Showing 1–50 of 77 results for author: Yuasa, S

Cryogenic-temperature Grain-to-grain Epitaxial Growth of High-quality Ultrathin CoFe Layer on MgO Tunnel Barrier for High-performance Magnetic Tunnel Junctions

Authors: Tomohiro Ichinose, Tatsuya Yamamoto, Takayuki Nozaki, Kay Yakushiji, Shingo Tamaru, Shinji Yuasa

Abstract: One candidate for ultimate non-volatile memory with ultralow power consumption is magneto-resistive random-access memory (VC-MRAM). To develop VC-MRAM, it is important to fabricate high-performance magnetic tunnel junctions (MTJs), which require the epitaxial growth of an ultrathin ferromagnetic electrode on a crystalline tunnel barrier using a mass-manufacturing-compatible process. In this study,… ▽ More One candidate for ultimate non-volatile memory with ultralow power consumption is magneto-resistive random-access memory (VC-MRAM). To develop VC-MRAM, it is important to fabricate high-performance magnetic tunnel junctions (MTJs), which require the epitaxial growth of an ultrathin ferromagnetic electrode on a crystalline tunnel barrier using a mass-manufacturing-compatible process. In this study, the grain-to-grain epitaxial growth of perpendicularly magnetized CoFe ultrathin films on polycrystalline MgO (001) was demonstrated using cryogenic-temperature sputtering on 300 mm Si wafers. Cryogenic-temperature sputtering at 100 K suppressed the island-like initial growth of CoFe on MgO without hampering epitaxy. Sub-nanometer-thick CoFe layers exhibited remarkable perpendicular magnetic anisotropy (PMA). An even larger PMA was obtained using an Fe-doped MgO (MgFeO) tunnel barrier owing to improved uniformity of the CoFe layer. A 0.8-nm-thick CoFe layer grown on MgFeO exhibited a magnetic damping constant as low as 0.008. The ultralow magnetic damping enables voltage-driven magnetization switching with a low write-error rate (WER) below 10^-6 at a pulse duration of 0.3 ns, and WER on the order of 10^-3 even for a relatively long pulse duration of 1.5 ns. These properties achieved using a mass-manufacturing deposition process can promote the development of VC-MRAM and other advanced spintronic devices based on MTJs. △ Less

Submitted 9 April, 2025; originally announced April 2025.

Comments: under review in NPG Asia Mater

Controlling encirclement of an exceptional point using coupled spintronic nano-oscillators

Authors: K. Ho, S. Perna, S. Wittrock, S. Tsunegi, H. Kubota, S. Yuasa, P. Bortolotti, M. d'Aquino, C. Serpico, V. Cros, R. Lebrun

Abstract: Exceptional points (EPs), branch singularities parameter space of non-Hermitian eigenvalue manifolds, display unique topological phenomena linked to eigenvalue and eigenvector switching: the parameter space states are highly sensitive to the system's parameter changes. Therefore, we suggest investigating the parameter space in the presence of an EP by experimentally accessing and exploiting the to… ▽ More Exceptional points (EPs), branch singularities parameter space of non-Hermitian eigenvalue manifolds, display unique topological phenomena linked to eigenvalue and eigenvector switching: the parameter space states are highly sensitive to the system's parameter changes. Therefore, we suggest investigating the parameter space in the presence of an EP by experimentally accessing and exploiting the topological nature of the coupled system around an EP. We demonstrate control over exceptional points in coupled vortex spin-transfer torque oscillators by adjusting the system's damping through the spin-transfer torque effect and their relative phase. This approach allows for precise manipulation of the coupling behavior in the vicinity of an exceptional point. We report the presence of both level attraction/repulsion by adjusting the system's parameters. Moreover, we evidence the topological nature of the EP by dynamically encircling it in the phase-current parameter space, leading to a switch of the eigenstates. Our study introduces a new method for exploring non-Hermitian physics in spintronic systems at room temperature. △ Less

Submitted 22 December, 2024; originally announced December 2024.

Comments: 6 pages and 4 figures

Rotational Grüneisen ratio: a probe for quantum criticality in anisotropic systems

Authors: Shohei Yuasa, Yohei Kono, Yuta Ozaki, Minoru Yamashita, Yasuyuki Shimura, Toshiro Takabatake, Shunichiro Kittaka

Abstract: The Grüneisen ratio $Γ$ and its magnetic analog, the magnetic Grüneisen ratio $Γ_H$, are powerful probes to study the nature of quantum phase transitions. Here, we propose a Grüneisen parameter, the rotational Grüneisen ratio $Γ_φ$, by introducing the orientation of the external field as a control parameter. We investigate $Γ_φ$ of the highly anisotropic paramagnets CeRhSn and CeIrSn by measuring… ▽ More The Grüneisen ratio $Γ$ and its magnetic analog, the magnetic Grüneisen ratio $Γ_H$, are powerful probes to study the nature of quantum phase transitions. Here, we propose a Grüneisen parameter, the rotational Grüneisen ratio $Γ_φ$, by introducing the orientation of the external field as a control parameter. We investigate $Γ_φ$ of the highly anisotropic paramagnets CeRhSn and CeIrSn by measuring the rotational magnetocaloric effect in a wide range of temperatures and magnetic fields. We find that the $Γ_φ$ data of both compounds are scaled by using the same critical exponents and the field-invariant critical field angle. Remarkably, the scaling function for the $Γ_φ$ data reveals the presence of highly anisotropic quantum criticality that develops as a function of the easy-axis component of the magnetic field from the quantum critical line. This paper provides a thermodynamic approach to detect and identify magnetic quantum criticality in highly anisotropic systems. △ Less

Submitted 10 January, 2025; v1 submitted 12 December, 2024; originally announced December 2024.

Comments: 6 pages, 3 figures (main text) + 12 pages, 10 figures (Supplemental Material)

Journal ref: Phys. Rev. B 111, 045123 (2025)

Coherent Driving of a Single Nitrogen Vacancy Center by a Resonant Magnetic Tunnel Junction

Authors: Gerald Q. Yan, Nathan McLaughlin, Tatsuya Yamamoto, Senlei Li, Takayuki Nozaki, Shinji Yuasa, Chunhui Rita Du, Hailong Wang

Abstract: Nitrogen-vacancy (NV) centers, atomic spin defects in diamond, represent an active contender for advancing transformative quantum information science (QIS) and innovations. One of the major challenges for designing NV-based hybrid systems for QIS applications results from the difficulty of realizing local control of individual NV spin qubits in a scalable and energyefficient way. To address this b… ▽ More Nitrogen-vacancy (NV) centers, atomic spin defects in diamond, represent an active contender for advancing transformative quantum information science (QIS) and innovations. One of the major challenges for designing NV-based hybrid systems for QIS applications results from the difficulty of realizing local control of individual NV spin qubits in a scalable and energyefficient way. To address this bottleneck, we introduce magnetic tunnel junction (MTJ) devices to establish coherent driving of an NV center by a resonant MTJ with voltage controlled magnetic anisotropy. We show that the oscillating magnetic stray field produced by a resonant micromagnet can be utilized to effectively modify and drive NV spin rotations when the NV frequency matches the corresponding resonance conditions of the MTJ. Our results present a new pathway to achieve all-electric control of an NV spin qubit with reduced power consumption and improved solid-state scalability for implementing cutting-edge QIS technological applications. △ Less

Submitted 15 October, 2024; originally announced October 2024.

Journal ref: Nano Letters 2024 24 (45), 14273-14278

A study of centaur (54598) Bienor from multiple stellar occultations and rotational light curves

Authors: J. L. Rizos, E. Fernández-Valenzuela, J. L. Ortiz, F. L. Rommel, B. Sicardy, N. Morales, P. Santos-Sanz, R. Leiva, M. Vara-Lubiano, R. Morales, M. Kretlow, A. Alvarez-Candal, B. J. Holler, R. Duffard, J. M. Gómez-Limón, J. Desmars, D. Souami, M. Assafin, G. Benedetti-Rossi, F. Braga-Ribas, J. I. B. Camargo, F. Colas, J. Lecacheux, A. R. Gomes-Júnior, R. Vieira-Martins , et al. (18 additional authors not shown)

Abstract: Centaurs, distinguished by their volatile-rich compositions, play a pivotal role in understanding the formation and evolution of the early solar system, as they represent remnants of the primordial material that populated the outer regions. Stellar occultations offer a means to investigate their physical properties, including shape, rotational state, or the potential presence of satellites and rin… ▽ More Centaurs, distinguished by their volatile-rich compositions, play a pivotal role in understanding the formation and evolution of the early solar system, as they represent remnants of the primordial material that populated the outer regions. Stellar occultations offer a means to investigate their physical properties, including shape, rotational state, or the potential presence of satellites and rings. This work aims to conduct a detailed study of the centaur (54598) Bienor through stellar occultations and rotational light curves from photometric data collected during recent years. We successfully predicted three stellar occultations by Bienor, which were observed from Japan, Eastern Europe, and the USA. In addition, we organized observational campaigns from Spain to obtain rotational light curves. At the same time, we develop software to generate synthetic light curves from three-dimensional shape models, enabling us to validate the outcomes through computer simulations. We resolve Bienor's projected ellipse for December 26, 2022, determine a prograde sense of rotation, and confirm an asymmetric rotational light curve. We also retrieve the axes of its triaxial ellipsoid shape as a = (127 $\pm$ 5) km, b = (55 $\pm$ 4) km, and c = (45 $\pm$ 4) km. Moreover, we refine the rotation period to 9.1736 $\pm$ 0.0002 hours and determine a geometric albedo of (6.5 $\pm$ 0.5) %, higher than previously determined by other methods. Finally, by comparing our findings with previous results and simulated rotational light curves, we analyze whether an irregular or contact-binary shape, the presence of an additional element such as a satellite, or significant albedo variations on Bienor's surface, may be present. △ Less

Submitted 27 May, 2024; originally announced May 2024.

