Understanding the interplay between spin states and lattice dynamics is crucial for developing next-generation spintronic devices. However, directly probing spin-phonon coupling in frustrated magnetic systems remains a challenge due to the lack of long-range magnetic order.

In a study published in Physical Review B, a research group led by Prof. SHENG Zhigao from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences reported the direct observation of strong spin-phonon coupling in the absence of long-range magnetic order.

Researchers focused on NaCrO₂, a typical two-dimensional layered triangular frustrated antiferromagnet with spin S=3/2. They employed the Steady High Magnetic Field Facility (SHMFF) and a temperature-dependent terahertz time-domain spectroscopy system.

Taking advantage of the low-frequency detection sensitivity of terahertz waves, researchers tracked temperature-dependent terahertz absorption spectra to sensitively capture the subtle phonon changes induced by spin-phonon coupling under short-range magnetic order.

They showed that short-range spin correlations in NaCrO₂ persisted well above the magnetic phase transition temperature. Near the critical point, these correlations gave rise to strong spin-phonon coupling, leading to abrupt changes in the frequency and intensity of low-frequency phonons. This effect was attributed to the renormalization of Cr–O bonds and resulted in an anomalous enhancement of infrared-active phonon modes.

"Our findings reveal the spin-phonon coupling under short-range magnetic order in frustrated magnetic materials, laying a foundation for developing next-generation spintronic devices," said Prof. SHENG.

Temperature dependence of phonon frequency and intensity. (Image by QIU Kang)