O. V. Prokopenko

Spin-Torque Nano-Oscillator as a Microwave Signal Source

We evaluate a possibility to use spin-torque nano-oscillators (STNOs) and oscillator arrays as new sources of microwave signals for telecommunication devices. The microwave signal generated by STNO can be received as oscillation of the device magnetoresistance (MR) or as direct electromagnetic emission of two effective magnetic dipoles. We calculate the dipolar microwave power emitted by STNO in free space and in several types of transmission lines and resonators. We demonstrate that although the power of a single STNO received through the MR effect is, typically, larger than the power of direct microwave emission of effective dipoles, the latter mechanism might have an advantage for sufficiently large arrays of coupled STNOs.

Spin-torque microwave detector with out-of-plane precessing magnetic moment

Operation of a spin-torque microwave detector (STMD) in a weak perpendicular bias magnetic field has been studied theoretically. It is shown that in this geometry a novel dynamical regime of STMD operation, characterized by large-angle out-of-plane magnetization precession, can be realized. The excitation of the large-angle precession has threshold character and is possible only for input microwave currents exceeding a certain frequency-dependent critical value. The output voltage of an STMD increases with the frequency of the input signal but is virtually independent of its power. An STMD working in the regime of large-amplitude out-of-plane precession functions as a non-resonant threshold detector of low frequency microwave signals, due to the large nonlinear shift of its operating frequency. Therefore, it is particularly suitable for applications in microwave energy harvesting.

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.

Spin-Torque Microwave Detectors

It is known that the spin-transfer torque (STT) effect provides a new method of manipulation of magnetization in nano-scale devices. According to the STT effect, bias DC current traversing magnetic multilayers can transfer angular magnetic moments from one layer to another, which can give rise to microwave dynamics of magnetization in the layer. However, it is clear that an inverse effect is also possible. This inverse effect leads to the so-called spin-torque diode effect, first originally observed experimentally in 2005. The spin-torque diode effect is a rectification effect of the input microwave current in a magnetoresistive junction. In this case, the resonance oscillations of the junction resistance can mix with the oscillations of the input microwave current and produce a large enough output DC voltage across the junction. The devices which utilize this effect are called the spin-torque microwave detectors (STMD). In this chapter, we review the general properties of STMDs and consider the performance of a STMD in two different dynamic regimes of detector operation: in the well-known traditional in-plane regime of STMD operation and in the recently discovered novel out-of-plane regime of STMD operation. We analyze the performance of a STMD and consider the typical applications for such detectors in both regimes.

Microwave detectors based on the spin-torque diode effect

The spin-transfer torque (STT) effect provides a new method of manipulation of magnetization in nanoscale objects. The STT effect manifests itself as a transfer of spin angular momentum between the parallel magnetic layers separated by a nonmagnetic spacer and traversed by a dc electric current. The transfer of the spin angular momentum from one layer to another could result in the excitation of the microwave-frequency magnetization dynamics in one of the magnetic layers. On the other hand, when a magnetization dynamics is excited in a magnetic layered structure by an external microwave signal both the structure electrical resistance and current through the structure will acquire microwave components resulting in the appearance of a rectified dc voltage on the magnetic structure. This “spin-torque diode effect” can be used for the development of ultra-sensitive spin-torque microwave detectors (STMD). Below we present a brief review of our recent work on the general properties of STMDs, analyze the perform...

Influence of Temperature on the Performance of a Spin-Torque Microwave Detector

We analyzed the influence of temperature on the main characteristics of a passive spin-torque microwave detector (STMD): volt-watt sensitivity, signal-to-noise ratio, and minimum detectable microwave power. We reveal that these parameters do not always improve with the decrease of temperature. The developed formalism can be used for the optimization of the practical parameters of a STMD in a wide range of temperatures.

Hysteresis regime in the operation of a dual-free-layer spin-torque nano-oscillator with out-of-plane counter-precessing magnetic moments

We studied the operation of a dual-free-layer (DFL) spin-torque nano-oscillator (STNO) and demonstrated that in a practically interesting regime when the magnetizations of the two free layers (FLs) precess in opposite directions along large-angle out-of-plane trajectories, thus doubling the generation frequency, the operation of the DFL STNO is strongly hysteretic as a function of a bias dc current. The stable magnetization dynamics starts at a rather large magnitude of the bias dc current density Jdc>Jthhigh when the bias current is increased, but the regime of stable counter-precession of the FLs persists till rather low magnitudes of the bias dc current density Jthlow<Jdc<Jthhigh when the bias current is decreased. This hysteresis is caused by the dipolar coupling between the FLs, and is especially pronounced for small distances between the FLs and the small magnetic damping in them. The discovered hysteretic behavior of the DFL STNO implies the possibility of application of a strong initial pulse of t...

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.

Electrodynamic Model of THz- Frequency Signal Source Based on Antiferromagnetic Spin Hall Auto-oscillator

Electrodynamic model of a THz-frequency signal source utilizing an antiferromagnetic spin Hall oscillator (SHO), where the magnetization vectors of the antiferromagnets (AFM) sublattices are canted inside the easy plane, resulting in the appearance of the small net magnetization vector, is proposed and analyzed. A bias dc electric current applied to the SHO creates a spin current that, being injected in the AFM layer, starts to rotate the net magnetization of the canted AFM with the THz frequency proportional to the injected spin current. This rotation of the small net magnetization results in the THz-frequency dipolar radiation that can be directly registered by an adjacent resonator. Our calculations show that the radiation power increases with the increase of frequency

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

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