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Ultrafast photo-magnetic recording in transparent films of the dielectric cobalt-substituted garnet has very low heat load and is much faster than existing alternatives. A cool feat of short-term memory The use of magnetic materials for the recording and storage of information is a mature technology, yet the search is still on for a means to both further accelerate the switching process and further reduce the detrimental production of heat that occurs during switching. Andrzej Stupakiewicz and colleagues describe a potential route to achieving this goal. They show that the optical stimulation of a ferromagnetic garnet film—using a laser pulse carefully tuned to a specific electronic transition associated with the dopant ions responsible for the magnetic properties of the material—can be used to write magnetic information at ultrafast speeds (less than 20 picoseconds), without the laser-induced heating that normally accompanies optically driven magnetic transitions. Discovering ways to control the magnetic state of media with the lowest possible production of heat and at the fastest possible speeds is important in the study of fundamental magnetism 1 , 2 , 3 , 4 , 5 , with clear practical potential. In metals, it is possible to switch the magnetization between two stable states (and thus to record magnetic bits) using femtosecond circularly polarized laser pulses 6 , 7 , 8 . However, the switching mechanisms in these materials are directly related to laser-induced heating close to the Curie temperature 9 , 10 , 11 , 12 . Although several possible routes for achieving all-optical switching in magnetic dielectrics have been discussed 13 , 14 , no recording has hitherto been demonstrated. Here we describe ultrafast all-optical photo-magnetic recording in transparent films of the dielectric cobalt-substituted garnet. A single linearly polarized femtosecond laser pulse resonantly pumps specific d − d transitions in the cobalt ions, breaking the degeneracy between metastable magnetic states. By changing the polarization of the laser pulse, we deterministically steer the net magnetization in the garnet, thus writing ‘0’ and ‘1’ magnetic bits at will. This mechanism outperforms existing alternatives in terms of the speed of the write–read magnetic recording event (less than 20 picoseconds) and the unprecedentedly low heat load (less than 6 joules per cubic centimetre).

Traditionally, magnetic solids are divided into two main classes—ferromagnets and antiferromagnets with parallel and antiparallel spin orders, respectively. Although normally the antiferromagnets have zero magnetization, in some of them an additional antisymmetric spin–spin interaction arises owing to a strong spin–orbit coupling and results in canting of the spins, thereby producing net magnetization. The canted antiferromagnets combine antiferromagnetic order with phenomena typical of ferromagnets and hold great potential for spintronics and magnonics 1 – 5 . In this way, they can be identified as closely related to the recently proposed new class of magnetic materials called altermagnets 6 – 9 . Altermagnets are predicted to have strong magneto-optical effects, terahertz-frequency spin dynamics and degeneracy lifting for chiral spin waves 10 (that is, all of the effects present in the canted antiferromagnets 11 , 12 ). Here, by utilizing these unique phenomena, we demonstrate a new functionality of canted spin order for magnonics and show that it facilitates mechanisms converting a magnon at the centre of the Brillouin zone into propagating magnons using nonlinear magnon–magnon interactions activated by an ultrafast laser pulse. Our experimental findings supported by theoretical analysis show that the mechanism is enabled by the spin canting. A study demonstrates a new functionality of canted spin order for magnonics and shows that it facilitates mechanisms for ultrafast nonlinear conversion of magnons.

Magnonics is a research field complementary to spintronics, in which the quanta of spin waves (magnons) replace electrons as information carriers, promising lower dissipation1–3. The development of ultrafast, nanoscale magnonic logic circuits calls for new tools and materials to generate coherent spin waves with frequencies as high and wavelengths as short as possible4,5. Antiferromagnets can host spin waves at terahertz frequencies and are therefore seen as a future platform for the fastest and least dissipative transfer of information6–11. However, the generation of short-wavelength coherent propagating magnons in antiferromagnets has so far remained elusive. Here we report the efficient emission and detection of a nanometre-scale wavepacket of coherent propagating magnons in the antiferromagnetic oxide dysprosium orthoferrite using ultrashort pulses of light. The subwavelength confinement of the laser field due to large absorption creates a strongly non-uniform spin excitation profile, enabling the propagation of a broadband continuum of coherent terahertz spin waves. The wavepacket contains magnons with a shortest detected wavelength of 125 nm that propagate into the material with supersonic velocities of more than 13 km s–1. This source of coherent short-wavelength spin carriers opens up new prospects for terahertz antiferromagnetic magnonics and coherence-mediated logic devices at terahertz frequencies.Ultrashort light pulses generate nanometre-scale wavepackets of magnons that propagate coherently and at high speed in an antiferromagnet. This pushes antiferromagnetic magnonics forward as a future platform for information processing.

