Observation and Analysis of Low-Energy Magnons with Raman Spectroscopy in Atomically Thin NiPS₃

NiPS₃ is one of the layered van der Waals magnetic materials, showing XXZ-type antiferromagnetic ordering below the Néel temperature (TN) of 155 K [1]. The antiferromagnetic ordering is suppressed in the monolayer [2]. Recently, several groups reported low-energy excitations in bulk NiPS₃, measured using different experimental tools such as THz spectroscopy, the pump-probe method, and electron-spin resonance (ESR), [3-5] and the signals have been identified as low-energy magnons gapped from the ground state. Here, we employed Raman scattering techniques to observe directly the low-lying magnon (~1 meV) even in bilayer NiPS₃. The unique advantage is that it offers excellent energy resolutions far better on low-energy sides than most inelastic neutron spectrometers can offer. More importantly, with appropriate theoretical analysis, the polarization dependence of the Raman scattering by those low-lying magnons also provides otherwise hidden information on the dominant spin-exchange scattering paths for different magnons. By comparing with high-resolution inelastic neutron scattering data, these low-energy Raman modes are confirmed to be indeed of magnon origin. Because of the different scattering mechanisms involved in inelastic neutron and Ramahn scattering, this new information is fundamental in pinning down the final spin Hamiltonian. This work demonstrates the capability of Raman spectroscopy to probe the genuine two-dimensional spin dynamics in atomically-thin vdW magnets, which can provide novel insights that are obscured in bulk spin dynamics.<br/><br/>[1] Balkanski, M. <i>et al</i>., <i>Pure and Applied Chemistry</i> <b>59</b> (10), 1247-1252 (1987).<br/>[2] Kim, K. <i>et al</i>., <i>Nat. Commun</i>., <b>10</b>, 345 (2019).<br/>[3] Belvin, C. A. <i>et al</i>., <i>Nat. Commun.</i>, <b>12</b> (1), 4837 (2021).<br/>[4] Afanasiev, D.<i> et al</i>., <i>Sci. Adv.</i>, <b>7</b> (23), eabf3096 (2021).<br/>[5] Mehlawat, K.<i> et al</i>., <i>Phys. Rev. B</i>, <b>105</b> (21), 214427 (2022).