††thanks: Correspond to<EMAIL_ADDRESS>
# Integrating 2D Magnets for Quantum Devices: from Materials and
Characterization to Future Technology

Han Zhong  Douglas Z. Plummer  Department of Electrical and Computer
Engineering, University of Florida, Gainesville, Florida 32611, USA.
Pengcheng Lu  Department of Materials Science and Engineering, University of
Florida, Gainesville, Florida 32611, USA.  Yang Li  Polina A. Leger
Department of Electrical and Computer Engineering, University of Florida,
Gainesville, Florida 32611, USA.  Yingying Wu  Department of Electrical and
Computer Engineering, University of Florida, Gainesville, Florida 32611, USA.

###### Abstract

The unveiling of 2D van der Waals magnetism in 2017 ignited a surge of
interest in low-dimensional magnetism. With dimensions reduced, research has
delved into facile electric control of 2D magnetism, high-quality
heterostructure design, and new device functionality. These atomically thin
magnetic materials have spawned a burgeoning field known as 2D spintronics,
holding immense promise for future quantum technologies. In this review, we
comprehensively survey the current advancements in 2D magnet-based quantum
devices, accentuating their role in manifesting exotic properties and enabling
novel functionalities. Topological states, spin torques, voltage control of
magnetic anisotropy, strain engineering, twistronics and designer interface
will be discussed. Furthermore, we offer an outlook to guide their development
in future CMOS and quantum hardware paradigms.

##  I Introduction

Two-dimensional (2D) magnets represent a groundbreaking class of materials
that leverage the unique properties of 2D structures, such as flexibility and
stackability, to advance the field of magnetism and spintronics. The discovery
of ferromagnetism in 2D materials in 2017 marked a pivotal moment1; 2. This
breakthrough demonstrated that 2D intrinsic magnetic anisotropy and long-range
magnetic order are achievable in certain van der Waals (vdW) materials,
unlocking new potentials for technological applications. One of the key
advantages of 2D magnets is the ability to manipulate layer thickness, which
offers layer-dependent magnetic properties. This tunability, combined with
interface and heterostructure engineering, allows for fine-tuning magnetic
properties to meet specific needs 3. The reduced dimensionality of 2D magnets
also facilitates the discovery of new phenomena that can enhance quantum
effects and functionality. For example, the sharp 2D interface enables
exploration of nanoscale quantum objects like 2D magnetic skyrmions4; 5; 6.
Thanks to the easy electric control of 2D ferroelectrics, tuning of skyrmion
size down to a few nanometers was proposed in a In2Se3/Fe3GeTe2 2D vdW
heterostructure7. Transitioning from conventional memory technologies (e.g.,
SRAM, DRAM) to 2D multiferroic devices could yield comparable performance
while introducing non-volatility, thereby reducing data retrieval time and
power consumption 8; 9.

The ongoing discovery of new 2D magnets, facilitated by predictive models such
as density functional theory, is broadening the scope of materials and their
potential applications. These materials offer prospects for non-volatile
memory, spin-based logic devices, and spin-dependent electronics.
Investigations in spintronics, magnonics, and microelectronics seek to
capitalize on novel insights into exchange interactions and magnetic order.
For example, antiferromagnetic materials can be manipulated by electric
currents to attain read/write capabilities akin to ferromagnetic materials,
while maintaining ultrafast speeds10. Furthermore, the coexistence of
magnetism and superconductivity in twisted 2D materials11; 2 hints at a
profound interconnection between these phenomena, offering valuable insights
for future quantum technologies3.

This review paper delves into recent advancements in 2D magnetism, exploring
its novel applications in non-volatile memories, magnetic sensors, and quantum
technology. A comprehensive grasp of the properties and fabrication techniques
of 2D magnets is indispensable for comprehending these applications. The paper
endeavors to bridge solid-state principles from 3D to 2D within the realm of
spintronics. Topics of discussion will encompass 2D topological orders in both
real and momentum spaces, spin torques such as spin orbit torque (SOT) and
spin transfer torque (STT), magnetoelectric coupling via voltage-controlled
magnetic anisotropy, strain engineering with and without external pressure,
twistronics applied to twisted 2D magnets for inducing exotic magnetic orders,
and the creation of designer heterostructures to attain diverse high-quality
interfaces. Beyond elucidating fundamental properties and effects, the review
will explore practical applications in binary magnetic memory, biomedical
devices, and quantum hardware. By offering a comprehensive overview of current
research in 2D spintronics, the aim is to unveil potential pathways for
technological innovation and provide insights into future hardware
advancements.

Figure 1: Emerging 2D spintronics, driven by 2D magnets, sparking interest in
various topics such as 2D topological orders, spin-torques, magnetoelectric
coupling, strain engineering, twistronics, and designer heterostructures.

