Gennadiy A. Melkov

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.

Noise properties of a resonance-type spin-torque microwave detector

Abstract : We analyze performance of a resonance-type spin-torque microwave detector (STMD) in the presence of noise and reveal two distinct regimes of STMD operation. In the first (high-frequency) regime the minimum detectable microwave power P(min) is limited by the low-frequency Johnson-Nyquist noise and the signal-to-noise ratio (SNR) of STMD is proportional to the input microwave power PRF. In the second (low-frequency) regime P(min) is limited by the magnetic noise, and the SNR is proportional to the square root of P(RF). The developed formalism can be used for the optimization of the practical noise-handling parameters of a STMD.

Microwave response of single crystal YBa2Cu3O7−δ films as a probe for pairing symmetry

Temperature dependences of the microwave surface impedance, Zs(T), are measured in the c-axis oriented single-crystal high-Tc superconducting cuprate YBa2Cu3O7−δ (YBCO) thin films deposited by the off-axis dc magnetron sputtering onto CeO2-buffered single-crystal sapphire substrates (film thickness is d≈150, 300, 480nm). Measurements are performed by a use of the coplanar resonator as well as the end-plate cylindrical cavity resonator techniques at several discrete frequencies within the range 5–134GHz. The measurements have revealed unexpected peculiarities on the Zs(T)-dependences for the most perfect films under study. The peculiarities appear to be most strongly pronounced on the temperature dependences of the film surface resistance Rs(T)=Re{Zs(T)}. The most important features of the unusual surface resistance behavior are: (i) the temperature dependence Rs(T) of the YBCO films under study at low temperatures obeys the exponential law: Rs(T)=Rres+R0exp[−Δs∕T] with a small energy gap Δs(Δs≈0.5Tc at f=...

Active magnetostatic wave delay line for microwave signals

Summary form only given. Three variants of a new active magnetostatic wave (MSW) delay line for pulsed microwave signals providing controlled delay times of up to 1 /spl mu/s has been developed and experimentally tested. The proposed active delay line consists of an input and output transducers, yttrium-iron garnet (YIG) film waveguide (where the microwave MSW pulse is propagating), and an open dielectric resonator which is situated between the transducers and is used to supply electromagnetic pumping pulses having frequency (9.4 GHz) twice as large as the carrier frequency of the input microwave radio-pulse (4.7 GHz). In contrast with the usual passive MSW delay line, where characteristics are controlled by the external bias magnetic field, the characteristics of the developed active delay line are controlled by the amplitude, duration, and the moment of application of the pumping pulse. While traditional passive MSW delay lines have substantial insertion losses proportional to the delay time (/spl sim/ 80 dB//spl mu/s), in the developed active delay line the amplitude of the output delayed pulse can be kept constant in the whole interval of possible time delays.

Surface wave HTS resonators

Nowdays microwave electronics and measuring technics use microstrip and parallel-plate resonators. They must contain two superconducting films to have high quality Q. Besides the manufacture of such resonators for short-wave part of millimeter of waveband is accompanied by both the difficulties of design and increase of losses in the input microstrip and coaxial circuits. For that reasons we have elabarated several microwave resonators using a single metal or HTS surface (existence of other surfaces only decreases Q). All resonators operate on the surface waves, running along metal. That can be surfase waves running along a solitary metal surface with a finite conductivity (Zennek waves) or surface waves along metal (even ideal) covered with dielectric. As a result, the microwave resonators can be formed with the properly sized pieces of an HTS wire, rod, plate, single-side film on a dielectric substrate. Theoretical analysis of the resonant frequencies has been carried out by decomposition method [3,4]. Resotator, which use two dielectrics is placed in waveguide has been considered. . Such resonator is shown on Fig. 1. Firstly, the own waves of waveguide which has section shown on Fig. 1 were described for determination of the resonant frequencies of such resonator. Waveguide has been devided into four partial areas: I. O

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.

Nonlinear Ferromagnetic Resonance in Nanostructures Having Discrete Spectrum of Spin-Wave Modes

Nonlinear ferromagnetic resonance (NFMR) at the nanoscale differs from the same phenomenon in bulk magnetic samples due to the nanosize-related discreteness of the spin wave spectrum. Because of the spectrum discreteness, the conservation laws for multiwave parametric interaction processes cannot be fulfilled exactly and become nonresonant. The presence of the nonresonant parametric processes in a strongly driven magnetic nanosystem can be detected experimentally as distortions on the NFMR curve caused by the nonresonant excitation of spin wave modes.

Parametric interaction of dipolar spin wave solitons with localized electromagnetic pumping

A new nonlinear method of effective amplification of single envelope solitons is discussed. This method combines amplification of a signal pulse with its simultaneous compression. It was found that the maximum single-soliton amplification gain can be two times larger than the pulse compression rate, and for large compression rates can significantly exceed the theoretical limit for an ideal linear amplifier. The proposed method was experimentally tested on envelope solitons of dipolar spin waves (backward volume magnetostatic waves), propagating in yttrium-iron garnet films and parametrically interacting with localised and pulsed electromagnetic pumping. The single-soliton amplification gain of 17 dB was obtained. The compression of spin wave pulses was realised both by amplification of only the central part of the pulse, and as a result of collective oscillations of parametrically coupled spin waves.

Double-wave-front reversal of dipole-exchange spin waves in yttrium-iron garnet films

The effect of double-wave-front reversal in the system of dipolar and dipole-exchange spin waves propagating in a ferrite film is studied experimentally and theoretically with the goal of development of a nonlinear microwave signal processor. In the proposed processor microwave signal pulse supplied to the input antenna excites in the ferrite film a long-wavelength dipolar spin wave (magnetostatic wave), which in the course of propagation is scattered on the film inhomogeneities and is transformed into short-wavelength long-lifetime dipole-exchange spin waves. Then, a sequence of two high-power double-frequency parametric pumping pulses, supplied to the film through an open dielectric resonator with a certain time delay between them, performs first, and then second, wave-front reversal for all the spin waves created by the input signal in the ferrite film. As a result of this double wave-front reversal process microwave output pulses with either reversed (input antenna) or nonreversed (output antenna) tim...

Irradiation of HTS Josephson junctions with the surface wave resonator

The new type of surface wave resonator (SWR) consists of HTS film on a dielectric substrate is developed. A possibility of optimal coupling between the electromagnetic field of the SWR and the HTS Josephson junction array is shown; The topologies for embedding the bicrystal HTS Josephson junctions in the SWR are developed. These topology ensure the junction synchronization, that can be used for creation voltage standards, microwave detectors and Josephson generators and so on; The microwave properties of the SWR with HTS Josephson junction arrays (up to 450 junctions) and nonlinear elements based on Shottky-barrier diodes have been investigated. For the array with 450 HTS Josephson junctions the voltage sequence with 10 mV step has been obtained, sensitivity of the detector based on this array is 0.1 A/W with dynamic range 10/sup 2/ dB; The Josephson generation with microwave power at several nW for the SWR with bicrystal HTS JJs is detected. There were two groups of junctions. These groups are formed at the time of fabrication due to imperfection of HTS junction fabrication technology.