Sumito Tsunegi

Improvement of structural, electronic, and magnetic properties of Co2MnSi thin films by He+ irradiation

The influence of 30 keV He+ ion irradiation on structural, electronic, and magnetic properties of Co2MnSi thin films with a partial B2 order was investigated. It was found that room temperature irradiation with light ions can improve the local chemical order. This provokes changes of the electronic structure and element-specific magnetization toward the bulk properties of a well-ordered Co2MnSi Heusler compound.

Large tunnel magnetoresistance in magnetic tunnel junctions using a Co2MnSi Heusler alloy electrode and a MgO barrier

A large tunnel magnetoresistance (TMR) ratio of 753% has been observed at 2 K in a magnetic tunnel junction (MTJ) using a Co2MnSi Heusler alloy electrode and a crystalline MgO tunnel barrier. This TMR ratio is the largest reported to date in MTJs using a Heusler alloy electrode. Moreover, we have observed a large TMR ratio of 217% at room temperature (RT). This TMR at RT is much larger than that of MTJs using an amorphous Al-oxide tunnel barrier. However, the temperature dependence of the TMR ratio is still large because of inelastic tunneling in the antiparallel magnetic configuration.

Half-metallicity and Gilbert damping constant in Co2FexMn1-xSi Heusler alloys depending on the film composition

Transport properties in magnetic tunnel junctions (MTJs) with Co2FexMn1−xSi (CFMS, x=0–1.0)/Al–O/Co75Fe25 structure and Gilbert damping constant in the epitaxial CFMS films were investigated. The tunnel magnetoresistance ratio is as high as 75% in MTJs with x=0.6 at room temperature. The Gilbert damping constant is minimal at x=0.4. Relations between half-metallicity and the Gilbert damping constant in CFMS films were examined, revealing that the damping constant is small in half-metallic CFMS films.

Neuromorphic computing with nanoscale spintronic oscillators

Neurons in the brain behave as nonlinear oscillators, which develop rhythmic activity and interact to process information. Taking inspiration from this behaviour to realize high-density, low-power neuromorphic computing will require very large numbers of nanoscale nonlinear oscillators. A simple estimation indicates that to fit 108 oscillators organized in a two-dimensional array inside a chip the size of a thumb, the lateral dimension of each oscillator must be smaller than one micrometre. However, nanoscale devices tend to be noisy and to lack the stability that is required to process data in a reliable way. For this reason, despite multiple theoretical proposals and several candidates, including memristive and superconducting oscillators, a proof of concept of neuromorphic computing using nanoscale oscillators has yet to be demonstrated. Here we show experimentally that a nanoscale spintronic oscillator (a magnetic tunnel junction) can be used to achieve spoken-digit recognition with an accuracy similar to that of state-of-the-art neural networks. We also determine the regime of magnetization dynamics that leads to the greatest performance. These results, combined with the ability of the spintronic oscillators to interact with each other, and their long lifetime and low energy consumption, open up a path to fast, parallel, on-chip computation based on networks of oscillators.

High emission power and Q factor in spin torque vortex oscillator consisting of FeB free layer

Microwave oscillation properties of spin torque vortex oscillators (STVOs) consisting of an FeB vortex free layer were investigated. Because of a high MR ratio and large DC current, a high emission power up to 3.6 µW was attained in the STVO with a thin FeB free layer of 3 nm. In STOs with a thicker FeB layer, e.g., 10 nm thick, we obtained a large Q factor greater than 6400 while maintaining a large integrated emission power of 1.4 µW. Such excellent microwave performance is a breakthrough for the mutual phase locking of STVOs by electrical coupling.

Enhancement in tunnel magnetoresistance effect by inserting CoFeB to the tunneling barrier interface in Co2MnSi/MgO/CoFe magnetic tunnel junctions

Tunnel magnetoresistance (TMR) effect was investigated in Co2MnSi/CoFeB(0–2 nm)/MgO/CoFe magnetic tunnel junctions (MTJs). TMR ratio was enhanced by inserting a thin CoFeB layer at the Co2MnSi/MgO interface. The MTJ with CoFeB thickness of 0.5 nm exhibited the highest TMR ratio. From the conductance-voltage measurements for the fabricated MTJs, we infer that the highly spin polarized electron created in Co2MnSi can conserve the polarization through the 0.5-nm-thick CoFeB layer. Furthermore, by insertion of the thin CoFeB layer, the temperature dependence of the TMR ratio was improved because of the suppression of the fluctuation of the magnetic moment at the Co2MnSi/MgO interface.

Critical Field of Spin Torque Oscillator with Perpendicularly Magnetized Free Layer

The oscillation properties of a spin torque oscillator consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer are studied based on an analysis of the energy balance between spin torque and damping. The critical value of an external magnetic field applied normal to the film plane is found, below which the controllable range of the oscillation frequency by the current is suppressed. The value of the critical field depends on the magnetic anisotropy, the saturation magnetization, and the spin torque parameter.

Self-Injection Locking of a Vortex Spin Torque Oscillator by Delayed Feedback.

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 integrated power and spectral linewidth are improved for optimal delays. Moreover by varying the phase difference between the emitted power and the re-injected one, we find a clear oscillatory dependence on the phase difference 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.

Nonlinear Behavior and Mode Coupling in Spin-Transfer Nano-Oscillators

By investigating thoroughly the tunable behavior of coupled modes, we highlight how it provides a means to tune the properties of spin-transfer nano-oscillators.We first demonstrate that the main features of the microwave signal associated with coupled vortex dynamics, i.e., frequency, spectral coherence, critical current, and mode localization, depend drastically on the relative vortex core polarities. Second, we report a large reduction of the nonlinear linewidth broadening obtained by changing the effective damping through the control of the core configuration. Such a level of control on the nonlinear behavior reinforces our choice to exploit the microwave properties of collective modes for applications in advanced spintronics devices for integrated telecommunication concerns.

Damping parameter and interfacial perpendicular magnetic anisotropy of FeB nanopillar sandwiched between MgO barrier and cap layers in magnetic tunnel junctions

We systematically investigated the Gilbert damping (α) and interfacial perpendicular magnetic anisotropy (Ki) in magnetic tunnel junctions with a MgO-barrier/FeB/MgO-cap layered structure using the spin-torque diode effect. By increasing the MgO cap thickness, α decreased, whereas Ki increased monotonically. Values down to 0.0054 for α and up to 3.3 erg/cm2 for Ki were obtained for a MgO cap thickness of 0.6 nm. The small α and large Ki suggest that MgO-capped FeB is a suitable free layer for spintronics devices such as spin-torque oscillators and spin-torque magnetic random access memories.