Kiwamu Kudo

Signal-to-noise ratios in high-signal-transfer-rate read heads composed of spin-torque oscillators

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.

Numerical Simulation on Temporal Response of Spin-Torque Oscillator to Magnetic Pulses

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.

Amplitude-phase coupling in a spin-torque nano-oscillator

The spin-torque nano-oscillator in the presence of thermal fluctuation is described by the normal form of the Hopf bifurcation with an additive white noise. By the application of the reduction method, the amplitude-phase coupling factor, which has a significant effect on the power spectrum of the spin-torque nano-oscillator, is calculated from the Landau–Lifshitz–Gilbert–Slonczewski equation with the nonlinear Gilbert damping. The amplitude-phase coupling factor exhibits a large variation depending on an in-plane anisotropy under the practical external fields.

Nanoscale layer-selective readout of magnetization direction from a magnetic multilayer using a spin-torque oscillator

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.

Measurement of nonlinear frequency shift coefficient in spin-torque oscillators based on MgO tunnel junctions

The nonlinear frequency shift coefficient, which represents the strength of the transformation of amplitude fluctuations into phase fluctuations of an oscillator, is measured for MgO-based spin-torque oscillators by analyzing the current dependence of the power spectrum. We have observed that linewidth against inverse normalized power plots show linear behavior below and above the oscillation threshold as predicted by the analytical theories for spin-torque oscillators. The magnitude of the coefficient is determined from the ratio of the linear slopes. Small magnitude of the coefficient (∼3) has been obtained for the device exhibiting narrow linewidth (∼10 MHz) at high bias current.

Real-Time Measurement of Temporal Response of a Spin-Torque Oscillator to Magnetic Pulses

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.

Decrease of nonlinearity and linewidth narrowing in spin-transfer oscillators under the external field applied near the hard axis

Angular dependence of power spectra is measured for tunnel-type spin-transfer oscillators by changing the direction of the external field in the plane. The oscillation frequency shows the red shift with increasing the current in the case that the field directions are away from the hard axis of the free layer. The frequency shift changes from red to blue at a specific direction near the hard axis, where the linewidth is much reduced. These results are explained by taking into account the nonlinearity arising from the in-plane anisotropy as well as that arising from the demagnetizing effect.

Resonant magnetization switching induced by spin-torque-driven oscillations and its use in three-dimensional magnetic storage applications

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 dots 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.

Simulations and Experiments Toward High-Data-Transfer-Rate Readers Composed of a Spin-Torque Oscillator

High-data-transfer-rate readers beyond 3 Gbit/s composed of spin-torque oscillators (STOs) are considered and the signal-to-noise ratios (SNRs) of the output signals under the thermal magnetization fluctuations are calculated by using the recent nonlinear theories. The “STO Reader” senses the media field as a modulation in the oscillation frequency, enabling high-data-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 expected even in nonlinear STOs less than 30×30 nm2 in size. The prompt response of the STO frequency (phase) to the media field and the high-data-transfer rate beyond 5 Gbit/s are shown by micromagnetic simulation. The frequency transition of STO in less than 1 ns under the pulse field is also confirmed by experiment.

Frequency transition of spin-torque oscillator under the magnetic-field pulse in nanosecond range

We report a time domain study of the frequency transition of spin-torque oscillator (STO) under the magnetic-field pulse in nanosecond range. We fabricated the pillar-structured STO devices consisting of MgO-based tunnel junctions with CoFeB free layers. Single-shot waveforms of the STO were obtained using a real-time oscilloscope (40 GS/sec). First, we measured current dependence of the waveform to investigate the time-domain stability of the oscillation. With the increase in the dc current applied to the STO, the oscillation state changed continuously in the following order: thermal fluctuation, intermittent unsteady oscillation, steady oscillation, and chaotic oscillation. Next, we measured the response of the STO to the magnetic-field pulse with a rise time of 0.5 ns, a duration time of 10 ns, and an amplitude of 60 Oe. In this measurement, the oscillation state was kept in the above-mentioned steady state with the frequency ∼3.5 GHz and the spectral linewidth ∼50 MHz. In the presence of the magnetic-...