Magnetic-field tuning of the spin dynamics in the quasi-2D van der Waals antiferromagnet CuCrP$_{2}$S$_{6}$

Abstract

The use of antiferromagnets in magnetoelectronic devices as counterparts of ferromagnets is a new, rapidly developing trend in spintronics that leverages antiferromagnetic (AFM) magnons for transmitting of spin currents. Van der Waals (vdW) antiferromagnets are particularly attractive in this respect as they possess tunable magnetic properties and can be easily integrated into spintronic devices. In this work we use electron spin resonance (ESR) spectroscopy to assess the potential of the vdW AFM compound CuCrP$_{2}$S$_{6}$ for magnonic applications by exploring the magnetic field ($H$) dependence of the spectrum of magnon excitations below its AFM ordering temperature $T_{\rm N} \approx 30$ K and the correlated spin dynamics above $T_{\rm N}$. ESR reveals prominent ferromagnetic (FM) spin correlations that persist far above $T_{\rm N}$ suggesting an intrinsically two-dimensional character of the spin dynamics in CuCrP$_{2}$S$_{6}$. Most interestingly, at $T < T_{\rm N}$, CuCrP$_{2}$S$_{6}$ features two non-degenerate, i.e., distinct in energy AFM magnon modes at $H = 0$ which can be tuned to the FM type of collective spin excitations with increasing $H$. These remarkable properties are favorable for the induction and control of unidirectional spin current in CuCrP$_{2}$S$_{6}$ and suggest it as a new functional material for magnetoelectronics.

Figures (15)

• Figure 1: Crystal structure of CuCrP$_{2}$S$_{6}$ as determined in Ref. maisonneuve1993. (a) View normal to the $ab$-plane, i.e., along $c^*$-axis, showing the structure at $T = 64$ K with Cr$^{3+}$ ions forming a 2D triangular magnetic lattice. (b) Quasi-2D layers stacked along the $c$-axis ($T = 64$ K). (c) Same as (b), at high temperature ($T = 300$ K) showing the distribution of Cu$^{1+}$ ions at two positions. The ratio of red and white colors denotes the site occupancy probability of Cu$^{1+}$ ions. Rectangle in (b) and (c) represents a unit cell.
• Figure 2: HF-ESR spectra of CuCrP$_{2}$S$_{6}$ at various temperatures for (a) $\textbf{H} \parallel {\bf b}$ configuration at 166 GHz and for (b) $\textbf{H} \parallel {\bf c^*}$ configuration at 170 GHz. The spectral shape was corrected to eliminate the dispersive component of the detected signal as explained in pal2024. The spectra are also normalized and shifted vertically for clarity.
• Figure 3: (a) Temperature dependence of the shift $\delta H (T) = H_{res} (T) - H_{res} (300\,{\rm K})$ of the resonance field from the paramagnetic position denoted by the horizontal dashed gray line for both orientations. (b) Evolution of the linewidth $\Delta H$ as a function of temperature. The vertical dashed gray line on both plots indicates the AFM transition temperature $T_{\rm N}$ of CuCrP$_{2}$S$_{6}$. Error bars less than the symbol size are omitted. Lines connecting data points are guides for the eye.
• Figure 4: Frequency dependence $\nu(H_{\rm res}$) at 300 K for (a) $\textbf{H} \parallel {\bf b}$ and (b) $\textbf{H} \parallel {\bf c^*}$ shown by open squares and triangles, respectively. The solid red line is the fit to the paramagnetic resonance condition (Equation (\ref{['eq:FDep_300K']})). Right vertical axis: representative ESR signals at different frequencies normalized and vertically shifted above the corresponding $\nu$ vs $H_{res}$ data points. The $g$-factor values are summarized in the table in Appendix \ref{['sec:abc_prop']}
• Figure 5: (a) $\nu(H_{\rm res})$ dependence of the resonance modes (branches) at 3 K for field configurations $\textbf{H} \parallel {\bf b}$ and $\textbf{H} \parallel {\bf c^*}$ (open symbols). Solid squares represent the results of the ESR measurements in the frequency domain for $\textbf{H} \parallel {\bf b}$ configuration (see Appendix \ref{['sec:FreqDom']}). Solid black and red lines denoted as L$_{\rm b}$ and L$_{\rm c^\ast}$ branches are the result of the modeling of the experimental data based on Hamiltonian (\ref{['Hamil']}). Dashed blue line represents the paramagnetic branch L$_{\rm par}$ described by Equation (\ref{['eq:FDep_300K']}). The dot-dashed line in green and the short-dashed line in magenta are modeled FMR branches calculated by setting $A_{\rm ex} = 0$ in Hamiltonian (\ref{['Hamil']}) (for details see the text). Right vertical axis: exemplary spectra (normalized and shifted vertically) for both field orientations at selected frequencies. (b) Zoomed-in low-field part of the $\nu(H_{\rm res})$ diagram in panel (a). Diamonds are the data from Wang et al.wang2023 for the magnetic field lying along easy axis axes_avsb at $T = 10$ K, open circles are the resonance fields for $\textbf{H} \parallel {\bf b}$ of the signals measured at the X-band frequency of 9.56 GHz which is indicated by the horizontal dashed line (see Section \ref{['sec:X-band']}). The notation of the solid black and the dashed blue lines is the same as in panel (a) (for details see the text).