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.

Magnetic susceptibilities and magnetization isotherms have been measured for single crystal FeTiO 3  The susceptibility parallcl to the c -axis has a large and sharp peak at T N =580±03 K, while the perpendicular susceptibility is almost independent of temperature below T N  Magnetization isotherm at 42 K with fields applied along the c -axis exhibits a typical metamagnetic transition at H c =803±01 kOe, being the first observation of metamagnetism in 3 d transition metal oxides The transition is of the first order up to about 35 K, above which it is of the second order The experimental data are well reproduced by molecular field calculations with the best fit parameters of the anisotropy field H A =150 kOe, the ferromagnetic intrasublattice exchange interaction of 2 z 1 J 1 =285 K, and the antiferromagnetic intersublattice interaction of 2 z 2 J 2 =-667 K

Metamagnetic Phase Transitions in FeTiO 3

The ground state of the triangular lattice gas model with up to 3rd neighbor interactions is investigated by use of the method of geometrical inequalities. The structure of the ground state at an arbitrary value of the particle density is determined in the full regimes of the interactions. Various ordered structures are obtained. Several examples of systems of adsorbates on the FCC(111) plane, of interstitial atoms in the transition metal carbides and nitrides and of the Ising spins, which may correspond to the present model, are discussed on the basis of the obtained results. A certain amount of information about the interactions in the systems is deduced from the observed ordered structures.

Ground State Structure of Triangular Lattice Gas Model with up to 3rd Neighbor Interactions

The magnetic excitations in FeBr2 have been studied between 2K and 9K using inelastic neutron scattering. The spin waves may be described by a simple S=1 Hamiltonian with an anisotropy field g mu BHA=0.428+or-0.016 THz, intra-plane isotropic interactions of 2J1=0.152+or-0.008 THz, 2J2=-0.051+or-0.006 THz and an inter-planar interaction 2J'1=-0.06+or-0.005 THz. The spin-wave spectrum is quite similar to FeCl2, although slightly less two-dimensional. At T>2K a second type of excitation is observed, which interacts strongly with the spin waves and is thought to correspond to the single ion transition from the Sz=0 to Sz=+1 state of the ground-state triplet.

Neutron scattering study of the magnetic excitations in FeBr2

Phase Transitions of Fel2 in high magnetic field parallel to the spin direction, static field up to 150 kOe, pulsed field up to 250 kOe

Magnon energies and phase transitions of ferrous iodide in a theoretical eight sublattice model

Phase transition of an antiferromagnetic system of triplet ions with uniaxial anisotropy in a parallel magnetic field

Nous presentons, pour les halogenures ferreux FeCl2, FeBr 2, FeI2, les isothermes d'aimantation obtenues, a basse temperature, en champ magnetique perpendiculaire (champ statique et champ pulse). Ces resultats experimentaux mettent en evidence un ecart a la loi lineaire d'aimantation. Nous donnons une interpretation naturelle de nos resultats par le traitement rigoureux d'un modele theorique ou l'on considere une assemblee d'ions triplets caracterises par une anisotropie uniaxiale dont l'amplitude D est du meme ordre de grandeur que les interactions d'echange, ferromagnetiques dans le plan (J 1) et antiferromagnetiques entre plans (J2) (approximation du champ moleculaire). Cette etude theorique permet d'evaluer l'ecart a la loi lineaire pour toute valeur de D, J1 et J 2 et montre le role fondamental joue par l'anisotropie. Ce modele theorique donne une explication plus satisfaisante des resultats obtenus pour FeCl2, FeBr2 et FeI2 que ceux donnes par les modeles precedents.

https://hal.archives-ouvertes.fr/jpa-00208140/document

J. Gelard

Papers

Phase Transitions of Fel2 in high magnetic field parallel to the spin direction, static field up to 150 kOe, pulsed field up to 250 kOe

Magnon energies and phase transitions of ferrous iodide in a theoretical eight sublattice model

Phase transition of an antiferromagnetic system of triplet ions with uniaxial anisotropy in a parallel magnetic field

Comportement non linéaire de l'aimantation de FeCl2, FeBr2 et FeI2 en champ perpendiculaire a l'axe d'anisotropie