##  II Fundamentals of 2D Magnets

2D magnets can be atomically thin, sometimes just a single layer of atoms
$\sim$ 0.8 nm, while preserving the magnetism. They exhibit remarkable
magnetic anisotropy, leading to quantum phenomena not observed in their bulk
counterparts. What sets them apart from traditional bulk magnetic materials
are their unique features. These include quantum confinement, which arises
from carriers being confined to a 2D plane, as well as mechanical flexibility
due to their ultrathin thickness. Additionally, they possess enhanced
electrical, thermal and optical sensitivity12; 13; 14, rendering them highly
responsive to external stimuli such as electric fields, laser control,
chemical alterations, and mechanical strain. Moreover, their electronic and
magnetic properties can be finely tuned through external adjustments like
electric gating15; 16 and strain17; 18. These distinctive characteristics open
up new avenues for 2D magnets in ultracompact devices. For instance, their 2D
properties enable applications in quantum computing19; 20 and the development
of highly sensitive magnetic sensors21; 22.

A diverse array of 2D magnets has been uncovered (Table 1), spanning a wide
spectrum of materials, from metals to semiconductors to insulators, each
exhibiting a range of magnetic behaviors including ferromagnetic,
antiferromagnetic, and multiferroic properties. Beginning with binary
transition metal halides like CrI3 and CrBr3, structured as layered vdW
solids, they are renowned for their either ferromagnetic or antiferromagnetic
interlayer order depending on layer stacking. In contrast, MnBi2Te4 showcases
interlayer antiferromagnetic order alongside topological properties,
extensively studied for phenomena such as the quantum anomalous Hall effect
23, which manifests quantized Hall resistivity at elevated temperatures up to
45 K 24; 25; 26 under applied magnetic fields. Ternary transition metal
compounds such as Cr2X2Te6 (X = Si, Ge, Sn) and MPX3 (M = transition metal; X
= S, Se, Te) represent materials with intricate interactions between
constituents, resulting in diverse magnetic and electronic properties. For
example, Cr2Ge2Te6 is reported as an insulator with a band gap of $\sim$0.7 eV
in bulk27 and order in a Heisenberg-type ferromagnetism below
T${}_{\textrm{C}}$ 61 K. The two-probe resistance of this material at room
temperature usually in the order of $10^{5}\,\Omega$ to $10^{6}\,\Omega$. This
electrically resistive nature poses challenges to transport characterization.
Furthermore, emerging 2D rare-earth and f-electron magnets show promise for
high-temperature and high-density magnetic storage, like DyOCl28 as an A-type
rare-earth antiferromagnet and gadolinium halide GdX2 (X = I, Br, Cl, F)as
high-temperature 4f-electron ferromagnets29.

Materials |  Bandgap (eV) |  Curie/Néel Tempe (K) | Magnetism |  Stability (air)
---|---|---|---|---
VSe2 |  .05530 | 400-600 | Ferro | Yes
Fe3GaTe2 | 0 |  350-38031 | Ferro | Yes
Fe5GeTe2 | 0 |  270-33032 | Ferro | Yes
CrTe2 | 0.37-0.50 |  31033 | Ferro | Yes
Fe3GeTe2 | 0 |  220-23034; 35 | Ferro | Yes
Ta3FeS6 | 0 |  8036 | Ferro | Yes
Cr2Ge2Te6 |  0.727 | 61 | Ferro | Yes
CrBr3 |  1.68-2.137 |  3038 | Ferro | No
CrI3 |  1.3-1.439 | 14-45 | Ferro | No
GdI2 | 1 |  25140 | Ferro | Yes
CrSBr |  1.42-1.4741 | 132 | Antiferro | Yes
FePS3 |  1.7742 | 117 | Antiferro | Yes
MnPSe3 |  2.3243 | 70 | Antiferro | Yes
CrPS4 |  1.444 | 40 | Antiferro | Yes
MnBi2Te4 |  0.5545 | 25 | Antiferro | No
CrCl3 |  2.646 | 14 | Antiferro | No
DrOCl | 5.72 | 10 | Antiferro | Yes
CuCrP2S6 |  1.247 |  14548 | Multiferroic | Yes
NiI2 |  1.11-1.2349 |  2150 | Multiferroic | No
Table 1: A list of 2D magnets and their properties.

