Two-dimensional terahertz magnetic resonance spectroscopy of collective spin waves

TL;DR

This paper introduces a novel two-dimensional terahertz magnetic resonance spectroscopy technique to observe nonlinear responses of collective spin waves, enabling insights into magnon interactions in high-frequency magnetic systems.

Contribution

The study develops the first 2D terahertz magnetic resonance spectroscopy method for direct observation of nonlinear magnon responses, expanding capabilities beyond traditional magnetic resonance techniques.

Findings

Observation of magnon spin echoes and 2-quantum signals

Detection of pairwise magnon correlations at the Brillouin zone center

Resonance-enhanced second-harmonic and difference-frequency signals

Abstract

Nonlinear manipulation of nuclear and electron spins is the basis for all advanced methods in magnetic resonance including multidimensional nuclear magnetic and electron spin resonance spectroscopies, magnetic resonance imaging, and in recent years, quantum control over individual spins. The methodology is facilitated by the ease with which the regime of strong coupling can be reached between radiofrequency or microwave magnetic fields and nuclear or electron spins respectively, typified by sequences of magnetic pulses that control the magnetic moment directions. The capabilities meet a bottleneck, however, for far-infrared magnetic resonances characteristic of correlated electron materials, molecular magnets, and proteins that contain high-spin transition metal ions. Here we report the development of two-dimensional terahertz magnetic resonance spectroscopy and its use for direct observation of the nonlinear responses of collective spin waves (magnons). The spectra show magnon spin echoes and 2-quantum signals that reveal pairwise correlations between magnons at the Brillouin zone center. They also show resonance-enhanced second-harmonic and difference-frequency signals. Our methods are readily generalizable to multidimensional magnetic resonance spectroscopy and nonlinear coherent control of terahertz-frequency spin systems in molecular complexes, biomolecules, and materials.