Journal ref: A&A 689, A82 (2024)

Magnetization dynamics driven by displacement currents across a magnetic tunnel junction

Authors: C. K. Safeer, Paul S. Keatley, Witold Skowroński, Jakub Mojsiejuk, Kay Yakushiji, Akio Fukushima, Shinji Yuasa, Daniel Bedau, Fèlix Casanova, Luis E. Hueso, Robert J. Hicken, Daniele Pinna, Gerrit van der Laan, Thorsten Hesjedal

Abstract: Understanding the high-frequency transport characteristics of magnetic tunnel junctions (MTJs) is crucial for the development of fast-operating spintronics memories and radio frequency devices. Here, we present the study of frequency-dependent capacitive current effect in CoFeB/MgO-based MTJs and its influence on magnetization dynamics using time-resolved magneto-optical Kerr effect technique. In… ▽ More Understanding the high-frequency transport characteristics of magnetic tunnel junctions (MTJs) is crucial for the development of fast-operating spintronics memories and radio frequency devices. Here, we present the study of frequency-dependent capacitive current effect in CoFeB/MgO-based MTJs and its influence on magnetization dynamics using time-resolved magneto-optical Kerr effect technique. In our device operating at gigahertz frequencies, we find a large displacement current of the order of mA's, which does not break the tunnel barrier of the MTJ. Importantly, this current generates an Oersted field and spin-orbit torque, inducing magnetization dynamics. Our discovery holds promise for building robust MTJ devices operating under high current conditions, also highlighting the significance of capacitive impedance in high frequency magnetotransport techniques. △ Less

Submitted 9 May, 2024; originally announced May 2024.

Journal ref: Phys. Rev. Applied 22, 024019 (2024)

High temperature spin selectivity in a quantum dot qubit using reservoir spin accumulation

Authors: R. Jansen, S. Yuasa

Abstract: Employing spins in quantum dots for fault-tolerant quantum computing in large-scale qubit arrays with on-chip control electronics requires high-fidelity qubit operation at elevated temperature. This poses a challenge for single spin initialization and readout. Existing schemes rely on Zeeman splitting or Pauli spin blockade with typical energy scales of 0.1 or 1 meV for electron-based qubits, so t… ▽ More Employing spins in quantum dots for fault-tolerant quantum computing in large-scale qubit arrays with on-chip control electronics requires high-fidelity qubit operation at elevated temperature. This poses a challenge for single spin initialization and readout. Existing schemes rely on Zeeman splitting or Pauli spin blockade with typical energy scales of 0.1 or 1 meV for electron-based qubits, so that sufficient fidelity is obtained only at temperatures around or below 0.1 or 1 K, respectively. Here we describe a method to achieve high temperature spin selectivity in a quantum dot using a reservoir with a spin accumulation, which deterministically sets the spin of a single electron on the dot. Since spin accumulation as large as 10 meV is achievable in silicon, spin selection with electrically adjustable error rates below $10^{-4}$ is possible even in a liquid He bath at 4 K. Via the reservoir spin accumulation, induced and controlled by a nearby ferromagnet, classical information (magnetization direction) is mapped onto a spin qubit. These features provide the prospect of spin qubit operation at elevated temperatures and connect the worlds of quantum computing and spintronics. △ Less

Submitted 29 March, 2024; originally announced March 2024.

Comments: Supplementary material available via the journal reference

Journal ref: npj Quantum Information 10, 21 (2024)

Interfacial Fe segregation and its influence on magnetic properties of CoFeB/MgFeO multilayers

Authors: Tomohiro Ichinose, Tatsuya Yamamoto, Takayuki Nozaki, Kay Yakushiji, Shingo Tamaru, Shinji Yuasa

Abstract: We investigated the effect of Fe segregated from partially Fe-substituted MgO (MgFeO) on the magnetic properties of CoFeB/MgFeO multilayers. X-ray photoelectron spectroscopy (XPS) as well as magnetic measurements revealed that the segregated Fe was reduced to metal and exhibited ferromagnetism at the CoFeB/MgFeO interface. The CoFeB/MgFeO multilayer showed more than 2-fold enhancement in perpendic… ▽ More We investigated the effect of Fe segregated from partially Fe-substituted MgO (MgFeO) on the magnetic properties of CoFeB/MgFeO multilayers. X-ray photoelectron spectroscopy (XPS) as well as magnetic measurements revealed that the segregated Fe was reduced to metal and exhibited ferromagnetism at the CoFeB/MgFeO interface. The CoFeB/MgFeO multilayer showed more than 2-fold enhancement in perpendicular magnetic anisotropy (PMA) energy density compared with a standard CoFeB/MgO multilayer. The PMA energy density was further enhanced by inserting an ultrathin MgO layer in between CoFeB and MgFeO layers. Ferromagnetic resonance measurement also revealed a remarkable reduction of magnetic damping in the CoFeB/MgFeO multilayers. △ Less

Submitted 17 August, 2023; originally announced August 2023.

Comments: 15 pages, 5 figures

Towards mutual synchronization of serially connected Spin Torque Oscillators based on magnetic tunnel junctions

Authors: Piotr Rzeszut, Jakub Mojsiejuk, Witold Skowroński, Sumito Tsunegi, Kay Yakushiji, Hitoshi Kubota, Shinji Yuasa

Abstract: Multiple neuromorphic applications require the tuning of two or more devices to a common signal. Various types of neuromorphic computation can be realized using spintronic oscillators, where the DC current induces magnetization precession, which turns into an AC voltage generator. However, in spintronics, synchronization of two oscillators using a DC signal is still a challenging problem because i… ▽ More Multiple neuromorphic applications require the tuning of two or more devices to a common signal. Various types of neuromorphic computation can be realized using spintronic oscillators, where the DC current induces magnetization precession, which turns into an AC voltage generator. However, in spintronics, synchronization of two oscillators using a DC signal is still a challenging problem because it requires a certain degree of similarity between devices that are to be synchronized, which may be difficult to achieve due to device parameter distribution during the fabrication process. In this work, we present experimental results on the mechanisms of synchronization of spin-torque oscillators. Devices are based on magnetic tunnel junction with a perpendicularly magnetized free layer and take advantage of a uniform magnetization precision in the presence of the magnetic field and a DC bias. By using an external microwave source, we show the optimal condition for the synchronization of the magnetic tunnel junctions. Finally, we present results on the in-series connection of two junctions and discuss the possible path towards improving oscillation power and linewidth. In addition, using numerical simulations of the coupled oscillators model, we aim to reproduce the conditions of the experiments and determine the tolerance for achieving synchronization. △ Less

Submitted 14 February, 2025; v1 submitted 20 June, 2023; originally announced June 2023.

Substantial reduction of write-error rate for voltage-controlled magnetoresistive random access memory by in-plane demagnetizing field and voltage-induced negative out-of-plane anisotropy field

Authors: Rie Matsumoto, Shiniji Yuasa, Hiroshi Imamura

Abstract: Voltage-controlled magnetoresistive random access memory (VC-MRAM) based on voltage-induced dynamic switching in magnetic tunnel junctions (MTJs) is a promising ultimate non-volatile memory with ultralow power consumption. However, the dynamic switching in a conventional MTJ is accompanied by a relatively high write error rate (WER), hindering the reliable operation of VC-MRAM. Here, we propose a… ▽ More Voltage-controlled magnetoresistive random access memory (VC-MRAM) based on voltage-induced dynamic switching in magnetic tunnel junctions (MTJs) is a promising ultimate non-volatile memory with ultralow power consumption. However, the dynamic switching in a conventional MTJ is accompanied by a relatively high write error rate (WER), hindering the reliable operation of VC-MRAM. Here, we propose a reliable writing scheme using the in-plane demagnetizing field (IDF) and voltage-induced negative out-of-plane anisotropy field (NOAF). Numerical simulations based on macrospin model demonstrate that the voltage-induced NOAF modifies the switching dynamics and increases the torque due to the IDF, thereby reducing the switching time. The IDF and voltage-induced NOAF also reduce the mean energy difference between the magnetization direction at the end of the pulse and the equilibrium direction. As a result, an appropriate combination of the IDF and voltage-induced NOAF reduces the WER by one order of magnitude compared with that of the dynamic switching in a conventional MTJ. △ Less

Submitted 16 May, 2023; originally announced May 2023.

