Ac Spin Current Research Articles

Enhanced magnon transport through an amorphous magnetic insulator

Conduction of spin currents in disordered insulating antiferromagnets has recently been at the center of scientific debate with both long-range spin transport or no spin transport at all observed experimentally. In this study, ferromagnetic resonance has been used to probe the transmission of ac spin current through thin amorphous yttrium iron garnet (YIG) layers. The spin current is found to be mediated by evanescent spin waves with a penetration length four times larger than that of previous studies of amorphous YIG, even exceeding the spin penetration length of crystalline NiO. Published by the American Physical Society 2024

Amplification of electron-mediated spin currents by stimulated spin pumping

Stimulated emission is a process during which an atomic system gives away energy to create a coherent photon. It is fundamental to the operation of the optical amplifier. Here we propose two mechanisms for amplifying AC spin currents in a solid-state magnetic medium by a stimulated spin pumping process. The first is synchronous and consists of phase-locked pulses that perturb a precessing magnetic moment. The second is asynchronous and is driven by DC spin currents. The amplification relies on a non-adiabatic interaction taking place in a ferromagnetic medium in which the magnetic moment emits spin angular momentum in the form of spin current before equilibrating with the environment. The pumped spin current amplifies or absorbs the injected AC spin current mimicking the operation of the optical gain medium as readily seen from the gain saturation profiles. The mechanisms we propose are a first step towards a realistic spin current amplifier.

Selective damping modulation in a synthetic antiferromagnet induced by spin–orbit torque

Magnetization dynamics in a synthetic antiferromagnet with an adjacent Pt layer are investigated. Using dc bias, magnetic damping can be effectively controlled by the spin–orbit torque from the Pt layer. In the spin-flop state, the acoustic mode is modulated by the dc bias, but the optical mode is not sufficiently modulated. In the saturation state, the optical mode is effectively modulated. By appropriately selecting acoustic and optical modes and magnetization states such as the spin-flop and saturation states, modulation of the mutual phases of the ac spin currents driven by the ac damping torques can be realized.

Direct and alternating magnon spin currents across a junction interface irradiated by linearly polarized laser

The developments in the field of quantum optics raise expectations that laser-matter coupling is a promising building block for magnonics. Here, we propose a method for the generation of direct and alternating spin currents of magnons across the junction interface irradiated by linearly polarized laser. In a junction of ferromagnetic insulators with a large electronic gap, the spin angular momentum is exchanged during the tunneling process of magnons across the junction interface. The advanced technology in the field of plasmonics and metamaterials realizes that spins irradiated by the laser field interact only with the magnetic component of the laser through the Zeeman coupling. Using an analytic perturbation theory, we provide a general formula for magnon transport induced by the inversion symmetry breaking across the junction interface. Then, we show that those spin currents are enhanced by the ferromagnetic resonance, and the period of the ac spin current is one-half of that of the laser magnetic field. Finally, we estimate the magnitude of the spin current, and find that it will be within experimental reach.

Chirality-selective easily adjustable spin current from uniaxial antiferromagnets

Spin current generation and injection in antiferromagnetic heterostructures opens up new perspectives in the fields of nanoelectronics, spintronics, and quantum computation. Here, we theoretically investigate terahertz spin current in an antiferromagnet (AFM)/normal metal (NM) heterostructure generated by spin pumping. Under the framework of linear response theory, we compute the alternating spin current (ac spin current) inside the NM of the heterostructure arising from the spin pumping of a resonant terahertz magnetic field inside the AFM. An external direct current (dc) magnetic field is also applied to tune the resonance frequency of the AFM, allowing the resonance between terahertz field and AFM to be achieved more easily. In both uniaxial antiferromagnets MnF2 and Cr2O3, we find that once the driving THz field is resonant and circular polarized according to the resonance mode of the AFM, the amplitude of ac spin current is independent of the external magnetic field. On the other hand, in the biaxial antiferromagnet NiO under the same pumping conditions, such a phenomenon is not observed, which can be ascribed to the complicated nondegenerate elliptical chiral mode in biaxial AFMs. More interestingly, for both types of AFMs, we find that the ac component of the spin current increases linearly with the applied terahertz magnetic field distinct from the dc component that increases quadratically. The aforementioned features in uniaxial AFMs provide an easily tunable guideline to optimize the generation of high-frequency spin current by a given terahertz pumping source.

Coherent Transfer of Spin Angular Momentum by Evanescent Spin Waves within Antiferromagnetic NiO.

Insulating antiferromagnets have recently emerged as efficient and robust conductors of spin current. Element-specific and phase-resolved x-ray ferromagnetic resonance has been used to probe the injection and transmission of ac spin current through thin epitaxial NiO(001) layers. The spin current is found to be mediated by coherent evanescent spin waves of GHz frequency, rather than propagating magnons of THz frequency, paving the way towards coherent control of the phase and amplitude of spin currents within an antiferromagnetic insulator at room temperature.

