Tunneling current-controlled spin states in few-layer van der Waals magnets.

Effective control of magnetic phases in two-dimensional magnets would constitute crucial progress in spintronics, holding great potential for future computing technologies. Here, we report a new approach of leveraging tunneling current as a tool for controlling spin states in CrI3. We reveal that a tunneling current can deterministically switch between spin-parallel and spin-antiparallel states in few-layer CrI3, depending on the polarity and amplitude of the current. We propose a mechanism involving nonequilibrium spin accumulation in the graphene electrodes in contact with the CrI3 layers. We further demonstrate tunneling current-tunable stochastic switching between multiple spin states of the CrI3 tunnel devices, which goes beyond conventional bi-stable stochastic magnetic tunnel junctions and has not been documented in two-dimensional magnets. Our findings not only address the existing knowledge gap concerning the influence of tunneling currents in controlling the magnetism in two-dimensional magnets, but also unlock possibilities for energy-efficient probabilistic and neuromorphic computing.

Nonvolatile Memristive Effect in Few-Layer CrI3 Driven by Electrostatic Gating.

The potential of memristive devices for applications in nonvolatile memory and neuromorphic computing has sparked considerable interest, particularly in exploring memristive effects in two-dimensional (2D) magnetic materials. However, the progress in developing nonvolatile, magnetic field-free memristive devices using 2D magnets has been limited. In this work, we report an electrostatic-gating-induced nonvolatile memristive effect in CrI3-based tunnel junctions. The few-layer CrI3-based tunnel junction manifests notable hysteresis in its tunneling resistance as a function of gate voltage. We further engineered a nonvolatile memristor using the CrI3 tunneling junction with low writing power and at zero magnetic field. We show that the hysteretic transport observed is not a result of trivial effects or inherent magnetic properties of CrI3. We propose a potential association between the memristive effect and the newly predicted ferroelectricity in CrI3 via gating-induced Jahn-Teller distortion. Our work illuminates the potential of 2D magnets in developing next-generation advanced computing technologies.

Controlling the Magneto-Optical Response in Ultrathin Films of EuO1-x via Interface Engineering with Ferroelectric BaTi2O5.

Utilizing pulsed laser deposition, a film of EuO1-x was deposited onto a Si(001) substrate with MgO buffer and compared to the same heterostructure with an additional BaTi2O5 thin film on top of the EuO1-x surface. X-ray diffraction (XRD) indicates the films crystallize into a preferred EuO(111) orientation; it also reveals the clear presence of EuSi2, which suggests Si or Eu diffuses across the MgO buffer layer. EuO1-x films exhibit a ferromagnetic (FM) signature and temperature-dependent exchange bias, indicated by MOKE measurements, suggesting the presence of a magnetic order well above the EuO Curie temperature with possible origins in charge carrier density near the interface. In comparison, an antiferromagnetic character persists well above the EuO Curie temperature of 69 K and the enhanced Curie temperature of 150 K for BaTi2O5 films grown on the EuO1-x films. The antiferromagnetic behavior is not seen in thicker EuO1-x thin films when integrated with other ferroelectric (FE) phases of the BaO-TiO2 system, suggesting an origin in the perturbed charge population at the BaTi2O5/EuO1-x interface.

Large magnetoelectric resistance in the topological Dirac semimetal α-Sn.

The spin-momentum locking of surface states in topological materials can produce a resistance that scales linearly with magnetic and electric fields. Such a bilinear magnetoelectric resistance (BMER) effect offers a new approach for information reading and field sensing applications, but the effects demonstrated so far are too weak or for low temperatures. This article reports the first observation of BMER effects in topological Dirac semimetals; the BMER responses were measured at room temperature and were substantially stronger than those reported previously. The experiments used topological Dirac semimetal α-Sn thin films grown on silicon substrates. The films showed BMER responses that are 106 times larger than previously measured at room temperature and are also larger than those previously obtained at low temperatures. These results represent a major advance toward realistic BMER applications. Significantly, the data also yield the first characterization of three-dimensional Fermi-level spin texture of topological surface states in α-Sn.

