Hirofumi Suto

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.

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.

Readout method from antiferromagnetically coupled perpendicular magnetic recording media using ferromagnetic resonance

We fabricate perpendicular magnetic recording media comprising two antiferromagnetically coupled Co/Pt multilayers and investigate its magnetic properties by ferromagnetic resonance (FMR) measurement. In such media, the magnetizations of the two perpendicular magnets are designed to compensate each other in the remanent state in order to reduce the dipolar field, which is a limiting factor in high-density magnetic recording devices. We measure FMR absorption spectra of the media and estimate the magnetic anisotropy and interlayer exchange coupling. We also demonstrate that FMR measurement can be employed to read out the magnetization direction. The principles behind this readout method are different from those behind the conventional method of detecting the stray field from the media by means of a magnetoresistive sensor; therefore, the proposed readout method is applicable to magnetic recording media having zero remanent magnetization. We expand this readout scheme to three-dimensional magnetic recording with several vertically stacked recording layers. By providing each recording layer with a different FMR frequency, we experimentally confirm that layer selective readout is possible.

Frequency stabilization of spin-torque-driven oscillations by coupling with a magnetic nonlinear resonator

The fundamental function of any oscillator is to produce a waveform with a stable frequency. Here, we show a method of frequency stabilization for spin-torque nano-oscillators (STNOs) that relies on coupling with an adjacent nanomagnet through the magnetic dipole–dipole interaction. It is numerically demonstrated that highly stable oscillations occur as a result of mutual feedback between an STNO and a nanomagnet. The nanomagnet acts as a nonlinear resonator for the STNO. This method is based on the nonlinear behavior of the resonator and can be considered as a magnetic analogue of an optimization scheme in nanoelectromechanical systems. The oscillation frequency is most stabilized when the nanomagnet is driven at a special feedback point at which the feedback noise between the STNO and resonator is completely eliminated.

Subnanosecond microwave-assisted magnetization switching in a circularly polarized microwave magnetic field

We study microwave-assisted magnetization switching (MAS) of a perpendicularly magnetized nanomagnet with a diameter of 50 nm in a circularly polarized microwave magnetic field. The MAS effect appears when the rotation direction of the microwave field matches that of the ferromagnetic resonance excitation, and a large switching field decrease from 7.1 kOe to 1.5 kOe is demonstrated. In comparison with a linearly polarized microwave magnetic field, the circularly polarized microwave field induces the same MAS effect at half the microwave field amplitude, thereby showing its efficiency. We also examine MAS in the subnanosecond region and show that the magnetization switching can be induced by a microwave field with the duration of 0.2 ns.

Transient magnetization dynamics of spin-torque oscillator and magnetic dot coupled by magnetic dipolar interaction: Reading of magnetization direction using magnetic resonance

We study the magnetization dynamics of a spin-torque oscillator (STO) and a magnetic dot coupled by a magnetic dipolar field using micromagnetic simulation with the aim of developing a read method in magnetic recording that uses magnetic resonance. We propose an STO with a perpendicularly magnetized free layer and an in-plane-magnetized fixed layer as a suitable STO for this resonance read method. When the oscillation frequency of the STO is near the ferromagnetic resonance (FMR) frequency of the magnetic dot, the oscillation amplitude of the STO decreases because FMR excited in the magnetic dot causes additional dissipation. To estimate the read rate of the resonance read method, we study the transient magnetization dynamics to the coupled oscillation state from an initial state where the STO is in a free-running state and the magnetic dot is in a stationary stable state. The STO shows transient dynamics within a time scale of 1 ns, which means that the STO can perform resonance reading with a response t...

Zero-dc-field rotation-direction-dependent magnetization switching induced by a circularly polarized microwave magnetic field

Magnetization switching of high-anisotropy nanomagnets by a small magnetic field is a key challenge in developing future magnetic nanodevices. In this paper, we experimentally demonstrate magnetization switching of a perpendicularly magnetized nanomagnet induced solely by an in-plane circularly polarized microwave magnetic field. Applying a microwave field with an amplitude below 5% of the anisotropy field induces large ferromagnetic resonance excitation, which results in magnetization switching even in the absence of a dc field. This kind of magnetization switching is induced by a microwave field with a duration of 0.5 ns and is clearly dependent on the rotation direction of the microwave field.

Switching field reduction of a perpendicular magnetic nanodot in a microwave magnetic field emitted from a spin-torque oscillator

We demonstrate microwave-assisted magnetization switching of a perpendicular magnetic nanodot in a microwave stray field from a spin-torque oscillator (STO). The switching field decreases when the STO is operated by applying a current. The switching field reduction is almost the same as that in a microwave magnetic field generated by a signal generator despite the fluctuations of the STO oscillation. The switching field distribution, however, is broader when the STO is used. We also examine the magnetization switching process in the nanosecond region by applying a nanosecond-order pulse current to the STO and measuring the STO signal waveform. The onset of the STO oscillation and subsequent assisted switching occur within a few nanoseconds.

Study of nonlinear ferromagnetic resonance in a nanoscale magnetic tunnel junction using diode effect

We use the diode effect caused by magnetization excitation in a microwave magnetic field to analyze the ferromagnetic resonance and magnetization switching in a nanoscale perpendicular magnetic tunnel junction. The cone angle and the lag angle with respect to the applied microwave field of the magnetization precession are accurately estimated by utilizing the homodyne nature of the diode effect. We observe a ferromagnetic resonance peak of the cone angle accompanied by an increase in the lag angle, and a nonlinear shift of the peak position with increasing the microwave field amplitude. We also reveal magnetization switching assisted by ferromagnetic resonance excitation.

Magnetization switching assisted by double-frequency component of an in-plane-magnetized spin-torque oscillator: Micromagnetic simulation study

Microwave-assisted magnetization switching (MAS) has been proposed as a writing method for magnetic recording [1, 2].