Synthesis techniques wield significant influence over the quality and
properties of 2D magnets, as listed in Table 2. Mechanical exfoliation, a
commonly employed method, typically yields the highest quality but faces
limitations in scalability, with the lateral size in the range of micrometers
to a hundred micrometers, with potential oxidation during the process posing
risks to material quality. New exfoliation methods, such as universal gold-
assisted exfoliation, extend fabrication capabilities by enabling the
production of monolayers on a millimeter to centimeter scale51. This method
has proven to be successful for over forty varieties of vdW materials,
including Fe3GeTe2.The exfoliated monolayer flakes can be intactly transferred
onto arbitrary substrates after removing gold layer by KI/I2 etchant. The
removal of gold layer may bring unexpected contamination in samples prepared
for electrical and optical measurements and thus reduce their performances
when removing gold films with chemical solvents in additional steps. The
alternative way is to keep gold layer thickness below 2-3 nm, in the
electrically insulating region. Additionally, Al2O3-coated exfoliation,
achieved through atomic layer deposition (ALD), provides enhanced thickness
and uniformity control while safeguarding material integrity 34. Removal of
the Al2O3 layer in solvent may cause contamination to the 2D surface. This
layer can be retained and serve as a dielectric layer for electric gating.
Liquid phase exfoliation (LPE) stands out for its simplicity and controllable
sizing, featuring efficient and stable three-stage sonication-assisted
exfoliation for mass production of high-integrity few- and single-layer
Fe3GeTe2 nanoflakes, with lateral sizes reaching up to 103 $\mu$m 52. This
method offers pathways for diverse applications, including printed electronics
and composite materials. Vapor deposition technique has been utilized for the
growth of NiI2 on substrates such as SiO2/Si and h-BN down to monolayer
thickness, enabling the production of high-purity solid materials through
vapor-phase precursor reactions on heated substrates, prized for its ability
to generate uniform, high-quality coatings 49. Molecular beam epitaxy (MBE), a
meticulously controlled vacuum deposition technique, facilitates atomic layer-
by-layer deposition of crystalline materials with low defect densities. MBE-
grown Fe3GeTe2 on Bi2Te3 substrates has demonstrated room-temperature
ferromagnetism, showcasing its potential for applications in quantum devices.
Later a high SOT efficiency at room temperature was reported in this
heterostructure, with details shown in Section IV.1.1. Sputtering, which
deposits thin films onto substrates using high-energy ion particles, is known
for its uniformity across various materials and substrate shapes. While
extensively used for 2D non-magnetic materials like MoS2 53, its potential
applications to 2D magnets are yet to be fully explored.

Method | Size | Thickness | Materials | Challenge
---|---|---|---|---
mechanical exfoliation |  $\mu$m | varied | universal | small scale monolayers
gold-assisted exfoliation | mm-cm | monolayer | universal | solvent to remove gold layer & contaminiation
Al2O3-coated exfoliation |  $\mu$m | monolayer |  Fe3GeTe2 |  Al2O3 attached
LPE |  $\mu$m | controllable |  Fe3GeTe2 | potential doping
vapor deposition |  $\mu$m | few-layer |  NiI2 | uniform monolayers
MBE | cm | few-layer |  Fe3GeTe2 | high-quality monolayer
Table 2: Comparison of methods using exfoliation or in-situ growth of 2D
magnets.

##  III Magnetic properties of 2D Materials

Due to their atomic thinness, 2D materials provide an opportunity to address
concerns regarding the dilution of magnetic interactions observed in bulk
materials, offering stronger and more precise magnetic responses across
various domains. The Mermin-Wagner limit 54 dictates that long-range magnetic
order is unattainable at finite temperatures in an ideal 2D Heisenberg model
with continuous symmetry and short-range interactions. Magnetic anisotropy
provides a solution. Referring to the directional dependence of a material’s
magnetic properties, magnetic anisotropy can stabilize magnetic order in a
low-dimensional system by breaking the continuous rotational symmetry.
Anisotropic properties can help create a magnon excitation gap 55, thereby
counteracting thermal fluctuations and facilitating long-range order. Ideally,
this results in a finite Curie temperature T${}_{\textrm{C}}$, ensuring the
existence of 2D magnetism.

The presence of strong magnetic anisotropy can facilitate the development of
room-temperature 2D magnets, which are pivotal for advancing various
technological applications. These magnets offer practical advantages,
including enhanced energy efficiency, simplified device design, and
compatibility with existing magnetic memory infrastructure. Furthermore, their
stability at room temperature enables seamless integration into everyday
electronics, eliminating the need for complex and costly cooling systems. This
makes such materials economically viable for widespread adoption in consumer
electronics, telecommunications, and beyond. Among the discovered room-
temperature 2D magnets depicted in Fig. 2, VSe2 is a high-temperature
ferromagnet. By varying the substrate56; 57 and strain58, its
T${}_{\textrm{C}}$ can be varied between 400 K and 600 K. Intrinsic magnets
like CrTe2 and Fe3GaTe2 were also studied. Their magnetic properties are
primarily characterized through anomalous Hall resistivity measurements (Figs.
2b & d-f) or SQUID measurements (Fig. 2c) on the magnetization. Fe3GaTe$2$,
for instance, is reported to be a 2D vdW ferromagnet with the high
T${}_{\textrm{C}}$ reaching up to 380 K depending on Fe vacancy, a temperature
suitable for ambient operation even considering heating in CMOS environments.
Conversely, Fe3GeTe2 exhibits ferromagnetism below room temperature, while its
Curie temperature can be adjusted through doping. Techniques such as ionic
liquid gating can enhance the Curie temperature of few-layer Fe3GeTe2 to above
room temperature, while MBE growth of this material onto Bi2Te3 can further
augment its Curie temperature.

