Two-dimensional (2D) ferromagnetic materials with unique magnetic properties have great potential for next-generation spintronic devices with high flexibility, easy controllability, and high heretointegrability. However, realizing magnetic switching with low power consumption at room temperature is challenging. Here, we demonstrate the room-temperature spin-orbit torque (SOT) driven magnetization switching in an all-van der Waals (vdW) heterostructure using an optimized epitaxial growth approach. The topological insulator Bi2Te3 not only raises the Curie temperature of Fe3GeTe2 (FGT) through interfacial exchange coupling but also works as a spin current source allowing the FGT to switch at a low current density of ~2.2×106 A/cm2. The SOT efficiency is ~2.69, measured at room temperature. The temperature and thickness-dependent SOT efficiency prove that the larger SOT in our system mainly originates from the nontrivial topological origin of the heterostructure. Our experiments enable an all-vdW SOT structure and provides a solid foundation for the implementation of room-temperature all-vdW spintronic devices in the future.
Despite recent advances in exfoliated vdW ferromagnets, the widespread application of 2D magnetism requires a Curie temperature (Tc) above room temperature as well as a stable and controllable magnetic anisotropy. Here we demonstrate a large-scale iron-based vdW material Fe4GeTe2 with the Tc reaching ~530 K. We confirmed the high-temperature ferromagnetism by multiple characterizations. Theoretical calculations suggested that the interface-induced right shift of the localized states for unpaired Fe d electrons is the reason for the enhanced Tc, which was confirmed by ultraviolet photoelectron spectroscopy. Moreover, by precisely tailoring Fe concentration we achieved arbitrary control of magnetic anisotropy between out-of-plane and in-plane without inducing any phase disorders. Our finding sheds light on the high potential of Fe4GeTe2 in spintronics, which may open opportunities for room-temperature application of all-vdW spintronic devices.
Two-dimensional hexagonal boron nitride (h-BN) is studied as a tunnel barrier in magnetic tunnel junctions (MTJs) as a possible alternative to MgO. The tunnel magnetoresistance (TMR) of such MTJs is calculated as a function of whether the interface involves the chemi- or physisorptive site of h-BN atoms on the metal electrodes, Fe, Co, or Ni. It is found that the physisorptive site on average produces higher TMR values, whereas the chemisorptive site has the greater binding energy but lower TMR. It is found that alloying the electrodes with an inert metal-like Pt can induce the preferred absorption site on Co to become a physisorptive site, enabling a higher TMR value. It is found that the choice of physisorptive sites of each element gives more Schottky-like dependence of their Schottky barrier heights on the metal work function.
We compare various calculation methods to determine the electronic structures and energy differences of the phases of VO2. We show that density functional methods in the form of GGA+U are able to describe the enthalpy difference (latent heat) between the rutile and M1 phases of VO2, and the effect of doping on the transition temperature and on the band gap of the M1 phase. An enthalpy difference of ΔE0 = -44.2 meV per formula unit, similar to the experimental value, is obtained if the randomly oriented spins of the paramagnetic rutile phase are treated by a non-collinear spin density functional calculation. The predicted change in the transition temperature of VO2 for Ge, Si or Mg doping is calculated and is in good agreement with the experiment data.
We report an effective approach for reducing the Schottky barrier height (SBH) in the source and drain (S/D) contacts of WS2 field-effect transistors (FETs) using an ultrathin Al2O3 interfacial layer between the metal and WS2. Al2O3 with various thicknesses was deposited to study the effect of the insulator thickness on contact resistances. The reduction of the SBH by inserting an ultrathin insulator was confirmed via both calculations and experiments, while further increasing the thickness of the insulator introduces a tunneling barrier that leads to the decrease in the current. By analyzing the metal/WS2 contact using four different metals, the density of gap states caused by defects at the interface is found to be the main reason for the Fermi level pinning, and this metal-insulator-semiconductor (MIS) contact structure shows its great advantage in the Fermi level depinning, a large improvement of the on-current, and enhanced field-effect mobilities in WS2 FETs due to an Al2O3 capping layer. This work demonstrates that the SBH can be effectively modulated by the optimization of the inserted interfacial material and its thickness.
A positive shift in the Dirac point in graphene field-effect transistors was observed with Hall-effect measurements coupled with Kelvin-probe measurements at room temperature. This shift can be explained by the asymmetrical behavior of the contact resistance by virtue of the electron injection barrier at the source contact. As an outcome, an intrinsic resistance is given to allow a retrieval of an intrinsic carrier mobility found to be decreased with increasing gate bias, suggesting the dominance of short-range scattering in a single-layer graphene field-effect transistor. These results analytically correlate the field-effect parameters with intrinsic graphene properties.