Cryogenic temperature deposition of high-performance CoFeB/MgO/CoFeB magnetic tunnel junctions on 300 mm wafers

Authors: Tomohiro Ichinose, Tatsuya Yamamoto, Takayuki Nozaki, Kay Yakushiji, Shingo Tamaru, Makoto Konoto, Shinji Yuasa

Abstract: We developed a cryogenic temperature deposition process for high-performance CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) on 300 mm thermally oxidized silicon wafers. The effect of the deposition temperature of the CoFeB layers on the nanostructure, magnetic and magneto-transport properties of the MTJs were investigated in detail. When CoFeB was deposited at 100 K, the MTJs exhibited a perpend… ▽ More We developed a cryogenic temperature deposition process for high-performance CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) on 300 mm thermally oxidized silicon wafers. The effect of the deposition temperature of the CoFeB layers on the nanostructure, magnetic and magneto-transport properties of the MTJs were investigated in detail. When CoFeB was deposited at 100 K, the MTJs exhibited a perpendicular magnetic anisotropy (PMA) of 214 uJ/m2 and a voltage-controlled magnetic anisotropy (VCMA) coefficient of -45 fJ/Vm, corresponding to 1.4- and 1.7-fold enhancements in PMA and VCMA, respectively, compared to the case of room-temperature deposition of CoFeB. The improvement in the MTJ properties was not simply due to the morphology of the MTJ films. The interface-sensitive magneto-transport properties indicated that interfacial qualities such as intermixing and oxidation states at the MgO/CoFeB interfaces were improved by the cryogenic temperature deposition. Cryogenic-temperature sputtering deposition is expected to be a standard manufacturing process for next-generation magnetoresistive random-access memory. △ Less

Submitted 12 January, 2023; originally announced January 2023.

Comments: Submitted to ACS Applied Materials and Interfaces

Heavily Damped Precessional Switching with Very Low Write-error Rate in Elliptical-cylinder Magnetic Tunnel Junction

Authors: Rie Matsumoto, Shinji Yuasa, Hiroshi Imamura

Abstract: Voltage-induced dynamic switching in magnetic tunnel junctions (MTJs) is a writing technique for voltage-controlled magnetoresistive random access memory (VCMRAM), which is expected to be an ultimate non-volatile memory with ultra-low power consumption. In conventional dynamic switching, the width of sub-nanosecond write voltage pulses must be precisely controlled to achieve a sufficiently low wri… ▽ More Voltage-induced dynamic switching in magnetic tunnel junctions (MTJs) is a writing technique for voltage-controlled magnetoresistive random access memory (VCMRAM), which is expected to be an ultimate non-volatile memory with ultra-low power consumption. In conventional dynamic switching, the width of sub-nanosecond write voltage pulses must be precisely controlled to achieve a sufficiently low write-error rate (WER). This very narrow tolerance of pulse width is the biggest technical difficulty in developing VCMRAM. Heavily damped precessional switching is a writing scheme for VCMRAM with a substantially high tolerance of pulse width although the minimum WER has been much higher than that of conventional dynamic switching with an optimum pulse width. In this study, we theoretically investigate the effect of MTJ shape and the direction of the applied magnetic field on the WER of heavily damped precessional switching. The results show that the WER in elliptical-cylinder MTJ can be several orders of magnitude smaller than that in usual circular-cylinder MTJ when the external magnetic field is applied parallel to the minor axis of the ellipse. The reduction in WER is due to the fact that the demagnetization field narrows the component of the magnetization distribution perpendicular to the plane direction immediately before the voltage is applied. △ Less

Submitted 14 November, 2022; originally announced November 2022.

The origin of enhanced interfacial perpendicular magnetic anisotropy in LiF-inserted Fe/MgO interface

Authors: Shoya Sakamoto, Takayuki Nozaki, Shinji Yuasa, Kenta Amemiya, Shinji Miwa

Abstract: The Fe/MgO interface is an essential ingredient in spintronics as it shows giant tunneling magnetoresistance and strong perpendicular magnetic anisotropy (PMA). A recent study demonstrated that the insertion of an ultra-thin LiF layer between the Fe and MgO layers enhances PMA significantly. In this study, we perform x-ray magnetic circular dichroism measurements on Fe/LiF/MgO multilayers to revea… ▽ More The Fe/MgO interface is an essential ingredient in spintronics as it shows giant tunneling magnetoresistance and strong perpendicular magnetic anisotropy (PMA). A recent study demonstrated that the insertion of an ultra-thin LiF layer between the Fe and MgO layers enhances PMA significantly. In this study, we perform x-ray magnetic circular dichroism measurements on Fe/LiF/MgO multilayers to reveal the origin of the PMA enhancement. We find that the LiF insertion increases the orbital-magnetic-moment anisotropy and thus the magnetic anisotropy energy. We attribute the origin of this orbital-magnetic-moment-anisotropy enhancement to the stronger electron localization and electron-electron correlation or the better interface quality with fewer defects. △ Less

Submitted 15 August, 2022; originally announced August 2022.

Electric-Field-Induced Coherent Control of Nitrogen Vacancy Centers

Authors: Gerald Q. Yan, Senlei Li, Tatsuya Yamamoto, Mengqi Huang, Nathan McLaughlin, Takayuki Nozaki, Hailong Wang, Shinji Yuasa, Chunhui Rita Du

Abstract: Enabling scalable and energy-efficient control of spin defects in solid-state media is desirable for realizing transformative quantum information technologies. Exploiting voltage-controlled magnetic anisotropy, we report coherent manipulation of nitrogen-vacancy (NV) centers by the spatially confined magnetic stray fields produced by a proximate resonant magnetic tunnel junction (MTJ). Remarkably,… ▽ More Enabling scalable and energy-efficient control of spin defects in solid-state media is desirable for realizing transformative quantum information technologies. Exploiting voltage-controlled magnetic anisotropy, we report coherent manipulation of nitrogen-vacancy (NV) centers by the spatially confined magnetic stray fields produced by a proximate resonant magnetic tunnel junction (MTJ). Remarkably, the coherent coupling between NV centers and the MTJ can be systematically controlled by a DC bias voltage, allowing for appreciable electrical tunability in the presented hybrid system. In comparison with current state-of-the-art techniques, the demonstrated NV-based quantum operational platform exhibits significant advantages in scalability, device compatibility, and energy-efficiency, further expanding the role of NV centers in a broad range of quantum computing, sensing, and communications applications. △ Less

Submitted 21 June, 2022; originally announced June 2022.

Comments: 13 pages, 4 figures

Journal ref: Phys. Rev. Applied 18, 064031 (2022)

Spin-torque Dynamics for Noise Reduction in Vortex-based Sensors

Authors: Mafalda Jotta Garcia, Julien Moulin, Steffen Wittrock, Sumito Tsunegi, Kay Yakushiji, Akio Fukushima, Hitoshi Kubota, Shinji Yuasa, Ursula Ebels, Myriam Pannetier-Lecoeur, Claude Fermon, Romain Lebrun, Paolo Bortolotti, Aurélie Solignac, Vincent Cros

Abstract: 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 sta… ▽ More 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. △ Less

Submitted 17 December, 2020; originally announced December 2020.

Influence of flicker noise and nonlinearity on the frequency spectrum of spin torque nano-oscillators

Authors: Steffen Wittrock, Philippe Talatchian, Sumito Tsunegi, Denis Crété, Kay Yakushiji, Paolo Bortolotti, Ursula Ebels, Akio Fukushima, Hitoshi Kubota, Shinji Yuasa, Julie Grollier, Gilles Cibiel, Serge Galliou, Enrico Rubiola, Vincent Cros

Abstract: 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… ▽ More 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. △ Less

Submitted 29 January, 2020; originally announced January 2020.

Binding events through the mutual synchronization of spintronic nano-neurons

Authors: Miguel Romera, Philippe Talatchian, Sumito Tsunegi, Kay Yakushiji, Akio Fukushima, Hitoshi Kubota, Shinji Yuasa, Vincent Cros, Paolo Bortolotti, Maxence Ernoult, Damien Querlioz, Julie Grollier

Abstract: 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 oscill… ▽ More 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. △ Less

Submitted 22 January, 2020; originally announced January 2020.

Microwave magnetic field modulation of spin torque oscillator based on perpendicular magnetic tunnel junctions

Authors: Witold Skowroński, Jakub Chęciński, Sławomir Ziętek, Kay Yakushiji, Shinji Yuasa

Abstract: Modulation of a spin-torque oscillator (STO) signal based on a magnetic tunnel junction (MTJ) with perpendicularly magnetized free layer is investigated. Magnetic field inductive loop was created during MTJ fabrication process, which enables microwave field application during STO operation. The frequency modulation by the microwave magnetic field of up to 3 GHz is explored, showing a potential for… ▽ More Modulation of a spin-torque oscillator (STO) signal based on a magnetic tunnel junction (MTJ) with perpendicularly magnetized free layer is investigated. Magnetic field inductive loop was created during MTJ fabrication process, which enables microwave field application during STO operation. The frequency modulation by the microwave magnetic field of up to 3 GHz is explored, showing a potential for application in high-data-rate communication technologies. Moreover, an inductive loop is used for self-synchronization of the STO signal, which after field-locking exhibits significant improvement of the linewidth and oscillation power. △ Less

Submitted 20 September, 2019; originally announced September 2019.

