Roman Verba

Fast switching of a ground state of a reconfigurable array of magnetic nano-dots

We show numerically that a ground state (ferromagnetic or chessboard antiferromagnetic) and microwave absorption frequency of a dipolarly coupled two-dimensional array of axially magnetized magnetic nano-dots can be switched by application of bias magnetic field pulses (duration 30–70 ns). Switching to the ferromagnetic state can be achieved by applying a rectangular field pulse along the dot axis while switching to the antiferromagnetic state requires the field pulse oriented in the dot plane and having a sufficiently long trailing edge (tail). Our results prove that arrays of magnetic nano-dots can be used as materials having rapidly reconfigurable magnetic and microwave properties.

Reduction of phase noise in nanowire spin orbit torque oscillators

Spin torque oscillators (STOs) are compact, tunable sources of microwave radiation that serve as a test bed for studies of nonlinear magnetization dynamics at the nanometer length scale. The spin torque in an STO can be created by spin-orbit interaction, but low spectral purity of the microwave signals generated by spin orbit torque oscillators hinders practical applications of these magnetic nanodevices. Here we demonstrate a method for decreasing the phase noise of spin orbit torque oscillators based on Pt/Ni80Fe20 nanowires. We experimentally demonstrate that tapering of the nanowire, which serves as the STO active region, significantly decreases the spectral linewidth of the generated signal. We explain the observed linewidth narrowing in the framework of the Ginzburg-Landau auto-oscillator model. The model reveals that spatial non-uniformity of the spin current density in the tapered nanowire geometry hinders the excitation of higher order spin-wave modes, thus stabilizing the single-mode generation regime. This non-uniformity also generates a restoring force acting on the excited self-oscillatory mode, which reduces thermal fluctuations of the mode spatial position along the wire. Both these effects improve the STO spectral purity.

Spin-Hall nano-oscillator with oblique magnetization and Dzyaloshinskii-Moriya interaction as generator of skyrmions and nonreciprocal spin-waves

Spin-Hall oscillators (SHO) are promising sources of spin-wave signals for magnonics applications, and can serve as building blocks for magnonic logic in ultralow power computation devices. Thin magnetic layers used as “free” layers in SHO are in contact with heavy metals having large spin-orbital interaction, and, therefore, could be subject to the spin-Hall effect (SHE) and the interfacial Dzyaloshinskii-Moriya interaction (i-DMI), which may lead to the nonreciprocity of the excited spin waves and other unusual effects. Here, we analytically and micromagnetically study magnetization dynamics excited in an SHO with oblique magnetization when the SHE and i-DMI act simultaneously. Our key results are: (i) excitation of nonreciprocal spin-waves propagating perpendicularly to the in-plane projection of the static magnetization; (ii) skyrmions generation by pure spin-current; (iii) excitation of a new spin-wave mode with a spiral spatial profile originating from a gyrotropic rotation of a dynamical skyrmion. These results demonstrate that SHOs can be used as generators of magnetic skyrmions and different types of propagating spin-waves for magnetic data storage and signal processing applications.

Parametric Resonance of Magnetization Excited by Electric Field

Manipulation of magnetization by electric field is a central goal of spintronics because it enables energy-efficient operation of spin-based devices. Spin wave devices are promising candidates for low-power information processing, but a method for energy-efficient excitation of short-wavelength spin waves has been lacking. Here we show that spin waves in nanoscale magnetic tunnel junctions can be generated via parametric resonance induced by electric field. Parametric excitation of magnetization is a versatile method of short-wavelength spin wave generation, and thus, our results pave the way toward energy-efficient nanomagnonic devices.

Excitation of propagating spin waves in ferromagnetic nanowires by microwave voltage-controlled magnetic anisotropy

The voltage-controlled magnetic anisotropy (VCMA) effect, which manifests itself as variation of anisotropy of a thin layer of a conductive ferromagnet on a dielectric substrate under the influence of an external electric voltage, can be used for the development of novel information storage and signal processing devices with low power consumption. Here it is demonstrated by micromagnetic simulations that the application of a microwave voltage to a nanosized VCMA gate in an ultrathin ferromagnetic nanowire results in the parametric excitation of a propagating spin wave, which could serve as a carrier of information. The frequency of the excited spin wave is twice smaller than the frequency of the applied voltage while its amplitude is limited by 2 mechanisms: (i) the so-called “phase mechanism” described by the Zakharov-L’vov-Starobinets “S-theory” and (ii) the saturation mechanism associated with the nonlinear frequency shift of the excited spin wave. The developed extension of the “S-theory”, which takes into account the second limitation mechanism, allowed us to estimate theoretically the efficiency of the parametric excitation of spin waves by the VCMA effect.

