Materials composed of two dimensional layers bonded to one another through weak van der Waals interactions often exhibit strongly anisotropic behaviors and can be cleaved into very thin specimens and sometimes into monolayer crystals. Interest in such materials is driven by the study of low dimensional physics and the design of functional heterostructures. Binary compounds with the compositions M X 2 and M X 3 where M is a metal cation and X is a halogen anion often form such structures. Magnetism can be incorporated by choosing a transition metal with a partially filled d-shell for M, enabling ferroic responses for enhanced functionality. Here a brief overview of binary transition metal dihalides and trihalides is given, summarizing their crystallographic properties and long-range-ordered magnetic structures, focusing on those materials with layered crystal structures and partially filled d-shells required for combining low dimensionality and cleavability with magnetism.

https://www.mdpi.com/2073-4352/7/5/121/pdf/1

Crystal and Magnetic Structures in Layered, Transition Metal Dihalides and Trihalides

Materials composed of two dimensional layers bonded to one another through weak van der Waals interactions often exhibit strongly anisotropic behaviors and can be cleaved into very thin specimens and sometimes into monolayer crystals. Interest in such materials is driven by the study of low dimensional physics and the design of functional heterostructures. Binary compounds with the compositions MX2 and MX3 where M is a metal cation and X is a halogen anion often form such structures. Magnetism can be incorporated by choosing a transition metal with a partially filled d-shell for M, enabling ferroic responses for enhanced functionality. Here a brief overview of binary transition metal dihalides and trihalides is given, summarizing their crystallographic properties and long-range-ordered magnetic structures, focusing on those materials with layered crystal structures and partially filled d-shells required for combining low dimensionality and cleavability with magnetism.

Full-potential linearized augmented plane-wave method has been used to calibrate isomer shift for the $14.4\text{\ensuremath{-}}\mathrm{keV}$ resonant transition in $^{57}\mathrm{Fe}$. Augmented plane waves and local orbitals were used for the valence electrons. For the correlation and exchange potentials, a generalized gradient approximation has been adopted. Calculations have been performed for the following compounds: $\mathrm{Fe}{\mathrm{F}}_{2}(P{4}_{2}∕mnm)$, $\mathrm{Fe}{\mathrm{Cl}}_{2}(R\overline{3}m)$, $\mathrm{Fe}{\mathrm{Br}}_{2}(P\overline{3}m1)$, $\mathrm{Fe}{\mathrm{I}}_{2}(P\overline{3}m1)$, $\mathrm{Fe}{\mathrm{F}}_{3}(R\overline{3}c)$, $\mathrm{Ti}\mathrm{Fe}(Pm\overline{3}m)$, and $\mathrm{Fe}(Im\overline{3}m)$. Strong on-site Coulomb interactions in the $\mathrm{Fe}\phantom{\rule{0.2em}{0ex}}3d$ shell of halides were taken into account by applying the Hubbard repulsion parameter $U$ and the on-site exchange interaction constant $J$. The isomer shift calibration constant of $\ensuremath{\alpha}=\ensuremath{-}0.291\phantom{\rule{0.3em}{0ex}}{\mathrm{a.u.}}^{3}\phantom{\rule{0.2em}{0ex}}\mathrm{mm}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ has been obtained. The nuclear quadrupole moment of the excited nuclear state involved was found to be $Q=+0.17\phantom{\rule{0.3em}{0ex}}\mathrm{b}$.

Calibration of the isomer shift for the 14.4 − keV transition in Fe 57 using the full-potential linearized augmented plane-wave method

Exploring two-dimensional (2D) materials with both ferromagnetic and ferroelectric properties is scientifically interesting and of great technical importance to numerous functionalities in nanoscale devices. In this work, we have demonstrated a strong magnetoelectric coupling that appeared in the 2D FeI2/In2Se3 van der Waals heterostructure. FeI2 layers undergo a transition from ferromagnetic to antiferromagnetic by reversing the direction of ferroelectric polarization. First-principles calculation predicts a new magnetoelectronic coupling mechanism which is completely different from the Dzyaloshinskii-Moriya (DM) effect in multiferroic materials. Because of the polarization discontinuity at the interface, the valence states of Fe ions change between +2 and +3 for two different polarization directions, leading to the magnetic interaction variation between the direct exchange and I ion mediated superexchange. Moreover, metallic 2D electron gas (2DEG) transfers from the surface of FeI2 to In2Se3 when the polarization reverses, which induces the spin polarization of the heterostructure varying from 93% to 0%. Our work is the first realization of manipulation magnetism by an electric field in full 2D van der Waals heterostructures.

https://pubs.rsc.org/en/content/articlepdf/2019/nr/c9nr01510h

Valence mediated tunable magnetism and electronic properties by ferroelectric polarization switching in 2D FeI 2 /In 2 Se 3 van der Waals heterostructures

Based on first-principles calculations, the evolution of the electronic and magnetic properties of transition metal dihalides $M{X}_{2}\phantom{\rule{4pt}{0ex}}(M=\text{V}$, Mn, Fe, Co, Ni; $X=\text{Cl}$, Br, I) is analyzed from the bulk to the monolayer limit. A variety of magnetic ground states is obtained as a result of the competition between direct exchange and superexchange. The results predict that $\mathrm{Fe}{X}_{2},\mathrm{Ni}{X}_{2},{\mathrm{CoCl}}_{2}$, and ${\mathrm{CoBr}}_{2}$ monolayers are ferromagnetic insulators with sizable magnetocrystalline anisotropies. This makes them ideal candidates for robust ferromagnetism at the single-layer level. Our results highlight the importance of spin-orbit coupling to obtain the correct ground state in these materials.

https://link.aps.org/accepted/10.1103/PhysRevMaterials.3.044001

Y. Allain

Papers

Magnetic structure of FeI2 by neutron diffraction experiments