Resonant ultrafast excitation of infrared-active phonons is a powerful technique with which to control the electronic properties of materials that leads to remarkable phenomena such as the 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, which defines the 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 . Non-thermal lattice control of exchange interactions allows for picosecond coherent switching between competing antiferromagnetic and weakly ferromagnetic order.

Strong light fields have created opportunities to tailor novel functionalities of solids 1 – 5 . Floquet–Bloch states can form under periodic driving of electrons and enable exotic quantum phases 6 – 15 . On subcycle timescales, lightwaves can simultaneously drive intraband currents 16 – 29 and interband transitions 18 , 19 , 30 , 31 , which enable high-harmonic generation 16 , 18 , 19 , 21 , 22 , 25 , 28 – 30 and pave the way towards ultrafast electronics. Yet, the interplay of intraband and interband excitations and their relation to Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this link by visualizing the ultrafast build-up of Floquet–Bloch bands with time-resolved and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator 32 , 33 with mid-infrared fields—strong enough for high-harmonic generation—and directly monitor the transient band structure with subcycle time resolution. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle; intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum non-equilibrium calculations explain the simultaneous occurrence of Floquet states with intraband and interband dynamics. Our joint experiment and theory study provides a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering. The build-up and dephasing of Floquet-–Bloch bands is visualized in both subcycle band-structure videography and quantum theory, revealing the interplay of strong-field intraband and interband excitations in a non-equilibrium Floquet picture.

Delocalized Bloch electrons and the low-energy correlations between them determine key optical 1 , electronic 2 and entanglement 3 functionalities of solids, all the way through to phase transitions 4 , 5 . To directly capture how many-body correlations affect the actual motion of Bloch electrons, subfemtosecond (1 fs = 10 −15  s) temporal precision 6 – 15 is desirable. Yet, probing with attosecond (1 as = 10 −18  s) high-energy photons has not been energy-selective enough to resolve the relevant millielectronvolt-scale interactions of electrons 1 – 5 , 16 , 17 near the Fermi energy. Here, we use multi-terahertz light fields to force electron–hole pairs in crystalline semiconductors onto closed trajectories, and clock the delay between separation and recollision with 300 as precision, corresponding to 0.7% of the driving field’s oscillation period. We detect that strong Coulomb correlations emergent in atomically thin WSe 2 shift the optimal timing of recollisions by up to 1.2 ± 0.3 fs compared to the bulk material. A quantitative analysis with quantum-dynamic many-body computations in a Wigner-function representation yields a direct and intuitive view on how the Coulomb interaction, non-classical aspects, the strength of the driving field and the valley polarization influence the dynamics. The resulting attosecond chronoscopy of delocalized electrons could revolutionize the understanding of unexpected phase transitions and emergent quantum-dynamic phenomena for future electronic, optoelectronic and quantum-information technologies. By forcing electron–hole pairs onto closed trajectories attosecond clocking of delocalized Bloch electrons is achieved, enabling greater understanding of unexpected phase transitions and quantum-dynamic phenomena.