Comments: 4 pages, 5 figures

Low offset frequency $1/f$ flicker noise in spin torque vortex oscillators

Authors: Steffen Wittrock, Sumito Tsunegi, Kay Yakushiji, Akio Fukushima, Hitoshi Kubota, Paolo Bortolotti, Ursula Ebels, Shinji Yuasa, Gilles Cibiel, Serge Galliou, Enrico Rubiola, Vincent Cros

Abstract: 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 TMR-based spin-torque vortex oscillator in the regime of large amplitude steady oscillations. We first phenomenologically expand the nonlinear auto-o… ▽ More 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 TMR-based 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. Hereby, 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 non-constant magnetic oscillation volume, which contributes to the magnetoresistance. △ Less

Submitted 26 June, 2019; v1 submitted 24 June, 2019; originally announced June 2019.

Comments: 10 pages

Journal ref: Phys. Rev. B 99, 235135 (2019)

Tunneling Magnetoresistance and Spin-Dependent Diode Performance in Fully Epitaxial Magnetic Tunnel Junctions with Rock-salt Type ZnO/MgO

Authors: Hidekazu Saito, Sai Krishna Narayananellore, Norihiro Matsuo, Naoki Doko, Shintaro Kon, Yukiko Yasukawa, Hiroshi Imamura, Shinji Yuasa

Abstract: We fabricate fully epitaxial Fe/ZnO/MgO/Fe magnetic tunnel junctions (MTJs) with a bilayer tunnel barrier, in which ZnO has a metastable rock-salt crystal structure. We observe a high magnetoresistance ratio up to 96% at room temperature (RT) and find that these MTJs have asymmetric current-voltage characteristics, and their rectifying performances are largely dependent on the magnetization alignm… ▽ More We fabricate fully epitaxial Fe/ZnO/MgO/Fe magnetic tunnel junctions (MTJs) with a bilayer tunnel barrier, in which ZnO has a metastable rock-salt crystal structure. We observe a high magnetoresistance ratio up to 96% at room temperature (RT) and find that these MTJs have asymmetric current-voltage characteristics, and their rectifying performances are largely dependent on the magnetization alignments of the Fe electrodes. Diode responsibilities at a zero-bias voltage ($β_{0}$), which is an important performance index for harvesting applications, are observed up to 1.3 A/W at RT in the antiparallel alignment of the magnetizations while maintaining rather low resistance-area (RA) products (a few tens of k${Ωμ}$m$^2$). Even with the same top and bottom electrodes (Fe), the obtained $β_{0}$ values are comparable to those of reported high-performance tunnel diodes consisting of amorphous bilayer tunnel barriers with polycrystalline dissimilar electrodes. This strongly suggests that the epitaxial ZnO/MgO bilayer tunnel barrier is effective for enhancing the $β_{0}$ without significant increase in the RA. In addition, we demonstrate that a zero-bias anomaly in thetunnel conductance, which originates from the magnon excitations at the Fe/barrier interfaces, plays a crucial role in observed spin-dependent diode performance. The results indicate that a fully epitaxial MTJ with a bilayer tunnel barrier is a promising candidate to establish a high-performance high-frequency rectifying system. △ Less

Submitted 13 June, 2019; originally announced June 2019.

Comments: 34 pages, 7 figures

Journal ref: Phys. Rev. Applied 11, 064032 (2019)

Role of non-linear data processing on speech recognition task in the framework of reservoir computing

Authors: Flavio Abreu Araujo, Mathieu Riou, Jacob Torrejon, Sumito Tsunegi, Damien Querlioz, Kay Yakushiji, Akio Fukushima, Hitoshi Kubota, Shinji Yuasa, Mark D. Stiles, Julie Grollier

Abstract: 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. However, 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 th… ▽ More 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. However, 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 may 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 benchmark 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. △ Less

Submitted 19 December, 2019; v1 submitted 10 May, 2019; originally announced June 2019.

Comments: 13 pages, 5 figures

Journal ref: Scientific Reports 10, 328 (2020)

High frequency voltage-induced ferromagnetic resonance in magnetic tunnel junctions

Authors: Witold Skowronski, Stanislaw Lazarski, Jakub Mojsiejuk, Jakub Checinski, Marek Frankowski, Takayuki Nozaki, Kay Yakushiji, Shinji Yuasa

Abstract: Voltage-induced ferromagnetic resonance (V-FMR) in magnetic tunnel junctions (MTJs) with a W buffer is investigated. Perpendicular magnetic anisotropy (PMA) energy is controlled by both thickness of a CoFeB free layer deposited directly on the W buffer and a post-annealing process at different temperatures. The PMA energy as well as the magnetization damping are determined by analysing field-depen… ▽ More Voltage-induced ferromagnetic resonance (V-FMR) in magnetic tunnel junctions (MTJs) with a W buffer is investigated. Perpendicular magnetic anisotropy (PMA) energy is controlled by both thickness of a CoFeB free layer deposited directly on the W buffer and a post-annealing process at different temperatures. The PMA energy as well as the magnetization damping are determined by analysing field-dependent FMR signals in different field geometries. An optimized MTJ structure enabled excitation of V-FMR at frequencies exceeding 30 GHz. The macrospin modelling is used to analyse the field- and angular-dependence of the V-FMR signal and to support experimental magnetization damping extraction. △ Less

Submitted 4 June, 2019; originally announced June 2019.

Comments: Manuscript containing 4 pages and 4 figures

Deterministic magnetization switching by voltage-control of magnetic anisotropy and Dzyaloshinskii-Moriya interaction under in-plane magnetic field

Authors: Hiroshi Imamura, Takayuki Nozaki, Shinji Yuasa, Yoshishige Suzuki

Abstract: Based on the micromagnetic simulations the magnetization switching in a triangle magnetic element by voltage-control of magnetic anisotropy and Dzyaloshinskii-Moriya interaction under in-plane magnetic field is proposed. The proposed switching scheme is not the toggle switching but the deterministic switching where the magnetic state is determined by the polarity of the applied voltage pulse. The… ▽ More Based on the micromagnetic simulations the magnetization switching in a triangle magnetic element by voltage-control of magnetic anisotropy and Dzyaloshinskii-Moriya interaction under in-plane magnetic field is proposed. The proposed switching scheme is not the toggle switching but the deterministic switching where the magnetic state is determined by the polarity of the applied voltage pulse. The mechanism and conditions for the switching are clarified. The results provide a fast and low-power writing method for magnetoresistive random access memories. △ Less

Submitted 4 June, 2019; originally announced June 2019.

Comments: 6 pages, 6 figures

Journal ref: Phys. Rev. Applied 10, 054039 (2018)

Temporal pattern recognition with delayed feedback spin-torque nano-oscillators

Authors: M. Riou, J. Torrejon, B. Garitaine, F. Abreu Araujo, P. Bortolotti, V. Cros, S. Tsunegi, K. Yakushiji, A. Fukushima, H. Kubota, S. Yuasa, D. Querlioz, M. D. Stiles, J. Grollier

Abstract: 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. In this study, we extend the memory of the spin-torque nano-oscillators through time-delayed feedback. We leverage this extrinsic memory to increase the efficiency… ▽ More 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. In this study, 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 non-linear oscillators allows us to control and optimize the delayed feedback memory using different operating conditions of applied current and magnetic field. △ Less

Submitted 7 May, 2019; originally announced May 2019.

Journal ref: Phys. Rev. Applied 12, 024049 (2019)

Microwave neural processing and broadcasting with spintronic nano-oscillators

Authors: P. Talatchian, M. Romera, S. Tsunegi, F. Abreu Araujo, V. Cros, P. Bortolotti, J. Trastoy, K. Yakushiji, A. Fukushima, H. Kubota, S. Yuasa, M. Ernoult, D. Vodenicarevic, T. Hirtzlin, N. Locatelli, D. Querlioz, J. Grollier

Abstract: Can we build small neuromorphic chips capable of training deep networks with billions of parameters? This challenge requires hardware neurons and synapses with nanometric dimensions, which can be individually tuned, and densely connected. While nanosynaptic devices have been pursued actively in recent years, much less has been done on nanoscale artificial neurons. In this paper, we show that spint… ▽ More Can we build small neuromorphic chips capable of training deep networks with billions of parameters? This challenge requires hardware neurons and synapses with nanometric dimensions, which can be individually tuned, and densely connected. While nanosynaptic devices have been pursued actively in recent years, much less has been done on nanoscale artificial neurons. In this paper, we show that spintronic nano-oscillators are promising to implement analog hardware neurons that can be densely interconnected through electromagnetic signals. We show how spintronic oscillators maps the requirements of artificial neurons. We then show experimentally how an ensemble of four coupled oscillators can learn to classify all twelve American vowels, realizing the most complicated tasks performed by nanoscale neurons. △ Less

Submitted 25 April, 2019; originally announced April 2019.

Neuromorphic Computing through Time-Multiplexing with a Spin-Torque Nano-Oscillator

Authors: M. Riou, F. Abreu Araujo, J. Torrejon, S. Tsunegi, G. Khalsa, D. Querlioz, P. Bortolotti, V. Cros, K. Yakushiji, A. Fukushima, H. Kubota, S. Yuasa, M. D. Stiles, J. Grollier

Abstract: Fabricating powerful neuromorphic chips the size of a thumb requires miniaturizing their basic units: synapses and neurons. The challenge for neurons is to scale them down to submicrometer diameters while maintaining the properties that allow for reliable information processing: high signal to noise ratio, endurance, stability, reproducibility. In this work, we show that compact spin-torque nano-o… ▽ More Fabricating powerful neuromorphic chips the size of a thumb requires miniaturizing their basic units: synapses and neurons. The challenge for neurons is to scale them down to submicrometer diameters while maintaining the properties that allow for reliable information processing: high signal to noise ratio, endurance, stability, reproducibility. In this work, we show that compact spin-torque nano-oscillators can naturally implement such neurons, and quantify their ability to realize an actual cognitive task. In particular, we show that they can naturally implement reservoir computing with high performance and detail the recipes for this capability. △ Less

Submitted 25 April, 2019; originally announced April 2019.