Figure 2: Reported room-temperature 2D magnets. (a) Current extensively
studied 2D magnet materials, with Fe3GaTe2 showing the highest Curie
temperature. (b) Anomalous Hall resistance characterization of few-layer
Fe3GaTe2, showing Curie temperature above 340 K. (c) Room-temperature
ferromagnetism in CrTe2, revealed by SQUID measurement59. (d) Pristine few-
layer Fe3GeTe2 showing a Curie temperature around 200 K4. (e) Ionic liquid
gating adopted to tune the carrier density in Fe3GeTe234, leading to room-
temperature magnetism. (f) Epitaxy growth of few-layer Fe3GeTe2 on Bi2Te3
substrates indicating an above-room-temperature ferromagnetism60.

###  III.1 Electrical characterizations

This process involves measuring various electrical parameters, such as
resistivity, conductivity, and Hall effect, to gain insights into the
material’s magnetic behavior and electronic structure. Techniques like
magneto-transport measurements can reveal how electrical resistance changes
with an applied magnetic field, providing information about phenomena such as
anisotropic magnetoresistance (AMR), tunneling magnetoresistance (TMR) and
anomalous Hall resistance (AHR).

A1: Curie temperature characterization Most materials-based work using
transport measurement on AHR showing a hysteresis loop of ferromagnets, where
the AHR is related to magnetization4; 61; 5. This was well established and
sometimes not applicable to antiferromagnet case.

A2: Voltage control of magnetic anisotropy (VCMA) Additionally, electrical
characterization helps in assessing the influence of external stimuli, such as
electric voltage on the magnetic state of the material. Multiferroic control
of 2D ferromagnetism was realized using $\pm$5 V across the Cr2Ge2Te6/polymer
heterostructure, where can magnetic hysteresis loop can be opened and closed.
The magnetic modulation is non-volatile, and is observed in bilayer, trilayer
and four-layer Cr2Ge2Te662.

A3: New transistors This also gives rise to new transistor types, like spin
tunnel field effect transistors63. The devices exhibit an ambipolar behaviour
and tunnel conductance that is dependent on the magnetic order in the CrI3
tunnel barrier. The gate voltage switches the tunnel barrier between
interlayer antiferromagnetic and ferromagnetic states under a constant
magnetic bias near the spin-flip transition, thus effectively and reversibly
altering the device between a low and a high conductance state, with large
hysteresis. Understanding these interactions is crucial for designing devices
that exploit spin-orbit coupling or spin-transfer torque for efficient data
storage and processing.

A4: Topological properties As we have mentioned in Fig. 1, topological
properties in both real and momentum space are allowed in 2D spintronics. As
shown in Section II, MnBi2Te2 is a Chern insulator64, showing topology with
momentum-space Berry phase, leading to quantization of AHR. Meanwhile,
topology in real space shown as magnetic skyrmions was also reported in 2D
magnets-based heterostructure4; 5; 9, extensively studied in Fe3GeTe2 and
Cr2Ge2Te2 case. The sharp nature of 2D interface can host two groups of
magnetic skyrmions at one interface, which is absent from epitaxy-grown or
sputtering films.

A5: Strain engineering Tensile strain is a proven method for tuning the
fundamental properties of three-dimensional bulk quantum materials and 2D thin
films. For vdW magnets, recent works have highlighted the tantalizing
opportunities for the strain control of magnetic properties such as ordering
temperature and coercive field from hysteresis loop in Fe3GeTe265, with a
dramatic increase of the coercive field by more than 150% with an applied
strain of 0.32%.

###  III.2 Optical properties

Since exfoliated 2D magnets often have a small lateral size around $\mu$m,
optical detection with a fine resolution are required to probe their optical
properties. MOKE measures the change in polarization of reflected light in
response to magnetization, with a resolution of 1-2 $\mu$m. It can help reveal
critical information about the materials magnetic domains66, anisotropy and
phase transitions4. Additionally, 2D magnets can exhibit unique excitonic
effects and spin-dependent optical transitions, which are influenced by their
reduced dimensionality and strong spin-orbit coupling. A reversible strain-
induced antiferromagnetic-to-ferromagnetic phase transition and strain control
of the out-of-plane spin-canting process were read through polar reflective
magnetic circular dichroism (RMCD) in CrBrS at zero magnetic field18. These
characteristics enable the development of novel optoelectronic devices, such
as spin-LEDs and magnetic photodetectors, which can operate with high
efficiency and sensitivity. Furthermore, the ability to manipulate magnetic
properties using light, known as all-optical switching67; 68, opens new
possibilities for ultrafast data storage and processing technologies.