Switching of a Magnet by Spin-Orbit Torque from a Topological Dirac Semimetal.

Recent experiments show that topological surface states (TSS) in topological insulators (TI) can be exploited to manipulate magnetic ordering in ferromagnets. In principle, TSS should also exist for other topological materials, but it remains unexplored as to whether such states can also be utilized to manipulate ferromagnets. Herein, current-induced magnetization switching enabled by TSS in a non-TI topological material, namely, a topological Dirac semimetal α-Sn, is reported. The experiments use an α-Sn/Ag/CoFeB trilayer structure. The magnetization in the CoFeB layer can be switched by a charge current at room temperature, without an external magnetic field. The data show that the switching is driven by the TSS of the α-Sn layer, rather than spin-orbit coupling in the bulk of the α-Sn layer or current-produced heating. The switching efficiency is as high as in TI systems. This shows that the topological Dirac semimetal α-Sn is as promising as TI materials in terms of spintronic applications.

Two-Dimensional 2M-WS2 Nanolayers for Superconductivity.

Recently, a newly discovered VIB group transition metal dichalcogenide (TMD) material, 2M-WS2, has attracted extensive attention due to its interesting physical properties such as topological superconductivity, nodeless superconductivity, and anisotropic Majorana bound states. However, the techniques to grow high-quality 2M-WS2 bulk crystals and the study of their physical properties at the nanometer scale are still limited. In this work, we report a new route to grow high-quality 2M-WS2 single crystals and the observation of superconductivity in its thin layers. The crystal structure of the as-grown 2M-WS2 crystals was determined by X-ray diffraction (XRD) and scanning tunneling microscopy (STM). The chemical composition of the 2M-WS2 crystals was determined by energy dispersive X-ray spectroscopy (EDS) analysis. At 77 K, we observed the spatial variation of the local tunneling conductance (dI/dV) of the 2M-WS2 thin flakes by scanning tunneling spectroscopy (STS). Our low temperature transport measurements demonstrate clear signatures of superconductivity of a 25 nm-thick 2M-WS2 flake with a critical temperature (T C) of ∼8.5 K and an upper critical field of ∼2.5 T at T = 1.5 K. Our work may pave new opportunities in studying the topological superconductivity at the atomic scale in simple 2D TMD materials.

Small energy gap revealed in CrBr3 by scanning tunneling spectroscopy.

CrBr3 is a layered van der Waals material with magnetic ordering down to the 2D limit. For decades, based on optical measurements, it is believed that the energy gap of CrBr3 is in the range of 1.68-2.1 eV. However, controversial results have indicated that the band gap of CrBr3 is possibly smaller than that. An unambiguous determination of the energy gap is critical to the correct interpretations of the experimental results of CrBr3. Here, we present the scanning tunneling microscopy and spectroscopy (STM/S) results of CrBr3 thin and thick flakes exfoliated onto highly ordered pyrolytic graphite (HOPG) surfaces and density functional theory (DFT) calculations to reveal the small energy gap (peak-to-peak energy gap to be 0.57 ± 0.04 eV; or the onset signal energy gap to be 0.29 ± 0.05 eV from dI/dV spectra). Atomic resolution topography images show the defect-free crystal structure and the dI/dV spectra exhibit multiple peak features measured at 77 K. The conduction band - valence band peak pairs in the multi-peak dI/dV spectrum agree very well with all reported optical transitions. STM topography images of mono- and bi-layer CrBr3 flakes exhibit edge degradation due to short air exposure (∼15 min) during sample transfer. The unambiguously determined small energy gap settles the controversy and is the key in better understanding CrBr3 and similar materials.

Topological Hall Effect in a Topological Insulator Interfaced with a Magnetic Insulator.

A topological insulator (TI) interfaced with a magnetic insulator (MI) may host an anomalous Hall effect (AHE), a quantum AHE, and a topological Hall effect (THE). Recent studies, however, suggest that coexisting magnetic phases in TI/MI heterostructures may result in an AHE-associated response that resembles a THE but in fact is not. This Letter reports a genuine THE in a TI/MI structure that has only one magnetic phase. The structure shows a THE in the temperature range of T = 2-3 K and an AHE at T = 80-300 K. Over T = 3-80 K, the two effects coexist but show opposite temperature dependencies. Control measurements, calculations, and simulations together suggest that the observed THE originates from skyrmions, rather than the coexistence of two AHE responses. The skyrmions are formed due to a Dzyaloshinskii-Moriya interaction (DMI) at the interface; the DMI strength estimated is substantially higher than that in heavy metal-based systems.