Symmetry and polarity of the voltage-controlled magnetic anisotropy studied by the Anomalous Hall effect

Authors: Vadym Zayets, Takayuki Nozaki, Hidekazu Saito, Akio Fukushima, Shinji Yuasa

Abstract: The voltage-controlled magnetic anisotropy (VCMA) effect in FeB and FeB/W films was measured by four independent methods. All measurements are consistent and show the same tendency. The coercive field, Hall angle, anisotropy field, the magnetization switching time and retention time linearly decrease when the gate voltage increases and they linearly increase when the gate voltage decreases. The voltage-controlled magnetic anisotropy (VCMA) effect in FeB and FeB/W films was measured by four independent methods. All measurements are consistent and show the same tendency. The coercive field, Hall angle, anisotropy field, the magnetization switching time and retention time linearly decrease when the gate voltage increases and they linearly increase when the gate voltage decreases. △ Less

Submitted 17 December, 2018; originally announced December 2018.

Comments: 5 pages

Reservoir computing with the frequency, phase and amplitude of spin-torque nano-oscillators

Authors: Danijela Marković, Nathan Leroux, Mathieu Riou, Flavio Abreu Araujo, Jacob Torrejon, Damien Querlioz, Akio Fukushima, Shinji Yuasa, Juan Trastoy, Paolo Bortolotti, Julie Grollier

Abstract: Spin-torque nano-oscillators can emulate neurons at the nanoscale. Recent works show that the non-linearity of their oscillation amplitude can be leveraged to achieve waveform classification for an input signal encoded in the amplitude of the input voltage. Here we show that the frequency and the phase of the oscillator can also be used to recognize waveforms. For this purpose, we phase-lock the o… ▽ More Spin-torque nano-oscillators can emulate neurons at the nanoscale. Recent works show that the non-linearity of their oscillation amplitude can be leveraged to achieve waveform classification for an input signal encoded in the amplitude of the input voltage. Here we show that the frequency and the phase of the oscillator can also be used to recognize waveforms. For this purpose, we phase-lock the oscillator to the input waveform, which carries information in its modulated frequency. In this way we considerably decrease amplitude, phase and frequency noise. We show that this method allows classifying sine and square waveforms with an accuracy above 99% when decoding the output from the oscillator amplitude, phase or frequency. We find that recognition rates are directly related to the noise and non-linearity of each variable. These results prove that spin-torque nano-oscillators offer an interesting platform to implement different computing schemes leveraging their rich dynamical features. △ Less

Submitted 1 November, 2018; originally announced November 2018.

Nonlinear Electrical Spin Conversion in a Biased Ferromagnetic Tunnel Contact

Authors: R. Jansen, A. Spiesser, H. Saito, Y. Fujita, S. Yamada, K. Hamaya, S. Yuasa

Abstract: The conversion of spin information into electrical signals is indispensable for spintronic technologies. Spin-to-charge conversion in ferromagnetic tunnel contacts is well-described using linear (spin-)transport equations, provided that there is no applied bias, as in nonlocal spin detection. It is shown here that in a biased ferromagnetic tunnel contact, spin detection is strongly nonlinear. As a… ▽ More The conversion of spin information into electrical signals is indispensable for spintronic technologies. Spin-to-charge conversion in ferromagnetic tunnel contacts is well-described using linear (spin-)transport equations, provided that there is no applied bias, as in nonlocal spin detection. It is shown here that in a biased ferromagnetic tunnel contact, spin detection is strongly nonlinear. As a result, the spin-detection efficiency is not equal to the tunnel spin polarization. In silicon-based 4-terminal spin-transport devices, even a small bias (tens of mV) across the Fe/MgO detector contact enhances the spin-detection efficiency to values up to 140 \% (spin extraction bias) or, for spin injection bias, reduces it to almost zero, while, parenthetically, the charge current remains highly spin polarized. Calculations reveal that the nonlinearity originates from the energy dispersion of the tunnel transmission and the resulting nonuniform energy distribution of the tunnel current, offering a route to engineer spin conversion. Taking nonlinear spin detection into account is also shown to explain a multitude of peculiar and puzzling spin signals in structures with a biased detector, including two- and three-terminal devices, and provides a unified, consistent and quantitative description of spin signals in devices with a biased and unbiased detector. △ Less

Submitted 26 October, 2018; originally announced October 2018.

Comments: 13 pages, 7 figures

Tunnel magnetoresistance angular and bias dependence enabling tuneable wireless communication

Authors: Ewa Kowalska, Akio Fukushima, Volker Sluka, Ciarán Fowley, Attila Kákay, Yuriy Aleksandrov, Jürgen Lindner, Jürgen Fassbender, Shinji Yuasa, Alina M. Deac

Abstract: Spin-transfer torques (STTs) can be exploited in order to manipulate the magnetic moments of nanomagnets, thus allowing for new consumer-oriented devices to be designed. Of particular interest here are tuneable radio-frequency (RF) oscillators for wireless communication. Currently, the structure that maximizes the output power is an Fe/MgO/Fe-type magnetic tunnel junction (MTJ) with a fixed layer… ▽ More Spin-transfer torques (STTs) can be exploited in order to manipulate the magnetic moments of nanomagnets, thus allowing for new consumer-oriented devices to be designed. Of particular interest here are tuneable radio-frequency (RF) oscillators for wireless communication. Currently, the structure that maximizes the output power is an Fe/MgO/Fe-type magnetic tunnel junction (MTJ) with a fixed layer magnetized in the plane of the layers and a free layer magnetized perpendicular to the plane. This structure allows for most of the tunnel magnetoresistance (TMR) to be converted into output power. Here, we experimentally and theoretically demonstrate that the main mechanism sustaining steady-state precession in such structures is the angular dependence of the magnetoresistance. The TMR of such devices is known to exhibit a broken-linear dependence versus the applied bias. Our results show that the TMR bias dependence effectively quenches spin-transfer-driven precession and introduces a non-monotonic frequency dependence at high applied currents. Thus we expect the bias dependence of the TMR to have an even more dramatic effect in MTJs with Mn-Ga-based free layers, which could be used to design wireless oscillators extending towards the THz gap, but have been experimentally shown to exhibit a non-trivial TMR bias dependence. △ Less

Submitted 17 August, 2018; originally announced August 2018.

Comments: Manuscript: 13 pages, 5 figures; Supplementary Information: 6 pages, 5 figures. The Manuscript and Supplementary Information submitted to Scientific Reports

Journal ref: Scientific Reports 9, 9541 (2019)

Voltage-induced precessional switching at zero-bias magnetic field in a conically magnetized free layer

Authors: R. Matsumoto, T. Nozaki, S. Yuasa, H. Imamura

Abstract: Voltage-induced magnetization dynamics in a conically magnetized free layer with an elliptic cylinder shape is theoretically studied on the basis of the macrospin model. It is found that an application of voltage pulse can induce the precessional switching of magnetization even at zero-bias magnetic field, which is of substantial importance for device applications such as voltage-controlled nonvol… ▽ More Voltage-induced magnetization dynamics in a conically magnetized free layer with an elliptic cylinder shape is theoretically studied on the basis of the macrospin model. It is found that an application of voltage pulse can induce the precessional switching of magnetization even at zero-bias magnetic field, which is of substantial importance for device applications such as voltage-controlled nonvolatile memory. Analytical expressions of the conditions for precessional switching are derived. △ Less

Submitted 23 January, 2018; originally announced January 2018.

Journal ref: Physical Review APPLIED 9, 014026 (2018)

Vowel recognition with four coupled spin-torque nano-oscillators

Authors: Miguel Romera, Philippe Talatchian, Sumito Tsunegi, Flavio Abreu Araujo, Vincent Cros, Paolo Bortolotti, Juan Trastoy, Kay Yakushiji, Akio Fukushima, Hitoshi Kubota, Shinji Yuasa, Maxence Ernoult, Damir Vodenicarevic, Tifenn Hirtzlin, Nicolas Locatelli, Damien Querlioz, Julie Grollier

Abstract: Substantial evidence indicates that the brain uses principles of non-linear dynamics in neural processes, providing inspiration for computing with nanoelectronic devices. However, training neural networks composed of dynamical nanodevices requires finely controlling and tuning their coupled oscillations. In this work, we show that the outstanding tunability of spintronic nano-oscillators can solve… ▽ More Substantial evidence indicates that the brain uses principles of non-linear dynamics in neural processes, providing inspiration for computing with nanoelectronic devices. However, training neural networks composed of dynamical nanodevices requires finely controlling and tuning their coupled oscillations. In this work, we show that the outstanding tunability of spintronic nano-oscillators can solve this challenge. We successfully train a hardware network of four spin-torque nano-oscillators to recognize spoken vowels by tuning their frequencies according to an automatic real-time learning rule. We show that the high experimental recognition rates stem from the high frequency tunability of the oscillators and their mutual coupling. Our results demonstrate that non-trivial pattern classification tasks can be achieved with small hardware neural networks by endowing them with non-linear dynamical features: here, oscillations and synchronization. This demonstration is a milestone for spintronics-based neuromorphic computing. △ Less

Submitted 18 October, 2018; v1 submitted 8 November, 2017; originally announced November 2017.

