Ivan Lisenkov

Antiferromagnetic THz-frequency Josephson-like Oscillator Driven by Spin Current

The development of compact and tunable room temperature sources of coherent THz-frequency signals would open a way for numerous new applications. The existing approaches to THz-frequency generation based on superconductor Josephson junctions (JJ), free electron lasers, and quantum cascades require cryogenic temperatures or/and complex setups, preventing the miniaturization and wide use of these devices. We demonstrate theoretically that a bi-layer of a heavy metal (Pt) and a bi-axial antiferromagnetic (AFM) dielectric (NiO) can be a source of a coherent THz signal. A spin-current flowing from a DC-current-driven Pt layer and polarized along the hard AFM anisotropy axis excites a non-uniform in time precession of magnetizations sublattices in the AFM, due to the presence of a weak easy-plane AFM anisotropy. The frequency of the AFM oscillations varies in the range of 0.1–2.0u2009THz with the driving current in the Pt layer from 108u2009A/cm2 to 109u2009A/cm2. The THz-frequency signal from the AFM with the amplitude exceeding 1u2009V/cm is picked up by the inverse spin-Hall effect in Pt. The operation of a room-temperature AFM THz-frequency oscillator is similar to that of a cryogenic JJ oscillator, with the energy of the easy-plane magnetic anisotropy playing the role of the Josephson energy.

Theoretical formalism for collective spin-wave edge excitations in arrays of dipolarly interacting magnetic nanodots

A general theory of edge spin wave excitations in semi-infinite and finite periodic arrays of magnetic nanodots existing in a spatially uniform magnetization ground state is developed. The theory is formulated using a formalism of multi-vectors of magnetization dynamics, which allows one to study edge excitations in arrays having arbitrary complex primitive cells and lattice geometry. The developed formalism can describe edge excitations localized both at the physical edges of the array and at the internal domain walls separating array regions existing in different static magnetization states. Using a perturbation theory in the framework of the developed formalism it is possible to calculate damping of edge modes and their excitation by external variable magnetic fields. The theory is illustrated on the following practically important examples: (i) calculation of the FMR absorption in a finite nanodot array having the shape of a right triangle, (ii) calculation of nonreciprocal spin wave spectra of edge modes, including modes at the physical edges of an array and modes at the domain walls inside an array, (iii) study of the influence of the domain wall modes on the FMR spectrum of an array existing in a non-ideal chessboard antiferromagnetic ground state.

Bias-free spin-wave phase shifter for magnonic logic

A design of a magnonic phase shifter operating without an external bias magnetic field is proposed. The phase shifter uses a localized collective spin wave mode propagating along a domain wall “waveguide” in a dipolarly-coupled magnetic dot array with a chessboard antiferromagnetic (CAFM) ground state. It is demonstrated numerically that the remagnetization of a single magnetic dot adjacent to the domain wall waveguide introduces a controllable phase shift in the propagating spin wave mode without significant change to the mode amplitude. It is also demonstrated that a logic XOR gate can be realized in the same system.

Edge rotational magnons in magnonic crystals

It is predicted that in 2D magnonic crystals the edge rotational magnons of forward volume magnetostatic spin waves can exist. Under certain conditions, locally bounded magnons may appear within the crystal consisting of the ferromagnetic matrix and periodically inserted magnetic/non-magnetic inclusions. It is also shown that interplay of different resonances in 2D magnonic crystal may provide conditions for spin wave modes existence with negative group velocity.

Review of phononic crystals, nonlinear processes, devices and prospects

The review of current state of the art of bulk and surface linear and nonlinear acoustic wave propagation in phononic crystals (PhC) is given. First, theoretical analysis of bulk acoustic waves propagation in 2D phononic crystals composed of elastic medium with periodic systems of air holes with different symmetry is considered. The properties of hypersonic bulk and surface acoustic waves (Love, Lamb) in PhC are considered theoretically and expermentally. We studied bulk waves propagation in microstructured optical fiber preforms made of quartz. The study of simultaneous propagation of acoustic waves and light in structures being both photonic and phononic crystals was done. We study also properties of guided waves (Lamb modes) propagating in layered structures containing the magnetic films with two‐dimensional periodic structures. In these structures we discovered a strong coupling between elastic and magnetic properties leading to effective waves transformation between magnetic and elastic systems. Such...

