Terahertz Time-Domain Observation of Spin Reorientation in Orthoferrite ${\mathrm{ErFeO}}_3$ through Magnetic Free Induction Decay

Terahertz time domain spectroscopy was performed on orthoferrite ${\mathrm{ErFeO}}_3$. Through the emission from the two magnetic resonance modes, we succeeded in observing the spin reorientation transition. Depending on the orientation of the single crystal, the reorientation can be detected as either mode switching between the two modes or polarization change of the emission. This method enables picosecond resolved observation of the reorientation without disturbances such as electronic excitation and heating, and it is expected to open the doorway to observe ultrafast reorientation with the terahertz pulse.

Figure 1

Schematic of spin configurations in ${\mathrm{ErFeO}}_3$ for high-temperature and low-temperature phases. The blue arrows indicate ${\mathrm{Fe}}^{3+}$ spins, and the red arrows indicate macroscopic magnetization, which results from canting of the ${\mathrm{Fe}}^{3+}$ spins.

Figure 2

(a) Temporal waveforms of transmitted THz wave through $c$ plane ${\mathrm{ErFeO}}_3$ single crystal. (b) Spectra obtained by Fourier transformation of temporal waveforms in (a). (c) Electric field component trajectory of emission at 0.377 THz calculated from (a).

Figure 3

Temperature dependence of F and AF mode resonant frequencies in ${\mathrm{ErFeO}}_3$. Circle markers show frequencies obtained with the sintered pellet, and triangle markers show frequencies in a single crystal sample with (001) surface. Two vertical lines indicate higher and lower transition temperatures at 96 K and 87 K.

Figure 4

(a) Intensity spectra of oscillatory components obtained with $H_{\mathrm{THz}}||a$-axis THz pulse excitation. (b) The relation of the incident THz polarization with excited resonance modes in each phase.

Figure 5

(a) Fourier spectra of oscillatory components in waveforms obtained from $H_{\mathrm{THz}}||b$-axis excitation. The red dashed line and the blue line show the oscillation in the polarization parallel and perpendicular to the incident THz pulse, respectively. (b) Schematic of spin precession motion and magnetic field component trajectory of radiation emitted from the precession in high temperature and low temperature phases. (c) Three-dimensional trajectory plot of temporal waveforms at 250 K and 50 K.

Figure 6

(a) Spectra obtained from Fourier transformation of waveforms in perpendicular polarization with a magnetized sintered pellet. The dashed spectra show the oscillatory component in the parallel polarization (arbitrary unit) for each temperature. (b) Magneto-optical spectra of the pellet sample at room temperature before and after cooling.