Excitation and detection of terahertz coherent spin waves in antiferromagnetic α-Fe2O3

The efficiency of the ultrafast excitation of spins in antiferromagnetic alpha-Fe2O3 using a 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 a magnetic-dipole interaction of the THz magnetic field with spins, and the mechanism implies that the efficiency of the coupling is proportional to the time derivative of the magnetic field.