An oscillator array has been proposed for associative memory, in which the synchronization of multiple oscillators is utilized for pattern-matching operations. An input pattern is represented by a set of frequency shifts of the oscillators and the matching result is attributed to the degree of synchronization. Here, we propose an electrically coupled spin-Hall oscillator (SHO) array in which multiple SHOs exhibit synchronization by interacting with each other through self-feedback spin torques. We numerically demonstrate the pattern matching functionality of the proposed SHO array.

Current-induced magnetization excitations are studied for a spin-torque oscillator (STO) composed of a nanopillar with a perpendicular polarizer layer (PL), a MgO barrier layer, and a planar free layer (FL). By applying direct current and perpendicular-to-plane magnetic field, we measure resistance and radio-frequency electrical signal of the STO, which reflect magnetization motions of both PL and FL. Examination of the experimental results reveals that large-cone-angle magnetization oscillation occurs in the FL regardless of the current direction, whereas the PL magnetization shows principally either synchronized excitation with the FL oscillation or thermal-induced ferromagnetic resonance (FMR), depending on the current direction. Utilizing macrospin simulations, we show that hybridization of the excitation modes of the PL and FL through mutual dipolar field explains the magnetization dynamics. When the current flows from the PL to the FL, large-cone-angle oscillation of the FL magnetization occurs with the same rotation direction as that of FMR of the PL magnetization, leading to emergence of the synchronized excitation modes. On the other hand, when the current flows from the FL to the PL, the magnetization motions of the two layers have opposite rotation directions, and consequently, the PL and FL show their respective intrinsic excitation modes.

For the application of the spin-torque oscillator to a high-data-transfer-rate read head, it is indispensable that the oscillation frequency responds promptly to the magnetic field from recorded bits. In this paper, we numerically exemplify the phase response to a short magnetic pulse. The phase basically follows the magnetic pulse although it takes several nanoseconds to return to the steady state because of the frequency nonlinearity. We also demonstrate the differential detection of recorded bits at the data-transfer rate beyond 5 Gbit/s.

An application of spin-torque oscillators (STOs) to high-signal-transfer-rate read heads beyond 3 Gbits/s is considered and the signal-to-noise ratios (SNRs) of the output signals under the thermal magnetization fluctuations are calculated by using the results of recent nonlinear theories. The STO head senses the media field as a modulation in the oscillation frequency, enabling high signal transfer rates beyond the limit of ferromagnetic relaxation. The output (digital) signal is obtained by frequency modulation (FM) detection, which is commonly used in communication technologies. As the problem of rapid phase diffusion in nonlinear STOs caused by the thermal fluctuations is overcome by employing a delay detection method, the sufficiently large SNRs are obtained even in nonlinear STOs less than 30×30 nm2 in size.

Technology for detecting the magnetization direction of nanoscale magnetic material is crucial for realizing high-density magnetic recording devices. Conventionally, a magnetoresistive device is used that changes its resistivity in accordance with the direction of the stray field from an objective magnet. However, when several magnets are near such a device, the superposition of stray fields from all the magnets acts on the sensor, preventing selective recognition of their individual magnetization directions. Here we introduce a novel readout method for detecting the magnetization direction of a nanoscale magnet by use of a spin-torque oscillator (STO). The principles behind this method are dynamic dipolar coupling between an STO and a nanoscale magnet, and detection of ferromagnetic resonance (FMR) of this coupled system from the STO signal. Because the STO couples with a specific magnet by tuning the STO oscillation frequency to match its FMR frequency, this readout method can selectively determine the magnetization direction of the magnet.

Control of magnetization switching is crucial for magnetic storage devices. We numerically show that magnetization switching of a magnetic dot can be induced by using only a spin-torque oscillator (STO). The magnetic-dot magnetization exhibits cooperative dynamics with the STO through dipolar coupling and reverses into a final switched state. This resonant switching occurs under a certain precession chirality and STO frequency range that depend on the magnetic dot’s initial direction and intrinsic resonance frequency. We also show that the resonant-switching characteristics are suitable for three-dimensional magnetic storage applications and propose a multi-bit memory cell based on the switching principle.

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A read head based on a spin-torque oscillator (STO) has been proposed for the high-density magnetic recording beyond 2 Tbit/in.2. To evaluate the stability and agility of the oscillation, we conduct a real-time measurement of the STO waveform under a magnetic pulse (∼60 Oe) in the nanosecond region, and estimate its instantaneous frequency. Stable oscillation is sustained in the presence of the magnetic pulse, and the frequency shift is clearly observed, indicating that the STO frequency responds to the change in magnetic field in less than 1 ns.