Complementary Lateral‐Spin–Orbit Building Blocks for Programmable Logic and In‐Memory Computing

Current-driven switching of nonvolatile spintronic materials and devices based on spin-orbit torques offer fast data processing speed, low power consumption, and unlimited endurance for future information processing applications. Analogous to conventional CMOS technology, it is important to develop a pair of complementary spin-orbit devices with differentiated magnetization switching senses as elementary building blocks for realizing sophisticated logic functionalities. Various attempts using external magnetic field or complicated stack/circuit designs have been proposed, however, plainer and more feasible approaches are still strongly desired. Here we show that a pair of two locally laser annealed perpendicular Pt/Co/Pt devices with opposite laser track configurations and thereby inverse field-free lateral spin-orbit torques (LSOTs) induced switching senses can be adopted as such complementary spin-orbit building blocks. By electrically programming the initial magnetization states (spin down/up) of each sample, four Boolean logic gates of AND, OR, NAND and NOR, as well as a spin-orbit half adder containing an XOR gate, were obtained. Moreover, various initialization-free, working current intensity-programmable stateful logic operations, including material implication (IMP) gate, were also demonstrated by regarding the magnetization state as a logic input. Our complementary LSOT building blocks provide a potentially applicable way towards future efficient spin logics and in-memory computing architectures.

Resonant thermal energy transfer to magnons in a ferromagnetic nanolayer

Energy harvesting is a concept which makes dissipated heat useful by transferring thermal energy to other excitations. Most of the existing principles are realized in systems which are heated continuously. We present the concept of high-frequency energy harvesting where the dissipated heat in a sample excites resonant magnons in a thin ferromagnetic metal layer. The sample is excited by femtosecond laser pulses with a repetition rate of 10 GHz which results in temperature modulation at the same frequency with amplitude ~0.1 K. The alternating temperature excites magnons in the ferromagnetic nanolayer which are detected by measuring the net magnetization precession. When the magnon frequency is brought onto resonance with the optical excitation, a 12-fold increase of the amplitude of precession indicates efficient resonant heat transfer from the lattice to coherent magnons. The demonstrated principle may be used for energy harvesting in various nanodevices operating at GHz and sub-THz frequency ranges

Contributions from coherent and incoherent lattice excitations to ultrafast optical control of magnetic anisotropy of metallic films

Spin-lattice coupling is one of the most prominent interactions mediating response of spin ensemble to ultrafast optical excitation. Here we exploit optically generated coherent and incoherent phonons to drive coherent spin dynamics, i.e. precession, in thin films of magnetostrictive metal Galfenol. We demonstrate unambiguously that coherent phonons, also seen as dynamical strain generated due to picosecond lattice temperature raise, give raise to magnetic anisotropy changes of the optically excited magnetic film; and this contribution may be comparable to or even dominate over the contribution from the temperature increase itself, considered as incoherent phonons

Enhanced Photon–Phonon Interaction in WSe2 Acoustic Nanocavities

Acoustic nanocavities (ANCs) with resonance frequencies much above 1 GHz are prospective to be exploited in sensors and quantum operating devices. Nowadays, acoustic nanocavities fabricated from van der Waals (vdW) nanolayers allow them to exhibit resonance frequencies of the breathing acoustic mode up to f ∼ 1 THz and quality factors up to Q ∼ 103. For such high acoustic frequencies, electrical methods fail, and optical techniques are used for the generation and detection of coherent phonons. Here, we study experimentally acoustic nanocavities fabricated from WSe2 layers with thicknesses from 8 up to 130 nm deposited onto silica colloidal crystals. The substrate provides a strong mechanical support for the layers while keeping their acoustic properties the same as in membranes. We concentrate on experimental and theoretical studies of the amplitude of the optically measured acoustic signal from the breathing mode, which is the most important characteristic for acousto-optical devices. We probe the acoustic signal optically with a single wavelength in the vicinity of the exciton resonance and measure the relative changes in the reflectivity induced by coherent phonons up to 3 × 10–4 for f ∼ 100 GHz. We reveal the enhancement of photon–phonon interaction for a wide range of acoustic frequencies and show high sensitivity of the signal amplitude to the photoelastic constants governed by the deformation potential and dielectric function for photon energies near the exciton resonance. We also reveal a resonance in the photoelastic response (we call it photoelastic resonance) in the nanolayers with thickness close to the Bragg condition. The estimates show the capability of acoustic nanocavities with an exciton resonance for operations with high-frequency single phonons at an elevated temperature

