Greatly Enhanced Emission from Spin Defects in Hexagonal Boron Nitride Enabled by a Low-Loss Plasmonic Nanocavity.

The negatively charged boron vacancy (VB-) defect in hexagonal boron nitride (hBN) with optically addressable spin states has emerged due to its potential use in quantum sensing. Remarkably, VB- preserves its spin coherence when it is implanted at nanometer-scale distances from the hBN surface, potentially enabling ultrathin quantum sensors. However, its low quantum efficiency hinders its practical applications. Studies have reported improving the overall quantum efficiency of VB- defects with plasmonics; however, the overall enhancements of up to 17 times reported to date are relatively modest. Here, we demonstrate much higher emission enhancements of VB- with low-loss nanopatch antennas (NPAs). An overall intensity enhancement of up to 250 times is observed, corresponding to an actual emission enhancement of ∼1685 times by the NPA, along with preserved optically detected magnetic resonance contrast. Our results establish NPA-coupled VB- defects as high-resolution magnetic field sensors and provide a promising approach to obtaining single VB- defects.

Electrically Active Domain Wall Magnons in Layered van der Waals Antiferromagnets.

We study, theoretically, domain wall (DW) magnons-elementary collective excitations of magnetic DWs-in easy-axis layered van der Waals (vdW) antiferromagnets, where they behave as normal modes of coupled spin superfluids. We uncover that, due to spin-charge coupling in vdW magnets, such DW magnons can be activated by voltage-induced torques, thereby providing a path for their low-dissipation and nanoscale excitation. Moreover, the electrical activation and the number of DW magnons at a frequency can be controlled by applying symmetry-breaking static magnetic field, adding tunability of signal transmission by them. Our results highlight that domain walls in vdW magnets provide a promising platform to route coherent spin information for a broad range of explorations in spintronics and magnetism.

High-Contrast Plasmonic-Enhanced Shallow Spin Defects in Hexagonal Boron Nitride for Quantum Sensing.

The recently discovered spin defects in hexagonal boron nitride (hBN), a layered van der Waals material, have great potential in quantum sensing. However, the photoluminescence and the contrast of the optically detected magnetic resonance (ODMR) of hBN spin defects are relatively low so far, which limits their sensitivity. Here we report a record-high ODMR contrast of 46% at room temperature and simultaneous enhancement of the photoluminescence of hBN spin defects by up to 17-fold by the surface plasmon of a gold film microwave waveguide. Our results are obtained with shallow boron vacancy spin defects in hBN nanosheets created by low-energy He+ ion implantation and a gold film microwave waveguide fabricated by photolithography. We also explore the effects of microwave and laser powers on the ODMR and improve the sensitivity of hBN spin defects for magnetic field detection. Our results support the promising potential of hBN spin defects for nanoscale quantum sensing.

Room-Temperature Skyrmion Shift Device for Memory Application.

Magnetic skyrmions are intensively explored for potential applications in ultralow-energy data storage and computing. To create practical skyrmionic memory devices, it is necessary to electrically create and manipulate these topologically protected information carriers in thin films, thus realizing both writing and addressing functions. Although room-temperature skyrmions have been previously observed, fully electrically controllable skyrmionic memory devices, integrating both of these functions, have not been developed to date. Here, we demonstrate a room-temperature skyrmion shift memory device, where individual skyrmions are controllably generated and shifted using current-induced spin-orbit torques. Particularly, it is shown that one can select the device operation mode in between (i) writing new single skyrmions or (ii) shifting existing skyrmions by controlling the magnitude and duration of current pulses. Thus, we electrically realize both writing and addressing of a stream of skyrmions in the device. This prototype demonstration brings skyrmions closer to real-world computing applications.

Magnetic Domain Wall Floating on a Spin Superfluid.

We theoretically investigate the transfer of angular momentum between a spin superfluid and a domain wall in an exchange coupled easy-axis and easy-plane magnetic insulator system. A domain wall in the easy-axis magnet absorbs spin angular momentum via disrupting the flow of a superfluid spin current in the easy-plane magnet. Focusing on an open geometry, where the spin current is injected electrically via a nonequilibrium spin accumulation, we derive analytical expressions for the resultant superfluid-mediated motion of the domain wall. The analytical results are supported by micromagnetic simulations. The proposed phenomenon extends the regime of magnon-driven domain-wall motion to the case where the magnons are condensed and exhibit superfluidity. Furthermore, by controlling the pinning of the domain wall, we propose a realization of a reconfigurable spin transistor. The long-distance dissipationless character of spin superfluids can thus be exploited for manipulating soliton-based memory and logic devices.

