CrI3 is one of the hot topics in the 2D community, ever since it was discovered to have layer-dependent magnetism (https://www.nature.com/articles/nature22391). In particular, it is ferromagnetic in monolayer, anti-ferromagnetic in bilayer, ferromagnetic in tri-, and so forth, due to inter-layer anti-ferromagnetic coupling.
But in bulk, it has been confirmed to be ferromagnetic. I never quite understood why this is so. As funny as it sounds, technically, shouldn't it be sometimes ferromagnetic and sometimes antiferromagnetic, depending on how many layers there are?
Among the most likely reasons are the existence of RKKY-type ( Ruderman-Kittel-Kasuya-Yoshida) exchange interactions between CrI3 layers.
The main specificity of this type of interaction is that it is alternative, i.e. it can change its sign from one layer to another, passing from a ferromagnetic character to an antiferromagnetic character, depending on the number of layers inserted.
So, depending on the number of CrI3 layers, this system can switch from ferro to antiferro.
That's a question I asked myself three years ago and tried to answer. You will find the answer in the attached paper (Ferromagnetism in CrI3 and Fe), which I have not yet published.
Magnetism in Van der Waals ferromagnet, Fe3GeTe2
In f-electron heavy-fermion systems, the localized f-spin is a necessary prerequisite for the appearance of magnetism and heavy-fermion state, which originate from the RKKY interaction and Kondo effect, respectively. Van der Waals ferromagnet, Fe3GeTe2 with a Tc of 230 K (1) is a non–f-electron systems but ferromagntism appears as a results of an effective electron mass enhancement meff= 5.29 me (1)
TC=(pi*h2/4kB)*1/(sqrt(5.29me*MFe)*(3.99Angström)2= 233 K.
where a=3.99 Anström is the lattice constant of Fe3GeTe2 (1) MFe atomic mass of iron and meff of 5.29 the mass of heavy electrons.
Reference:
(1) Yun Zhang et al. Vol. 4, no. 1, eaao6791, Science Advances 12 Jan 2018
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