Evolution of interlayer and intralayer magnetism in three atomically thin chromium trihalides

The recent discoveries of magnetism in the monolayer limit have opened a new avenue for two-dimensional (2D) materials research. Already, several groups have reported a giant magnetoresistance effect in CrI$_3$ tunnel junctions as well as electric field control of its magnetic properties. Overall, these remarkable effects arise from strong magnetic anisotropy combined with weak interlayer magnetic interactions, which allow for spins in individual CrI$_3$ layers to be flipped by relatively small external fields. A quantitative understanding of these properties is thus crucial for future applications of such materials. Here, we conduct a comprehensive study of three different magnetic semiconductors, CrI$_3$, CrBr$_3$, and CrCl$_3$, by incorporating both few- and bi-layer samples in van der Waals tunnel junctions. We find that the interlayer magnetic ordering, exchange gap, magnetic anisotropy, as well as magnon excitations evolve systematically with changing halogen atom. By fitting to a spin wave theory that accounts for nearest neighbor exchange interactions, we are able to further determine the microscopic spin Hamiltonian for all three systems. These results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin classes in a single material family. Using magneto-optical measurements, we additionally demonstrate that ferromagnetism can be stabilized down to monolayer in nearly isotropic CrBr$_3$, with transition temperature close to that of the bulk.