The brain displays many features typical of non-linear dynamical networks, such as synchronization or chaotic behaviour. These observations have inspired a whole class of models that harness the power of complex non-linear dynamical networks for computing. In this framework, neurons are modeled as non-linear oscillators, and synapses as the coupling between oscillators. These abstract models are very good at processing waveforms for pattern recognition or at generating precise time sequences useful for robotic motion. However there are very few hardware implementations of these systems, because large numbers of interacting non-linear oscillators are indeed. In this talk, I will show that coupled spin-torque nano-oscillators are very promising for realizing cognitive computing at the nanometer and nanosecond scale, and will present our first results in this direction.
We propose an experimental scheme to determine the spin-transfer torque efficiency excited by the spin-orbit interaction in ferromagnetic bilayers from the measurement of the longitudinal magnetoresistace. Solving a diffusive spin-transport theory with appropriate boundary conditions gives an analytical formula of the longitudinal charge current density. The longitudinal charge current has a term that is proportional to the square of the spin-transfer torque efficiency and that also depends on the ratio of the film thickness to the spin diffusion length of the ferromagnet. Extracting this contribution from measurements of the longitudinal resistivity as a function of the thickness can give the spin-transfer torque efficiency.
Spintronics aims to utilize the coupling between charge transport and magnetic dynamics to develop improved and novel memory and logic devices. Future progress in spintronics may be enabled by exploiting the spin-orbit coupling present at the interface between thin film ferromagnets and heavy metals. In these systems, applying an in-plane electrical current can induce magnetic dynamics in single domain ferromagnets, or can induce rapid motion of domain wall magnetic textures. There are multiple effects responsible for these dynamics. They include spin-orbit torques and a chiral exchange interaction (the Dzyaloshinskii-Moriya interaction) in the ferromagnet. Both effects arise from the combination of ferromagnetism and spin-orbit coupling present at the interface. There is additionally a torque from the spin current flux impinging on the ferromagnet, arising from the spin hall effect in the heavy metal. Using first principles calculations, we identify spin-orbit hybridization at the ferromagnet-heavy metal interface as central to the spin-orbit torques present in Co-Pt bilayers. We additionally propose that the transverse spin current (from the spin hall effect) is locally enhanced over its bulk value due to scattering at an interface which is oriented normal to the charge current direction.
Spin-transfer torques (STT) provides a new mechanism to alter the magnetic configurations in magnetic heterostructures, a
feat previously only achieved by an external magnetic field. A current flowing perpendicular through a magnetic
noncollinear spin structure can induce torques on the magnetization, depending on the polarity of the current. This is
because an electron carries angular momentum, or spin, part of which can be transferred to the magnetic layer as a torque.
A spin-polarized current of a substantial current density (e.g., 10<sup>8</sup> A/cm<sup>2</sup>) is required to observe the effect of the spin
transfer torques. Consequently, switching by spin-polarized currents is often realized in small structures with sub-micron
cross sections made by nanolithography. Here we demonstrate spin transfer torque effects using point-contact spin
injection involving no lithography. In a continuous Co/Cu/Co trilayer, we have observed hysteretic reversal of sub-100 nm
magnetic elements by spin injection through a metal tip both at low temperature and at room temperature. A small
magnetic domain underneath the tip in the top Co layer can be manipulated to align parallel or anti-parallel to the bottom
Co layer with a unique bias voltage. In an exchange-biased single ferromagnetic layer, we have observed a new form of
STT effect which is the inverse effect of domain wall magnetoresistance effect rather than giant magnetoresistance effect.
We further show that in granular solids, the STT effect that can be exploited to induce a large spin disorder when combined
with a large magnetic field. As a result, we have obtained a spectacular MR effect in excess of 400%, the largest ever
reported in any metallic systems.