Integer, fractional and side band injection locking of spintronic feedback nano-oscillator to microwave signal

Authors: Hanuman Singh, K. Konishi, S. Bhuktare, A. Bose, S. Miwa, A. Fukushima, K. Yakushiji, S. Yuasa, H. Kubota, Y. Suzuki, A. A. Tulapurkar

Abstract: In this article we demonstrate the injection locking of recently demonstrated spintronic feedback nano oscillator to microwave magnetic fields at integers as well fractional multiples of its auto oscillation frequency. Feedback oscillators have delay as a new degree of freedom which is absent for spin-transfer torque based oscillators, which gives rise to side peaks along with a main peak. We show… ▽ More In this article we demonstrate the injection locking of recently demonstrated spintronic feedback nano oscillator to microwave magnetic fields at integers as well fractional multiples of its auto oscillation frequency. Feedback oscillators have delay as a new degree of freedom which is absent for spin-transfer torque based oscillators, which gives rise to side peaks along with a main peak. We show that it is also possible to lock the oscillator on its side band peaks, which opens a new avenue to phase locked oscillators with large frequency differences. We observe that for low driving fields, side band locking improves the quality factor of the main peak, whereas for higher driving fields the main peak is suppressed. Further, measurements at two field angles provide some insight into the role of symmetry of oscillation orbit in determining the fractional locking. △ Less

Submitted 2 November, 2017; originally announced November 2017.

Comments: 16 pages, 6 figure(Manuscript), 3 figure( supplementary information),Accepted ( Physical Review Applied)

Journal ref: Phys. Rev. Applied 8, 064011 (2017)

Giant spin accumulation in silicon nonlocal spin-transport devices

Authors: A. Spiesser, H. Saito, Y. Fujita, S. Yamada, K. Hamaya, S. Yuasa, R. Jansen

Abstract: Although the electrical injection, transport and detection of spins in silicon have been achieved, the induced spin accumulation was much smaller than expected and desired, limiting the potential impact of Si-based spintronic devices. Here, using non-local spin-transport devices with an n-type Si channel and Fe/MgO magnetic tunnel contacts, we demonstrate that it is possible to create a giant spin… ▽ More Although the electrical injection, transport and detection of spins in silicon have been achieved, the induced spin accumulation was much smaller than expected and desired, limiting the potential impact of Si-based spintronic devices. Here, using non-local spin-transport devices with an n-type Si channel and Fe/MgO magnetic tunnel contacts, we demonstrate that it is possible to create a giant spin accumulation in Si, with the spin splitting reaching 13 meV at 10 K and 3.5 meV at room temperature. The non-local spin signals are in good agreement with a numerical evaluation of spin injection and diffusion that explicitly takes the size of the injector contact into account. The giant spin accumulation originates from the large tunnel spin polarization of the Fe/MgO contacts (53 % at 10 K and 18 % at 300 K), and the spin density enhancement achieved by using a spin injector with a size comparable to the spin-diffusion length of the Si. The ability to induce a giant spin accumulation enables the development of Si spintronic devices with a large magnetic response. △ Less

Submitted 21 October, 2017; originally announced October 2017.

Journal ref: Phys. Rev. Applied 8, 064023 (2017)

Low-Energy Truly Random Number Generation with Superparamagnetic Tunnel Junctions for Unconventional Computing

Authors: Damir Vodenicarevic, Nicolas Locatelli, Alice Mizrahi, Joseph S. Friedman, Adrien F. Vincent, Miguel Romera, Akio Fukushima, Kay Yakushiji, Hitoshi Kubota, Shinji Yuasa, Sandip Tiwari, Julie Grollier, Damien Querlioz

Abstract: Low-energy random number generation is critical for many emerging computing schemes proposed to complement or replace von Neumann architectures. However, current random number generators are always associated with an energy cost that is prohibitive for these computing schemes. In this paper, we introduce random number bit generation based on specific nanodevices: superparamagnetic tunnel junctions… ▽ More Low-energy random number generation is critical for many emerging computing schemes proposed to complement or replace von Neumann architectures. However, current random number generators are always associated with an energy cost that is prohibitive for these computing schemes. In this paper, we introduce random number bit generation based on specific nanodevices: superparamagnetic tunnel junctions. We experimentally demonstrate high quality random bit generation that represents orders-of-magnitude improvements in energy efficiency compared to current solutions. We show that the random generation speed improves with nanodevice scaling, and investigate the impact of temperature, magnetic field and crosstalk. Finally, we show how alternative computing schemes can be implemented using superparamagentic tunnel junctions as random number generators. These results open the way for fabricating efficient hardware computing devices leveraging stochasticity, and highlight a novel use for emerging nanodevices. △ Less

Submitted 24 November, 2017; v1 submitted 16 June, 2017; originally announced June 2017.

Comments: Supplementary Materials provided

Journal ref: Phys. Rev. Applied 8, 054045 (2017)

Neuromorphic computing with nanoscale spintronic oscillators

Authors: Jacob Torrejon, Mathieu Riou, Flavio Abreu Araujo, Sumito Tsunegi, Guru Khalsa, Damien Querlioz, Paolo Bortolotti, Vincent Cros, Akio Fukushima, Hitoshi Kubota, Shinji Yuasa, M. D. Stiles, Julie Grollier

Abstract: Neurons in the brain behave as non-linear oscillators, which develop rhythmic activity and interact to process information. Taking inspiration from this behavior to realize high density, low power neuromorphic computing will require huge numbers of nanoscale non-linear oscillators. Indeed, a simple estimation indicates that, in order to fit a hundred million oscillators organized in a two-dimensio… ▽ More Neurons in the brain behave as non-linear oscillators, which develop rhythmic activity and interact to process information. Taking inspiration from this behavior to realize high density, low power neuromorphic computing will require huge numbers of nanoscale non-linear oscillators. Indeed, a simple estimation indicates that, in order to fit a hundred million oscillators organized in a two-dimensional array inside a chip the size of a thumb, their lateral dimensions must be smaller than one micrometer. However, despite multiple theoretical proposals, there is no proof of concept today of neuromorphic computing with nano-oscillators. Indeed, nanoscale devices tend to be noisy and to lack the stability required to process data in a reliable way. Here, we show experimentally that a nanoscale spintronic oscillator can achieve spoken digit recognition with accuracies similar to state of the art neural networks. We pinpoint the regime of magnetization dynamics leading to highest performance. These results, combined with the exceptional ability of these spintronic oscillators to interact together, their long lifetime, and low energy consumption, open the path to fast, parallel, on-chip computation based on networks of oscillators. △ Less

Submitted 14 April, 2017; v1 submitted 25 January, 2017; originally announced January 2017.

Enhancement of perpendicular magnetic anisotropy and its electric field-induced change through interface engineering in Cr/Fe/MgO

Authors: A. Kozioł-Rachwał, T. Nozaki, K. Freindl, J. Korecki, S. Yuasa, Y. Suzuki

Abstract: Recently, perpendicular magnetic anisotropy (PMA) and its voltage control (VC) was demonstrated for Cr/Fe/MgO (Physical Review Applied 5, 044006 (2016)). In this study, we shed a light on the origin of large voltage-induced anisotropy change in Cr/Fe/MgO. Analysis of the chemical structure of Cr/Fe/MgO revealed the existence of Cr atoms in the proximity of the Fe/MgO interface, which can affect bo… ▽ More Recently, perpendicular magnetic anisotropy (PMA) and its voltage control (VC) was demonstrated for Cr/Fe/MgO (Physical Review Applied 5, 044006 (2016)). In this study, we shed a light on the origin of large voltage-induced anisotropy change in Cr/Fe/MgO. Analysis of the chemical structure of Cr/Fe/MgO revealed the existence of Cr atoms in the proximity of the Fe/MgO interface, which can affect both magnetic anisotropy (MA) and its VC. We showed that PMA and its VC can be enhanced by controlled Cr doping at the Fe/MgO interface. For Cr/Fe (5.9 Å)/Cr (0.7 Å)/MgO with an effective PMA of 0.8 MJ/m3, a maximum value of the voltage-controlled magnetic anisotropy (VCMA) effect of 370 fJ/Vm was demonstrated. △ Less

Submitted 30 December, 2016; originally announced January 2017.

