Magnon Polarons in the Spin Seebeck Effect

Area of Science:

• Condensed matter physics
• Materials science
• Spintronics

Background:

• The spin Seebeck effect (SSE) is a fundamental phenomenon in spintronics, enabling the conversion of heat gradients into spin currents.
• Understanding the interplay between magnetic excitations (magnons) and lattice vibrations (phonons) is crucial for optimizing spintronic devices.
• Pt/Y3Fe5O12 interfaces are model systems for studying spin-caloritronic effects due to their distinct magnetic and transport properties.

Purpose of the Study:

• To investigate the origin of sharp anomalies observed in the magnetic field-dependent spin Seebeck effect (SSE) in Pt/Y3Fe5O12 at low temperatures.
• To elucidate the role of magnon-phonon interactions and magnetoelastic coupling in shaping SSE voltage signals.
• To establish a theoretical framework for predicting and understanding SSE behavior under specific magnetic field conditions.

Main Methods:

• Experimental measurements of the spin Seebeck effect voltage in Pt/Y3Fe5O12 heterostructures under varying magnetic fields and low temperatures.
• Theoretical modeling using a Boltzmann transport equation incorporating magnetoelastic coupling effects.
• Analysis of experimental data to identify correlations between SSE anomalies and theoretical predictions.

Main Results:

• Sharp, distinct structures were observed in the magnetic field-dependent SSE voltages at low temperatures.
• These anomalies were successfully reproduced by a Boltzmann theory that explicitly included magnetoelastic coupling.
• The observed SSE anomalies were found to coincide with magnetic field values corresponding to the threshold of magnon-polaron formation.

Conclusions:

• The magnon-phonon interaction, mediated by magnetoelastic coupling, is identified as the primary cause of the sharp SSE voltage structures.
• The study provides a theoretical explanation for the observed anomalies, linking them to the formation of magnon-polaron quasiparticles.
• The findings offer valuable insights into the quality of the material's lattice and the dynamics of its magnetization, crucial for advancing spintronic device design.