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Two-dimensional materials-based ultrafast spintronics are expected to surpass conventional data storage and manipulation technologies, that are now reaching their fundamental limits. The newly discovered van der Waals (VdW) magnets provide a new platform for ultrafast spintronics since their magnetic and electrical properties can be tuned by many external factors, such as strain, voltage, magnetic field, or light absorption for instance. Here, we report on the direct relationship between magnetic order and Terahertz (THz) electrodynamics in FenGeTe2 (n = 3, 5) (FGT) films after being illuminated by a femtosecond optical pulse, studying their ultrafast THz response as a function of the optical pump-THz probe temporal delay. In Fe5GeTe2, we find clear evidence that light-induced electronic excitations directly influence THz electrodynamics similarly to a demagnetization process, contrasting with the effects observed in Fe3GeTe2, which are characterized by a thermal energy transfer among electrons, magnons, and phonons. We address these effects as a function of the pump fluence and pump-probe delay, and by tuning the temperature across the magnetic ordering Curie temperature, highlighting the microscopic mechanisms describing the out-of-equilibrium evolution of the THz conductivity. Finally, we find evidence for the incoherent-coherent crossover predicted by the Kondo-Ising scenario in Fe3GeTe2 and successfully simulate its light-driven electrodynamics through a three-temperature model. As indicated by these results, FGT surpasses conventional metals in terms of modulating their properties using an optical lever. [ABSTRACT FROM AUTHOR]

In recent years, two-dimensional van der Waals materials have emerged as an important platform for the observation of long-range ferromagnetic order in atomically thin layers. Although heterostructures of such materials can be conceived to harness and couple a wide range of magneto-optical and magneto-electrical properties, technologically relevant applications require Curie temperatures at or above room temperature and the ability to grow films over large areas. Here we demonstrate the large-area growth of single-crystal ultrathin films of stoichiometric Fe5GeTe2 on an insulating substrate using molecular beam epitaxy. Magnetic measurements show the persistence of soft ferromagnetism up to room temperature in 12 nm-thick films, with a Curie temperature of 293 K, and a weak out-of-plane magnetocrystalline anisotropy. The ferromagnetic order is preserved in bilayer Fe5GeTe2, with Curie temperature decreasing to 229 K. Surface, chemical, and structural characterizations confirm the layer-by-layer growth, 5:1:2 Fe:Ge:Te stoichiometric elementary composition, and single-crystalline character of the films. [ABSTRACT FROM AUTHOR]

We present a comparative investigation of the influence of crystallinity and film thickness on the acoustic and thermal properties of layered PtSe2 films of varying thickness (1–40 layers) using frequency-domain thermo-reflectance, low-frequency Raman, and pump-probe coherent phonon spectroscopy. We find ballistic cross-plane heat transport up to ~30 layers PtSe2 and a 35% reduction in the cross-plane thermal conductivity of polycrystalline films with thickness larger than 20 layers compared to the crystalline films of the same thickness. First-principles calculations further reveal a high degree of thermal conductivity anisotropy and a remarkable large contribution of the optical phonons to the thermal conductivity in bulk (~20%) and thin PtSe2 films (~30%). Moreover, we show strong interlayer interactions in PtSe2, short acoustic phonon lifetimes in the range of picoseconds, an out-of-plane elastic constant of 31.8 GPa, and a layer-dependent group velocity ranging from 1340 ms−1 in bilayer to 1873 ms−1 in eight layers of PtSe2. The potential of tuning the lattice thermal conductivity of layered materials with the level of crystallinity and the real-time observation of coherent phonon dynamics open a new playground for research in 2D thermoelectric devices and provides guidelines for thermal management in 2D electronics. [ABSTRACT FROM AUTHOR]

In recent years, two-dimensional van der Waals materials have emerged as an important platform for the observation of long-range ferromagnetic order in atomically thin layers. Although heterostructures of such materials can be conceived to harness and couple a wide range of magneto-optical and magneto-electrical properties, technologically relevant applications require Curie temperatures at or above room temperature and the ability to grow films over large areas. Here we demonstrate the large-area growth of single-crystal ultrathin films of stoichiometric Fe5GeTe2 on an insulating substrate using molecular beam epitaxy. Magnetic measurements show the persistence of soft ferromagnetism up to room temperature in 12 nm-thick films, with a Curie temperature of 293 K, and a weak out-of-plane magnetocrystalline anisotropy. The ferromagnetic order is preserved in bilayer Fe5GeTe2, with Curie temperature decreasing to 229 K. Surface, chemical, and structural characterizations confirm the layer-by-layer growth, 5:1:2 Fe:Ge:Te stoichiometric elementary composition, and single-crystalline character of the films. [ABSTRACT FROM AUTHOR]

