Coupling up of spins and lattice: The development of spintronics and magnetic data storage relies on understanding and controlling the dynamics of magnetic excitations within a material. Of crucial importance for practical applications is how fast the magnetization can be switched. Mashkovich et al . report the use of ultrafast terahertz radiation to create magnon excitations in the antiferromagnet cobalt difluoride that can then be coupled with phonon excitations (see the Perspective by Juraschek and Narang). Using light to control coupling between the spins and the lattice provides a route to manipulate magnetization in antiferromagnetic materials on ultrafast time scales. —ISO

A nearly single cycle intense terahertz (THz) pulse with peak electric and magnetic fields of $0.5\mathrm{MV}/\mathrm{cm}$ and 0.16 T, respectively, excites both modes of spin resonances in the weak antiferromagnet ${\mathrm{FeBO}}_3$. The high frequency quasiantiferromagnetic mode is excited resonantly and its amplitude scales linearly with the strength of the THz magnetic field, whereas the low frequency quasiferromagnetic mode is excited via a nonlinear mechanism that scales quadratically with the strength of the THz electric field and can be regarded as a THz inverse Cotton-Mouton effect. THz optomagnetism is shown to be more energy efficient than similar effects reported previously for the near-infrared spectral range.

THz magnetization dynamics of antiferromagnetically coupled spins in ferrimagnetic ${\mathrm{Tm}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ is excited by a picosecond single-cycle pulse of a magnetic field and probed with the help of the magneto-optical Faraday effect. Data analysis combined with numerical modeling shows that the dynamics corresponds to the exchange mode excited by the Zeeman interaction of the THz magnetic field with the spins. We argue that THz-pump–IR-probe experiments on ferrimagnets offer a unique tool for quantitative studies of dynamics and mechanisms to control antiferromagnetically coupled spins.

Finding efficient and ultrafast ways to control antiferromagnets is believed to be instrumental in unlocking their potential for magnetic devices operating at THz frequencies. Still, it is challenged by the absence of net magnetization in the ground state. Here, we show that the magnetization emerging from a state of coherent spin precession in antiferromagnetic iron borate ${\mathrm{FeBO}}_3$ can be used to enable the nonlinear coupling of light to another, otherwise weakly susceptible, mode of spin precession. This nonlinear mechanism can facilitate conceptually new ways of controlling antiferromagnetism.

A theory of terahertz emission from a femtosecond laser pulse with a tilted intensity front propagating through a prism-shaped electro-optic crystal is developed. The theory accounts for transient effects at the entrance boundary of the crystal and allows us to explore the dynamics of terahertz generation in the crystal. In particular, transverse walkoff length is introduced as an important parameter of the terahertz field formation process. Two typical experimental situations—${\mathrm{LiNbO}}_3$ excited with a Ti:sapphire laser ($0.8\mathrm{\mu m}$ wavelength) at room and cryogenic temperatures—are considered, and new schemes, in which GaAs at room temperature is excited at 1.8 and $3.5\mathrm{\mu m}$, are proposed and analyzed. The parameters of the laser pulse (transverse size, tilt angle, and pulse duration) and crystal size maximizing the terahertz yield are calculated.

The efficiency of the ultrafast excitation of spins in antiferromagnetic $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ 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.