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Ultrafast magnetization switching induced by a single femtosecond laser pulse, under no applied magnetic field has attracted a lot of attention in the last 10 years because of its high potential for low-energy and ultrafast memory applications. Single-pulse helicity-independent switching has mostly been demonstrated for $\mathrm{Gd}$-based materials. It is now necessary to optimize the pulse duration and the energy needed to switch a $\mathrm{Gd}$-$\mathrm{Fe}$-$\mathrm{Co}$ magnet depending on the alloy thickness and composition. Here we experimentally report state diagrams showing the magnetic state obtained after one single pulse depending on the laser pulse duration and fluence for various $\mathrm{Gd}$-$\mathrm{Fe}$-$\mathrm{Co}$ thin films with different compositions and thicknesses. We demonstrate that these state diagrams share similar characteristics: the fluence window for switching narrows for longer pulse duration and for the considered pulse-duration range the critical fluence for single-pulse switching increases linearly as a function of the pulse duration while the critical fluence required for creating a multidomain state remains almost constant. Calculations based on the atomistic spin model qualitatively reproduce the experimental state diagrams and their evolution. By studying the effect of the composition and the thickness on the state diagram, we demonstrate that the best energy efficiency and the longest pulse duration for switching are obtained for composition around the magnetic compensation.

Recent demonstration of magnetization manipulation has been focused on the utilization of pure spin current converted by charge current in nonmagnetic materials with strong spin-orbit coupling (SOC), which stimulates intensive studies on the exploration of materials with larger SOC, such as heavy metals and topological insulators. We suggest an alternative approach for enhancing the effective charge-to-spin conversion efficiency by applying strain in Ta/Fe/Pt films grown on flexible mica substrates. We experimentally demonstrate a large and tunable charge-to-spin conversion efficiency in Ta/Fe/Pt films by applying compressive strain within a flexible substrate, and over 50% enhancement of effective spin Hall angle eff SHE of up to approximately 0.2 is achieved in a 6.26‰ strained film. Our findings may spur further work on the integration of flexible electronics and SOC and may potentially lead to the innovation of alternative flexible spintronics devices.

International audience; We investigate spin-wave resonance modes and their damping in insulating thin films of bismuth-substituted yttrium iron garnet by performing femtosecond magneto-optical pump-probe experiments. For large magnetic fields in the range below the magnetization saturation, we find that the damping of high-order standing spin-wave (SSW) modes is about 40 times lower than that for the fundamental one. The observed phenomenon can be explained by considering different features of magnetic anisotropy and exchange fields that, respectively, define the precession frequency for fundamental and high-order SSWs. These results provide further insight into SSWs in iron garnets and may be exploited in many new photomagnonic devices.

All-optical helicity-dependent switching in ferromagnetic Co/Pt multilayers is investigated using magneto-optical microscopy and anomalous Hall effect measurements. A state diagram is built by studying the effect of pulse duration, fluence, and spot size. We use numerical solutions of the three-temperature model to explain that the all-optical helicity-dependent switching mechanism relies on the spin bath reaching temperatures close to the Curie point. Further insights into the reversal process are provided by the experimental demonstration of significant helicity-dependent reversal after a single laser pulse that reveals the involvement of direct angular momentum transfer. Moreover, based on the observation that longer pulse durations and larger spot sizes lead to enhanced reversal efficiency, we identify experimental conditions that lead to saturated magnetization reversal after just a few tens of laser pulses.

International audience; Nonvolatile magnetic random-access memory is one of the most promising memory candidates to meet the requirements of high-density and low-power data storage. Downsize scalability and energy efficiency of conventional memory-unit cells with elliptic shape, however, remain matters of great concern. The development of alternative memory-unit-cell architecture that would potentially enhance the performance of practical devices is thus particularly interesting for applications. Here the magnetic configurations for nanoring magnetic tunnel junctions with an in-plane magnetic storage layer are studied by micromagnetic simulation, revealing that, for an appropriate ring width, the outer diameter can be scaled down to less than 20 nm, where the magnetic onion configuration becomes energetically favored. We also study the spin-transfer-torque switching process and dynamic resistance state diagram with respect to the applied magnetic field and spin-polarized current. A low switching-current density is demonstrated. The results indicate the advantage of using nanoring magnetic tunnel junctions as memory units, which may provide an alternative solution for high-storage-density and low-power-consumption nonvolatile memory.

We report on the growth and study of Co-Fe-B/MgO/Ge(001) spin photodiode by using a combination of both molecular beam epitaxy (MBE) and sputtering methods. An epitaxial growth of MgO on Ge in MBE is achieved by the deposition of MgO at room temperature (RT) followed by a post-growth anneal-ing at 300 °C. The spin detector, which consists of 1.1-nm ultrathin Co-Fe-B layer capped with 5-nm Ta is subsequently grown by sputtering at RT. After a post-growth annealing of the whole structure at 250 °C, we obtain clear evidence of a strong perpendicular magnetic anisotropy in the temperature range 10-300 K. Co-Fe-B/MgO/Ge(001) heterojunctions are then processed into spin photodiodes demonstrating at zero magnetic field a photocurrent helicity asymmetry of about 0.9% at 9 K and 0.1% at RT at the telecommunication wavelength of 1310 nm. The demonstration of a spin photodiode working at a telecommunication wavelength with zero applied magnetic field is of great interest for future applications of the optical transport of spin information.

We investigate the influence of the MgO growth process on the bias dependence of the electrical spin injection from a Co-Fe-B=MgO spin injector into a GaAs-based light-emitting diode (spin LED). With this aim, textured MgO tunnel barriers are fabricated either by sputtering or molecular-beam-epitaxy (MBE) methods. For the given growth parameters used for the two techniques, we observe that the circular polarization of the electroluminescence emitted by spin LEDs is rather stable as a function of the injected current or applied bias for the samples with sputtered tunnel barriers, whereas the corresponding circular polarization decreases abruptly for tunnel barriers grown by MBE. We attribute these different behaviors to the different kinetic energies of the injected carriers linked to differing amplitudes of the parasitic hole current flowing from GaAs to Co-Fe-B in both cases.