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Spin torque oscillators (STOs) that generate a persistent magnetization oscillation by spin-transfer torque are promising spintronic devices for various applications. In this study, we propose and experimentally demonstrate a method to characterize the magnetization dynamics of STOs using injection locking. By placing the STO near an antenna that generates a microwave magnetic field of a variable frequency ( f M W ), injection locking of the STO to the microwave field occurs when f M W is in the locking range around the intrinsic oscillation frequency. During injection locking, the dc resistance of the STO exhibits a peak-and-valley dependence on f M W , which originates from the modification of the magnetization trajectory induced by injection locking. Based on this principle, the oscillation frequency can be estimated by measuring the dc resistance change. Because this method does not require measuring the high-frequency magnetoresistance signal from the STO, which is used in the conventional method, it is advantageous in cases where the high-frequency magnetoresistance signal is undetectable or exhibits additional frequency components different from the oscillation frequency. [ABSTRACT FROM AUTHOR]

Neuromorphic computing using spintronic devices, such as spin-torque oscillators (STOs), has been intensively studied for energy-efficient data processing. One of the critical issues in this application is stochasticity in magnetization dynamics, which limits the accuracy of computation. Such stochastic behavior, however, plays a key role in stochastic computing and machine learning. It is therefore important to develop methods for both suppressing and enhancing stochastic response in spintronic devices. We report on experimental investigations on control of stochastic quantity, such as the width of a distribution of transient time in magnetization dynamics in vortex-type STO. The spin-transfer effect can suppress stochasticity in transient dynamics from a non-oscillating to oscillating state, whereas an application of a radio-frequency magnetic field is effective in reducing stochasticity on the time evolution of the oscillating state. [ABSTRACT FROM AUTHOR]

In this study, we investigated the effect of Co film thickness and Ru insertion on the magnetic domain structure of a Pt/Co/Ru/MgO system during the magnetization reversal process. It was observed that with increasing Co thickness, the magnetization process systematically changed from the dendrite-like domain growth region to an isolated skyrmion bubble generation region through the suppression of domain wall motion and the decrease in the nucleation field. In addition, by Ru insertion, a similar systematic change in the magnetization reversal process was observed in a thinner Co film while maintaining the Co thickness dependent trend. It was also found during our study that the change in the magnetization reversal process could be inferred by observing the shape of the magnetization curve. Moreover, our experiment showed that once skyrmion bubbles were generated, the Ru inserted Pt/Co/MgO system was stable even after the removal of the magnetic field. This magnetic-field insensitivity of skyrmion bubbles is a pivotal characteristic and could be of great advantage for many spintronics applications. [ABSTRACT FROM AUTHOR]

The critical current of the self-oscillation of a spin torque oscillator consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer was studied by solving the Landau–Lifshitz–Gilbert equation. We found that the critical current diverged at certain field directions, indicating that the self-oscillation does not occur at these directions. It was also found that the sign of the critical current changed depending on the applied field direction. [ABSTRACT FROM AUTHOR]

We systematically measured the spin torque diode spectra of a magnetic tunnel junction nanoelement as a function of external magnetic field and field angle using a homemade quasi-omnidirectional broadband electrical spectrometer. The electrical spectroscopies reveal a significant higher-order spin wave modes excited in the free layer, which is substantiated by an analytical model. [ABSTRACT FROM PUBLISHER]

We investigated gain and fan-out in a current-field driven spin transistor that is driven by a current-induced magnetic field. The basic structure of the transistor consisted of a coplanar waveguide for magnetic field application and an MgO-based magnetic tunnel junction with high magnetoresistance and low resistance area product. Under an assisting ac magnetic field and a dc bias voltage of 0.4 V, we demonstrated substantial power gain of 130, current gain of 4.9, and a fan-out value of 5.7. The scaling of the fan-out value in this device is discussed in detail. The current-field driven spin transistor is proven to be a promising candidate as a basic component of a nonvolatile logic device. [ABSTRACT FROM PUBLISHER]

Statistical variance in the switching probability of spin-torque switching was studied on conventional in-plane MgO-MTJs. Spin-torque switching events by a current pulse longer than inverse of the attempt frequency (typically \sim1 nsec) are well described by the thermal activation model. This model provides the relationship between the switching probability and the current amplitude. If the switching probability is unchanged for a large number of switching events, the statistical variance in the switching probability can be obtained analytically. However, we found the statistical variance in the switching probability varies widely on measured in-plane type CoFeB/MgO/CoFeB-MTJs. Contrary to a simple expectation, the slightly negative correlation was observed between the thermal stability parameter \Delta and the statistical variance in the switching probability. We also measured the external magnetic field dependence along the easy axis, and found that the external field which assists the spin-torque switching dramatically increases the statistical variance of the switching probability. This behavior is considered one of the evidence of the effect of sub-volume excitation. [ABSTRACT FROM PUBLISHER]

Fully epitaxial Co(001)/MgO(001)/Co(001) magnetic tunnel junctions (MTJs) with metastable bcc Co(001) electrodes were fabricated with molecular beam epitaxy. The MTJs exhibited giant magnetoresistance (MR) ratios up to 410% at room temperature, the highest value reported to date. Temperature dependence of the MR ratio was observed to be very small compared with fully epitaxial Fe/MgO/Fe and textured CoFeB/MgO/CoFeB MTJs. The MR ratio of the Co/MgO/Co MTJ showed larger bias voltage dependence than that of the epitaxial Fe/MgO/Fe MTJs, which probably reflects the band structures of bcc Co and Fe for the k∥=0 direction. [ABSTRACT FROM AUTHOR]

