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Register a new userLow Power Microwave Signal Detection With a Spin-Torque Nano-Oscillator in the Active Self-Oscillating Regime
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A spin-torque nano-oscillator (STNO) driven by a ramped bias current can perform spectrum analysis quickly over a wide frequency bandwidth. The STNO spectrum analyzer operates by injection locking to external microwave signals and produces an output DC voltage $V_{rm dc}$ that temporally encodes the input spectrum. We found, via numerical analysis with a macrospin approximation, that an STNO is able to scan a $10~rm GHz$ bandwidth in less than $100~rm ns$ (scanning rate $R$ exceeds $100~rm MHz/ns$). In contrast to conventional quadratic microwave detectors, the output voltage of the STNO analyzer is proportional to the amplitude of the input microwave signal $I_{rm rf}$ with sensitivity $S = dV_{rm dc}/dI_{rm rf} approx 750~rm mV/mA$. The minimum detectable signal of the analyzer depends on the scanning rate $R$ and, at low $R approx 1~rm MHz/ns$, is about $1~rm pW$.
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Spin torque and spin Hall effect nanooscillators generate high intensity spin wave auto oscillations on the nanoscale enabling novel microwave applications in spintronics, magnonics, and neuromorphic computing. For their operation, these devices require externally generated spin currents either from an additional ferromagnetic layer or a material with a high spin Hall angle. Here we demonstrate highly coherent field and current tunable microwave signals from nanoconstrictions in single 15 and 20 nm thick permalloy layers. Using a combination of spin torque ferromagnetic resonance measurements, scanning microBrillouin light scattering microscopy, and micromagnetic simulations, we identify the autooscillations as emanating from a localized edge mode of the nanoconstriction driven by spin orbit torques. Our results pave the way for greatly simplified designs of auto oscillating nanomagnetic systems only requiring a single ferromagnetic layer.
A theoretical study of delayed feedback in spin-torque nano-oscillators is presented. A macrospin geometry is considered, where self-sustained oscillations are made possible by spin transfer torques associated with spin currents flowing perpendicular to the film plane. By tuning the delay and amplification of the self-injected signal, we identify dynamical regimes in this system such as chaos, switching between precession modes with complex transients, and oscillator death. Such delayed feedback schemes open up a new field of exploration for such oscillators, where the complex transient states might find important applications in information processing.
We investigated the dynamics of a novel design of spin torque oscillator (STO) for microwave assisted magnetic recording. Using Ni$_{80}$Fe$_{20}$ (NiFe) as the polarizer and Fe$_{67}$Co$_{33}$ (FeCo) as the field generating layer, we experimentally observed the magnetization reversal of NiFe, followed by multiple signals in the power spectra as the bias voltage increased. The signals reflected the out-of-plane precession (OPP) mode oscillation of both FeCo and NiFe, as well as the magnetoresistance effect of the STO device, which had the frequency equal to the difference between the oscillation frequency of NiFe and FeCo. Such dynamics were reproduced by micromagnetic simulation. In addition to the merit of realizing the OPP mode oscillation with a simple and thin structure suitable for a narrow gap recording head, the experimental results using this design suggested that a large cone angle of $sim$ 70$^{circ}$ for the OPP mode oscillation of FeCo was achieved, which was estimated based on the macrospin model.
We investigate the spectral characteristics of spin torque oscillator (STO) excited by the spin Hall-induced spin current. We observe that the modest spin current injection triggers the conventional single peak oscillating behavior of STO. As the spin current is further increased to enter the non-linear regime, we find the transition of the spectrum from a single peak to multipeak structure whose frequency spacing is constant. This behavior can be primarily explained by the extremely broadened peak of the STO, which is accompanied by the frequency-dependent filtering by the transmission line. To explain the observation more quantitatively, we also discuss that the multipeak may reflect the characteristics of the intrinsic dynamics of STO in the non-linear regime.
Energy loss due to ohmic heating is a major bottleneck limiting down-scaling and speed of nano-electronic devices, and harvesting ohmic heat for signal processing is a major challenge in modern electronics. Here we demonstrate that thermal gradients arising from ohmic heating can be utilized for excitation of coherent auto-oscillations of magnetization and for generation of tunable microwave signals. The heat-driven dynamics is observed in $mathrm{Y_{3}Fe_{5}O_{12}/Pt}$ bilayer nanowires where ohmic heating of the Pt layer results in injection of pure spin current into the $mathrm{Y_{3}Fe_{5}O_{12}}$ layer. This leads to excitation of auto-oscillations of the $mathrm{Y_{3}Fe_{5}O_{12}}$ magnetization and generation of coherent microwave radiation. Our work paves the way towards spin caloritronic devices for microwave and magnonic applications.
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