Generation of DC, AC, and Second-Harmonic Spin Currents by Electromagnetic Fields in an Inversion-Asymmetric Antiferromagnet

Manipulating spin currents in magnetic insulators is a key technology in spintronics. We theoretically study a simple inversion-asymmetric model of quantum antiferromagnets, where both the exchange interaction and the magnetic field are staggered. We calculate spin currents generated by external electric and magnetic fields by using a quantum master equation. We show that an ac electric field with amplitude E 0 leads, through exchange-interaction modulation, to the dc and second-order harmonic spin currents proportional to E 0 2 . We also show that dc and ac staggered magnetic fields B 0 generate the dc and ac spin currents proportional to B 0 , respectively. We elucidate the mechanism by an exactly solvable model, and thereby propose the ways of spin current manipulation by electromagnetic fields.

Coherent ac spin current transmission across an antiferromagnetic CoO insulator

The recent discovery of spin current transmission through antiferromagnetic insulating materials opens up vast opportunities for fundamental physics and spintronics applications. The question currently surrounding this topic is: whether and how could THz antiferromagnetic magnons mediate a GHz spin current? This mismatch of frequencies becomes particularly critical for the case of coherent ac spin current, raising the fundamental question of whether a GHz ac spin current can ever keep its coherence inside an antiferromagnetic insulator and so drive the spin precession of another ferromagnet layer coherently? Utilizing element- and time-resolved x-ray pump-probe measurements on Py/Ag/CoO/Ag/Fe75Co25/MgO(001) heterostructures, here we demonstrate that a coherent GHz ac spin current pumped by the Py ferromagnetic resonance can transmit coherently across an antiferromagnetic CoO insulating layer to drive a coherent spin precession of the Fe75Co25 layer. Further measurement results favor thermal magnons rather than evanescent spin waves as the mediator of the coherent ac spin current in CoO.

Frequency dependence of the longitudinal spin Seebeck effect

The spin Seebeck effect provides an easy way to create and manipulate spin current. Schreier et al. experimentally observed a decrease of spin Seebeck voltage at high frequencies in the yttrium iron garnet/Pt system [Phys. Rev. B 93, 224430 (2016)]. The frequency dependence was attributed to the magnon spectrum. Here we provide a different explanation using the frequency dependence of the ac spin current. By combining a phenomenological two-temperature model with the ac-temperature method, our calculation shows that the frequency dependence of the spin current is determined by the thermal wavelength and the thermal boundary conditions of the phonons and the magnons.

Antiferromagnetic resonance excited by oscillating electric currents

In antiferromagnetic materials the order parameter exhibits resonant modes at frequencies that can be in the terahertz range, making them interesting components for spintronic devices. Here, it is shown that antiferromagnetic resonance can be excited using the inverse spin-Hall effect in a system consisting of an antiferromagnetic insulator coupled to a normal-metal waveguide. The time-dependent interplay between spin-torque, ac spin-accumulation and magnetic degrees of freedom is studied. It is found that the dynamics of the antiferromagnet effects the frequency-dependent conductivity of the normal metal. Further, it is shown that in antiferromagnetic insulators, the resonant excitation by ac spin-currents can be orders of magnitude more efficient than excitation by the current-induced Oersted field.

Analytical theory and possible detection of the ac quantum spin Hall effect

We develop an analytical theory of the low-frequency ac quantum spin Hall (QSH) effect based upon the scattering matrix formalism. It is shown that the ac QSH effect can be interpreted as a bulk quantum pumping effect. When the electron spin is conserved, the integer-quantized ac spin Hall conductivity can be linked to the winding numbers of the reflection matrices in the electrodes, which also equal to the bulk spin Chern numbers of the QSH material. Furthermore, a possible experimental scheme by using ferromagnetic metals as electrodes is proposed to detect the topological ac spin current by electrical means.

Antiferromagnetic spin current rectifier

It is shown theoretically, that an antiferromagnetic dielectric with bi-axial anisotropy, such as NiO, can be used for the rectification of linearly-polarized AC spin current. The AC spin current excites two evanescent modes in the antiferromagnet, which, in turn, create DC spin current flowing back through the antiferromagnetic surface. Spin diode based on this effect can be used in future spintronic devices as direct detector of spin current in the millimeter- and submillimeter-wave bands. The sensitivity of such a spin diode is comparable to the sensitivity of modern electric Schottky diodes and lies in the range 102-103 V/W for 30×30 nm2 structure.

