We demonstrate in situ 90° electric field-induced uniform magnetization rotation in single domain submicron ferromagnetic islands grown on a ferroelectric single crystal using x-ray photoemission electron microscopy. The experimental findings are well correlated with micromagnetic simulations, showing that the reorientation occurs by the strain-induced magnetoelectric interaction between the ferromagnetic nanostructures and the ferroelectric crystal. Specifically, the ferroelectric domain structure plays a key role in determining the response of the structure to the applied electric field, resulting in three strain-induced regimes of magnetization behavior for the single domain islands.

The original observation of the Einstein-de Haas effect was a landmark experiment in the early history of modern physics that illustrates the relationship between magnetism and angular momentum 1, 2 . Today the effect is still discussed in elementary physics courses to demonstrate that the angular momentum associated with the aligned electron spins in a ferromagnet can be converted to mechanical angular momentum by reversing the direction of magnetisation using an external magnetic field. In recent times, a related problem in magnetism concerns the time-scale over which this angular momentum transfer can occur. It is known experimentally for several metallic ferromagnets that intense photoexcitation leads to a drop in the magnetisation on a time scale shorter than 100 fs, a phenomenon called ultrafast demagnetisation 3-5 . The microscopic mechanism for this process has been hotly debated, with one key question still unanswered: where does the angular momentum go on these femtosecond time scales? Here we show using femtosecond time-resolved x-ray diffraction that a majority of the angular momentum lost from the spin system on the laser-induced demagnetisation of ferromagnetic iron is transferred to the lattice on sub-picosecond timescales, manifesting as a transverse strain wave that propagates from the surface into the bulk. By fitting a simple model of the x-ray data to simulations and optical data, we estimate that the angular momentum occurs on a time scale of 200 fs and corresponds to 80% of the angular momentum lost from the spin system. Our results show that interaction with the lattice plays an essential role in the process of ultrafast demagnetisation in this system. 2Broadly speaking, proposed mechanisms for ultrafast demagnetisation fall into two categories: spin-flip scattering mechanisms and spin transport mechanisms. The first category explains the demagnetisation process as a sudden increase in scattering processes that ultimately result in a decrease of spin order. These scattering processes can include electron-electron, electron-phonon, electron-magnon and even direct spin-light interactions. On average, such scattering must necessarily involve a transfer of angular momentum from the electronic spins to some other subsystem(s). Candidates include the lattice, the electromagnetic field, and the orbital angular momentum of the electrons. Numerical estimates and experiments using circularly polarised light strongly suggest that the amount of angular momentum given to the electromagnetic field interaction is negligible 6 , and experiments using femtosecond x-ray magnetic dichroism (XMCD) indicate that the angular momentum of both electronic spins and orbitals decrease in magnitude nearly simultaneously 7-9 . The only remaining possibility for a spin-flip induced change in angular momentum therefore appears to be a transfer to the lattice via spin-orbit coupling, but this remains to be experimentally verified.The second category of proposed mechanisms relies on the idea that laser excitation causes a ...

The properties of organic conductors are often tuned by the application of chemical or external pressure, which change orbital overlaps and electronic bandwidths while leaving the molecular building blocks virtually unperturbed. Here, we show that, unlike any other method, light can be used to manipulate the local electronic properties at the molecular sites, giving rise to new emergent properties. Targeted molecular excitations in the charge-transfer salt κ-ðBEDTÀTTFÞ 2 Cu½NðCNÞ 2 Br induce a colossal increase in carrier mobility and the opening of a superconducting optical gap. Both features track the density of quasiparticles of the equilibrium metal and can be observed up to a characteristic coherence temperature T Ã ≃ 50 K, far higher than the equilibrium transition temperature T C ¼ 12.5 K. Notably, the large optical gap achieved by photoexcitation is not observed in the equilibrium superconductor, pointing to a light-induced state that is different from that obtained by cooling. First-principles calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photomolecular superconductivity.

Optical excitation at terahertz frequencies has emerged as an effective means to dynamically manipulate complex materials. In the molecular solid K3C60, short mid-infrared pulses transform the high-temperature metal into a non-equilibrium state with the optical properties of a superconductor. Here we tune this effect with hydrostatic pressure and find that the superconducting-like features gradually disappear at around 0.3 GPa. Reduction with pressure underscores the similarity with the equilibrium superconducting phase of K3C60, in which a larger electronic bandwidth induced by pressure is also detrimental for pairing. Crucially, our observation excludes alternative interpretations based on a high-mobility metallic phase. The pressure dependence also suggests that transient, incipient superconductivity occurs far above the 150 K hypothesised previously, and rather extends all the way to room temperature.

Excitation of high-Tc cuprates and certain organic superconductors with intense far-infrared optical pulses has been shown to create non-equilibrium states with optical properties that are consistent with transient high-temperature superconductivity. These non-equilibrium phases have been generated using femtosecond drives, and have been observed to disappear immediately after excitation, which is evidence of states that lack intrinsic rigidity. Here we make use of a new optical device to drive metallic K3C60 with mid-infrared pulses of tunable duration, ranging between one picosecond and one nanosecond. The same superconducting-like optical properties observed over short time windows for femtosecond excitation are shown here to become metastable under sustained optical driving, with lifetimes in excess of ten nanoseconds. Direct electrical probing, which becomes possible at these timescales, yields a vanishingly small resistance with the same relaxation time as that estimated by terahertz conductivity. We provide a theoretical description of the dynamics after excitation, and justify the observed slow relaxation by considering randomization of the order-parameter phase as the rate-limiting process that determines the decay of the light-induced superconductor.

A phenomenon that can be exploited for the manipulation of magnetization without the conventional current-generated magnetic fields is magnetoelastic coupling, which might, thus, pave the way for lowpower data-storage devices. Here, we report a quantitative analysis of the magnetic uniaxial anisotropy induced by piezoelectric strain in Ni nanostructured squares. By applying strain, the magnetic domains in Ni nanostructured squares can be manipulated by the magnetoelastic effect in the Ni. The strain-induced anisotropy displaces the domain walls in the square leading to changes in the domain sizes. By comparing the experiments with micromagnetic simulations, the resulting uniaxial anisotropy is quantified. We find a good agreement for a magnetostrictive constant of λ s ¼ −26 ppm, confirming a full strain transfer from the piezoelectric to the Ni nanostructures and the retainment of a bulklike λ s .

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Optical pulses are routinely used to drive dynamical changes in the properties of solids. In quantum materials, many new phenomena have been discovered, including ultrafast transitions between electronic phases, switching of ferroic orders and nonequilibrium emergent behaviors such as photo-induced superconductivity.Understanding the underlying non-equilibrium physics requires detailed measurements of multiple microscopic degrees of freedom at ultrafast time resolution. Femtosecond x-rays are key to this endeavor, as they can access the dynamics of structural, electronic and magnetic degrees of freedom. Here, we cover a series of representative experimental studies in which ultrashort x-ray pulses from free electron lasers have been used, opening up new horizons for materials research.