On-chip phonon-magnon reservoir for neuromorphic computing

Reservoir computing is a concept involving mapping signals onto a high-dimensional phase space of a dynamical system called "reservoir" for subsequent recognition by an artificial neural network. We implement this concept in a nanodevice consisting of a sandwich of a semiconductor phonon waveguide and a patterned ferromagnetic layer. A pulsed write-laser encodes input signals into propagating phonon wavepackets, interacting with ferro-magnetic magnons. The second laser reads the output signal reflecting a phase-sensitive mix of phonon and magnon modes, whose content is highly sensitive to the write-and read-laser positions. The reservoir efficiently separates the visual shapes drawn by the write-laser beam on the nanodevice surface in an area with a size comparable to a single pixel of a modern digital camera. Our finding suggests the phonon-magnon interaction as a promising hardware basis for realizing on-chip reservoir computing in future neuro-morphic architectures

Bi2Se3 interlayer treatments affecting the Y3Fe5O12 (YIG) platinum spin Seebeck effect

In this work, we present a method to enhance the longitudinal spin Seebeck effect at platinum/yttrium iron garnet (Pt/YIG) interfaces. The introduction of a partial interlayer of bismuth selenide (Bi2Se3, 2.5% surface coverage) interfaces significantly increases (by ∼380%–690%) the spin Seebeck coefficient over equivalent Pt/YIG control devices. Optimal devices are prepared by transferring Bi2Se3 nanoribbons, prepared under anaerobic conditions, onto the YIG (111) chips followed by rapid over-coating with Pt. The deposited Pt/Bi2Se3 nanoribbon/YIG assembly is characterized by scanning electron microscope. The expected elemental compositions of Bi2Se3 and YIG are confirmed by energy dispersive x-ray analysis. A spin Seebeck coefficient of 0.34–0.62 μV/K for Pt/Bi2Se3/YIG is attained for our devices, compared to just 0.09 μV/K for Pt/YIG controls at a 12 K thermal gradient and a magnetic field swept from −50 to +50 mT. Superconducting quantum interference device magnetometer studies indicate that the magnetic moment of Pt/Bi2Se3/YIG treated chips is increased by ∼4% vs control Pt/YIG chips (i.e., a significant increase vs the ±0.06% chip mass reproducibility). Increased surface magnetization is also detected in magnetic force microscope studies of Pt/Bi2Se3/YIG, suggesting that the enhancement of spin injection is associated with the presence of Bi2Se3 nanoribbons

Coherent Phononics of van der Waals Layers on Nanogratings

Strain engineering can be used to control the physical properties of two-dimensional van der Waals (2D-vdW) crystals. Coherent phonons, which carry dynamical strain, could push strain engineering to control classical and quantum phenomena in the unexplored picosecond temporal and nanometer spatial regimes. This intriguing approach requires the use of coherent GHz and sub-THz 2D phonons. Here, we report on nanostructures that combine nanometer thick vdW layers and nanogratings. Using an ultrafast pump-probe technique, we generate and detect in-plane coherent phonons with frequency up to 40 GHz and hybrid flexural phonons with frequency up to 10 GHz. The latter arises from the periodic modulation of the elastic coupling of the vdW layer at the grooves and ridges of the nanograting. This creates a new type of a tailorable 2D periodic phononic nanoobject, a flexural phononic crystal, offering exciting prospects for the ultrafast manipulation of states in 2D materials in emerging quantum technologies

Protected Long-Distance Guiding of Hypersound Underneath a Nanocorrugated Surface

In nanoscale communications, high-frequency surface acoustic waves are becoming effective data carriers and encoders. On-chip communications require acoustic wave propagation along nanocorrugated surfaces which strongly scatter traditional Rayleigh waves. Here, we propose the delivery of information using subsurface acoustic waves with hypersound frequencies of ∼20 GHz, which is a nanoscale analogue of subsurface sound waves in the ocean. A bunch of subsurface hypersound modes are generated by pulsed optical excitation in a multilayer semiconductor structure with a metallic nanograting on top. The guided hypersound modes propagate coherently beneath the nanograting, retaining the surface imprinted information, at a distance of more than 50 μm which essentially exceeds the propagation length of Rayleigh waves. The concept is suitable for interfacing single photon emitters, such as buried quantum dots, carrying coherent spin excitations in magnonic devices and encoding the signals for optical communications at the nanoscale

