Magnetic Field Tuning of Modulated Magnetic Orders in CrOCl at the Two-Dimensional Limit.

Chromium oxychloride is a van der Waals magnet with intrinsic competing exchange interactions, including a strong antiferromagnetic one, source of a very rich magnetic phase diagram, with ferrimagnetic, antiferromagnetic, and canted states, up to high magnetic fields. We investigate the sequence of these magnetic phases in thin layers of CrOCl using magneto-Raman scattering spectroscopy. We identify phases whose magnetic order is commensurate with the atomic lattice, and find signatures of strong magneto-striction, presumably of exchange origin. The coupling of the spin and atomic degrees of freedom in the crystal is observed down to the single-layer limit - phonon modes significantly soften or stiffen, in a complex way due to the competition of interactions. The domains existence of the different phases change with the number of layers.