Room-Temperature Creation and Spin-Orbit Torque Manipulation of Skyrmions in Thin Films with Engineered Asymmetry.

Magnetic skyrmions, which are topologically protected spin textures, are promising candidates for ultralow-energy and ultrahigh-density magnetic data storage and computing applications. To date, most experiments on skyrmions have been carried out at low temperatures. The choice of available materials is limited, and there is a lack of electrical means to control skyrmions in devices. In this work, we demonstrate a new method for creating a stable skyrmion bubble phase in the CoFeB-MgO material system at room temperature, by engineering the interfacial perpendicular magnetic anisotropy of the ferromagnetic layer. Importantly, we also demonstrate that artificially engineered symmetry breaking gives rise to a force acting on the skyrmions, in addition to the current-induced spin-orbit torque, which can be used to drive their motion. This room-temperature creation and manipulation of skyrmions offers new possibilities to engineer skyrmionic devices. The results bring skyrmionic memory and logic concepts closer to realization in industrially relevant and manufacturable thin film material systems.

Magnetization switching through giant spin-orbit torque in a magnetically doped topological insulator heterostructure.

Recent demonstrations of magnetization switching induced by in-plane current in heavy metal/ferromagnetic heterostructures (HMFHs) have drawn great attention to spin torques arising from large spin-orbit coupling (SOC). Given the intrinsic strong SOC, topological insulators (TIs) are expected to be promising candidates for exploring spin-orbit torque (SOT)-related physics. Here we demonstrate experimentally the magnetization switching through giant SOT induced by an in-plane current in a chromium-doped TI bilayer heterostructure. The critical current density required for switching is below 8.9 × 10(4) A cm(-2) at 1.9 K. Moreover, the SOT is calibrated by measuring the effective spin-orbit field using second-harmonic methods. The effective field to current ratio and the spin-Hall angle tangent are almost three orders of magnitude larger than those reported for HMFHs. The giant SOT and efficient current-induced magnetization switching exhibited by the bilayer heterostructure may lead to innovative spintronics applications such as ultralow power dissipation memory and logic devices.

Proximity induced high-temperature magnetic order in topological insulator--ferrimagnetic insulator heterostructure.

Introducing magnetic order in a topological insulator (TI) breaks time-reversal symmetry of the surface states and can thus yield a variety of interesting physics and promises for novel spintronic devices. To date, however, magnetic effects in TIs have been demonstrated only at temperatures far below those needed for practical applications. In this work, we study the magnetic properties of Bi2Se3 surface states (SS) in the proximity of a high Tc ferrimagnetic insulator (FMI), yttrium iron garnet (YIG or Y3Fe5O12). Proximity-induced butterfly and square-shaped magnetoresistance loops are observed by magneto-transport measurements with out-of-plane and in-plane fields, respectively, and can be correlated with the magnetization of the YIG substrate. More importantly, a magnetic signal from the Bi2Se3 up to 130 K is clearly observed by magneto-optical Kerr effect measurements. Our results demonstrate the proximity-induced TI magnetism at higher temperatures, an important step toward room-temperature application of TI-based spintronic devices.

Competing weak localization and weak antilocalization in ultrathin topological insulators.

We demonstrate evidence of a surface gap opening in topological insulator (TI) thin films of (Bi(0.57)Sb(0.43))(2)Te(3) below six quintuple layers through transport and scanning tunneling spectroscopy measurements. By effective tuning the Fermi level via gate-voltage control, we unveil a striking competition between weak localization and weak antilocalization at low magnetic fields in nonmagnetic ultrathin films, possibly owing to the change of the net Berry phase. Furthermore, when the Fermi level is swept into the surface gap of ultrathin samples, the overall unitary behaviors are revealed at higher magnetic fields, which are in contrast to the pure WAL signals obtained in thicker films. Our findings show an exotic phenomenon characterizing the gapped TI surface states and point to the future realization of quantum spin Hall effect and dissipationless TI-based applications.

Manipulating surface-related ferromagnetism in modulation-doped topological insulators.