Driven energy transfer between coupled modes in spin-torque oscillators

Authors: R. Lebrun, J. Grollier, P. Bortolotti, V. Cros, A. Hamadeh, X. de Milly, Y. Li, G. de Loubens, O. Klein, S. Tsunegi, H. Kubota, K. Yakushiji, A. Fukushima, S. Yuasa

Abstract: The mutual interaction between the different eigenmodes of a spin-torque oscillator can lead to a large variety of physical mechanisms from mode hopping to multi-mode generation, that usually reduce their performances as radio-frequency devices. To tackle this issue for the future applications, we investigate the properties of a model spin-torque oscillator that is composed of two coupled vortices… ▽ More The mutual interaction between the different eigenmodes of a spin-torque oscillator can lead to a large variety of physical mechanisms from mode hopping to multi-mode generation, that usually reduce their performances as radio-frequency devices. To tackle this issue for the future applications, we investigate the properties of a model spin-torque oscillator that is composed of two coupled vortices with one vortex in each of the two magnetic layers of the oscillator. In such double-vortex system, the remarkable properties of energy transfer between the coupled modes, one being excited by spin transfer torque while the second one being damped, result into an alteration of the damping parameters. As a consequence, the oscillator nonlinear behavior is concomitantly drastically impacted. This efficient coupling mechanism, driven mainly by the dynamic dipolar field generated by the spin transfer torque induced motion of the vortices, gives rise to an unexpected dynamical regime of self-resonance excitation. These results show that mode coupling can be leveraged for controlling the synchronization process as well as the frequency tunability of spin-torque oscillators. △ Less

Submitted 31 October, 2016; originally announced October 2016.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B 95, 134444 (2017)

Neural-like computing with populations of superparamagnetic basis functions

Authors: Alice Mizrahi, Tifenn Hirtzlin, Akio Fukushima, Hitoshi Kubota, Shinji Yuasa, Julie Grollier, Damien Querlioz

Abstract: In neuroscience, population coding theory demonstrates that neural assemblies can achieve fault-tolerant information processing. Mapped to nanoelectronics, this strategy could allow for reliable computing with scaled-down, noisy, imperfect devices. Doing so requires that the population components form a set of basis functions in terms of their response functions to inputs, offering a physical subs… ▽ More In neuroscience, population coding theory demonstrates that neural assemblies can achieve fault-tolerant information processing. Mapped to nanoelectronics, this strategy could allow for reliable computing with scaled-down, noisy, imperfect devices. Doing so requires that the population components form a set of basis functions in terms of their response functions to inputs, offering a physical substrate for calculating. For this purpose, the responses of the nanodevices should be non-linear, and each tuned to different values of the input. These strong requirements have prevented a demonstration of population coding with nanodevices. Here, we show that nanoscale magnetic tunnel junctions can be assembled to meet these requirements. We demonstrate experimentally that a population of nine junctions can implement a basis set of functions, providing the data to achieve, for example, the generation of cursive letters. We design hybrid magnetic-CMOS systems based on interlinked populations of junctions and show that they can learn to realize non-linear variability-resilient transformations with a low imprint area and low power. △ Less

Submitted 15 March, 2018; v1 submitted 27 October, 2016; originally announced October 2016.

Comments: 5 figures

Coherent microwave generation by spintronic feedback oscillator

Authors: Dinesh Kumar, K. Konishi, Nikhil Kumar, S. Miwa, A. Fukushima, K. Yakushiji, S. Yuasa, H. Kubota, C. V. Tomy, A. Prabhakar, Y. Suzuki, A. Tulapurkar

Abstract: The transfer of spin angular momentum to a nanomagnet from a spin polarized current provides an efficient means of controlling the magnetization direction in nanomagnets. A unique consequence of this spin torque is that the spontaneous oscillations of the magnetization can be induced by applying a combination of a dc bias current and a magnetic field. Here we experimentally demonstrate a different… ▽ More The transfer of spin angular momentum to a nanomagnet from a spin polarized current provides an efficient means of controlling the magnetization direction in nanomagnets. A unique consequence of this spin torque is that the spontaneous oscillations of the magnetization can be induced by applying a combination of a dc bias current and a magnetic field. Here we experimentally demonstrate a different effect, which can drive a nanomagnet into spontaneous oscillations without the need of external spin torque injection. For the demonstration of this effect, we use a nano-pillar of magnetic tunnel junction (MTJ) powered by a dc current and connected to a coplanar waveguide (CPW) lying above the free layer of the MTJ. Any fluctuation of the free layer magnetization is converted into oscillating voltage via the tunneling magneto-resistance effect and is fed back into the MTJ by the CPW through inductive coupling. As a result of this feedback, the magnetization of the free layer can be driven into a continual precession. The combination of MTJ and CPW behaves similar to a laser system and outputs a stable rf power with quality factor exceeding 10,000. △ Less

Submitted 12 August, 2016; originally announced August 2016.

Comments: 15 Pages, 6 Figures

Journal ref: Scientific Reports 6, 30747 (2016)

Inside the perpendicular spin-torque memristor

Authors: Steven Lequeux, Joao Sampaio, Vincent Cros, Kay Yakushiji, Akio Fukushima, Rie Matsumoto, Hitoshi Kubota, Shinji Yuasa, Julie Grollier

Abstract: Memristors are non-volatile nano-resistors. Their resistance can be tuned by applied currents or voltages and set to a large number of levels between two limit values. Thanks to these properties, memristors are ideal building blocks for a number of applications such as multilevel non-volatile memories and artificial nano-synapses, which are the focus of this work. A key point towards the developme… ▽ More Memristors are non-volatile nano-resistors. Their resistance can be tuned by applied currents or voltages and set to a large number of levels between two limit values. Thanks to these properties, memristors are ideal building blocks for a number of applications such as multilevel non-volatile memories and artificial nano-synapses, which are the focus of this work. A key point towards the development of large scale memristive neuromorphic hardware is to build these neural networks with a memristor technology compatible with the best candidates for the future mainstream non-volatile memories. Here we show the first experimental achievement of a memristor compatible with Spin-Torque Magnetic Random Access Memory. The resistive switching in our spin-torque memristor is linked to the displacement of a magnetic domain wall by spin-torques in a perpendicularly magnetized magnetic tunnel junction. We demonstrate that our magnetic synapse has a large number of intermediate resistance states, sufficient for neural computation. Moreover, we show that engineering the device geometry allows leveraging the most efficient spin torque to displace the magnetic domain wall at low current densities and thus to minimize the energy cost of our memristor. Our results pave the way for spin-torque based analog magnetic neural computation. △ Less

Submitted 24 May, 2016; originally announced May 2016.

Comments: 9 pages, 3 figures

Controlling the phase locking of unstable magnetic bits for ultra-low power computation

Authors: A. Mizrahi, N. Locatelli, R. Lebrun, V. Cros, A. Fukushima, H. Kubota, S. Yuasa, D. Querlioz, J. Grollier

Abstract: When fabricating magnetic memories, one of the main challenges is to maintain the bit stability while downscaling. Indeed, for magnetic volumes of a few thousand nm3, the energy barrier between magnetic configurations becomes comparable to the thermal energy at room temperature. Then, switches of the magnetization spontaneously occur. These volatile, superparamagnetic nanomagnets are generally con… ▽ More When fabricating magnetic memories, one of the main challenges is to maintain the bit stability while downscaling. Indeed, for magnetic volumes of a few thousand nm3, the energy barrier between magnetic configurations becomes comparable to the thermal energy at room temperature. Then, switches of the magnetization spontaneously occur. These volatile, superparamagnetic nanomagnets are generally considered useless. But what if we could use them as low power computational building blocks? Remarkably, they can oscillate without the need of any external dc drive, and despite their stochastic nature, they can beat in unison with an external periodic signal. Here we show that the phase locking of superparamagnetic tunnel junctions can be induced and suppressed by electrical noise injection. We develop a comprehensive model giving the conditions for synchronization, and predict that it can be achieved with a total energy cost lower than 10-^13 J. Our results open the path to ultra-low power computation based on the controlled synchronization of oscillators. △ Less

Submitted 23 May, 2016; originally announced May 2016.

Comments: 19 pages, 3 figures

Temperature dependence of spin-orbit torques in W/CoFeB bilayers

Authors: Witold Skowronski, Monika Cecot, Jaroslaw Kanak, Slawomir Zietek, Tomasz Stobiecki, Lide Yao, Sebastiaan van Dijken, Takayuki Nozaki, Kay Yakushiji, Shinji Yuasa

Abstract: We report on the temperature and layer thickness variation of spin-orbit torques in perpendicularly magnetized W/CoFeB bilayers. Harmonic Hall voltage measurements reveal dissimilar temperature evolutions of longitudinal and transverse effective magnetic field components. The transverse effective field changes sign at 250 K for a 2 nm thick W buffer layer, indicating a much stronger contribution f… ▽ More We report on the temperature and layer thickness variation of spin-orbit torques in perpendicularly magnetized W/CoFeB bilayers. Harmonic Hall voltage measurements reveal dissimilar temperature evolutions of longitudinal and transverse effective magnetic field components. The transverse effective field changes sign at 250 K for a 2 nm thick W buffer layer, indicating a much stronger contribution from interface spin-orbit interactions compared to, for example, Ta. Transmission electron microscopy measurements reveal that considerable interface mixing between W and CoFeB is primarily responsible for this effect. △ Less

Submitted 18 April, 2016; originally announced April 2016.

