumulation at the edges of the current carrying channel with opposite orientation. However, this spin accumulation cannot be detected by a voltmeter, as there is no net difference in the number of charges associated with the spin accumulation. Thus direct electrical detection of spin Hall voltage is an experimental challenge. Spin Hall effect is significant in materials with high spin-orbit coupling strength, for example, in metals with high atomic number, and semiconductors. We present a new device scheme and appropriate analysis protocol that allows us to directly measure the spin accumulation potential arising due to SHE. Our device consists of one FM and one HM electrode in contact with only the top surface of a HM channel, for measuring a transverse voltage directly proportional to SH voltage. We have prepared a series of tungsten channels as the medium for generating SHE, of room temperature resistivity varying from 14 to 800 μΩ cm and have studied the temperature dependence of RSHE for all the different samples above which has not been reported so far. In this work, we have chosen β−tungsten as the material to generate Spin Hall effect, based on previously reported experiments. β−Tungsten is a metastable phase observed in tungsten thin films grown under special conditions. By varying the growth conditions, we have deposited a series of samples of β−tungsten, as confirmed by X-ray diffraction, with resistivity varying from 30−800 μΩ cm and α−tungsten with resistivity 14μΩ cm. Temperature dependence of resistivity of the individual tungsten channels, shows metallic behavior for the alpha (14 μΩ cm), 30 μΩ cm and 100μΩ cm sample and increasing insulating behavior for the rest of the samples with higher resistivity. Using standard device fabrication methods, we fabricated 300 micron long, 30 micron wide and 20 nm thick channels of β−tungsten films indented inside insulating Silicon substrates, so that only the top face of the channel is exposed and rest of the three faces are protected. In case of α−tungsten, the channel thickness is 30 nm. We then deposit a strip of Ferromagnetic metal(FM) electrode and a reference normal metal(NM) electrode of beta tungsten of the same dimensions on the top surface of the channel. The pair of FM/NM electrodes are approximately aligned in a line along the width of the channel without touching each other and connected to contact pads on opposite sides of the channel. We apply in-plane magnetic field along the channel width (along the FM/NM electrode pair). The magnetic field defines the direction of magnetization of the FM and the quantization axis of the carrier spins in β−tungsten. When current flows along the channel length, due to spin Hall effect, a pure spin current is generated in a direction perpendicular to both the current flow and spin quantization axis. Thus, spins of opposite orientations accumulate at the entire top and bottom surface of the β−tungsten channel. The FM/NM electrode pair is connected to a nanovoltmeter. The NM electrode will be insensitive to the presence of any spin accumulation. But the FM electrode will develop an extra voltage depending on the relative orientation of its magnetization and net spin accumulation of the top surface. When the direction of applied in plane magnetic field is reversed , so that the magnetization of the FM is reversed , the sign of voltage appearing due to spin Hall effect is also reversed. The difference of the two voltages measured at the FM/NM pairs for two opposite directions of the applied magnetic field is a measure of the spin accumulation potentials, factored by spin polarization of the FM. Our measurement scheme is based on a “Hall-effect like” configuration where we measure voltage in the direction transverse to the current flow and extract directly an electrical signal that is proportional to spin Hall voltage. All our devices (except α-tungsten whose thickness is 30nm and is obtained after annealing) are fabricated with the same parameters except the resistivity of the tungsten channel, which was systematically varied. For each sample we have measured the spin Hall resistance with temperature varying from 5-290K. Further, with the set of samples with almost a decade of resistivity variation of tungsten channel, we hope to shed light on the nature of spin dependent scattering mechanism: skew scattering or side jump, associated with the spin Hall effect in β-tungsten. Further we have done a control experiment on a titanium channel which is reported to have low spin orbit scattering strength (atomic number 20). Measurement on titanium device with exact same parameters as that of the tungsten devices confirm the absence of typical behaviour as shown in the tungsten channel due to spin Hall effect.

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