記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

Artificial spin ice (ASI) consisted of submicron-sized magnetic cells, which showed emergent behavior of the magnetizations in the cells. To apply the magnetization dynamics of the ASI to spintronics devices, it is important to understand the magnetization process of the cells. In the ASI samples, stadium shaped (SS) cells were widely used, however, whose magnetization process has not been well understood. In this study we investigated the magnetization process of the SS cells by magnetoresistance measurement in magnetic tunnel junctions as well as micromagnetic simulation. It was found that the SS cells showed magnetization switching at a low field, which was triggered by the annihilation of the magnetic vortex.

DOI： 10.1109/INTERMAGShortPapers58606.2023.10228518

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記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

Physical reservoir computing using spin-torque oscillator succeeded human-voice recognition with high accuracy. It accompanies, however, large energy dissipation because the magnetization dynamics in the oscillator is driven by electric current and an external magnetic field is also necessary. Here, we show that the magnetization dynamics driven by voltage controlled magnetic anisotropy (VCMA) effect can be applied to physical reservoir computing. It does not accompany Joule heating because of the absence of current. An external magnetic field is also unnecessary because physical reservoir computing does not requires magnetization switching. Instead, a second-order magnetic anisotropy is required for a deterministic computing. Solving the Landau-Lifshitz-Gilbert equation with the second-order magnetic anisotropy field, we find that the computational capability of a magnetic tunnel junction can be tuned by the VCMA effect.

DOI： 10.1109/INTERMAGShortPapers58606.2023.10228213

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The brain naturally binds events from different sources in unique concepts. It is hypothesized that this process occurs through the transient mutual synchronization of neurons located in different regions of the brain when the stimulus is presented. This mechanism of ‘binding through synchronization’ can be directly implemented in neural networks composed of coupled oscillators. To do so, the oscillators must be able to mutually synchronize for the range of inputs corresponding to a single class, and otherwise remain desynchronized. Here we show that the outstanding ability of spintronic nano-oscillators to mutually synchronize and the possibility to precisely control the occurrence of mutual synchronization by tuning the oscillator frequencies over wide ranges allows pattern recognition. We demonstrate experimentally on a simple task that three spintronic nano-oscillators can bind consecutive events and thus recognize and distinguish temporal sequences. This work is a step forward in the construction of neural networks that exploit the non-linear dynamic properties of their components to perform brain-inspired computations.

DOI： 10.1038/s41467-022-28159-1

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Recent studies have shown that nonlinear magnetization dynamics excited in nanostructured ferromagnets are applicable to brain-inspired computing such as physical reservoir computing. The previous works have utilized the magnetization dynamics driven by electric current and/or magnetic field. This work proposes a method to apply the magnetization dynamics driven by voltage control of magnetic anisotropy to physical reservoir computing, which will be preferable from the viewpoint of low-power consumption. The computational capabilities of benchmark tasks in single MTJ are evaluated by numerical simulation of the magnetization dynamics and found to be comparable to those of echo-state networks with more than 10 nodes.

DOI： 10.1038/s41598-022-14738-1

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記述言語：英語   掲載種別：研究論文（学術雑誌）

A new research topic in spintronics relating to the operation principles of brain-inspired computing is input-driven magnetization dynamics in nanomagnet. In this paper, the magnetization dynamics in a vortex spin-torque oscillator driven by a series of random magnetic field are studied through a numerical simulation of the Thiele equation. It is found that input-driven synchronization occurs in the weak perturbation limit, as found recently. As well, chaotic behavior is newly found to occur in the vortex core dynamics for a wide range of parameters, where synchronized behavior is disrupted by an intermittency. Ordered and chaotic dynamical phases are examined by evaluating the Lyapunov exponent. The relation between the dynamical phase and the computational capability of physical reservoir computing is also studied.

DOI： 10.1038/s41598-022-26018-z

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Noise-induced synchronization is a phenomenon where several oscillators with different initial conditions show synchronized motion, even in the absence of a coupling between them, when common stochastic input signals are injected. The phenomenon has attracted attention from nonlinear science, as well as applied physics, because it enables environmental noise to be used to synchronize many oscillators and is a necessary condition for brain-inspired computing. Here we develop a theoretical analysis of noise-induced synchronization in spin-torque oscillators (STOs). The analytical form for the Lyapunov exponent for the present model we derive indicates that there are two contributions from input signal to noise-induced synchronization in STOs. The first is that the input signal directly aligns the phases of the magnetizations, and the second is that the input signal changes the oscillating amplitude, and the amplitude-phase coupling results in synchronization. The validity of the analytical results was qualitatively confirmed by numerical simulation. We also show the existence of on-off intermittency at finite temperature, whose statistical properties are similar to those of other oscillator systems.

