Ultrafast terahertz spin dynamics : from phonon-induced spin order to coherent magnon control

Abstract

Ultrashort pulses in the terahertz (THz) spectral range allow us to study and control spin dynamics on time scales faster than a single oscillation cycle of light. In a first set of experiments, we harness an optically triggered coherent lattice vibration to induce a transient spin-density wave in BaFe2As2, the parent compound of pnictide superconductors. The time-dependent multi-THz response of the non-equilibrium phases shows that the ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. The results suggest important implications for unconventional superconductivity. In a second step, we utilize the magnetic field component of intense THz transients to directly switch on and off coherent spin waves in the antiferromagnetic nickel oxide NiO. A femtosecond optical probe traces the magnetic dynamics in the time domain and verifies that the THz field addresses spins selectively via Zeeman interaction. This concept provides a universal ultrafast handle on magnetic excitations in the electronic ground state.