Elastic wave propagation in a microstructured acoustic fiber

Elastic wave propagation along the structure of hollow cylinders in a linear isotropic medium is investigated. The multipole method for modeling elastic waves propagation in such structures is formulated and implemented. Using the multipole method, dispersion dependencies of the structures (microstructured fibers) containing 3, 6, and 7 hollow cylinders are calculated. Comparison with wave dispersion properties along one cylinder is made. Also, an approximate physical model based on an equivalent coaxial waveguide and multipole method is proposed. Exploiting this model, wave dispersion of the wave propagating along a structure with 18 hollow cylinders is calculated. Validation of the model is also proposed.

Acoustic wave propagation in fluid metamaterial with solid inclusions

Acoustic wave propagation in a composite of water with embedded double-layered silicone resin/silver rods is considered. Approximate values of effective dynamical constitutive parameters are obtained. Frequency ranges of simultaneous negative constitutive parameters are found. Localized surface states on the interface between metamaterial and “normal” material are found. The Doppler effect in metamaterial is considered. The presence of anomalous modes is shown.

Interaction of Microwave Photons with Nanostructured Magnetic Metasurfaces

A theoretical formalism for the description of the interaction of microwave photons with a thin (compared to the photon wavelength) magnetic metasurface comprised of dipolarly interacting nano-scale magnetic elements is developed. A scattering matrix describing the processes of photon transmission and reflection at the metasurface boundary is derived. As an example of the use of the developed formalism, it is demonstrated, that the introduction of a magnetic metasurface inside a microstrip electromagnetic waveguide quantitatively changes the dispersion relation of the fundamental waveguide mode, opening a non-propagation frequency band gap in the waveguide spectrum. The frequency position and the width of the band gap are dependent on the waveguide thickness, and can be controlled dynamically by switching the magnetic ground state of the metasurface. For sufficiently thin waveguides the position of the band gap is shifted from the resonance absorption frequency of the metasurface. In such a case, the magnetic metasurface inside a waveguide works as an efficient reflector, as the energy absorption in the metasurface is small, and most of the electromagnetic energy inside the non-propagation band gap is reflected.

Ultra-fast artificial neuron: generation of picosecond-duration spikes in a current-driven antiferromagnetic auto-oscillator

We demonstrate analytically and numerically, that a thin film of an antiferromagnetic (AFM) material, having biaxial magnetic anisotropy and being driven by an external spin-transfer torque signal, can be used for the generation of ultra-short “Dirac-delta-like” spikes. The duration of the generated spikes is several picoseconds for typical AFM materials and is determined by the inplane magnetic anisotropy and the effective damping of the AFM material. The generated output signal can consist of a single spike or a discrete group of spikes (“bursting”), which depends on the repetition (clock) rate, amplitude, and shape of the external control signal. The spike generation occurs only when the amplitude of the control signal exceeds a certain threshold, similar to the action of a biological neuron in response to an external stimulus. The “threshold” behavior of the proposed AFM spike generator makes possible its application not only in the traditional microwave signal processing but also in the future neuromorphic signal processing circuits working at clock frequencies of tens of gigahertz.

Edge Rotational Magnons in Magnonic Crystals

It is predicted that in 2D magnonic crystals the edge rotational magnons of forward vol-ume magnetostatic spin waves can exist. Under certain conditions locally bounded magnons may ap-pear within the crystal consisting of the ferromagnetic matrix and periodically inserted magnetic/non-magnetic inclusions. It is also shown that interplay of different resonances in 2D magnonic crystalmay provide conditions for spin wave modes existence with negative group velocity.