###  III.3 Magnetic imaging

Magnetic imaging encompasses various techniques to visualize and understand
magnetic structures at different scales and resolutions (Fig. 3a & Table 3).
Among most techniques, MOKE (Fig. 3a) was extensively adopted for magnetic
domain imaing, with a low resolution of 1-2 $\mu$m. Lorentz transmission
electron microscopy (L TEM) exploits electron phase shifts caused by
magnetization within samples (Fig. 3b), allowing the observation of magnetic
domain structures with high spatial resolution. It is regarded as the most
efficient method because of its ultrahigh magnetic spatial resolution (down to
2 nm)69; 70. The fine magnetic structure of a skyrmion was first observed in
real-space using L TEM71. It was also used in the first observation of 2D
skyrmions in vdW WTe2/Fe3GeTe2 heterostructure4. 4D scanning transmission
electron microscopy (STEM), a variant of TEM, provides detailed atomic-level
imaging by scanning a focused electron beam over the sample (Fig. 3c), useful
in correlating magnetic properties with atomic structure. Semi-quantitative
maps of the magnetic structures can be obtained with a spatial resolution
nearly equal to the size of the electron probe or 10-20 nm. Aberration
corrected STEM can have a spatial resolution lower than 1 nm72. It was
proposed machine learning can be integrated with 4D STEM to automate
experiments and novel scanning modes73. Fully realizing this potential
necessitates significant developments on multiple levels, from instrumental
platforms to common workflows, shared data and codes. In magnetic force
microscopy (MFM), the magnetic stray field from nanoscale features is detected
by a scanning nanoscale magnetic tip (Fig. 3d). This technique has been
applied to image magnetic domain and skyrmions in Fe3GeTe2 and Cr2Ge2Te6 thin
films5. Small-Angle Neutron Scattering (SANS) probes magnetic correlations in
materials by scattering neutrons at small angles, suitable for studying bulk
magnetic structures and dynamics on the nanoscale (Fig. 3e). It was used to
show the formation of short-range magnetic regions in CrSBr with correlation
lengths at the antiferromagnetic ordering temperature (TN $\sim$ 140 K)74.
Nitrogen-Vacancy (NV) centers (Fig. 3f) in diamond offer a quantum-based
approach to magnetic imaging, enabling high-sensitivity detection of magnetic
fields at the nanoscale through the interaction of NV electron spins with
external magnetic fields. This technique has been applied to image magnetic
domains in CrI3, showing a spatial resolution of $\sim$50 nm75. Other
techniques like scanning transmission X-ray microscopy (STXM) and X-ray
magnetic circular dichroism (XMCD) are adopted for imaging in Fe3GeTe2 and
Fe5GeTe2 system76; 77.

Figure 3: Magnetic imaging methods. (a) MOKE setup35, with an examples showing mapping of magnetization on CrI3 layers1. (b) L TEM exhibiting contrast of dark and bright, examples including magnetic domain and magnetic skrymions imaging4. (c) 4D STEM integrated with machine learning offering high resolution78; 73. (d) MFM using a magnetic tip, with examples showing magnetic domains of Cr2Ge2Te6 and Fe3GeTe25. (e) SANS mapping of magnetic textures79, with an example of CrSBr74. (f) NV center mapping of magnetization, showing an example on CrI3 magnetization75.  Method | spatial resolution | Pros | Best used for | vdW magnets studied
---|---|---|---|---
MOKE |  1-2 $\mu$m | easy access | large-scale domain |  CrI368
L TEM | 2 nm |  High resolution easy acces | thin film |  Fe3GeTe24; 80; 81 Cr2Ge2Te682; 83
4D STEM | 1 nm |  High resolution no post image process | thin film |  Co-doped Fe5GeTe284
MFM | 5-10 nm | Electrical signal allowed | thin film |  Cr2Ge2Te65; 85 Fe2GeTe286
SANS | 0.5-1 nm | High resolution | bulk or film |  DyTe387
NV center | 50 nm | Sentitive | Ultrathin film |  VI388 CrI389; 75
Table 3: Comparison of magnetic imaging techniques.

###  III.4 Magnon-phonon interaction

The interaction between magnons and phonons in 2D magnets is a fascinating
area of study that has significant implications for both fundamental physics
and practical applications90; 91. Magnons, which are quasiparticles
representing collective excitations of electron spins, and phonons, which are
quasiparticles representing lattice vibrations, can strongly influence each
other’s behavior in 2D materials due to their reduced dimensionality and
enhanced surface effects. This magnon-phonon coupling can affect the
thermal92; 93 and magnetic properties94 of 2D magnets, leading to phenomena
such as spin Seebeck effect, where a temperature gradient generates a spin
current. Understanding these interactions is crucial for developing efficient
spintronic devices, as they can impact the propagation of spin waves and the
dissipation of energy. Moreover, manipulating magnon-phonon interactions could
enable new ways to control magnetic states and develop low-power, high-speed
information processing technologies. Advances in this field could thus provide
novel insights into the dynamic processes within 2D magnets and drive
innovations in areas ranging from quantum computing to thermoelectrics.

##  IV Integration of 2D Magnets in Quantum Devices

###  IV.1 Spintronics and magnetic memory devices

Spintronics, arising at the intersection of magnetism and electronics, has
experienced significant advancements through the integration of 2D magnets.
Spin-orbit torque (SOT) and spin-transfer torque (STT) mechanisms have played
pivotal roles in enhancing the functionality and efficiency of spin-based
logic devices, such as spin field-effect transistors (FETs). By harnessing 2D
magnets in these devices, improved controllability, capacity, and reduced
energy consumption have been achieved, revolutionizing the capabilities of
modern data storage and processing. This section endeavors to elucidate the 2D
magnetic properties that underpin this transformative progress in magnetic
memory, offering insights into their potential for future applications.