Magnetization switching using topological surface states.

Topological surface states (TSSs) in a topological insulator are expected to be able to produce a spin-orbit torque that can switch a neighboring ferromagnet. This effect may be absent if the ferromagnet is conductive because it can completely suppress the TSSs, but it should be present if the ferromagnet is insulating. This study reports TSS-induced switching in a bilayer consisting of a topological insulator Bi2Se3 and an insulating ferromagnet BaFe12O19. A charge current in Bi2Se3 can switch the magnetization in BaFe12O19 up and down. When the magnetization is switched by a field, a current in Bi2Se3 can reduce the switching field by ~4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature; it is ~30 times higher than in Pt/BaFe12O19. These strong effects originate from the presence of more pronounced TSSs at low temperatures due to enhanced surface conductivity and reduced bulk conductivity.

Room temperature d0 ferromagnetism in PbS films: nonuniform distribution of Pb vacancies.

Because of the importance of ferromagnetism at room temperature, we search for new materials that can exhibit a non-vanishing magnetic moment at room temperature and at the same time can be used in spintronics. The experimental results indicate that d0 ferromagnetism without any magnetic impurities takes place in PbS films made of close-packed lead sulfide nanoparticles of 30 nm. To explain the existence of the d0 ferromagnetism, we propose a model where various PbS bulk and surface configurations of Pb-vacancies are analyzed. The bulk configurations have a zero magnetic moment while the two surface configurations with Pb vacancies with the same non-vanishing magnetic moments and lowest ground state energies contribute to the total magnetization. Based on the experimental value of the saturation magnetization, 0.2 emu g-1, we have found that the calculated Pb vacancy concentration should be about 3.5%, which is close to typical experimental values. Besides being very important for applications, there is one feature of PbS d0 ferromagnetism that makes this material special for fundamental research: PbS ferromagnetism can exhibit topologically driven spatial magnetic moment distributions (e.g., magnetic skyrmions) due to large spin-orbit coupling.

Magnetic properties and photovoltaic applications of ZnO:Mn nanocrystals.

A simple and large-scale synthetic method of Mn doped ZnO (ZnO:Mn) was developed in this work. ZnO:Mn nanocrystals with hexagonal structure were prepared by thermal decomposition of zinc acetate and manganese acetate in the presence of oleylamine and oleic acid with different temperatures, ligand ratios, and Mn doping concentrations. The particle size (47-375 nm) and morphology (hexagonal nanopyramid, hexagonal nanodisk and irregular nanospheres) of ZnO:Mn nanocrystals can be controlled by the ratio of capping ligand, reaction temperature, reaction time and Mn doping concentration. The corresponding optical and magnetic properties were systemically studied and compared. All samples were found to be paramagnetic with antiferromagnetic (AFM) exchange interactions between the Mn moments in the ZnO lattice, which can be affected by the reaction conditions. The quantum dot sensitized solar cells (QDSSCs) were fabricated based on ZnO:Mn nanocrystals and CdS quantum dots, and the device performance affected by Mn doping concentration was also studied and compared.

Origin of the Ultrafast Response of the Lateral Photovoltaic Effect in Amorphous MoS2/Si Junctions.