A new class of devices based on topological insulators (TI) can be achieved by the direct engineering of the time-reversal-symmetry (TRS) protected surface states. In the meantime, a variety of interesting phenomena are also expected when additional ferromagnetism is introduced to the original topological order. In this Letter, we report the magnetic responses from the magnetically modulation-doped (Bi(z)Sb(1-z))2Te3/Cr(x)(Bi(y)Sb(1-y))2Te3 bilayer films. By electrically tuning the Fermi level across the Dirac point, we show that the top TI surface carriers can effectively mediate the magnetic impurities and generate robust ferromagnetic order. More importantly, such surface magneto-electric effects can be either enhanced or suppressed, depending on the magnetic interaction range inside the TI heterostructures. The manipulation of surface-related ferromagnetism realized in our modulation-doped TI device is important for the realization of TRS-breaking topological physics, and it may also lead to new applications of TI-based multifunctional heterostructures.

Direct imaging of thermally driven domain wall motion in magnetic insulators.

Thermally induced domain wall motion in a magnetic insulator was observed using spatiotemporally resolved polar magneto-optical Kerr effect microscopy. The following results were found: (i) the domain wall moves towards hot regime; (ii) a threshold temperature gradient (5 K/mm), i.e., a minimal temperature gradient required to induce domain wall motion; (iii) a finite domain wall velocity outside of the region with a temperature gradient, slowly decreasing as a function of distance, which is interpreted to result from the penetration of a magnonic current into the constant temperature region; and (iv) a linear dependence of the average domain wall velocity on temperature gradient, beyond a threshold thermal bias. Our observations can be qualitatively explained using a magnonic spin transfer torque mechanism, which suggests the utility of magnonic spin transfer torque for controlling magnetization dynamics.

Voltage-induced ferromagnetic resonance in magnetic tunnel junctions.

We demonstrate excitation of ferromagnetic resonance in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) by the combined action of voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque (ST). Our measurements reveal that GHz-frequency VCMA torque and ST in low-resistance MTJs have similar magnitudes, and thus that both torques are equally important for understanding high-frequency voltage-driven magnetization dynamics in MTJs. As an example, we show that VCMA can increase the sensitivity of an MTJ-based microwave signal detector to the sensitivity level of semiconductor Schottky diodes.

Emergence of electric-field-tunable interfacial ferromagnetism in 2D antiferromagnet heterostructures.

Van der Waals (vdW) magnet heterostructures have emerged as new platforms to explore exotic magnetic orders and quantum phenomena. Here, we study heterostructures of layered antiferromagnets, CrI3 and CrCl3, with perpendicular and in-plane magnetic anisotropy, respectively. Using magneto-optical Kerr effect microscopy, we demonstrate out-of-plane magnetic order in the CrCl3 layer proximal to CrI3, with ferromagnetic interfacial coupling between the two. Such an interlayer exchange field leads to higher critical temperature than that of either CrI3 or CrCl3 alone. We further demonstrate significant electric-field control of the coercivity, attributed to the naturally broken structural inversion symmetry of the heterostructure allowing unprecedented direct coupling between electric field and interfacial magnetism. These findings illustrate the opportunity to explore exotic magnetic phases and engineer spintronic devices in vdW heterostructures.

Correlative Nanoscale Imaging of Strained hBN Spin Defects.

Spin defects like the negatively charged boron vacancy color center (VB-) in hexagonal boron nitride (hBN) may enable new forms of quantum sensing with near-surface defects in layered van der Waals heterostructures. Here, the effect of strain on VB- color centers in hBN is revealed with correlative cathodoluminescence and photoluminescence microscopies. Strong localized enhancement and redshifting of the VB- luminescence is observed at creases, consistent with density functional theory calculations showing VB- migration toward regions with moderate uniaxial compressive strain. The ability to manipulate spin defects with highly localized strain is critical to the development of practical 2D quantum devices and quantum sensors.

Synthesis of high-Curie-temperature Fe0.02Ge0.98 quantum dots.

Self-assembled Fe(0.02)Ge(0.98) dilute magnetic quantum dots show a high Curie temperature above 400 K. Such extraordinary magnetic properties can potentially resolve the critical problem of power dissipation in today's integrated circuits and lead to the realization of a new class of spintronics devices.

Voltage-controlled ferromagnetic order in MnGe quantum dots.

