Generating and controlling spin currents at magnetic/nonmagnetic layer interfaces using ultrashort laser pulses has triggered the development of new high- operational frequency spintronic devices. Recent studies showed that laser-driven spin currents and opto-magnetic torques acting on spins are most effective when an interface is created between ferromagnetic Co and nonmagnetic Pt thin films. Our study focuses on the role of the Co-Pt interface on laser-induced optical torques in the strongly spin-orbit coupled Co/Pt model system. We varied the average roughness at the interface, in the range of 0.1-1.0 nm, by tuning the deposition pressure conditions during fabrication. With the aid of time-resolved THz-emission spectroscopy we detected both the laser-induced helicity-independent(HI) and helicity-dependent(HD) THz-emission due to spin-Hall and spin-orbit torque effects, respectively. We reveal a dramatic change in the detected THz-signals when the interface roughness is varied. For example, the HD-THz emission is observed only when the roughness is 0.3 nm or above. To study the role of intermixing a CoPt spacer layer, with varying compositions, is introduced at the interface. However, the detected THz-emission signals rule out the intermixing effects in determining the helicity-dependence. Moreover, static spin-hall conductivity measurements provide with new insights in understanding the role of spin-orbit coupling, at the Co/Pt interfaces, in laser-induced optical torques on net magnetization.
This research is funded by DOE:DE-SC001823
We review recent experiments on the fast and ultrafast all-optical control of light in bulk nematic and smectic-A liquid
crystals. Ultrafast optical control at sub-picosecond time scalecan be achieved via the optical Kerr response of a nematic
liquid crystal. We show that the refractive index changes are of the order of 10<sup>-4</sup> in 5CB nematic liquid crystal and can be
optically induced by applying 100 fs pulses of 4 mJ/cm<sup>2</sup> fluence. We discuss stimulated emission depletion of
fluorescence in a smectic-A liquid crystal and demonstrate nanosecond light control of fluorescent pulse shaping. Both
methods could be applied to control light by light in future photonic devices based on liquid crystals.
We investigate the light-induced magnetization reversal in samples of rare-earth transition metal alloys, where we aim to
spatially confine the switched region at the nanoscale, with the help of nano-holes in an Al-mask covering the sample.
First of all, an optimum multilayer structure is designed for the optimum absorption of the incident light. Next, using
finite difference time domain simulations we investigate light penetration through nano-holes of different diameter. We
find that the holes of 200 nm diameter combine an optimum transmittance with a localization better than λ/4. Further,
we have manufactured samples with the help of focused ion beam milling of Al-capped TbCoFe layers. Finally,
employing magnetization-sensitive X-ray holography techniques, we have investigated the magnetization reversal with
extremely high resolution. The results show severe processing effects on the switching characteristics of the magnetic
Revealing the ultimate speed limit at which magnetic order can be controlled, is a fundamental challenge of modern
magnetism having far reaching implications for magnetic recording industry. Exchange interaction is the strongest force
in magnetism, being responsible for ferromagnetic or antiferromagnetic spin order. How do spins react after being
optically perturbed on an ultrashort timescales pertinent to the characteristic time of the exchange interaction? Here we
demonstrate that femtosecond measurements of X-ray magnetic circular dichroism provide revolutionary new insights
into the problem of ultrafast magnetism. In particular, we show that upon femtosecond optical excitation the ultrafast
spin reversal of Gd(FeCo) - a material with antiferromagnetic coupling of spins - occurs via a transient ferromagnetic
state. The latter one emerges due to different dynamics of Gd and Fe magnetic moments: Gd switches within 1.5 ps
while it takes only 300 fs for Fe. Thus, by using a single fs laser pulse one can force the spin system to evolve via an
energetically unfavorable way and temporary switch from an antiferromagnetic to ferromagnetic type of ordering. These
observations supported by atomistic simulations, present a novel concept of manipulating magnetic order on different
classes of magnetic materials on timescales of the exchange interaction.
This review summarizes the recent progress in the study of ultrafast nonthermal effects of light on magnetic materials.
Fundamental aspects of interaction between photons and spins, magneto-optical and opto-magnetic phenomena,
microscopical mechanisms responsible for laser control of magnetism are discussed. Our recent experiments on laser
excitation of magnetic resonances, quantum control of magnons, ultrafast phase transitions and femtosecond laser-induced
switching of magnetization are reviewed.