Skyrmion dynamics in a chiral magnet driven by periodically varying spin currents*

In this work, we investigated the spin dynamics in a slab of chiral magnets induced by an alternating (ac) spin current. Periodic trajectories of the skyrmion in real space are discovered under the ac current as a result of the Magnus and viscous forces, which originate from the Gilbert damping, the spin transfer torque, and the β-nonadiabatic torque effects. The results are obtained by numerically solving the Landau-Lifshitz-Gilbert equation and can be explained by the Thiele equation characterizing the skyrmion core motion.

Proposal for an ac spin current source

We propose an ac current source that can be tuned from a pure charge to a pure spin current source. The device consists of two mesoscopic capacitors attached to a two-dimensional strip of a topological insulator. The change from charge to spin current is controlled by an offset in the top gate potentials that drive the capacitors. In addition to this setup, which anticipates the experimental realisation of quantum point contacts in topological insulators, we propose an analogous source in the quantum Hall regime which only relies on presently available building blocks. To this end, we calculate the band structure of a topological insulator in a magnetic field. The intrinsic spin-orbit coupling, together with a split gate, allows for an analogous source, where charge and spin current can be manipulated. The realisation of the device as well as the detection of the ac spin current are within reach of the present experimental technology.

Electrical detection of direct and alternating spin current injected from a ferromagnetic insulator into a ferromagnetic metal

We report room temperature electrical detection of spin injection from a ferromagnetic insulator (YIG) into a ferromagnetic metal (Permalloy, Py). Non-equilibrium spins with both static and precessional spin polarizations are dynamically generated by the ferromagnetic resonance of YIG magnetization, and electrically detected by Py as dc and ac spin currents, respectively. The dc spin current is electrically detected via the inverse spin Hall effect of Py, while the ac spin current is converted to a dc voltage via the spin rectification effect of Py which is resonantly enhanced by dynamic exchange interaction between the ac spin current and the Py magnetization. Our results reveal a new path for developing insulator spintronics, which is distinct from the prevalent but controversial approach of using Pt as the spin current detector.

Swing switching of spin-torque valves

We propose a method for inducing magnetization reversal using an AC spin current polarized perpendicular to the equilibrium magnetization of the free magnetic layer. We show that the critical AC spin current is significantly smaller than the corresponding DC one. The effect is understood as a consequence of the underdamped nature of the spin-torque oscillators. It allows to use the kinetic inertia to overcome the residual energy barrier, rather than suppressing the latter by a large spin current. The effect is similar to a swing which may be set into high amplitude motion by a weak near-resonant push. The optimal AC frequency is identified as the upper bifurcation frequency of the corresponding driven nonlinear oscillator. Together with fast switching times it makes the perpendicular AC method to be the most efficient way to realize spin-torque memory valve.

Andreev Current Induced by Ferromagnetic Resonance

We study charge transport through a metallic dot coupled to a superconducting and a ferromagnetic lead with a precessing magnetization due to ferromagnetic resonance. Using the quasiclassical theory, we find that the magnetization precession induces a dc current in the subgap regime even in the absence of a bias voltage. This effect is due to the rectification of the ac spin currents at the interface with the ferromagnet; it exists in the absence of spin current in the superconductor. When the dot is strongly coupled to the superconductor, we find a strong enhancement in a wide range of parameters as compared to the induced current in the normal state.

Transport Properties of Pure Spin Currents in a Polycrystalline Gd Wire

We have investigated the magneto- and spin-current-transport properties in patterned Gd wires in order to understand the interplay between the spin current and the spin entropy. From the magneto-transport measurements, the electrical resistance was found to decrease with decreasing the spin entropy because of the suppression of the spin fluctuation. We also found that the Gd acts as a strong absorber both for the ac and dc pure spin currents from the ferromagnetic resonance and nonlocal spin injection experiments.

Dynamics of Spin Transport in Voltage-Biased Josephson Junctions

We investigate spin transport in voltage-biased spin-active Josephson junctions. The interplay of spin filtering, spin mixing, and multiple Andreev reflection leads to nonlinear voltage dependence of the dc and ac spin current. We compute the voltage characteristics of the spin current (IS) for superconductor-ferromagnet-superconductor Josephson junctions. The subharmonic gap structure of IS(V) is shown to be sensitive to the degree of spin mixing generated by the ferromagnetic interface, and exhibits a pronounced even-odd effect associated with spin transport during multiple Andreev reflection processes. For strong spin mixing both the magnitude and the direction of the dc spin current can be sensitively controlled by the bias voltage.

Optical injection of ac spin currents in a semiconductor via a quantum interference between exciton states of different spatial symmetry

Optical injection of ac spin currents in a semiconductor via a quantum interference between exciton states of different spatial symmetry