####  IV.1.1 Spin-Orbit Torque (SOT)

In conventional information systems, signals are switched between binary
states 0 and 1, generated by different magnetic fields. Such an approach
relies on magnets or current coils that occupy a considerable amount of space
and energy. Under the trend of minimizing device fabrication, SOT, serves as a
fast switch based on magnetic structures. The SOT structure consists of a non-
magnetic conductor and a ferromagnetic heterostructure (Fig. 5a). In such a
system, the spin current generated by the spin Hall effect (SHE) will pass
through the adjacent ferromagnetic layer when an electrical current is
applied, inducing SOT. This technology allows the device to become non-
volatile, allowing it to store the data when voltage is off. SOT can receive
angular momentum through four different ways, i.e. magnetic order, carrier
spins, orbital moments, and atomic lattice. SOT usually can be categrorized
into Spin Hall Torque and Rashba-Edelstein Torque. The magnetization due to
spin Hall torque runs toward the interface plane, and Rashba-Edelstein Torque
rotates about the interface plane. Fig. 4 listed reported SOT efficiency in
some structures. Compared to non-vdW system, SOT in vdW heterostructures are
in high efficiency and low dissipation power region. Room-temperature SOT
driven magnetization switching in an all-vdW heterostructure was realized
using an optimized epitaxial growth. The topological insulator Bi2Te3 not only
raises the Curie temperature of Fe3GeTe2 through interfacial exchange coupling
but also works as a spin current source allowing the Fe3GeTe2 to switch at a
low current density of $\sim$2.2$\times$106 A/cm2. The SOT efficiency is
$\sim$2.69, measured at room temperature95. In the case of WTe2/Fe3GeTe2, a
high SOT efficiency of 4.6 and a switching current density of 3.90 $\times$
106 A/cm2 at 150 K was reported96.

Figure 4: SOT in 2D magnet-based heterostructure showing a high efficiency
with relatively low dissipation power density, compared to other system.

####  IV.1.2 Spin-Transfer Torque (STT)

A STT device consists of two ferromagnetic layers separated by a spacer97
(Fig. 5b). The first ferromagnetic layer has a fixed magnetization direction
and the second ferromagnetic layer has a free magnetization that can be
adjusted by the spin-polarized current. The non-magnetic layer serves as a
spacer, allows the flow of spin-polarized electrons without altering their
spin direction. When the electrons enter the device, they first pass through
the fixed ferromagnetic layer and become spin-polarized. This means that most
electrons will have their spins aligned along the direction of the first
ferromagnetic layer. Later, these spin-polarized electrons enter the free
ferromagnetic layer, and they exert a torque on its magnetization97. STT
device has huge potential in advanced memory technologies, particularly
magnetic random-access memory (MRAM). The direction of these magnetic domains
represents binary data (0 and 1) and remains stable when there are no external
power supplies, called non-volatility. The key component of STT-MRAM is the
magnetic tunnel junction (MTJ), which consists of an insulating layer
sandwiched by two ferromagnetic layers. After the current is applied, the
spin-polarized current will pass through the MTJ and exert torque on the
magnetic moments in the free layer. This will force them to align with the
direction of the current. Adoption of 2D magnets in STT allows ultracompact
devices, considering their uniform thickness down to $\sim$ 0.8 nm. Examples98
have been shown on using vdW magnets as spacer99; 100; 101 (like CrI3) or
ferromagnetic metal102; 103; 104 (like Fe5GeTe2).

Figure 5: Illustration of SOT, STT, magnetism sensor mechanism and example.
(a) 2D SOT made with vdW materials. (b) STT using vdw materials for
ferromagnetic layer and non-magnetic spacer. (c) Traditional magnetic sensor
mechanism. (d) Sensitive photodector on CrI3105.

###  IV.2 Magnetic sensors

The integration of 2D magnets has profoundly influenced the landscape of
magnetic sensing technologies; enabling the realization of highly sensitive
and compact sensors including AMR, GMR, and TMR which have been leveraged to
enhance the performance of magnetic sensors(Fig. 5c), facilitating diverse
applications ranging from automotive systems to biomedical diagnostics.
Additionally, the emergence of vdW room-temperature ferromagnetic materials
has spurred related advancements in nano-sensor technology. For example, the
vdW magnetic semiconductor CrI3 in a tunnelling geometry has been used to
create a photodetector that is sensitive to the helicity of circularly
polarized light105. This offers unprecedented opportunities for
miniaturization and integration with emerging platforms.

