Quantifying spin for future spintronics
Spin-momentum locking induced anisotropic magnetoresistance in monolayer
WTe2

Date:
November 3, 2021
Source:
ARC Centre of Excellence in Future Low-Energy Electronics
Technologies
Summary:
An international collaboration quantifies spin in a 2D quantum spin
Hall insulator (QSHI), a promising option for future low-energy
nano- electronic and spintronic devices. The team used anisotropic
magnetoresistance (AMR) to reveal the relationship between the
electrons' spin and momentum when the current is spin-polarized,
demonstrating the promising potential of QSHI for novel spintronic
devices, and proving the value of AMR for design and development
of QSHI-based spintronic devices.



FULL STORY ==========================================================================
A RMIT-led, international collaboration published this week has observed
large in-plane anisotropic magnetoresistance (AMR) in a quantum spin
Hall insulator and the spin quantization axis of the edge states can
be well-defined.


==========================================================================
A quantum spin Hall insulator (QSHIs) is a two-dimensional state of
matter with an insulating bulk and non-dissipative helical edge states
that display spin- momentum locking, which are promising options for
developing future low-energy nano-electronic and spintronic devices.

The FLEET collaboration of researchers at RMIT, UNSW and South China
Normal University (China) confirm for the first time the existence of
large in-plane AMR in monolayer WTe2 which is a novel QSHI with higher
critical temperatures.

By allowing electrical conduction without wasted dissipation of energy,
such materials could form the basis of a new future generation of
ultra-low energy electronics.

FABRICATING MONOLAYER WTE2 DEVICES The rise of topological insulators
has offered significant hope for researchers seeking non-dissipative
transport, and thus a solution to the already observed plateauing of
Moore's law.



========================================================================== Unlike previously-reported quantum-well systems, which could only exhibit quantized edge transport at low temperatures, the recent observation of quantized edge transport at 100 K in a predicted large band-gap QSHI,
monolayer WTe2 , has shed more light on the applications of QSHI.

"Although we had gained much experience in stacking van der Waals (vdW) heterostructures, fabricating monolayer vdW devices was still challenging
for us," the study's first author Dr Cheng Tan says.

"Because monolayer WTe2 nanoflakes are difficult to obtain, we firstly
focused on a more mature material, graphene, to develop the best way
for fabricating monolayer WTe2 vdW devices" says Cheng, who is a FLEET
Research Fellow at RMIT University in Melbourne.

As the monolayer WTe2 nanoflakes are also very sensitive to the air,
protective 'suits of amours' made of inert hBN nanoflakes should be
utilized to encapsulate them. Additional, the assembly was carried out in
an oxygen- and water-free glove box before series of tests outside. After
some effort, the team then successfully fabricated the monolayer WTe2
devices with gate electrodes and observed typical transport behaviours
of gated monolayer WTe2.

"For materials to be used in future spintronic devices, we need a method
to determine spin characteristics, in particular the direction of spin,"
says Dr Guolin Zheng (also at RMIT).



========================================================================== "However, in monolayer WTe2, spin-momentum locking (an essential
property of QSHI) and whether spin quantization axis in its helical edge
states could be determined had yet to be experimentally demonstrated." Anisotropic magnetoresistance (AMR) is an effective transport measurement method to reveal the relationship between the electrons' spin and momentum
when the current is spin-polarized.

Considering that the edge states of a QSHI only allow the transport of
spin- polarized electrons, the team then used AMR measurements to explore
the potential spin-momentum locking in the edge states of monolayer WTe2.

"Fortunately, we found the proper method to deal with the monolayer
WTe2 nanoflakes," says co-author Dr Feixiang Xiang (UNSW). "So then
we performed angular-dependent transport measurements to explore the
potential spin features in the edge states." PERFORMING ANISOTROPIC MAGNETORESISTANCE AND DEFINING THE SPIN QUANTIZATION AXIS However, the topological edge states are not the only possible cause for spin- momentum locking and in-plane AMR effects in a QSHI. Rashba splitting could also generate similar effects, which may make the experimental results unclear.

"Fortunately, topological edge states and Rashba splitting induce very different gate-dependent in-plane AMR behaviours, because the band
structure under these two situations are still very different." says
co-author Prof Alex Hamilton (also at UNSW).

"Most of the samples show that minimum of in-plane AMR happens
when the magnetic field is nearly perpendicular to the edge current
direction." says Cheng.

Further theoretical calculations by collaborators at South China Normal University further confirmed that electrons' spins in the edge states
of monolayer WTe2should be always perpendicular to their propagation directions, so-called 'spin-momentum locking'.

"The amplitudes of the in-plane AMR observed in monolayer WTe2 is very
large, up to 22%" says co-author A/Prof Lan Wang (also at RMIT).

"While the previous amplitudes of in-plane AMR in other 3D topological insulators are only around 1%. By AMR measurements, we can also
precisely determine the spin quantization axis of the spin polarized
electrons in the edge states." "Again, this work demonstrates
the promising potential of QSHI for designing and developing novel
spintronic devices and prove AMR as a useful tool for the design and development of QSHI-based spintronic devices, which are one of the
promising routes for FLEET to realize low-energy devices in future." ========================================================================== Story Source: Materials provided by ARC_Centre_of_Excellence_in_Future_Low-Energy_Electronics
Technologies. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Cheng Tan, Ming-Xun Deng, Guolin Zheng, Feixiang Xiang, Sultan
Albarakati, Meri Algarni, Lawrence Farrar, Saleh Alzahrani,
James Partridge, Jia Bao Yi, Alex R. Hamilton, Rui-Qiang Wang, Lan
Wang. Spin- Momentum Locking Induced Anisotropic Magnetoresistance
in Monolayer WTe2.

Nano Letters, 2021; DOI: 10.1021/acs.nanolett.1c02329 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211103115424.htm

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