Here, we speculate that room temperature voltage-controlled ferromagnetic ordering may become a founding phenomenon for the next generation of low-power spintronics nanodevices and discuss the special role of dilute magnetic semiconductors as the most reliable material basis to date. Then, we report on our latest experimental achievements in the voltage manipulation of the ferromagnetism in MnGe quantum dots, experimentally demonstrating the capacity of pushing the Curie temperature further above room temperature for technological applications.

Direct observation of valley-coupled topological current in MoS2.

The valley degree of freedom of electrons in two-dimensional transition metal dichalcogenides has been extensively studied by theory (1-4), optical (5-9), and optoelectronic (10-13) experiments. However, generation and detection of pure valley current without relying on optical selection have not yet been demonstrated in these materials. Here, we report that valley current can be electrically induced and detected through the valley Hall effect and inverse valley Hall effect, respectively, in monolayer molybdenum disulfide. We compare temperature and channel length dependence of nonlocal electrical signals in monolayer and multilayer samples to distinguish the valley Hall effect from classical ohmic contributions. Notably, valley transport is observed over a distance of 4 µm in monolayer samples at room temperature. Our findings will enable a new generation of electronic devices using the valley degree of freedom, which can be used for future novel valleytronic applications.

Role of dimensional crossover on spin-orbit torque efficiency in magnetic insulator thin films.

Magnetic insulators (MIs) attract tremendous interest for spintronic applications due to low Gilbert damping and the absence of Ohmic loss. Spin-orbit torques (SOTs) on MIs are more intriguing than magnetic metals since SOTs cannot be transferred to MIs through direct injection of electron spins. Understanding of SOTs on MIs remains elusive, especially how SOTs scale with the MI film thickness. Here, we observe the critical role of dimensionality on the SOT efficiency by studying the MI layer thickness-dependent SOT efficiency in tungsten/thulium iron garnet (W/TmIG) bilayers. We show that the TmIG thin film evolves from two-dimensional to three-dimensional magnetic phase transitions as the thickness increases. We report the significant enhancement of the measured SOT efficiency as the TmIG thickness increases, which is attributed to the increase of the magnetic moment density. We demonstrate the current-induced SOT switching in the W/TmIG bilayers with a TmIG thickness up to 15 nm.

Control and local measurement of the spin chemical potential in a magnetic insulator.

The spin chemical potential characterizes the tendency of spins to diffuse. Probing this quantity could provide insight into materials such as magnetic insulators and spin liquids and aid optimization of spintronic devices. Here we introduce single-spin magnetometry as a generic platform for nonperturbative, nanoscale characterization of spin chemical potentials. We experimentally realize this platform using diamond nitrogen-vacancy centers and use it to investigate magnons in a magnetic insulator, finding that the magnon chemical potential can be controlled by driving the system's ferromagnetic resonance. We introduce a symmetry-based two-fluid theory describing the underlying magnon processes, measure the local thermomagnonic torque, and illustrate the detection sensitivity using electrically controlled spin injection. Our results pave the way for nanoscale control and imaging of spin transport in mesoscopic systems.

Competing effect of spin-orbit torque terms on perpendicular magnetization switching in structures with multiple inversion asymmetries.

Current-induced spin-orbit torques (SOTs) in structurally asymmetric multilayers have been used to efficiently manipulate magnetization. In a structure with vertical symmetry breaking, a damping-like SOT can deterministically switch a perpendicular magnet, provided an in-plane magnetic field is applied. Recently, it has been further demonstrated that the in-plane magnetic field can be eliminated by introducing a new type of perpendicular field-like SOT via incorporating a lateral structural asymmetry into the device. Typically, however, when a current is applied to such devices with combined vertical and lateral asymmetries, both the perpendicular field-like torque and the damping-like torque coexist, hence jointly affecting the magnetization switching behavior. Here, we study perpendicular magnetization switching driven by the combination of the perpendicular field-like and the damping-like SOTs, which exhibits deterministic switching mediated through domain wall propagation. It is demonstrated that the role of the damping-like SOT in the deterministic switching is highly dependent on the magnetization direction in the domain wall. By contrast, the perpendicular field-like SOT is solely determined by the relative orientation between the lateral structural asymmetry and the current direction, regardless of the magnetization direction in the domain wall. The experimental results further the understanding of SOTs-induced switching, with implications for spintronic devices.