Anisotropic Magnetoresistance AMR is a phenomenon where the resistance of a
specific material is influenced by the angle between an applied current and
the direction of magnetization in the material being used. It is important to
note that the type of ferromagnetic material also plays a key role in the
effect, with metals like nickel experiencing over three times the
magnetoresistance when compared to iron. This is because the effect is a
byproduct of scattering that occurs in conducting electrons with uncompensated
spins. It is also be paramount to recognize that the resistance of these
simple ferromagnets varies in a nonlinear dependence on the applied field 106.
This is compounded with the fact that when the applied field (separate from
the total magnetic anisotropy field) is small, the sensitivity of the sensor
suffers, with the sensor much more likely to produce false negatives or
misidentify what it is observing107. In the case of Cr2GeTe6, the field-
dependent resistance shows a positive or negative magnetoresistance effect
when magnetic field H is switched to different directions108. Cr1.2Te2 flake
reveals a room-temperature remarkable 4-fold AMR effect, resulting from the
material high-term lattice symmetry109.

Giant Magnetoresistance GMR is comprised of three distinct layers. A thin film
of non-magnetic conducting metal which is placed between two magnetic layers
whose resistance varies depending on the relative orientation of magnetization
within the layers. This effect is caused by spin dependent electron
scattering; that means when the magnetization between the two layers is
aligned only one spin of electrons is able to scatter and thus the overall
resistance is minimized. This effect is reversed when the magnetization
between the layers is mismatched107. Spin-dependent transport in vdW GMR
junctions with the structure Fe3GeTe2/XTe2/Fe3GeTe2 (X = Pt, Pd) is
investigated using first-principles calculations. A GMR of around 2000% is
obtained110.

Figure 6: GMR ratio of varied systems, including 2D manget system.

Tunnel Magnetoresistance TMR also referred to as MTJ has a setup similar to
GMR except the non-magnetic conducting metal is replaced with an insulator
referred to as the tunnel. This tunnel layer acts as a barrier so when the
relative orientation of magnetization is aligned between magnetic plates the
resistance is low. Conversely, when the relative orientation of magnetization
is antiparallel the resistance reaches a peak107. Fig. 7 shows reported TMR
ratio since 1995. The best on-off ratio is typically limited to 2-6, posing a
fundamental challenge in spintronics. The fundamental limit is from (1)
selection of right magnetic materials and insulator. (2) interface quality
between the materials. Thus, vdW interface would be very promising. Among
reported values, vdW heterostructures relatively high TMR ratio around 2-3.
Further progress can be made using ultra-clean vdW interface.

Figure 7: Reported TMR ratio since 1995. 2D magnets-based TMR showing relative
high ratio.

###  IV.3 Quantum computing

Quantum computing, poised to revolutionize computational paradigms, has been a
focal point for integrating 2D magnets to realize quantum information
processing capabilities. The unique properties of 2D magnets, such as long-
range order and tunable magnetic anisotropy, have been harnessed to engineer
qubit architectures with enhanced coherence and scalability. One approach is
through forming topological superconductivity on 2D magnets, since it was
proposed that the dispersing Majorana states could be created at the edges of
an island of magnetic adatoms on the surface of an s-wave superconductor 111;
112. Considering the high-quality interface and strong orbital hybridization
35 in vdW heterostructures, low-temperature scanning tunneling microscopy and
spectroscopy were used to reveal the signatures of one-dimensional Majorana
edge modes in vdW superconductor/magnet structure 19. On the other hand, as
exemplified by the recent discoveries of various correlated electronic states
in twisted vdW materials, moiré patterns can act as a powerful knob to create
artificial electronic structures. It was proposed that moiré pattern between a
vdW superconductor113; 114 and a monolayer ferromagnet creates a periodic
potential modulation that enables the realization of a topological
superconducting state that would not be accessible in the absence of the
moiré115. More experiments are needed to carry out to verify these proposals.

###  IV.4 Quantum sensing and metrology

In the realm of quantum sensing and metrology, the integration of 2D magnets
has ushered in a new era of precision and sensitivity, enabling the
measurement of physical quantities such as temperature and time with quantum
precision. Harnessing quantum phenomena like spin-dependent transport and
quantum entanglement, sensors based on 2D magnets have demonstrated remarkable
performance in detecting magnetic fields, gravitational waves, and
electromagnetic fields with unprecedented accuracy. As an example, NV centres
in diamond have been utilized as quantum sensors owing to the high sensitivity
of their spin state to environmental conditions, and defects that are
localized near or at the surface of the host material are likely to provide
improved sensitivity and accessibility. However, despite progress in the
growth of diamond thin films and nanowires, the difficulty of integrating
diamond components into devices using traditional lithographic approaches
motivates the exploration of defect states in 2D material systems, like h-BN
and transition metal dischalcogenides. Moreover, this high-sensitivity quantum
sensing holds promise for characterizing nanoscale 2D magnetism. For instance,
the NV center in diamond has emerged as a potent probe for static magnetism in
2D vdW materials, offering quantitative imaging at nanoscale spatial
resolutions. By employing coherent control of the NV center’s spin precession,
researchers achieved ultrasensitive, quantitative ac susceptometry of a 2D
ferromagnet, CrBr3 116.

##  V Challenges and opportunities

Figure 8: Challenges and opportunities in 2D magnets-based quantum devices.
Challenges existing in materials and devices level like farication and
scalability, stability and nanoscale magnetization conntrol, and large-scale
integration. Future applications of these 2D magnets-based quantum devices are
topological superconductivity for quantum computing, novel magnetic sensor,
non-volatile memory and ultracompact logic devices.