The lateral photovoltaic (LPV) effect has attracted much attention for a long time because of its application in position-sensitive detectors (PSD). Here, we report the ultrafast response of the LPV in amorphous MoS2/Si (a-MoS2/Si) junctions prepared by the pulsed laser deposition (PLD) technique. Different orientations of the built-in field and the breakover voltages are observed for a-MoS2 films deposited on p- and n-type Si wafers, resulting in the induction of positive and negative voltages in the a-MoS2/n-Si and a-MoS2/p-Si junctions upon laser illumination, respectively. The dependence of the LPV on the position of the illumination shows very high sensitivity (183 mV mm-1) and good linearity. The optical relaxation time of LPV with a positive voltage was about 5.8 µs in a-MoS2/n-Si junction, whereas the optical relaxation time of LPV with a negative voltage was about 2.1 µs in a-MoS2/p-Si junction. Our results clearly suggested that the inversion layer at the a-MoS2/Si interface made a good contribution to the ultrafast response of the LPV in a-MoS2/Si junctions. The large positional sensitivity and ultrafast relaxation of LPV may promise the a-MoS2/Si junction's applications in fast position-sensitive detectors.

Magnetic hard gap due to bound magnetic polarons in the localized regime.

We investigate the low temperature electron transport properties of manganese doped lead sulfide films. The system shows variable range hopping at low temperatures that crosses over into an activation regime at even lower temperatures. This crossover is destroyed by an applied magnetic field which suggests a magnetic origin of the hard gap, associated with bound magnetic polarons. Even though the gap forms around the superconducting transition temperature of lead, we do not find evidence of this being due to insulator-superconductor transition. Comparison with undoped PbS films, which do not show the activated transport behavior, suggests that bound magnetic polarons create the hard gap in the system that can be closed by magnetic fields.

Near-ultraviolet lateral photovoltaic effect in Fe3O4/3C-SiC Schottky junctions.

In this paper, we report a sensitive lateral photovoltaic effect (LPE) in Fe3O4/3C-SiC Schottky junctions with a fast relaxation time at near-ultraviolet wavelengths. The rectifying behavior suggests that the large build-in electric field was formed in the Schottky junctions. This device has excellent position sensitivity as high as 67.8 mV mm-1 illuminated by a 405 nm laser. The optical relaxation time of the LPE is about 30 µs. The fast relaxation and high positional sensitivity of the LPE make the Fe3O4/3C-SiC junction a promising candidate for a wide range of ultraviolet/near-ultraviolet optoelectronic applications.

Spin-orbit torque-assisted switching in magnetic insulator thin films with perpendicular magnetic anisotropy.

As an in-plane charge current flows in a heavy metal film with spin-orbit coupling, it produces a torque on and thereby switches the magnetization in a neighbouring ferromagnetic metal film. Such spin-orbit torque (SOT)-induced switching has been studied extensively in recent years and has shown higher efficiency than switching using conventional spin-transfer torque. Here we report the SOT-assisted switching in heavy metal/magnetic insulator systems. The experiments used a Pt/BaFe12O19 bilayer where the BaFe12O19 layer exhibits perpendicular magnetic anisotropy. As a charge current is passed through the Pt film, it produces a SOT that can control the up and down states of the remnant magnetization in the BaFe12O19 film when the film is magnetized by an in-plane magnetic field. It can reduce or increase the switching field of the BaFe12O19 film by as much as about 500 Oe when the film is switched with an out-of-plane field.

Origin of colossal dielectric permittivity of rutile Ti0.9In0.05Nb0.05O2: single crystal and polycrystalline.

In this paper, we investigated the dielectric properties of (In + Nb) co-doped rutile TiO2 single crystal and polycrystalline ceramics. Both of them showed colossal, up to 10(4), dielectric permittivity at room temperature. The single crystal sample showed one dielectric relaxation process with a large dielectric loss. The voltage-dependence of dielectric permittivity and the impedance spectrum suggest that the high dielectric permittivity of single crystal originated from the surface barrier layer capacitor (SBLC). The impedance spectroscopy at different temperature confirmed that the (In + Nb) co-doped rutile TiO2 polycrystalline ceramic had semiconductor grains and insulating grain boundaries, and that the activation energies were calculated to be 0.052 eV and 0.35 eV for grain and grain boundary, respectively. The dielectric behavior and impedance spectrum of the polycrystalline ceramic sample indicated that the internal barrier layer capacitor (IBLC) mode made a major contribution to the high ceramic dielectric permittivity, instead of the electron-pinned defect-dipoles.

The optical and magnetic properties of CoO and Co nanocrystals prepared by a facile technique.