Reconfigurable nanoscale spin-wave directional coupler

We propose a nanoscale spin-wave directional coupler that allows the realization of magnonic integrated circuits. Spin waves, and their quanta magnons, are prospective data carriers in future signal processing systems because Gilbert damping associated with the spin-wave propagation can be made substantially lower than the Joule heat losses in electronic devices. Although individual spin-wave signal processing devices have been successfully developed, the challenging contemporary problem is the formation of two-dimensional planar integrated spin-wave circuits. Using both micromagnetic modeling and analytical theory, we present an effective solution of this problem based on the dipolar interaction between two laterally adjacent nanoscale spin-wave waveguides. The developed device based on this principle can work as a multifunctional and dynamically reconfigurable signal directional coupler performing the functions of a waveguide crossing element, tunable power splitter, frequency separator, or multiplexer. The proposed design of a spin-wave directional coupler can be used both in digital logic circuits intended for spin-wave computing and in analog microwave signal processing devices.

Localized Defect Modes in a Two-Dimensional Array of Magnetic Nanodots

The microwave properties of an array of magnetic nanodots in a ferromagnetic state having a point defect-a dot with inverted magnetization or different material parameters-are considered. The existence of a single-point defect in a dot array may lead to the appearance of several localized modes: one “defect eigenmode” and several “well modes,” the number and structure of which strongly depend on the magnetostatic interactions between the dots. By performing a ferromagnetic resonance experiment on an array of magnetic dots containing a small number of defects, it is possible to obtain information about the entire spin-wave spectrum of the array.

Nonreciprocal Spin Waves in a Magnonic Crystal with In-Plane Static Magnetization

It is shown that spin waves propagating in a planar periodic array of dipolarly coupled magnetic strips or dots with in-plane static magnetization can be nonreciprocal. The conditions of the spin waves nonreciprocity are: (i) breaking of the mirror symmetry of an array with respect to its plane, and (ii) complex structure of the array lattice. These conditions can be satisfied, e.g., if an array consists of at least two groups of strips (dots) having different thicknesses (heights). The largest nonreciprocity can be achieved if the static magnetic state of an array is antiferromagnetic, and the difference in the strip thicknesses is about two times. A significant dependence of the spin wave nonreciprocity on the static magnetic state of a strip array allows one to create magnonic crystals with reconfigurable nonreciprocal properties.

Amplification and stabilization of large-amplitude propagating spin waves by parametric pumping

The interaction of a localized parametric pumping with spin waves of different amplitudes, propagating in a ferromagnetic nanowire, is studied analytically and by micromagnetic simulations. It is shown that parametric amplification of spin waves by localized pumping becomes less efficient with an increase in the spin wave amplitude due to the influence of nonlinear 4-magnon processes. In a certain range of spin wave amplitudes, the parametric amplifier acts as a stabilizer of the spin wave amplitude, as its action significantly reduces the spread of the spin wave amplitude in the vicinity of a certain mean value. The stabilization effect becomes more pronounced for higher pumping strength and larger relative lengths of the pumping localization region, compared to the spin wave mean free path. In contrast, the use of relatively short pumping localization regions allows one to efficiently amplify large-amplitude nonlinear spin waves.The interaction of a localized parametric pumping with spin waves of different amplitudes, propagating in a ferromagnetic nanowire, is studied analytically and by micromagnetic simulations. It is shown that parametric amplification of spin waves by localized pumping becomes less efficient with an increase in the spin wave amplitude due to the influence of nonlinear 4-magnon processes. In a certain range of spin wave amplitudes, the parametric amplifier acts as a stabilizer of the spin wave amplitude, as its action significantly reduces the spread of the spin wave amplitude in the vicinity of a certain mean value. The stabilization effect becomes more pronounced for higher pumping strength and larger relative lengths of the pumping localization region, compared to the spin wave mean free path. In contrast, the use of relatively short pumping localization regions allows one to efficiently amplify large-amplitude nonlinear spin waves.