Identifying efficient pathways to control and modify the order parameter of a macroscopic phase in materials is an important ongoing challenge. One way to do this is via the excitation of a high-frequency mode that couples to the order, and this is the ultimate goal of the field of ultrafast phase transitions1,2. This is an especially interesting research direction in magnetism, where the coupling between spin and lattice excitations is required for magnetization reversal3,4. However, previous attempts5,6 have not demonstrated switching between magnetic states via resonant pumping of phonon modes. Here we show how an ultrafast resonant excitation of the longitudinal optical phonon modes in magnetic garnet films switches magnetization into a peculiar quadrupolar magnetic domain pattern, revealing the magneto-elastic mechanism of the switching. In contrast, the excitation of strongly absorbing transverse phonon modes results in a thermal demagnetization effect only.Resonant excitation of phonons by a laser pulse switches the magnetization of a thin yttrium iron garnet film. This particular combination of longitudinal optical phonons results in a quadrupolar pattern, but this could be tailored in the future.

When intense lightwaves accelerate electrons through a solid, the emerging high-order harmonic (HH) radiation offers key insights into the material 1 – 11 . Sub-optical-cycle dynamics—such as dynamical Bloch oscillations 2 – 5 , quasiparticle collisions 6 , 12 , valley pseudospin switching 13 and heating of Dirac gases 10 —leave fingerprints in the HH spectra of conventional solids. Topologically non-trivial matter 14 , 15 with invariants that are robust against imperfections has been predicted to support unconventional HH generation 16 – 20 . Here we experimentally demonstrate HH generation in a three-dimensional topological insulator—bismuth telluride. The frequency of the terahertz driving field sharply discriminates between HH generation from the bulk and from the topological surface, where the unique combination of long scattering times owing to spin–momentum locking 17 and the quasi-relativistic dispersion enables unusually efficient HH generation. Intriguingly, all observed orders can be continuously shifted to arbitrary non-integer multiples of the driving frequency by varying the carrier-envelope phase of the driving field—in line with quantum theory. The anomalous Berry curvature warranted by the non-trivial topology enforces meandering ballistic trajectories of the Dirac fermions, causing a hallmark polarization pattern of the HH emission. Our study provides a platform to explore topology and relativistic quantum physics in strong-field control, and could lead to non-dissipative topological electronics at infrared frequencies. High-harmonic generation from the Dirac-like surface state of a topological insulator is separated from bulk contributions and continuously tuned by the carrier-envelope phase of the driving lightwave.

The question of how, and how fast, magnetization can be reversed is a topic of great practical interest for the manipulation and storage of magnetic information. It is generally accepted that magnetization reversal should be driven by a stimulus represented by time-non-invariant vectors such as a magnetic field, spin-polarized electric current, or cross-product of two oscillating electric fields. However, until now it has been generally assumed that heating alone, not represented as a vector at all, cannot result in a deterministic reversal of magnetization, although it may assist this process. Here we show numerically and demonstrate experimentally a novel mechanism of deterministic magnetization reversal in a ferrimagnet driven by an ultrafast heating of the medium resulting from the absorption of a sub-picosecond laser pulse without the presence of a magnetic field. The dynamics of spin ordering in magnetic materials is of interest both from a fundamental and an applied point of view. Using a combination of numerical and experimental techniques, Ostler et al . show that the magnetization of a ferrimagnet can be reversed on a timescale of picoseconds solely by heating it.

Reciprocal interactions between B and follicular T helper (Tfh) cells orchestrate the germinal center (GC) reaction, a hallmark of humoral immunity. Abnormal GC responses could lead to the production of pathogenic autoantibodies and the development of autoimmunity. Here we show that miR-146a controls GC responses by targeting multiple CD40 signaling pathway components in B cells; by contrast, loss of miR-146a in T cells does not alter humoral responses. However, specific deletion of both miR-146a and its paralog, miR-146b, in T cells increases Tfh cell numbers and enhanced GC reactions. Thus, our data reveal differential cell-intrinsic regulations of GC B and Tfh cells by miR-146a and miR-146b. Together, members of the miR-146 family serve as crucial molecular brakes to coordinately control GC reactions to generate protective humoral responses without eliciting unwanted autoimmunity. In the germinal center (GC), B and T cells interact to induce the production of protective antibodies against threats. Here the authors show that microRNA miR-146a modulates CD40 signaling in GC B cells, while both miR-146a and miR-146b synergize to control GC T cell responses, thereby implicating intricate controls of GC response by miR-146.