Excitation and Detection of THz Coherent Spin Waves in Antiferromagnetic $\mathrm{\alpha-Fe_2O_3}$

TL;DR

This study investigates how ultrafast THz pulses excite coherent spin waves in antiferromagnetic $ ext{ extalpha-Fe}_2 ext{O}_3$, revealing the dependence on pulse polarization, temperature, and the underlying magnetic-dipole interaction mechanism.

Contribution

It provides new insights into the excitation mechanism of spin waves in antiferromagnets using THz pulses, highlighting the role of magnetic-dipole interactions and polarization dependence.

Findings

Maximum excitation efficiency occurs when the THz magnetic field is perpendicular to spins above the Morin point.

The excitation mechanism is driven by the magnetic-dipole interaction proportional to the time derivative of the magnetic field.

Efficiency varies with temperature, especially around the Morin transition.

Abstract

The efficiency of ultrafast excitation of spins in antiferromagnetic $\mathrm{\alpha-Fe_{2}O_{3}}$ using nearly single-cycle THz pulse is studied as a function of the polarization of the THz pulse and the sample temperature. Above the Morin point the most efficient excitation is achieved when the magnetic field of the THz pulse is perpendicular to the antiferromagnetically coupled spins. Using the experimental results and equations of motion for spins, we show that the mechanism of the spin excitation above and below the Morin point relies on magnetic-dipole interaction of the THz magnetic field with spins and the efficiency of the coupling is proportional to the time derivative of the magnetic field.