Hybrid coherent control of magnons in a ferromagnetic phononic resonator excited by laser pulses

We propose and demonstrate the concept of hybrid coherent control (CC) whereby a quantum or classical harmonic oscillator is excited by two excitations: one is quasi-harmonic (i.e. harmonic with a finite lifetime) and the other is a pulsed broadband excitation. Depending on the phase relation between the two excitations, controlled by the detuning of the oscillator eigenfrequencies and the waveforms of the quasi-harmonic and broadband excitations, it is possible to observe Fano-like spectra of the harmonic oscillator due to the interference of the two responses to the simultaneously acting excitations. Experimentally, as an example, the hybrid CC is implemented for magnons in a ferromagnetic grating where GHz coherent phonons act as the quasi-harmonic excitation and the broadband impact arises from pulsed optical excitation followed by spin dynamics in the ferromag-netic nanostructure. Coherent control (CC) is well established as a powerful method to manipulate the amplitude and phase of quantum states. First used for chemical reactions [1, 2], CC has been demonstrated for single electrons [3], spins [4, 5], nanoelectromechanical oscillators [6], magnons [7, 8] and other systems [9]. The basic phenomenon governing CC is the interference of the responses of a quantum system to specific excitations, which determine the phase of the wavefunction. One of the common technical solutions for realizing CC is to use two optical pulses from ultrafast lasers with adjustable time separation or more sophisticated laser pulse shaping [10]. For CC of magnons, two microwave pulses may be used [11]. Traditionally , the excitations that lead to the interfering responses have the same origin, e.g. transitions between the ground and an excited quantum state are induced by a resonant electromagnetic field. However, there are quantum systems that may be excited by a pair of exci-tations of different origins. For example, one excitation may be broad-band and the other harmonic. Exploiting a combination of various types of excitations for hybrid CC would broaden a diversity of CC applications for quantum computing and communications. The idea of hybrid CC in the spectral domain is illustrated in Figs. 1(a) and 1(b) for a linear tunable quantum or classical oscillator with eigenfrequency ω 0 and finite lifetime. Figures 1(a) and 1(b) show the amplitude spectra of the oscillator's responses to two types of excitation: (1) quasi-harmonic (i.e. harmonic with finite lifetime) excitation with central frequency ω R detuned relative to ω 0 ; and (2) broad band excitation. Two cases of detun-ing are considered: negative (ω 0 ω R) in Fig. 1(b). The top blue curves show the spectra when only quasi-harmonic excitation is present. In this case the phase ϕ of the oscillator at ω = ω 0 changes by π when the oscillator eigenfrequency is tuned through the resonance ω = ω R , say from −π/2 to π/2 as demonstrated in the comparison of the blue spectra in Figs. 1(a) and 1(b). The middle red curves are spectral responses when the oscillator is excited by a broadband excitation (2). The oscillator's phase ϕ, e.g. ϕ = π/2, at ω = ω 0 in this case does not depend on ω 0. The lower black curves are the spectra when the two ex-citations, (1) and (2), operate together. Clearly, we get destructive [ Fig. 1(a)] or constructive [Fig. 1(b)] interference of the oscillator's responses at ω = ω 0 depending on the detuning of the oscillator eigenfrequency relative to the central frequency of the quasi-harmonic excitation, ω 0 ω R respectively. For negative detuning (ω 0 ω R) the spectral amplitude at ω = ω 0 increases by a factor of two. The interference effects represent an example of hybrid CC where two excitations have different spectra and are of different nature, for example (1) could be a coherent phonon wavepacket and (2) could be a short microwave or laser pulse. By varying the detuning, amplitudes and phases of excitations (1) and (2), it is possible to model various Fano-like spectral shapes similar to Fano spectra which appear as a result of interference of broad-and narrow-band eigenstates [12]. In the present Letter we demonstrate an example where CC is realized for the case of magnons. Magnons are a typical example for which a diversity of quantum excitations exists [13]. The quasi-harmonic excitation of magnons is coming from quasi-monochromatic surface phonons. They drive the spectrally isolated magnon mode at the frequency ω R. The broadband excitation is based on ultrafast modulation of the ferromagnet mag-netization. Both excitations are triggered optically by a femtosecond laser pulse. The magnon eigenfrequency ω 0 is tuned by the external magnetic field B. Monitoring the magnon spectrum, we observe destructive or constru