DOI： 10.1103/PhysRevB.105.224407

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Excitation of chaotic magnetization dynamics in nanomagnets is of great interest because it bridges the condensed matter physics and nonlinear science and has a potential to emerging technologies such as neuromorphic computing. However, it has been difficult to observe and identify chaos in spintronics devices because the excitation of chaos requires dynamics in a large-dimensional phase space, according to the Poincaré-Bendixson theorem. An efficient way to overcome this issue is using feedback, which enables the dynamical degrees of freedom to be increased even in a single device. Here, we experimentally demonstrate the excitation of chaos in a vortex spin-torque oscillator by utilizing a feedback circuit. The radio-frequency current emitted by the oscillator flows in the feedback circuit and is converted into an oscillating magnetic field. The oscillating field generates a torque acting on the vortex and modulates its dynamics, resulting in chaotic dynamics which can be tuned by electrical means.

DOI： 10.1103/PhysRevResearch.3.043216

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The performance of magnetoresistive sensors is today mainly limited by their 1/f low-frequency noise. Here, we study this noise component in vortex-based TMR sensors. We compare the noise level in different magnetization configurations of the device, i.e., vortex state or uniform parallel or antiparallel states. We find that the vortex state is at least an order of magnitude noisier than the uniform states. Nevertheless, by activating the spin-transfer-induced dynamics of the vortex configuration, we observe a reduction of the 1/f noise, close to the values measured in the AP state, as the vortex core has a lower probability of pinning into defect sites. Additionally, by driving the dynamics of the vortex core by a non-resonant rf field or current, we demonstrate that the 1/f noise can be further decreased. The ability to reduce the 1/f low-frequency noise in vortex-based devices by leveraging their spin-transfer dynamics thus enhances their applicability in the magnetic sensors' landscape.

DOI： 10.1063/5.0040874

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The correlation of phase fluctuations in any type of oscillator fundamentally defines its spectral shape. However, in nonlinear oscillators, such as spin torque nano-oscillators, the frequency spectrum can become particularly complex. This is specifically true when not only considering thermal but also colored 1/f flicker noise processes, which are crucial in the context of the oscillator’s long term stability. In this study, we address the frequency spectrum of spin torque oscillators in the regime of large-amplitude steady oscillations experimentally and as well theoretically. We particularly take both thermal and flicker noise into account. We perform a series of measurements of the phase noise and the spectrum on spin torque vortex oscillators, notably varying the measurement time duration. Furthermore, we develop the modelling of thermal and flicker noise in Thiele equation based simulations. We also derive the complete phase variance in the framework of the nonlinear auto-oscillator theory and deduce the actual frequency spectrum. We investigate its dependence on the measurement time duration and compare with the experimental results. Long term stability is important in several of the recent applicative developments of spin torque oscillators. This study brings some insights on how to better address this issue.

DOI： 10.1038/s41598-020-70076-0

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The reservoir computing neural network architecture is widely used to test hardware systems for neuromorphic computing. One of the preferred tasks for bench-marking such devices is automatic speech recognition. This task requires acoustic transformations from sound waveforms with varying amplitudes to frequency domain maps that can be seen as feature extraction techniques. Depending on the conversion method, these transformations sometimes obscure the contribution of the neuromorphic hardware to the overall speech recognition performance. Here, we quantify and separate the contributions of the acoustic transformations and the neuromorphic hardware to the speech recognition success rate. We show that the non-linearity in the acoustic transformation plays a critical role in feature extraction. We compute the gain in word success rate provided by a reservoir computing device compared to the acoustic transformation only, and show that it is an appropriate bench-mark for comparing different hardware. Finally, we experimentally and numerically quantify the impact of the different acoustic transformations for neuromorphic hardware based on magnetic nano-oscillators.

DOI： 10.1038/s41598-019-56991-x

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Spintronics devices generate diverse nonlinear dynamics and have been studied as promising candidates for physical reservoir computing systems. However, the dynamic properties of spintronics reservoirs driven by input streams are largely yet to be uncovered. This study reveals that two types of bifurcation, from order to chaos and from chaos to order, can be induced by increasing the strength of input signals to the spintronics reservoir, and the information processing capacity of the reservoir changes drastically according to these bifurcations. The significant contributions of input-induced diversity in magnetization dynamics are demonstrated through numerical experiments, which include a real-world sensor emulation task. Our results suggest that modulating input settings can generate a diverse repertoire of magnetization dynamics without tuning the physical platform itself, providing valuable insights into neuromorphic applications.

