Altermagnetism & Magnon-Phonon Coupling

Overview

In ordered magnets, spin waves (magnons) and lattice vibrations (phonons) are often treated as independent channels. In real materials, however, spin–orbit coupling, exchange striction, and magnetoelastic interactions mix these channels, producing hybridized quasiparticles whose properties differ qualitatively from either parent mode. My research uses inelastic neutron scattering (INS) and resonant inelastic X-ray scattering (RIXS) to directly resolve how magnon branches hybridize with optical phonons in itinerant antiferromagnets and correlated insulators.

By tracking dispersion renormalization, linewidth broadening, and spectral-weight transfer as functions of momentum, energy, temperature, and applied field, we extract the microscopic spin–lattice coupling constants that govern this hybridization. These are then fed into model Hamiltonians to understand how the hybrid excitation spectrum connects to measurable macroscopic quantities such as thermal conductivity, spin Seebeck coefficient, and magnon lifetime.

Case Study: Magnon Gap Tuning in Li-doped MnTe

Hexagonal MnTe is an altermagnetic semiconductor with a large magnon gap set by single-ion anisotropy and exchange. Using INS on the HYSPEC and ARCS spectrometers at the Spallation Neutron Source (SNS, ORNL), we measured the full magnon dispersion in a series of Li_xMn_1−xTe samples across a wide doping range. Li substitution on the Mn site introduces both hole doping and local structural distortions that modify the crystal-field environment around the remaining Mn²⁺ ions.

We found that the magnon gap is continuously and systematically tunable with Li content — increasing by over 50% at moderate doping levels — without destroying long-range antiferromagnetic order. By comparing the measured dispersion to spin-wave theory with a magnetoelastic coupling term, we demonstrated that the gap evolution is driven primarily by a renormalization of the single-ion anisotropy through spin–lattice interaction, rather than by changes in the exchange integrals. This work establishes Li-doped MnTe as a model system for magnon-gap engineering via chemical tuning.

Case Study: Lattice Dynamics and Magnetism in RuO₂

RuO₂ has attracted intense recent attention as a candidate altermagnet — a material with zero net magnetization but spin-split bands, promising for spin-transport applications. However, the existence and magnitude of magnetic order in RuO₂ remain actively debated, with conflicting results from different probes. We brought together phonon spectroscopy (NRIXS) and Mössbauer spectroscopy to provide independent, phonon-based constraints on the magnetic and electronic state of RuO₂.

Our NRIXS measurements resolve the ⁵⁷Fe partial phonon density of states in Fe-doped RuO₂ with high energy resolution, allowing us to detect subtle changes in the force constants that would accompany magnetic ordering. Combined with Mössbauer hyperfine parameters, our data place quantitative upper bounds on the size of any ordered moment in RuO₂ and provide clear constraints on the degree of electron correlation. These results demonstrate that scattering-based phonon probes can serve as precision sensors for magnetism in systems where conventional magnetic measurements are ambiguous.

Broader Implications

Understanding magnon–phonon hybridization has direct consequences for the design of magnonic and thermoelectric devices. In magnonic systems, strong spin–lattice coupling can open avoided crossings that act as frequency-selective gates for spin-wave propagation. In thermoelectrics, the same coupling modifies phonon lifetimes and thermal conductivity in ways that are not captured by purely electronic models. Our work aims to provide the quantitative microscopic foundation — extracted from direct scattering measurements — needed to engineer these effects rather than merely observe them.

Related Publications