Pressure-dependent magnetic properties of bulk CrBr3 single crystals.
The van der Waals class of materials offer an approach to two-dimensional magnetism as their spin fluctuations can be tuned upon exfoliation of layers. Moreover, it has recently been shown that spin-lattice coupling and long-range magnetic ordering can be modified with pressure in van der Waals materials.
In this publication, the magnetic properties of quasi two-dimensional CrBr3 are reported applying hydrostatic pressure. The application of pressure up to 0.844 GPa shows a 1.77% decrease in saturation magnetization with a decrease in the Curie temperature from 33.05 to 30.41 K. Density functional theory calculations with pressure up to 1 GPa show a reduction in volume and interplanar distance as pressure increases.
To further understand the magnetic properties with applied pressure, the magnetocrystalline anisotropy energy (MAE) and exchange coupling parameter(s) (J) are calculated. Overall, CrBr3 displays ferromagnetic interlayer coupling and the calculated exchange coupling and MAE parameters match well with the observations from the experimental work.
Single crystal samples of CrBr3 were prepared using chemical vapor transport by placing powders of Cr metal and TeBr4 inside a quartz tube and maintaining the hot and cold zones at 700 and 600 °C for 5 days. The green plate-like crystals were a few millimeters across and 50 µm thick. Magnetic measurements were performed using a Quantum Design MPMS 3 SQUID magnetometer. Isothermal magnetization measurements were taken at 2 K with a ± 7 T magnetic field. Zero-field cool (ZFC) temperatures dependent magnetization measurements were performed from 2 - 400 K. Hydrostatic pressure was applied using a Quantum Design piston cell.
The Density functional theory (DFT) calculations of vdW layered CrBr3 were carried out with Vienna ab initio (VASP) code within projector augmented-wave (PAW) method. General gradient approximation (GGA) in the Perdew-Burke-Enzerhof (PBE) were used for the exchange correlation functional. The plane-wave cut-off energy was 500 eV and an 8×8×3 k-point was used to sample the Brillouin zone.
For the relaxation of the crystal structure, the non-local vdW functional in form of optB88-vdW was incorporated to account for the interlayer vdW force, and all the lattice constants and ionic coordinates were relaxed until the maximum force on all ions was less than 5 × 10−3 meV/Å.
For the magnetic property calculations, the on-site Coulomb interactions are taken into account using LDA+U to improve the description of theinteractions between localized d electrons of transition atoms. The hydrostatic pressure effectcalculation was done by adding the PSTRESS tag, which adds the stress to the stress tensor and an energy generated from the external pressure.
This work explored the magnetic characteristics of the less studied chromium halide, CrBr3, with pressure as the tuning parameter. It was revealed through experiment that Ms and TC both decrease upon increasing the pressure up to 0.8 GPa. Moreover, through computations we realize that the role of pressure on J values is very complicated and gives rise to further questioning, especially considering that there are a very few pressure-induced calculations and experiments performed on the bulk CrBr3 system. Additionally, the behavior of MAE as function of increasing pressure directly relates to the decrease in TC our work. Although, TC was not drastically altered for the bulk CrBr3 case, recent works have shown the importance of carrying out pressure dependent studies at significantly high pressures, thus, it is important to fully characterize the pressure dependent magnetic properties of CrBr3 at pressures above 1 GPa in the future.