Magnetic order dynamics in optically excited multiferroicTbMnO3

Abstract

We performed ultrafast time-resolved near-infrared pump, resonant soft X-ray diffraction probe measurements to investigate the coupling between the photoexcited electronic system and the spin cycloid magnetic order in multiferroic TbMnO3 at low temperatures. We observe melting of the long range antiferromagnetic order at low excitation fluences with a decay time constant of 22.3 ± 1.1 ps, which is much slower than the ~1 ps melting times previously observed in other systems. To explain the data we propose a simple model of the melting process where the pump laser pulse directly excites the electronic system, which then leads to an increase in the effective temperature of the spin system via a slower relaxation mechanism. Despite this apparent increase in the effective spin temperature, we do not observe changes in the wavevector q of the antiferromagnetic spin order that would typically correlate with an increase in temperature under equilibrium conditions. We suggest that this behavior results from the extremely low magnon group velocity that hinders a change in the spin-spiral wavevector on these time scales.2

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

“…In ferromagnets, the temperature dependence of τ sp follows that of C s (T ) very closely, so that g is deduced to be approximately temperature independent. In contrast, our measurements on HoMnO 3 , as well as other ultrafast optical measurements on AFMs [7,9,19], show that the spin-phonon thermalization time does not simply follow the spin specific heat, but also decreases monotonously with increasing temperature. We find that our data still fits to τ sp (T ) = C s /g(T ), if temperature dependence is incorporated into g. Fitting our data to a polynomial of the form T Q reveals that Q = -0.5, as shown in Fig.…”

“…Although the pump penetration depth is about 1000 times smaller than the THz probe penetration depth, our optical-pump, THz-probe (OPTP) signal comes only from the 200 nm thick pumped region. Comparison of our time scales to expected diffusion times (>20 nanoseconds) as well as all-optical pump-probe measurements on our crystal and opticalpump, x-ray diffraction probe measurements on a similar system [19], where the penetration depths match, confirm this (see the Supplemental Material at [URL will be inserted by publisher] for more details). Absorption of our pump pulses excites on-site Mn d − d transitions [24,30].…”

“…To learn more about the microscopic origin of this heating, we compare our results, as well as measurements in similar AFM manganites [7,9,19], to previous observations in FM manganites [22]. While the microscopic mechanism governing spin-lattice thermalization in FM systems originates from spin-orbit coupling [23], our study reveals very different trends in AFM systems, for which the underlying mechanism is not yet known.…”

“…For example, even thermal processes involving spins, although relatively well understood in FMs, show very different trends in AFM compounds that have not been extensively studied (compare, e.g. [9,11,19] to [21]). …”

Directly probing spin dynamics in insulating antiferromagnets using ultrashort terahertz pulses

“…Optical second harmonic generation (SHG) is sensitive to AFM spin order and its dynamics [6,7], but when applied to multiferroics it must be distinguished from the larger SHG signal originating from ferroelectric order. The other option for probing ultrafast AFM spin dynamics is to use resonant x-ray diffraction with femtosecond x-ray pulses from large scale free electron lasers or synchrotrons [8][9][10], which are difficult to gain access to.…”