Femtosecond (fs) coherent control of collective order parameters is important for non-equilibrium phase dynamics in correlated materials. Here we propose a possible scheme for fs control of a ferromagnetic order parameter based on non-adiabatic optical manipulation of electron-hole (e-h) photoexcitations between spin-orbit-coupled bands that are exchange-split by magnetic interaction with local spins. We photoexcite fs carrier spin-pulses with controllable direction and time profile without using circularly-polarized light, via time-reversal symmetry-breaking by non-perturbative interplay between spin-orbit and magnetic exchange coupling of coherent photocarriers. We manipulate photoexcited fs spin-orbit torques to control complex switching pathways of the magnetization between multiple magnetic memory states. We calculate the photoinduced fs magnetic anisotropy in the time domain by using density matrix equations of motion rather than the quasi-equilibrium free energy. By comparing to pump-probe experiments, we identify a "sudden" magnetization canting induced by laser excitation, which displays magnetic hysteresis absent in static magneto-optical measurements and agrees with switchings measured by Hall magnetoresistivity. The fs magnetization canting switches direction with magnetic state and laser frequency, which distinguishes it from nonlinear optical and demagnetization longitudinal effects. By shaping two-color laser-pulse sequences analogous to multi-dimensional Nuclear Magnetic Resonance (NMR) spectroscopy, we show that sequences of clockwise or counter-clockwise fs spin-orbit torques can enhance or suppress magnetic ringing and switching rotation at any desired time. We propose protocols that can provide controlled access to four magnetic states via consequative 90 o switchings.

We describe a mechanism for insulator-to-metal transition triggered by spin-canting following fs laserexcitation of insulating anti-ferromagnetic (AFM) states of colossal magneto-resistive (CMR) manganites. We show that photoexcitation of composite fermion quasi-particles dressed by spin fluctuations results in the population of a broad metallic conduction band due to canting of the AFM background spins via strong electron-spin local correlation. By inducing spin-canting, photoexcitation can increase the quasi-particle energy dispersion and quench the charge excitation energy gap. This increases the critical Jahn-Teller (JT) lattice displacement required to maintain an insulating state. We present fs-resolved pump-probe measurements showing bi-exponential relaxation of the differential reflectivity below the AFM transition temperature. We observe a nonlinear dependence of the ratio of the fs and ps relaxation component amplitudes at the same pump fluence threshold where we observe femtosecond magnetization photoexcitation. We attribute this correlation between nonlinear fs spin and charge dynamics to spin/charge/lattice coupling and population inversion between the polaronic majority carriers and metallic quasi-electron minority carriers as the lattice displacement becomes smaller than the critical value required to maintain an insulating state following laser-induced spin canting.

Using density matrix equations of motion and a tight-binding band calculation, we predict alloptical switching between four metastable magnetic states of (III,Mn)As ferromagnets. This switching is initiated non-thermally within 100fs, during nonlinear coherent photoexcitation. For a single optical pulse, magnetization reversal is completed after ∼100 ps and controlled by the coherent femtosecond photoexcitation. Our predicted switching comes from magnetic nonlinearities triggered by a femtosecond magnetization tilt that is sensitive to un-adiabatic light-induced spin interactions. The goal of THz magnetic switches underlies the entire field of spin-electronics and challenges our understanding of fundamental non-equilibrium spin processes. The reading and writing of bits rely on reversing the magnetization direction between "up" and "down". In conventional switching, the magnetization moves out of equilibrium via laser heating. A magnetic field then exerts a torque that reverses the magnetization within few ns [1,2]. This speed can be improved by using coherent spin rotation, via precession of the entire memory cell around a magnetic field pulse-precessional/ballistic switching [1][2][3][4]. This pulsed field must have duration of at least half the precession period (100's of ps), which sets a fundamental limit of the magnetization reversal time. This speed is further limited by randomness [4] or weak precession damping allowing back-switching of magnetic elements ("ringing") [2,3]. Faster switching, within 100fs, could be explored by using laser pulses to inject spin-polarized carriers [5]. This is important for meeting the demand for improved read/write speeds, bit density, and reliability of current magnetic devices.Magnetic properties of (III,Mn)V ferromagentic semiconductors exhibit sensitive response to carrier density tuning via light, electrical gates, or spin currents. This holds promise for high-speed magnetic switches that combine information processing and storage on a single chip device with low power consumption [6]. The femtosecond photoexcitation of GaMnAs revealed distinct transient magneto-optical responses: (i) ultrafast decrease of the magnetization amplitude [7] within ∼ 100fs (demagnetization) [8][9][10], (ii) enhancement of magnetic order on ps timescale [11], (iii) magnetization re-orientation within ∼100fs, followed by a distinct ps regime of coherent precession [12,13]. Such non-equilibrium magnetic effects appear to be universal [7,14,15]. The pioneering work of Bigot and collaborators [7], who observed demagnetization on a ∼100fs timescale much shorter than the spin-phonon relaxation time, thus evolved into a new field of femto-magnetism. However, the many-body theory of femto-magnetism remains controversial [5,15] and must ultimately engage the elements of transient coherence, correlation, and nonlinearity on an equal footing.Here we present such a mean field theory and propose a nonthermal mechanism for achieving ultrafast all-optical magnetization switching in ferromagnetic (Ga,Mn)A...

Light–wave quantum electronics utilizes the oscillating carrier wave to control electronic properties with intense laser pulses. Without direct light–spin interactions, however, magnetic properties can only be indirectly affected by the light electric field, mostly at later times. A grand challenge is how to establish a universal principle for quantum control of charge and spin fluctuations, which can allow for faster-than-THz clock rates. Using quantum kinetic equations for the density matrix describing non–equilibrium states of Hubbard quasiparticles, here we show that time–periodic modulation of electronic hopping during few cycles of carrier–wave oscillations can dynamically steer an antiferromagnetic insulating state into a metalic state with transient magnetization. While nonlinearities associated with quasi-stationary Floquet states have been achieved before, magneto–electronics based on quasiparticle acceleration by time–periodic multi–cycle fields and quantum femtosecond/attosecond magnetism via strongly–coupled charge–spin quantum excitations represents an alternative way of controlling magnetic moments in sync with quantum transport.