###  V.1 Fabrication and scalability of 2D magnets

The fabrication and scalability of 2D magnets present significant challenges
despite their promising properties. One major hurdle lies in achieving high-
quality, large-area 2D materials with uniform magnetic properties (Fig. 8).
Current techniques such as conventional mechanical exfoliation using scotch
tape and chemical vapor deposition often struggle to consistently produce 2D
magnets with controlled thickness and desired magnetic characteristics across
a scalable format. Progress has been in MBE grown mm to cm sized Fe3GeTe2 thin
films117; 60. Moreover, integrating these materials into functional devices
requires precise control over layer stacking, interface quality, and
maintaining magnetic order at reduced dimensions. Another critical issue is
ensuring the stability and durability of 2D magnets under real-world
conditions, including environmental factors and operational stresses, which
can affect their performance and longevity. Addressing these challenges
demands innovative fabrication methods, advanced characterization techniques,
and comprehensive understanding of the fundamental physics governing 2D
magnetism, ultimately paving the way for practical applications in future
technologies.

###  V.2 Stability and control of magnetic properties at the nanoscale

The stability and control of magnetic properties at the nanoscale in 2D
magnets represent pivotal challenges and opportunities (Fig. 8) in the field
of nanomagnetism. In the case of CrI3, which was extensively studied for its
antiferromagnetic coupling and semiconducting properties, its thin films can
easily and rapidly disapprear in the air due to the oxidization. Achieving
stable magnetic properties requires precise control over factors such as layer
thickness, defects, and strain, which can significantly impact magnetic
anisotropy and coercivity. Moreover, the ability to manipulate these
properties dynamically, for instance, through external stimuli like electric
fields or spin currents, opens avenues for developing next-generation magnetic
devices with enhanced functionality and energy efficiency. Yet, due to either
metallic or insulating properties of most 2D magnets, tuning of carrier
density in the system is not easy to achieve. Many more semiconducting magnets
are yet to be discovered, which will propel this field forward. Overcoming
these challenges involves advancing fabrication techniques to achieve defect-
free and reproducible 2D magnet structures, as well as exploring novel
approaches to tailor magnetic properties at the nanoscale, thereby unlocking
the full potential of 2D magnets in future nanoelectronics and spintronic
applications.

###  V.3 Integration with existing quantum technologies

The integration of 2D magnets with existing quantum technologies presents an
exciting frontier in materials science and quantum engineering (Fig. 8). By
combining the unique magnetic properties of 2D materials118; 119 with
superconductivity or spintronics, researchers aim to create novel devices with
enhanced functionalities9. The coexistence of 2D magnetism and
superconductivity in layered materials offers intriguing prospects for
exploring new quantum phenomena and potentially uncovering mechanisms that
could advance quantum computing and communication technologies. For example,
recent progress has been made on topological superconductivity in vdW
hetersotructures, where low-temperature scanning tunnelling microscopy and
spectroscopy were used to reveal the signatures of one-dimensional Majorana
edge modes19. Moreover, 2D topological spin texture like skyrmion can be
incorporated to carry the Majorana edge mode120. This interdisciplinary
approach promises to unlock new capabilities and applications at the
intersection of magnetism and quantum science, paving the way towards more
efficient, versatile, and powerful quantum technology.

###  V.4 Opportunities for further research and development

Opportunities for further research and development in 2D magnets abound (Fig.
8), driven by their unique properties and potential applications across
various fields. Exploring new 2D materials and understanding their magnetic
behaviors at atomic scales could uncover novel magnetic phenomena and enhance
our ability to engineer customized magnetic properties. This can be done
integrating artificial intelligence, where the materials properties can be
predicted by supervised/unsupervised learning121; 122. Integrating 2D magnets
with other nanoscale materials, such as superconductors or topological
insulators, offers exciting prospects for discovering emergent quantum states
and advancing quantum information technologies. Additionally, investigating
dynamic control mechanisms for manipulating magnetic states in 2D materials,
such as SOT or magnetoelectric effects, promises to unlock new functionalities
for ultrafast and energy-efficient magnetic devices. For the industry side, 2D
magnets offer innovative solutions and advancements across multiple sectors.
In electronics, these non-volatile memories based on 2D magnets could
significantly enhance the performance and scalability of storage solutions,
reducing power consumption and improving data access speeds. Moreover, in
sensor technologies, the unique magnetic properties of 2D materials can be
leveraged to create highly sensitive and miniaturized magnetic sensors for
diverse applications, including biomedical diagnostics, automotive systems,
and industrial monitoring. Additionally, the integration of 2D magnets into
spintronic devices holds promise for realizing next-generation computing
architectures with enhanced processing capabilities and reduced energy
consumption. Furthermore, advancements in 2D magnet research could lead to
breakthroughs in quantum technologies, enabling the development of more robust
and versatile platforms for quantum computing, communication, and sensing.

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