Probing ultrafast spin dynamics through a magnon resonance in the antiferromagnetic multiferroicHoMnO3

Abstract

We demonstrate a new approach for directly tracking antiferromagnetic (AFM) spin dynamics by measuring ultrafast changes in a magnon resonance. We test this idea on the multiferroic HoMnO3 by optically photoexciting electrons, after which changes in the spin order are probed with a THz pulse tuned to a magnon resonance. This reveals a photoinduced change in the magnon line shape that builds up over 5-12 picoseconds, which we show to be the spin-lattice thermalization time, indicating that electrons heat the spins via phonons. We compare our results to previous studies of spin-lattice thermalization in ferromagnetic (FM) manganites, giving insight into fundamental differences between the two systems. Our work sheds light on the microscopic mechanism governing spin-phonon interactions in AFMs and demonstrates a powerful approach for directly monitoring ultrafast spin dynamics.

Directly probing spin dynamics in insulating antiferromagnets using ultrashort terahertz pulses

“…1); the optical absorption thus determines the effective crystal length for our OPTP measurements. As discussed in our previous work on HoMnO 3 , lateral diffusion (either heat or transport) is too slow to have any significant effect on our time constants (which might result from the pump and probe penetration depth mismatch) [11]. The fact that our THz transmission probe gives a very similar time constant to that measured with x-rays in reflection (discussed in more detail below), where the probe penetration depth is less than that of the pump, confirms this.…”

“…In our previous work on HoMnO 3 [11] this was more obvious since the photoinduced changes occurred only at the magnon resonance, while in TbMnO 3 , the spectral changes happen across the whole THz spectrum (Fig. 2(b)).…”

“…4(c) and 5(b), the spins and lattice are in thermal equilibrium, allowing us to ascribe τ R to spin-lattice thermalization (as in ref. [11]). We suggest that the importance of short-range magnetic order in TbMnO 3 [17,24], apparent also from the fact that our OPTP signal persists above T N 1 , as well as the temperature dependence of the broad two-magnon excitation, could account for the slower spin-lattice relaxation in TbMnO 3 as compared to HoMnO 3 [11], where such a feature was not present.…”

“…[11]). We suggest that the importance of short-range magnetic order in TbMnO 3 [17,24], apparent also from the fact that our OPTP signal persists above T N 1 , as well as the temperature dependence of the broad two-magnon excitation, could account for the slower spin-lattice relaxation in TbMnO 3 as compared to HoMnO 3 [11], where such a feature was not present. This is also consistent with all-optical pump-probe measurements on Eu 0.75 Y 0.35 MnO 3 , a system also known to have short range magnetic order above T N [18], which showed a relaxation time similar to that seen here that also slowly decreases in amplitude above T N [13].…”

“…Considering that it is still not straightforward to measure ultrafast spin dynamics in AFMs, we recently introduced a simple, table-top method that probes AFM spin dynamics through a magnon resonance using terahertz (THz) pulses [11]. Applying this to the AFM multiferroic HoMnO 3 , after photoexciting electrons with an optical pulse, we observed an induced transparency for the THz probe pulse only at the magnon resonance, clearly indicating a direct sensitivity to spin order.…”