Future information technology demands ultimately fast, low-loss quantum control. Intense light fields have facilitated important milestones, such as inducing novel states of matter 1-3 , accelerating electrons ballistically 4-7 , or coherently flipping the valley pseudospin 8 . These dynamics leave unique signatures, such as characteristic bandgaps or high-order harmonic radiation. The fastest and least dissipative way of switching the technologically most important quantum attribute -the spin -between two states separated by a potential barrier is to trigger an all-coherent precession. Pioneering experiments and theory with picosecond electric and magnetic fields have suggested this possibility 9-11 , yet observing the actual dynamics has remained out of reach. Here, we show that terahertz (1 THz = 10 12 Hz) electromagnetic pulses allow coherent navigation of spins over a potential barrier and we reveal the corresponding temporal and spectral fingerprints. This goal is achieved by coupling spins in antiferromagnetic TmFeO3 with the locally enhanced THz electric field of custom-tailored antennas. Within their duration of 1 ps, the intense THz pulses abruptly change the magnetic anisotropy and trigger a large-amplitude ballistic spin motion. A characteristic phase flip, an asymmetric splitting of the magnon resonance, and a long-lived offset of the Faraday signal are hallmarks of coherent spin switching into adjacent potential minima, in agreement with a numerical simulation. The switchable spin states can be selected by an external magnetic bias. The low dissipation and the antenna's sub-wavelength spatial definition could facilitate scalable spin devices operating at THz rates.

Resonant ultrafast excitation of infrared-active phonons is a powerful technique to control the electronic properties of materials, leading to remarkable phenomena such as light-induced enhancement of superconductivity 1 , 2 , switching of ferroelectric polarization 3 , 4 and ultrafast insulator to metal transitions 5 . Here we show that light-driven phonons can be utilized to coherently manipulate macroscopic magnetic states. Intense mid-infrared electric field pulses, tuned to resonance with a phonon mode of the archetypical antiferromagnet DyFeO 3 , induce ultrafast and long-living changes of the fundamental exchange interaction between rare-earth orbitals and transition metal spins. Non-thermal lattice control of the magnetic exchange, defining the very stability of the macroscopic magnetic state, allows us to perform picosecond coherent switching between competing antiferromagnetic and weakly ferromagnetic spin orders. Our discovery emphasizes the potential of resonant phonon excitation for the manipulation of ferroic order on ultrafast timescales 6 .

Single-shot helicity-independent all-optical switching of magnetization in ferrimagnets represents the fastest known approach for deterministic data recording. Recently, it was shown that 15-ps-long optical pulses could suffice in triggering the magnetic switching in certain Gd-Fe-Co alloys, generating enormous controversy about the underlying mechanism. Here, we demonstrate how the exact composition of the ferrimagnet affects the kinetics of the reversal process and facilitates the use of thermal pulses with a duration spanning all relevant timescales within the nonadiabatic limit. By modelling a generic ferrimagnet as two coupled macrospins, we show that the magnetization reversal can occur via distinctly different pathways, depending on the duration of the heater. We experimentally reveal that pulses with a duration below and above a critical pulse width respectively enable and disable the capability of all-optical magnetization switching in Gd-Fe-Co alloys, and that modest change of the alloy composition leads to drastic variation of the critical pulse width, by almost 2 orders of magnitude. Our interpretation and results resolve an urgent and outstanding technologically relevant controversy, and provide crucial but previously overlooked guidelines for how to engineer deterministic all-optical switching of magnetization in suitable ferrimagnets.

Ever since the first observation of all-optical switching of magnetization in the ferrimagnetic alloy GdFeCo using femtosecond laser pulses, there has been significant interest in exploiting this process for data-recording applications. in particular, the ultrafast speed of the magnetic reversal can enable the writing speeds associated with magnetic memory devices to be potentially pushed towards tHz frequencies. this work reports the development of perpendicular magnetic tunnel junctions incorporating a stack of tb/co nanolayers whose magnetization can be all-optically controlled via helicity-independent single-shot switching. toggling of the magnetization of the tb/co electrode was achieved using either 60 femtosecond-long or 5 picosecond-long laser pulses, with incident fluences down to 3.5 mJ/cm 2 , for co-rich compositions of the stack either in isolation or coupled to a cofeBelectrode/Mgo-barrier tunnel-junction stack. Successful switching of the cofeB-[tb/co] electrodes was obtained even after annealing at 250 °c. After integration of the [tb/co]-based electrodes within perpendicular magnetic tunnel junctions yielded a maximum tunneling magnetoresistance signal of 41% and RxA value of 150 Ωμm 2 with current-in-plane measurements and ratios between 28% and 38% in nanopatterned pillars. these results represent a breakthrough for the development of perpendicular magnetic tunnel junctions controllable using single laser pulses, and offer a technologically-viable path towards the realization of hybrid spintronic-photonic systems featuring tHz switching speeds. Ferrimagnetic systems based on rare earth (RE)-transition metal (TM) alloys and multilayers have been extensively studied in recent decades, largely due to their potential application in the field of magneto-optical recording 1. The strong perpendicular magnetocrystalline anisotropy inherent to amorphous RE-TM systems have allowed these alloys to play a key role in the historical transition from longitudinal to perpendicular magnetic recording structures 2 , and made them ideal for handling magnetic bit instabilities arising from superparamagnetic effects 3. Binary and ternary RE-TM systems such as GdFeCo, GdCo, TbCo or GdFe are still driving forward new developments pertaining to spintronic devices, including spin valves for magnetic read heads 4 , perpendicular magnetic tunnel junctions (p-MTJs) 5,6 or spin-orbit-torque phenomena 7. Recent works in this field have also revealed that RE-TM-based films (amorphous or multilayered) represent ideal materials for the observation and study of the phenomena of all-optical switching (AOS) 8-10. In these systems, it is possible to switch the magnetization using suitable laser pulses without the application of any external magnetic field. Depending on whether the laser-pulses need to be circularly-polarized, AOS of magnetization can be classed as either helicity-dependent (HD-AOS) or helicity-independent (HI-AOS). Furthermore, subject to the material in question, the switching process can be achieved with either a...

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.

Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. However, the spatial evolution of the magnetisation is not yet well understood, in part due to the difficulty in experimentally probing such rapid dynamics. Here, we find evidence of a universal rapid magnetic order recovery in ferrimagnets with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify magnon localisation and coalescence processes, whereby localised magnetic textures nucleate and subsequently interact and grow in accordance with a power law formalism. A hydrodynamic representation of the numerical simulations indicates that the appearance of noncollinear magnetisation via optical pumping establishes exchange-mediated spin currents with an equivalent 100% spin polarised charge current density of 10 7 A cm −2 . Such large spin currents precipitate rapid recovery of magnetic order after optical pumping. The magnon processes discussed here provide new insights for the stabilization of desired meta-stable states.

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Magnon-polaritons are shown to play a dominant role in the propagation of terahertz (THz) waves through TmFeO3 orthoferrite, if the frequencies of the waves are in the vicinity of the quasi-antiferromagnetic spin resonance mode. Both time-domain THz transmission and emission spectroscopies reveal clear beatings between two modes with frequencies slightly above and slightly below this resonance, respectively. Rigorous modeling of the interaction between the spins of TmFeO3 and the THz light shows that the frequencies correspond to the upper and lower magnon-polariton branches. Our findings reveal the previously ignored importance of propagation effects and polaritons in such heavily debated areas as THz magnonics and THz spectroscopy of electromagnons. It also shows that future progress in these areas calls for an interdisciplinary approach at the interface between magnetism and photonics.