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Electronic structure and magnetic exchange interactions of Cr-based van der Waals ferromagnets. A comparative study between CrBr3 and Cr2Ge2Te6

Low dimensional magnetism has been powerfully boosted as a promising candidate for numerous applications. The stability of the long-range magnetic order is directly dependent on the electronic structure and the relative strength of the competing magnetic exchange constants. Here, we report a comparative pressure-dependent theoretical and experimental study of the electronic structure and exchange interactions of two-dimensional ferromagnets CrBr3 and Cr2Ge2Te6 . While CrBr3 is found to be a Mott-Hubbard-like insulator, Cr2Ge2Te6 shows a charge-transfer character due to the broader character of the Te 5p bands at the Fermi level. This different electronic behaviour is responsible of the robust insulating state of CrBr3 , in which the magnetic exchange constants evolve monotonically with pressure, and the proximity to a metal-insulator transition predicted for Cr2Ge2Te6 , which causes a non-monotonic evolution of its magnetic ordering temperature. We provide a microscopic understanding for the pressure evolution of the magnetic properties of the two systems.