Perpendicular Magnetic Anisotropy and Dzyaloshinskii-Moriya Interaction at an Oxide/Ferromagnetic Metal Interface.

We report on the study of both perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii-Moriya interaction (DMI) at an oxide/ferromagnetic metal (FM) interface, i.e., BaTiO_{3} (BTO)/CoFeB. Thanks to the functional properties of the BTO film and the capability to precisely control its growth, we are able to distinguish the dominant role of the oxide termination (TiO_{2} vs BaO) from the moderate effect of ferroelectric polarization in the BTO film, on the PMA and DMI at an oxide/FM interface. We find that the interfacial magnetic anisotropy energy of the BaO-BTO/CoFeB structure is 2 times larger than that of the TiO_{2}-BTO/CoFeB, while the DMI of the TiO_{2}-BTO/CoFeB interface is larger. We explain the observed phenomena by first principles calculations, which ascribe them to the different electronic states around the Fermi level at oxide/ferromagnetic metal interfaces and the different spin-flip process. This study paves the way for further investigation of the PMA and DMI at various oxide/FM structures and thus their applications in the promising field of energy-efficient devices.

Vacancy-Induced Synaptic Behavior in 2D WS2 Nanosheet-Based Memristor for Low-Power Neuromorphic Computing.

Memristors with nonvolatile memory characteristics have been expected to open a new era for neuromorphic computing and digital logic. However, existing memristor devices based on oxygen vacancy or metal-ion conductive filament mechanisms generally have large operating currents, which are difficult to meet low-power consumption requirements. Therefore, it is very necessary to develop new materials to realize memristor devices that are different from the mechanisms of oxygen vacancy or metal-ion conductive filaments to realize low-power operation. Herein, high-performance and low-power consumption memristors based on 2D WS2 with 2H phase are demonstrated, which show fast ON (OFF) switching times of 13 ns (14 ns), low program current of 1 µA in the ON state, and SET (RESET) energy reaching the level of femtojoules. Moreover, the memristor can mimic basic biological synaptic functions. Importantly, it is proposed that the generation of sulfur and tungsten vacancies and electron hopping between vacancies are dominantly responsible for the resistance switching performance. Density functional theory calculations show that the defect states formed by sulfur and tungsten vacancies are at deep levels, which prevent charge leakage and facilitate the realization of low-power consumption for neuromorphic computing application.

Modulating Multiferroic Control of Magnetocrystalline Anisotropy Using 5d Transition Metal Capping Layers.

Electric-field control of magnetocrystalline anisotropy energy (MAE) is important for the optimal performance of the tunnel junction components of the STT-MRAM. In such a device, a high MAE of the free magnetic layer improves storage robustness, whereas a low MAE is also useful to keep energy expenditure in the switching process at a minimum. Using the frozen potential method to calculate the MAE of the CoFe layer, the electric-field control of MAE in the BaTiO3/CoFe/(Hf, Ta, W, Re, Os, Ir, Pt, or Au) heterostructure is studied. Electric field tuning of MAE is determined to be possible through switching the direction of BaTiO3 ferroelectric polarization, although both the tuning effect and the MAE depend strongly on the choice of the 5d transition metal element in the capping layer. The results predict a complicated behavior of both MAE and the underlayer polarization effect as we progress down the 5d series of elements as the choice of the capping layer element. Using the second-order perturbation theoretical framework, this behavior can nevertheless be explained by mechanisms including CoFe/capping layer interface hybridization and 5d band-filling trends in the capping layer.

Thickness and Ferroelectric Polarization Influence on Film Magnetic Anisotropy across a Multiferroic Material Interface.

The ferroelectric switching effect on perpendicular magnetic anisotropy is examined for the case of the BaTiO3/L10-CoFe interface through first-principles calculations of film magnetocrystalline anisotropy energy (MAE), both with the frozen-potential method and the second-order perturbation theory. The ferroelectric switching-MAE relationship is shown to have opposite trends for BaO- and TiO2-terminated interfaces because of the distinct orbital interaction mechanisms predominant in each termination configuration. The ferroelectric switching effect, changes in Fe-O bond lengths, and termination constitute three different contributors to MAE change, each with a different penetration depth into the CoFe film. The top surface CoFe atoms are shown to feature a high density of minority-spin 3dxz states, which could play a role in influencing the ferroelectric switching-MAE relationship in cases where the top surface undergoes modifications.

A carbon-based memristor design for associative learning activities and neuromorphic computing.

Carbon quantum dots (QDs) have attracted significant interest due to their excellent electronic properties and wide application prospects. However, the application of carbon QDs has been rarely reported in memristors. Here, a memristor model with carbon conductive filaments (CFs) is proposed for the first time based on carbon quantum dots. The CF-based devices exhibited excellent resistive switching performance, in particular a narrow range of SET and RESET voltages and good power efficiency and retention properties. These devices could also emulate important biological synapse performances, such as the transition from short-term plasticity (STP) to long-term potentiation (LTP) behaviors, long-term depression (LTD) behavior, and four types of spike-timing-dependent plasticity (STDP) learning rules. Interestingly, Pavlovian associative learning functions were also reliably demonstrated in the memristor device (MD). The digit recognition ability of the MDs was evaluated though a single-layer perceptron model, in which the recognition accuracy of digits reached 92.63% after 250 training iterations. The transmission electron microscopy (TEM) results evidenced that the carbon CF was found in the MD at the "ON" state. Thus, this new carbon CF-based mechanism for memristors provides a new idea for achieving better neuromorphic MDs and applications.