In this project, we plan to investigate the coupling of ordered spins in magnetic solids to the electronic and lattice degrees of freedom on the ultrafast time scales of elementary interaction processes. For this purpose, we will make use of a highly selective pump-probe technique with sub-picosecond time resolution: an electromagnetic pump pulse (with center frequency tunable from the terahertz (THz) to the ultraviolet spectral range) excites specific low-energy resonances (e.g. magnons, phonons, electron-hole pairs). The impact of this perturbation on other degrees of freedom is monitored by a subsequently arriving probe pulse that is sensitive to observables such as magnetization, electronic conductivity and phonon occupation. To reduce complexity of the dynamics triggered by the pump, we focus on insulators in which the electron-orbital degrees of freedom are frozen out at room temperature, but can selectively be excited by a suitable optical pump pulse.
Using this approach, we expect to gain new and specific insights into both spin-electron and spin-lattice interactions, a central goal of ultrafast magnetism research. Examples of the questions addressed are: how does magnetic order in a given material respond to selectively excited electron-hole pairs, crystal-field transitions, optical or acoustic phonons? How fast are energy and angular momentum transferred between spins and the other subsystems? What is the dominant spin coupling underlying these processes? How can we transiently modify properties of the material, in particular the strength with which spins couple to the other degrees of freedom?
To implement this project, a versatile THz pump-probe experiment will be set up, with challenging requirements in terms of THz pulse energy (>1 μJ), signal-to-noise ratio, widely tunable pump frequency (from THz to ultraviolet), variable sample temperature and access to different probing schemes (magnetooptical effects at optical and THz frequencies, incoherent Raman scattering and THz emission). Regarding materials, we will study electrically insulating model magnets, which are relevant for spintronics applications, but exhibit complementary
magnetic order: ferromagnetic EuO, antiferromagnetic NiO and ferrimagnetic yttrium iron garnet (YIG). Their orbital electronic degrees of freedom are frozen out at room temperature, but can selectively be excited by an optical pump pulse. Finally, we will apply our methodology to transition-metal dichalcogenides such as WSe2 to study transiently spin-polarized electrons following generation by a circularly polarized optical pump pulse.
Dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation
S. F. Maehrlein, I. Radu, P. Maldonado, A. Paarmann, M. Gensch, A. M. Kalashnikova, R. V. Pisarev, M. Wolf, P. M. Oppeneer, J. Barker, and T. Kampfrath
Sci. Adv. 4, eaar5164 (2018) - DOI: 10.1126/sciadv.aar5164