Magnetic field tuning of modulated magnetic orders in CrOCl at the two-dimensional limit

Abstract

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 existence domains of the different phases change with the number of layers.

Figures (6)

• Figure 1: Air-stable 2D flakes of CrOCl exfoliated down to the monolayer. (a) Ball-and-stick models of CrOCl's atomic structure (iso-structural to that of CrSBr), from different perspectives. The $\alpha$ angle goes off-90$^\circ$ in the orthorhombic phase. (b) Optical micrograph of a CrOCl flake exfoliated on SiO$_2$/Si. Inset: Optical contrast as function of the flake thickness (number of layers $N$). (c,d) AFM topographs of bilayer and monolayer regions, together with height profile across the step edges (along the white lines). (e) Raman scattering spectra (5 K, laser wavelength 515 nm) of a single layer, on the day of exfoliation and six days after.
• Figure 2: Interlayer vibrational coupling in few-layer CrOCl. (a) Raman scattering spectra (5 K) of bulk and few-layer exfoliated CrOCl. The zone-folded phonon modes (labeled with a *) are related to the antiferromagnetic superstructure. (b) Frequency of the $A_{\mathrm{g}}^1$, $A_{\mathrm{g}}^2$ and $A_{\mathrm{g}}^3$ modes as function of the number of layers, from experiments (solid symbols) and from DFT calculations (gray crosses/lines; a scale factor of 5-6% has been applied for wavenumber values to ease the comparison with the experimental data). Insets illustrate the atomic displacements associated with each phonon mode.
• Figure 3: Inelastic scattering signature of 2D magnetism. Frequency of the $A_{\mathrm{g}}^1$ phonon mode as function of temperature for bulk CrOCl and for a bilayer and a single-layer. The vertical dashed lines mark the Néel temperature $T_\mathrm{N\acute{e}el}$ to an antiferromagnetic (AFM) state and the magnetic incommensurate (IC) / paramagnet (PM) phase transition ($T_\mathrm{mag}$) in the bulk.
• Figure 4: Spin-phonon coupling and zone-folding from a commensurate phase to the next one. (a) Waterfall 2D maps of the Raman scattering spectra (5 K), around the $A_{\mathrm{g}}^3$ mode, acquired at increasing magnetic fields, for (from left to right) bulk CrOCl, six-, four-, three-, two- and single-layer flakes. (b) Selected spectra (arbitrary units) for the commensurate antiferromagnetic (1/4, low field) and ferrimagnetic (1/5, high field) phases, as well as for the intermediate field region, for the bulk (left), a six-layer (center) and a bilayer (right). (c) Field-domains of existence, derived from the Raman scattering data, of the 1/4-incommensurate (antiferromagnetic, AFM), incommensurate (IC) and 1/5-commensurate (ferrimagnetic, FiM) phases (0-10 T range), as a function of the number of layers. The spin-flop transition is indicated with a solid gray line. The spin arrangement for the two commensurate phases is sketched.
• Figure 5: Spin-phonon coupling in a canted magnetic phase. (a,b) Waterfall 2D maps of the Raman scattering spectra (5 K), around the $A_{\mathrm{g}}^1$ (a) and $A_{\mathrm{g}}^3$ (b) modes, in the 10-30 T range, for (left to right) 50- (i.e. bulk), six- , three-, two- and single-layer flakes. The red arrows mark the field values at which the $F_{1/5}$ folded mode vanishes. (c,d) Field-induced evolution of the frequency of the $A_{\mathrm{g}}^1$ ($\omega_1$) (c) and $A_{\mathrm{g}}^3$ ($\omega_3$) modes, with respect to their frequency at 10 T (d). Solid lines are quadratic fits to the data. In (d), thick lines connect the data points around the minimum of $\omega_3(10)-\omega_3(H)$. Inset: sketch of the canted ferrimagnetic (c-FiM) and antiferromagnetic (c-AFM) spin arrangements, with an intermediate, unassigned state in the intermediate field range. Note that the field was applied with a superconductive coil below 14 T and with a resistive one above.