CoO and Co nanocrystals with cubic crystal structures were prepared by thermal decomposition of cobalt(II) acetate tetrahydrate in a mixture of oleylamine and oleic acid under the protection of nitrogen gas at 300 °C for 2 h. The products of CoO or Co nanocrystals are determined by the relative amount of oleylamine due to its reducibility. The sizes and shapes of CoO or Co can be controlled by the ratio of cobalt : oleylamine : oleic acid due to different binding capabilities of the two capping ligands (oleylamine and oleic acid). A modification of the surface state by surface passivation arising from the capping ligands for CoO nanocrystals leads to the blue shift of the ligand-metal charge transfer (LMCT) absorption. Room temperature ferromagnetism originating from uncompensated surface spins, as well as magnetic moments weakly exchange coupled to the CoO lattice due to defects inside CoO nanoparticles, are observed. The magnetic behaviors of CoO and Co nanoparticles also shed light on the synthesis and the magnetic properties of the antiferromagnetic and ferromagnetic nanomaterials.

Watermelon-like iron nanoparticles: Cr doping effect on magnetism and magnetization interaction reversal.

Cr-doped core-shell iron/iron-oxide nanoparticles (NPs) containing 0, 2, 5, and 8 at.% of Cr dopant were synthesized via a nanocluster deposition system and their structural and magnetic properties were investigated. We observed the formation of a σ-FeCr phase in 2 at.% of Cr doping in core-shell NPs. This is unique since it was reported in the past that the σ-phase forms above 20 at.% of Cr. The large coercive field and exchange bias are ascribed to the antiferromagnetic Cr2O3 layer formed with the Fe-oxide shell, which also acts as a passivation layer to decrease the Fe-oxide shell thickness. The additional σ-phase in the core and/or Cr2O3 in the shell cause the hysteresis loop to appear tight waisted near the zero-field axis. The exchange interaction competes with the dipolar interaction with the increase of σ-FeCr grains in the Fe-core. The interaction reversal has been observed in 8 at.% of Cr. The observed reversal mechanism is confirmed from the Henkel plot and delta M value, and is supported by a theoretical watermelon model based on the core-shell nanostructure system.

A magnetic polaron model for the enhanced Curie temperature of EuO(1-x).

The investigation of a series of oxygen-deficient EuO thin films provided strong evidence that the doped electrons form magnetic polarons with the nearby Eu2+ 4f spins; this is responsible for the enhanced Curie temperature observed near 140 K. Unlike in the previous magnetic polaron models proposed for the metal-to-insulator transition in EuO, the exchange coupling J between the doped electron and its neighboring 4f spins is antiferromagnetic. The model explains satisfactorily the fact that the ordering temperature of the magnetic polarons occurs at ~140 K, independently of the oxygen vacancy concentration, and the contradiction that electron doping increases T(c) and yet reduces the red shift in the optical absorption. The magnetic polarons are coupled antiferromagnetically to the Eu2+ local moments that are ordered in the Heisenberg ferromagnet below 69 K. This coupling was observable in the vicinity of 69 K. We discuss how, with increasing concentration of the oxygen vacancies, their behaviors evolve from those of isolated superparamagnetic polarons to those of percolating magnetic polarons with a finite coercivity.

UV-Vis-NIR luminescence properties and energy transfer mechanism of LiSrPO4:Eu2+, Pr3+ suitable for solar spectral convertor.

An efficient near-infrared (NIR) phosphor LiSrPO(4):Eu(2+), Pr(3+) is synthesized by solid-state reaction and systematically investigated using x-ray diffraction, diffuse reflection spectrum, photoluminescence spectra at room temperature and 3 K, and the decay curves. The UV-Vis-NIR energy transfer mechanism is proposed based on these results. The results demonstrate Eu(2+) can be an efficient sensitizer for harvesting UV photon and greatly enhancing the NIR emission of Pr(3+) between 960 and 1060 nm through efficient energy feeding by allowed 4f-5d absorption of Eu(2+) with high oscillator strength. Eu(2+)/Pr(3+) may be an efficient donor-acceptor pair as solar spectral converter for Si solar cells.