Characterizing the SDL and spin polarization

• We fabricated a series of FM pairs, where in each pair the distance

between the FM is different.

• By doing NLSV measurement on each of the junctions, we can plot

the NLSV signal as a function of the distance between FM pairs.

Spin Injection and Propagation in 2D monolayer Molybdenum Disulfides

E.Zion1,2, S.Wissberg1,2, D.Naveh3, A.Sharoni1,2

1Department of Physics ; 2Bar Ilan Institute of Nanotechnology and Advanced Materials; 3Faculty of Engineering

Acknowledgements This work was supported by the EU Mary Currie IRG #268357

MoS2

The vast research and remarkable properties of graphene have ignited interest in other 2D materials that may

have unique electronic and optical characteristics. Transition metal dichalcogenides (TMDs) are 2D layered

materials which possess sizable band gaps (around 1-2 eV), promising interesting field-effects and spin

properties.

An electrical current composed of a majority of

one type of spin population (up or down) is

called a spin polarized current. Since there are

different densities of states for spin-up and spin-

down electrons in a ferromagnet (FM), when a

FM contact is used to inject spin-polarized

carriers into a semiconductor (SC), a spin-

polarization of the current is expected from the

different conductivities (one for each kind of

spin). This appears as a splitting in the spin

dependent chemical potential and leads to spin

accumulation in the vicinity of the FM/SC

interface.

The charge current IC in the FM component

provides a spin current 𝐼𝑠 = 𝛼𝐹 ∙ 𝐼𝑐, where 𝛼𝐹 is

the spin polarization in the FM. In our research

we are interested in a spin diffusion length

(SDL) which is defined as the distance over

which a non-equilibrium spin population can

propagate, until it loses its polarization.

An elegant

Method to measure

the SDL and spin polarization

is by using non-local spin valve (NLSV).

In the NLSV geometry, one FM electrode

is the spin injector and the other FM is the

detector. By placing a spin detection

electrode outside the path of the charge

current, we minimize the background

effects.

Fabrication Results fa

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Photolitography and thermal evaporation are used for

fabricating matrix of coordinates on top of Si\SiO2

substrates:

Monolayer

Few Layers

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Until now, we have successfully

demonstrated an example of measurements

on one particular flake. After we detected an

appropriate MoS2 flake by optical

microscopy, we verified the flake thickness

(number of layers) using Atomic-Force

Microscopy scanning. When we confirmed

that the particle is thin enough, we

characterized the current on\off ratio of the

source-drain current for different gate

voltages.

Molybdenum disulfide, MoS2, is a member of the TMDs family that is

receiving much attention recently. Large spin-orbit coupling could lead to

Enhanced spin lifetimes and long Spin Diffusion length. Monolayer MoS2 is a

favorable Candidate for the field of spintronics and for demonstrating the

Data-Dass spin field-effect transistor.

• Monolayer MoS2 in a Field-Effect transistor (FET) setup demonstrates Large 𝑰 𝒐𝒏\𝒐𝒇𝒇 ratio at room-temperatures (exceeding 108).

• MoS2 predicted to be an excellent material for spintronics devices Due to strong intrinsic Spin-Orbit coupling.

• Possible strategies of improvement in efficient injection of spin current into MoS2.

• Resolving the spin related properties in MoS2 and their dependence on the number of layers.

• By controlling the Rashba spin-orbit coupling, spin FETs will become feasible.

Here, the injected

spin accumulation

diffuses to the right

of the device,

resulting in the flow

of a pure spin

current. The signal

is then picked up by

another electrode as

a voltage.

By measuring the voltage on the second

FM we can deduce the current

polarization and the SDL.

MoS2 flakes are exfoliated from bulk natural

molybdenite crystals, using mechanical cleavage

exfoliation technique (the scotch tape method).

We locate appropriate MoS2 flakes rapidly and non-

destructively using optical microscopy.

This followed by deposition of

the desired material and lift off,

For spin injection and detection, electrodes are prepared by E-beam lithography:

We design an appropriate

electrode layout for each flake

We write the electrodes using

E-beam lithography.

Source-drain current versus bottom

gate voltage. Measurements were

performed at room temperature.

The device can be completely

turned off by changing the bottom

gate bias.

Atomic-force microscopy (AFM)

image with its height profile of a

few layers MoS2 flake.

In NLSV measurements, the spin related voltages may be in

the range of a few mV. We carefully apply accurate

measurement techniques, such as Lock-In or Delta-Mode

amplification, which enables us to measure these signals.

Magneto-transport measurements

• PPMS - high field cryostat, possible of

variating temperature (2K-400K) and

magnetic fields (up to 9 Tesla).

• closed cycle cryostat, equipped with

external electromagnet (5K-350K, up to 0.3

Tesla field), which enables us to connect up

to 28 electrodes to the devices, expose

samples to polarized light or rotate the

sample in the magnetic field.

Source-drain-gate characterization

A measurement of the current between the

source and drain for different voltages, as a

function of gate voltage. We use two source-

meters with a common low electrode:

1. First one is used to apply a gate voltage.

2. Second one measures the I vs. V

characteristic.

From these current-voltage-gate curves we

can extract the current on/off ratios, the

mobility and the charge carrier sign of the

device for various temperatures and as

function of number of MoS2 layers.

5.7 𝑛𝑚