Terahertz Spectroscopy of Femtosecond Spin Dynamics in Orthoferrites R.V. Mikhaylovskiy1, E. Hendry2, V.V. Kruglyak2, A. Wu3, R.V. Pisarev4, Th. Rasing1 and A.V. Kimel1 1 Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, the Netherlands 2
School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK
3
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
4
Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia
[email protected]
Abstract Here we report experiments performed on orthoferrites by means of THz time-domain emission spectroscopy. We show that the femtosecond laser pulses excite both iron and rare-earth sub-systems on sub-picosecond timescale. In particular, the novel route to the optical control of the exchange interaction has been discovered.
Introduction Although a remarkable progress in understanding of ultrafast magnetic phenomena has been achieved, the ultimate challenge has not been overcome: the femtosecond control of the magnetic state of matter by employing the exchange interaction which is the strongest, fastest and ubiquitous force in the magnetism, eventually governing the very existence of the magnetic substances. At the same time laser manipulation of the exchange interaction must be feasible in any material in which the magnetic order is governed by indirect exchange via ligand ions (super-exchange). Indeed, since the super-exchange interaction is defined by virtual charge-transfer excitations from ligand to magnetic ion, one can anticipate the feasibility of laser control of magnetism via optical pumping of the electronic transitions mediating the exchange. The effect of the optical modification of the magnetic state has been most conveniently detected by recording the motion of spins triggered by the associated transient torque. Orthoferrites are an excellent model system for investigation of laser-induced magnetic phenomena, in general, and observing the ultrafast optical control of the exchange interaction, in particular. These materials are a family of canted Ó Springer International Publishing Switzerland 2015 J.-Y. Bigot et al. (eds.), Ultrafast Magnetism I, Springer Proceedings in Physics 159 DOI 10.1007/978-3-319-07743-7_102
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antiferromagnets which show an unusual variety of magnetic properties [1]. The spin dynamics of these compounds governed by the exchange interaction belongs to terahertz (THz) range [2] thereby being two orders of magnitude faster than in conventional ferromagnets like permalloy or iron garnets. This fact makes orthoferrites particularly attractive for ultrafast opto-magnetic recording. Indeed, recently laser-induced ultrafast spin reorientation [3] and ultrafast inverse Faraday effect [4] have been successfully demonstrated in orthoferrites. To date, the ultrafast spin dynamics in orthoferrites has been investigated by all optical pump-probe spectroscopy. In order to understand better the interaction between intense femtosecond pulses and both iron and rare-earth magnetic subsystems of orthoferrites we employed the THz emission spectroscopy. This technique combines sub-picosecond time resolution with high sensitivity with respect to any dipole active modes. By measuring the electro-magnetic emissions arising from the illumination of the orthoferrites by femtosecond optical pulses of Ti:Sapphire laser we were able to demonstrate that these pulses instantaneously excite many resonances which were not visible with the help of all optical pump-probe spectroscopy at all.
Results and Discussion a)
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Fig. 1. a. The emitted from TmFeO3 signal different temperatures below 55 K. b. Below 55 K the magnetization M = S1+S2 lies along x-axis in TmFeO3 cut perpendicular to z-axis. The optical pump is focused onto the sample along z-axis, while THz emission is collected along this direction at other side of the sample. The THz emission arises from the quasi-antiferromagnetic oscillation m(t). c. The amplitude of the quasi-antiferromagnetic mode in TmFeO3 at 35 K vs the pump intensity with linear fit. d. The THz waveforms generated in TmFeO3 optical pulses with linear and circular polarization.
By measuring the THz emission arising from optically excited coherent quasiantiferromagnetic spin oscillations [Fig. 1 (a) and (b)] we demonstrate the evidence for sub-picosecond optical modulation of the energy of the exchange spin-
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spin interaction in this compound. Distinct features of such an optical control of the exchange interaction are linear dependence of the effect on the intensity [Fig. 1 (c)] and independence on the direction of propagation and polarization of light [Fig. 1 (d)]. The effect has been observed in several orthoferrites such as YFeO 3, TmFeO3 and ErFeO3. From the strength of the electric field of the emitted THz wave we have been able to estimate that the amplitude of the emitting magnetic dipole is ~ 1 μA cm2 showing that the sub-picosecond laser pulse with the fluence ~ 1 mJ/cm2 was able to change the energy of the exchange interaction over ~ 0.01 % that in turn exerts on spins the effective magnetic field of 0.1 T. Additionally, we observed the emission arising from the high frequency (1 – 2 THz) dipoles excited between the sub-levels of the 4I15/2 ground multiplet of rareearth ion in ErFeO3. To the best of our knowledge it is the first proven manifestation of the direct optical excitation of the rare-earth sub-system in the orthoferrites.
Summary This work demonstrates potential power of the THz techniques in the area of femtomagnetism. Particularly, the THz emission spectroscopy of orthoferrites allowed us to demonstrate the feasibility of sub-picosecond optical control of the exchange interaction that opens a novel means of optical magnetization control. Acknowledgments This work was partially supported by The Netherlands Organization for Scientific Research (NWO), the Foundation for Fundamental Research on Matter (FOM), the European Union’s Seventh Framework Program (FP7/2007-2013) Grants No. NMP3-LA-2010246102 (IFOX), No. 280555 (Go-Fast), No. 214810 (FANTOMAS), the European Research Council under the European Union’s Seventh Framework Program (FP7/2007-2013)/ERC Grant Agreement No. 257280 (Femtomagnetism) as well as the program “Invited Scientist” funded by the Russian Ministry of Education and Science (Grant Agreement 14.B37.21.0899).
References [1] R. L. White, “Review of Recent Work on the Magnetic and Spectroscopic Properties of the RareEarth Orthoferrites” J. Appl. Phys. 40, 1061 (1969) [2] G. Srinivasan (Ed.), A. N. Slavin (Ed.) “High frequency processes in magnetic materials” (World Scientific Publishing, 1995), Chap. 2, pp. 56-98. [3] A.V. Kimel, A. Kirilyuk, A. Tsvetkov, R.V. Pisarev, and Th. Rasing, “Laser-induced ultrafast spin reorientation in the antiferromagnet TmFeO3” Nature 429, 850 (2004). [4] A. V. Kimel, A. Kirilyuk, P. A. Usachev, R. V. Pisarev, A. M. Balbashov, and Th. Rasing, “Ultrafast non-thermal control of magnetization by instantaneous photomagnetic pulses” Nature 435, 655 (2005).