Comments: 4 pages, 5 figures

Twist in the bias-dependence of spin-torques in magnetic tunnel junctions

Authors: Sören Boyn, João Sampaio, Vincent Cros, Julie Grollier, Akio Fukushima, Hitoshi Kubota, Kay Yakushiji, Shinji Yuasa

Abstract: The spin-torque in magnetic tunnel junctions possesses two components that both depend on the applied voltage. Here, we develop a new method for the accurate extraction of this bias-dependence from experiments over large voltage ranges. We study several junctions with different magnetic layer structures of the top electrode. Our results obtained on junctions with symmetric CoFeB electrodes agree w… ▽ More The spin-torque in magnetic tunnel junctions possesses two components that both depend on the applied voltage. Here, we develop a new method for the accurate extraction of this bias-dependence from experiments over large voltage ranges. We study several junctions with different magnetic layer structures of the top electrode. Our results obtained on junctions with symmetric CoFeB electrodes agree well with theoretical calculations. The bias-dependences of asymmetric samples, with top electrodes containing NiFe, however, are twisted compared to the quadratic form generally assumed. Our measurements reveal the complexity of spin-torque mechanisms at large bias. △ Less

Submitted 15 April, 2016; originally announced April 2016.

Journal ref: Phys. Rev. B 93, 224427 (2016)

Mutual synchronization of spin torque nano-oscillators through a non-local and tunable electrical coupling

Authors: R. Lebrun, S. Tsunegi, P. Bortolotti, H. Kubota, A. S. Jenkins, M. Romera, K. Yakushiji, A. Fukushima, J. Grollier, S. Yuasa, V. Cros

Abstract: The concept of spin torque driven high frequency magnetization dynamics has opened up the field of spintronics to non-linear physics, potentially in complex networks of dynamical systems. In the scarce demonstrations of synchronized spin-torque oscillators, the local nature of the magnetic coupling that is used have largely hampered a good understanding and thus the control of the synchronization… ▽ More The concept of spin torque driven high frequency magnetization dynamics has opened up the field of spintronics to non-linear physics, potentially in complex networks of dynamical systems. In the scarce demonstrations of synchronized spin-torque oscillators, the local nature of the magnetic coupling that is used have largely hampered a good understanding and thus the control of the synchronization process. Here we take advantage of the non-local nature of an electrical coupling to mutually synchronize spin-torque oscillators through their self-emitted microwave currents. The control of the synchronized state is achieved at the nanoscale through two active components of spin transfer torques, but also externally through an electrical delay line. These additional levels of control of the synchronization capability provide new approaches to underlie a large variety of nanoscale collective dynamics in complex networks. △ Less

Submitted 6 January, 2016; originally announced January 2016.

Comments: 8 pages, 3 figures

Journal ref: Nature Communications 8, 15825 (2017)

Self-injection locking of a vortex spin torque oscillator by delayed feedback

Authors: S. Tsunegi, E. Grimaldi, R. Lebrun, H. Kubota, A. S. Jenkins, K. Yakushiji, A. Fukushima, P. Bortolotti, J. Grollier, S. Yuasa, V. Cros

Abstract: The self-synchronization of spin torque oscillators is investigated experimentally by re-injecting its radiofrequency (rf) current after a certain delay time. We demonstrate that the emission power and the spectral linewidth are improved for optimal delay times. Moreover by varying the phase difference between the emitted power and the re-injected one, we find a clear oscillatory dependence with a… ▽ More The self-synchronization of spin torque oscillators is investigated experimentally by re-injecting its radiofrequency (rf) current after a certain delay time. We demonstrate that the emission power and the spectral linewidth are improved for optimal delay times. Moreover by varying the phase difference between the emitted power and the re-injected one, we find a clear oscillatory dependence with a 2π periodicity of the frequency of the oscillator as well as its power and linewidth. Such periodical behavior within the self-injection regime is well described by the general model of nonlinear auto-oscillators including not only a delayed rf current but also all spin torque forces responsible for the self-synchronization. Our results reveal new approaches for controlling the non-autonomous dynamics of spin torque oscillators, a key issue for rf spintronics applications as well as for the development of neuro-inspired spin-torque oscillators based devices. △ Less

Submitted 18 September, 2015; originally announced September 2015.

Increased magnetic damping of a single domain wall and adjacent magnetic domains detected by spin torque diode in a nanostripe

Authors: Steven Lequeux, Joao Sampaio, Paolo Bortolotti, Thibaut Devolder, Rie Matsumoto, Kay Yakushiji, Hitoshi Kubota, Akio Fukushima, Shinji Yuasa, Kazumasa Nishimura, Yoshinori Nagamine, Koji Tsunekawa, Vincent Cros, Julie Grollier

Abstract: We use spin-torque resonance to probe simultaneously and separately the dynamics of a magnetic domain wall and of magnetic domains in a nanostripe magnetic tunnel junction. Thanks to the large associated resistance variations we are able to analyze quantitatively the resonant properties of these single nanoscale magnetic objects. In particular, we find that the magnetic damping of both domains and… ▽ More We use spin-torque resonance to probe simultaneously and separately the dynamics of a magnetic domain wall and of magnetic domains in a nanostripe magnetic tunnel junction. Thanks to the large associated resistance variations we are able to analyze quantitatively the resonant properties of these single nanoscale magnetic objects. In particular, we find that the magnetic damping of both domains and domain walls is doubled compared to the damping value of their host magnetic layer. We estimate the contributions to damping arising from dipolar couplings between the different layers in the junction and from the intralayer spin pumping effect. We find that they cannot explain the large damping enhancement that we observe. We conclude that the measured increased damping is intrinsic to large amplitudes excitations of spatially localized modes or solitons such as vibrating or propagating domain walls △ Less

Submitted 17 August, 2015; originally announced August 2015.

Spin torque resonant vortex core expulsion for an efficient radio-frequency detection scheme

Authors: A. S. Jenkins, R. Lebrun, E. Grimaldi, S. Tsunegi, P. Bortolotti, H. Kubota, K. Yakushiji, A. Fukushima, G. de Loubens, O. Klein, S. Yuasa, V. Cros

Abstract: Spin-polarised radio-frequency currents, whose frequency is equal to that of the gyrotropic mode, will cause an excitation of the core of a magnetic vortex confined in a magnetic tunnel junction. When the excitation radius of the vortex core is greater than that of the junction radius, vortex core expulsion is observed, leading to a large change in resistance, as the layer enters a predominantly u… ▽ More Spin-polarised radio-frequency currents, whose frequency is equal to that of the gyrotropic mode, will cause an excitation of the core of a magnetic vortex confined in a magnetic tunnel junction. When the excitation radius of the vortex core is greater than that of the junction radius, vortex core expulsion is observed, leading to a large change in resistance, as the layer enters a predominantly uniform magnetisation state. Unlike the conventional spin-torque diode effect, this highly tunable resonant effect will generate a voltage which does not decrease as a function of rf power, and has the potential to form the basis of a new generation of tunable nanoscale radio-frequency detectors. △ Less

Submitted 20 May, 2015; originally announced May 2015.

Understanding of phase noise squeezing under fractional synchronization of non-linear spin transfer vortex oscillator

Authors: R. Lebrun, A. Jenkins, A. Dussaux, N. Locatelli, S. Tsunegi, E. Grimaldi, H. Kubota, P. Bortolotti, K. Yakushiji, J. Grollier, A. Fukushima, S. Yuasa, V. Cros

Abstract: We investigate experimentally the synchronization of a vortex based spin transfer oscillator to an external rf current whose frequency is at multiple integers, as well as half integer, of the oscillator frequency. Through a theoretical study of the locking process, we highlight both the crucial role of the symmetries of the spin torques acting on the magnetic vortex and the nonlinear properties of… ▽ More We investigate experimentally the synchronization of a vortex based spin transfer oscillator to an external rf current whose frequency is at multiple integers, as well as half integer, of the oscillator frequency. Through a theoretical study of the locking process, we highlight both the crucial role of the symmetries of the spin torques acting on the magnetic vortex and the nonlinear properties of the oscillator on the phase locking process. Through the achievement of a perfect injection locking state, we report a record phase noise reduction down to -90dBc/Hz at 1 kHz offset frequency. The phase noise of these nanoscale oscillators is demonstrating as being low and controllable which is of significant importance for real applications using spin transfer devices. △ Less

Submitted 11 February, 2015; originally announced February 2015.

Journal ref: Phys. Rev. Lett. 115, 017201 (2015)

A method for reduction of propagation loss of surface plasmons. Experimental demonstration of the loss reduction for Fe/MgO/AlGaAs plasmonic structure integrated with AlGaAs/GaAs optical waveguide

Authors: V. Zayets, H. Saito, K. Ando, S. Yuasa

Abstract: A method for the substantial reduction of propagation loss of surface plasmons was proposed and experimentally demonstrated. The method is based on the fact that the propagation loss of the surface plasmons depends significantly on the optical confinement of the plasmon. A plasmonic structure, which contains a metal and two dielectric layers of different refractive indexes, is proposed in order to… ▽ More A method for the substantial reduction of propagation loss of surface plasmons was proposed and experimentally demonstrated. The method is based on the fact that the propagation loss of the surface plasmons depends significantly on the optical confinement of the plasmon. A plasmonic structure, which contains a metal and two dielectric layers of different refractive indexes, is proposed in order to optimize optical confinement and to reduce propagation loss of the surface plasmons. A low propagation loss of 0.17 dB/um for a surface plasmon in a Fe/MgO/AlGaAs plasmonic structure was achieved. A good coupling efficiency of 2.2 dB/facet between a surface plasmon in Fe/MgO/AlGaAs and a waveguide mode in AlGaAs/GaAs optical waveguide was demonstrated. △ Less

Submitted 12 January, 2015; originally announced January 2015.

Comments: 6 pages, 4 figures

Journal ref: Optic Express 23, 12834-12839 (2015)