Fully epitaxial bcc Fe1-xCox(001)/MgO(001)/Fe(001) magnetic tunnel junctions (x=0, 0.5, 1) were fabricated with molecular-beam epitaxy and microfabrication techniques. While the bcc Fe(001) and Fe0.5Co0.5(001) electrodes had similar magnetoresistance (MR) ratios of about 180% at room temperature, the bcc Co(001) electrode exhibited a higher MR ratio up to 271% at room temperature (353% at 20 K). The fact that the MR ratio for a bcc Co electrode is much higher than that for a bcc Fe electrode is consistent with first-principle calculations, indicating the importance of electrode band structure in the k∥=0 direction. [ABSTRACT FROM AUTHOR]

The bottom shift in a parabolic baseline of resistance-current (R-I) curves in trilayer current perpendicular to plane-giant magnetoresistance (CPP-GMR) elements is often observed in current induced magnetization reversal (CIMR) experiments. We fabricated CPP-metallic junction elements with various metal combinations (Co/Au, Cr/Au, Cr/Co, Cr/Cr), and measured the R-I curves. Our results reveal that the direction of the shift is coincident with the sign of the Peltier coefficient of the CPP structure, and the amount of the shift is proportional to the Peltier coefficient. We estimated the Peltier coefficients by using the current where the Joule heating and the Peltier cooling compensates, and found that the Peltier coefficients are larger than those obtained from bulk materials. The expected cooling power of the CPP-Co/Cr elements was about 100 μW, which results in relatively large areal cooling power (∼105 W/cm²) because of the large current density (∼107 A/cm²). [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions with the structure of Al2O3 (0001)/Pt (111) 20 nm/Ni80Fe20 (111) 50 nm/Al 1.6 nm–O/Co75Fe25 4 nm/Ir22Mn78 10 nm/Ni80Fe20 30 nm were fabricated using UHV sputtering and photolithography process. As the annealing temperature increased up to 250 °C, tunnel magnetoresistance (TMR) ratio at 1 mV bias increased from 28% to 43% for tox=180 s plasma oxidation and the V±1/2, at which the zero bias TMR value is halved, is +640 mV and-650 mV for positive and negative bias voltages, respectively. The bias-voltage dependence of TMR could be explained in terms of the relationship with V±1/2 and the interface of the ferromagnetic electrode and the Al–O insulating layer. V+1/2, which reflects the bottom ferromagnetic electrode-barrier interface state, changes with plasma oxidation time, while V-1/2, which corresponds to top ferromagnetic electrode-barrier interface, hardly changes. [ABSTRACT FROM AUTHOR]

We investigated systematically the spin torque diode spectrum of a ferromagnetically coupled (FeB/CoFe)/Ru/(CoFe/FeB) synthetic free layer in an MgO-based magnetic tunnel junction. In the spectra, we observed single peaks shifted to higher frequency with increasing the in-plane magnetic fields, as expected from the ferromagnetic resonance of the FeB/CoFe adjacent to the MgO tunnel barrier. On the other hand, under the perpendicular fields, we observed several peaks below 6 GHz and around 10 GHz, which were rather insensitive to the field. These behaviors are different from our simple calculation taking account the interlayer coupling, suggesting that the excitation of magnetizations occurs in a complicated manner for the synthetic free layers. [ABSTRACT FROM AUTHOR]

Abstract 1 1 Ferromagnetic resonance (FMR), magnetic tunnel junctions (MTJ). We studied the rectification function of microwaves in CoFeB/MgO-based magnetic tunnel junctions using voltage-induced ferromagnetic resonance (FMR). Our findings reveal that the shape of the structure of the spectrum depends on the rotation angle of the external magnetic field, providing clear evidence that FMR dynamics are excited by voltage-induced magnetic anisotropy changes. Further, enhancement of the rectified voltage was demonstrated under a DC bias voltage. In our experiments, the highest microwave detection sensitivity obtained was 350 mV/mW, at an RF frequency of 1.0 GHz and field angle of θ H =80°, ϕ H =0°. The experimental results correlated with those obtained via simulation, and the calculated results revealed the magnetization dynamics at the resonance state. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the annealing temperature dependence of differential tunneling conductance spectra ( as a function of ) in CoFeB/textured MgO(001)/CoFeB magnetic tunnel junctions (MTJs) that exhibit the giant tunneling magnetoresistance (TMR) effect at room temperature. The spectra were strongly affected by annealing the MTJs. A reduction in at around ±300 mV was observed only in annealed MTJs in which the CoFeB electrodes were crystallized in a bcc(001) structure. Because the reduction in conductance was observed in both MTJs that have a 1.8-nm-thick MgO barrier and MTJs that have a 3.2-nm-thick MgO barrier, we concluded that and evanescent states, which rapidly decay in the MgO tunneling barrier, are not the cause of the reduction in conductance. We believe the cause of the reduction is either the electronic structure of the interfaces between MgO(001)/bcc CoFeB(001) after annealing or a particular feature of states in MgO(001) or bcc CoFeB(001). [Copyright &y& Elsevier]