Magnetic-field-induced magnon portfolio in a van der Waals magnet

Abstract

Magnonic excitations are investigated in chromium oxychloride (CrOCl), a van der Waal (vdW) antiferromagnet prone to a multitude of magnetic phase transitions, with absorption experiments in a broad continuous energy range. At low magnetic fields, the magnon spectra show a strong bi-axial anisotropy and inform on the relative weights of the effective exchange coupling and the system anisotropies. As the magnetic field increases, magnons characteristic of a canted phase are first observed, with peculiarities attributed to in-plane anisotropies and magnon-magnon coupling. Subsequently, a hysteretic magnon spectrum appears as the system transitions to a ferrimagnetic state, with two new magnon branches partly coexisting with the lower energy canted phase branch, indicating the formation of spatially separated magnetic phases. Further changes in the magnon spectrum in higher magnetic fields accompany transitions to the different canted magnetic phases previously reported. Our experiments show that competing exchange interactions and ground states broaden the options to generate different kinds of magnonic excitations in the same vdW material upon the variation of external parameters.

Figures (3)

• Figure 1: (color online) (a) Crystal structure (2 layers, only half of the bottom layer is represented). (b) Definition of exchange coupling parameters between Cr spins. (c) Different phases reported in the literature (see text) with their schematic Cr spin configurations. The magnetic field boundary of each phase is approximately positioned based on previous experiments Reuvekamp2014Gu2022Pawbake2025 performed with upward magnetic fields sweeps at $T\sim 4$ K. Insets: representative raw data of magnetic field sweep-induced resonances at fixed microwave frequencies (specified to the first digit in GHz near each traces) in different magnetic phases. Relative changes of the photo-response phase $\phi$ are expressed with respect to their "off-resonant" value $\phi_{0}$. Data in black (blue) were taken with upward (downward) magnetic fields sweeps. The signal at $B\sim$11 T is obtained by Fourier Transform spectroscopy.
• Figure 2: (color online) (a) Magnetization traces for an upward (black) and downward (blue) magnetic field applied along the $c$-axis, at T$=4.2$K. (b) Magnetic resonances frequencies as a function of the magnetic field $B$ along the c-axis, for upward sweeps (open black up-triangles), or downward sweeps (open blue down-triangles). The sample temperature lies in the range $T = 3.9-4.8$ K depending on the microwave frequency. Triangles with a middle vertical dash were obtained at $T =4.2$ K by a FT spectrometer operating above $f\sim$ 250 GHz in the employed configuration. Additional data obtained with an EPR spectrometer at $T \sim5$ K are reported as half-filled triangles, with the same upward/downward sweeps convention. The location and extent of the different magnetic phases (AFM, cAFM, FiM, cFiM, see text) are indicated with double-sided horizontal arrows in black for upward sweeps, and blue for downward sweeps whenever hysteresis is observed.
• Figure 3: (color online) Frequency of the main absorption observed in the FIR range, as a function of magnetic field (up triangles, left scale). Magnetization curves from Ref. Pawbake2025 (dark green solid (dashed) line for upward (downward) sweep, right scale). Magnetic phases identified in Fig. \ref{['Fig2']} (black text) and inferred from Ref. Pawbake2025 (dark green text).