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

Technical Deep Dive: Magnetic-field tuning of the spin dynamics in CuCrP${2}$S${6}$

Overview

The work investigates the low-energy spin dynamics in the van der Waals antiferromagnet CuCrP${2}$S${6}$, which features interpenetrating antiferroelectric Cu$^{1+}$ and antiferromagnetic Cr$^{3+}$ lattices. Using electron spin resonance (ESR) spectroscopy, the authors explore the magnetic field and temperature dependence of the spin excitations in the paramagnetic regime above the antiferromagnetic (AFM) ordering temperature ($TN \approx 30$ K) as well as in the long-range AFM ordered state below $TN$.

Methodology

• Experimental methods:
  • ESR spectroscopy at frequencies up to 330 GHz, magnetic fields up to 16 T, and temperatures 3 - 300 K
  • Density functional theory (DFT) calculations to analyze magnetic exchange and anisotropy
  • Analysis using linear spin wave theory (LSWT) to model the AFMR branches

Results

1. Paramagnetic regime above $T_N$:
   • ESR data show persistent two-dimensional ferromagnetic (FM) spin correlations in the planes well above $T_N$, evidenced by field-dependent shifts and anisotropic linewidths of the ESR signal.
   • No signatures of magneto-electric coupling were observed within the experimental accuracy.
2. AFM ordered state below $T_N$:
   • LSWT modeling of the AFMR branches reveals CuCrP${2}$S${6}$ behaves as an easy-plane biaxial antiferromagnet, with the $b$-axis as the easy axis and the $c^*$-axis as the hard axis.
   • Two distinct AFM magnon modes with a large energy difference ($\Delta2 = 84$ GHz, $\Delta1 = 13$ GHz) are observed at zero magnetic field.
3. Field-tuning of spin excitations:
   • A remarkable effect is observed where the character of the spin excitations can be tuned from AFM-type at low fields to FM-type at high fields ($\sim 3.5 - 7$ T).
   • This is attributed to the relative closeness of the interlayer exchange and uniaxial anisotropy energy scales.

Implications and Functionalities

• The non-degenerate magnon modes at zero field and the ability to tune the spin excitation character with magnetic field make CuCrP${2}$S${6}$ an interesting candidate for magnon-based spintronic devices:
  • The distinct magnon energies can be used to generate and control unidirectional spin currents without an external field.
  • The switchable AFM-FM character of the spin dynamics provides different "channels" for spin current transmission.

Limitations and Uncertainties

• The study focuses on bulk CuCrP${2}$S${6}$ crystals; the behavior of thin films or heterostructures remains to be explored.
• While the DFT calculations support the experimental findings, the exact nature of the Cu$^{1+}$ sublattice ordering is still not fully resolved.

Future Work

• Detailed investigations of the momentum-dependent spin wave dispersion using inelastic neutron scattering are called for to further elucidate the anisotropic spin dynamics in CuCrP${2}$S${6}$.
• Exploring the spin transport properties and magnon propagation in this material could provide valuable insights for magnonic applications.

Sources:

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