Spin-orbit torques, including the Rashba and spin Hall effects, have been widely observed and investigated in various systems. Since interesting spin-orbit torque (SOT) arises at the interface between heavy nonmagnetic metals and ferromagnetic metals, most studies have focused on the ultra-thin ferromagnetic layer with interface perpendicular magnetic anisotropy. Here, we measured the effective longitudinal and transverse fields of bulk perpendicular magnetic anisotropy Pd/FePd (1.54 to 2.43 nm)/MgO systems using harmonic methods with careful correction procedures. We found that in our range of thicknesses, the effective longitudinal and transverse fields are five to ten times larger than those reported in interface perpendicular magnetic anisotropy systems. The observed magnitude and thickness dependence of the effective fields suggest that the SOT do not have a purely interfacial origin in our samples. [ABSTRACT FROM AUTHOR]

Using resonant phenomena to exert coherent control over electron spin dynamics could enable more-energy-efficient spintronic devices and related technologies. Yet, achieving collective spin resonant control by ferromagnetic resonance excitation most often requires radiofrequency magnetic fields or the injection of spin-polarized currents that consume relatively large amounts of power. Inducing ferromagnetic resonances directly with an electric field offers a lower-power alternative to these, but doing so requires strong electric and magnetic coupling that is difficult to realize owing to screening effects, particularly in metallic materials. Here, we demonstrate electric-field-induced ferromagnetic resonance excitation by means of voltage control over the magnetic anisotropy in a few monolayers of FeCo at room temperature. The technique provides a low-power, highly localized and coherent means to manipulate electron spin dynamics. [ABSTRACT FROM AUTHOR]

The magnetization direction of a metallic magnet has generally been controlled by a magnetic field or by spin-current injection into nanosized magnetic cells. Both these methods use an electric current to control the magnetization direction; therefore, they are energy consuming. Magnetization control using an electric field is considered desirable because of its expected ultra-low power consumption and coherent behaviour. Previous experimental approaches towards achieving voltage control of magnetization switching have used single ferromagnetic layers with and without piezoelectric materials, ferromagnetic semiconductors, multiferroic materials, and their hybrid systems. However, the coherent control of magnetization using voltage signals has not thus far been realized. Also, bistable magnetization switching (which is essential in information storage) possesses intrinsic difficulties because an electric field does not break time-reversal symmetry. Here, we demonstrate a coherent precessional magnetization switching using electric field pulses in nanoscale magnetic cells with a few atomic FeCo (001) epitaxial layers adjacent to a MgO barrier. Furthermore, we demonstrate the realization of bistable toggle switching using the coherent precessions. The estimated power consumption for single switching in the ideal equivalent switching circuit can be of the order of 104kBT, suggesting a reduction factor of 1/500 when compared with that of the spin-current-injection switching process. [ABSTRACT FROM AUTHOR]

Electric fields at interfaces exhibit useful phenomena, such as switching functions in transistors, through electron accumulations and/or electric dipole inductions. We find one potentially unique situation in a metal-dielectric interface in which the electric field is atomically inhomogeneous because of the strong electrostatic screening effect in metals. Such electric fields enable us to access electric quadrupoles of the electron shell. Here we show, by synchrotron X-ray absorption spectroscopy, electric field induction of magnetic dipole moments in a platinum monatomic layer placed on ferromagnetic iron. Our theoretical analysis indicates that electric quadrupole induction produces magnetic dipole moments and provides a large magnetic anisotropy change. In contrast with the inability of current designs to offer ultrahigh-density memory devices using electric-field-induced spin control, our findings enable a material design showing more than ten times larger anisotropy energy change for such a use and highlight a path in electric-field control of condensed matter. [ABSTRACT FROM AUTHOR]