After the discovery of graphene in 2004, the family of 2D materials has grown rapidly.
But since 2016 there have been numerous reports of 2D magnetic materials that can be exfoliated with ease, similarly to graphene.
The first few-layered van der Waals magnetism was reported in 2017 (Cr2Ge2Te6,[1] and CrI3[2]).
[3] One reason for this seemingly late discovery is that thermal fluctuations tend to destroy magnetic order for 2D magnets more easily compared to 3D bulk.
It is also generally accepted in the community that low dimensional materials have different magnetic properties compared to bulk.
This academic interest that transition from 3D to 2D magnetism can be measured has been the driving force behind much of the recent works on van der Waals magnets.
Much anticipated transition of such has been since observed in both antiferromagnets and ferromagnets: FePS3,[4] Cr2Ge2Te6,[1] CrI3,[2] NiPS3,[5] MnPS3,[6] Fe3GeTe2[7] Although the field has been only around since 2016, it has become one of the most active fields in condensed matter physics and materials science and engineering.
There have been several review articles written up to highlight its future and promise.
The special feature of these new materials is that they exhibit a magnetic ground state, either antiferromagnetic or ferromagnetic, when they are thinned down to very few sheets or even one layer of materials.
Another, probably more important, feature of these materials is that they can be easily produced in few layers or monolayer form using simple means such as scotch tape, which is rather uncommon among other magnetic materials like oxide magnets.
[4][12] The field was expanded further with the publication of similar observations in ferromagnetism the following year.
[1][2] Since then, several new materials discovered and several review papers have been published.
[8][9][10] Magnetic materials have their (spins) aligned over a macroscopic length scale.
), causing a phase transition to a non-magnetic state.
[13] For 2D systems, the transition temperature depends on the spin dimensionality (
, described by the isotropic Heisenberg model, does not display magnetism at any finite temperature.
The long range ordering of the spins for an infinite system is prevented by the Mermin-Wagner theorem stating that spontaneous symmetry breaking required for magnetism is not possible in isotropic two dimensional magnetic systems.
Spin waves in this case have finite density of states and are gapless and are therefore easy to excite, destroying magnetic order.
The intrinsic anisotropy in CrI3 and Fe3GeTe2 is caused by strong spin–orbit coupling, allowing them to remain magnetic down to a monolayer, while Cr2Ge2Te6 has only exhibit magnetism as a bilayer or thicker.
It was reported that the theoretical predictions of the XY model are consistent with those experimental observations of NiPS3.
In this system, there is no transition between the ordered and unordered states because of the Mermin-Wagner theorem.
[14][6] The above systems can be described by a generalized Heisenberg spin Hamiltonian: Where
[9] Magnetic properties of two-dimensional materials are usually measured using Raman spectroscopy, Magneto-optic Kerr effect, Magnetic circular dichroism or Anomalous Hall effect techniques.
[9] The dimensionality of the system can be determined by measuring the scaling behaviour of magnetization (
The critical exponents depend on the system and its dimensionality, as demonstrated in Table 1.
Therefore, an abrupt change in any of the critical exponents indicates a transition between two models.
Furthermore, the Curie temperature can be measured as a function of number of layers (
[17] Magnetic 2D materials can be used as a part of van der Waals heterostructures.
This structure can have significant spin valve effect,[18] and thus they can have many applications in the field of spintronics.
Another newly emerging direction came from the rather unexpected observation of magnetic exciton in NiPS3.