DOI： 10.1103/PhysRevResearch.2.043303

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Physical reservoir computing is a type of recurrent neural network that applies the dynamical response from physical systems to information processing. However, the relation between computation performance and physical parameters/phenomena still remains unclear. This study reports our progress regarding the role of current-dependent magnetic damping in the computational performance of reservoir computing. The current-dependent relaxation dynamics of a magnetic vortex core results in an asymmetric memory function with respect to binary inputs. A fast relaxation caused by a large input leads to a fast fading of the input memory, whereas a slow relaxation by a small input enables the reservoir to keep the input memory for a relatively long time. As a result, a step-like dependence is found for the short-term memory and parity-check capacities on the pulse width of input data, where the capacities remain at 1.5 for a certain range of the pulse width, and drop to 1.0 for a long pulse-width limit. Both analytical and numerical analyses clarify that the step-like behavior can be attributed to the current-dependent relaxation time of the vortex core to a limit-cycle state.

DOI： 10.1038/s41598-020-76142-x

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The role of the feedback effect on physical reservoir computing is studied theoretically by solving the vortex-core dynamics in a nanostructured ferromagnet. Although the spin-transfer torque due to the feedback current makes the vortex dynamics complex, it is clarified that the feedback effect does not always contribute to the enhancement of the memory function in a physical reservoir. The memory function, characterized by the correlation coefficient between the input data and the dynamical response of the vortex core, becomes large when the delay time of the feedback current is not an integral multiple of the pulse width. On the other hand, the memory function remains small when the delay time is an integral multiple of the pulse width. As a result, a periodic behavior for the short-term memory capacity is observed with respect to the delay time, the phenomenon of which can be attributed to correlations between the virtual neurons via the feedback current.

DOI： 10.1103/PhysRevResearch.2.023389

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記述言語：英語   掲載種別：研究論文（学術雑誌）

A theoretical analysis is developed on spin-torque diode effect in nonlinear region. An analytical solution of the diode voltage generated from spin-torque oscillator (STO) by the rectification of an alternating current is derived. The diode voltage is revealed to depend nonlinearly on the phase difference between the oscillator and the alternating current. The validity of the analytical prediction is confirmed by numerical simulation of the Landau-Lifshitz-Gilbert equation. The results indicate that the spin-torque diode effect is useful to evaluate the phase of a STO in forced synchronization state.

DOI： 10.35848/1347-4065/ab6a28

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Synchronization and chaos caused by alternating current and microwave field in a spin torque oscillator consisting of a perpendicularly magnetized free layer and an in-plane magnetized reference layer is comprehensively studied theoretically. A forced synchronization by the alternating current is observed in numerical simulation over wide ranges of its amplitude and frequency. An analytical theory clarifies that the nonlinear frequency shift, as well as the spin-transfer torque asymmetry, plays a key role in determining locking range and phase difference between the oscillator and current. Chaos caused by the alternating current is identified for a region of large alternating current by evaluating the Lyapunov exponent. Similar results are also obtained for microwave field, although the parameter regions causing chaos are narrower than those by the alternating current.

DOI： 10.1103/PhysRevB.100.224422

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The recent demonstration of neuromorphic computing with spin-torque nano-oscillators has opened a path to energy efficient data processing. The success of this demonstration hinged on the intrinsic short-term memory of the oscillators. We extend the memory of the spin-torque nano-oscillators through time-delayed feedback. We leverage this extrinsic memory to increase the efficiency of solving pattern recognition tasks that require memory to discriminate different inputs. The large tunability of these nonlinear oscillators allows us to control and optimize the delayed-feedback memory using different operating conditions of applied current and magnetic field.

DOI： 10.1103/PhysRevApplied.12.024049

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Low-frequency noise close to the carrier remains little explored in spin-torque nano-oscillators. However, it is crucial to investigate as it limits the oscillator's frequency stability. This work addresses the low offset frequency flicker noise of a spin-torque vortex oscillator in the regime of large-amplitude steady oscillations. We first phenomenologically expand the nonlinear auto-oscillator theory, aiming to reveal the properties of this noise. We then present a thorough experimental study of the oscillator's 1/f flicker noise and discuss the results based on the theoretical predictions. Thereby, we connect the oscillator's nonlinear dynamics with the concept of flicker noise and furthermore refer to the influence of a standard 1/f noise description based on the Hooge formula, taking into account the nonconstant magnetic oscillation volume, which contributes to the magnetoresistance.

DOI： 10.1103/PhysRevB.99.235135

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