Taro Kanao

Three-dimensional magnetic recording using ferromagnetic resonance

To meet the ever-increasing demand for data storage, future magnetic recording devices will need to be made three-dimensional by implementing multilayer recording. In this article, we present methods of detecting and manipulating the magnetization direction of a specific layer selectively in a vertically stacked multilayer magnetic system, which enable layer-selective read and write operations in three-dimensional magnetic recording devices. The principle behind the methods is ferromagnetic resonance excitation in a microwave magnetic field. By designing each magnetic recording layer to have a different ferromagnetic resonance frequency, magnetization excitation can be induced individually in each layer by tuning the frequency of an applied microwave magnetic field, and this selective magnetization excitation can be utilized for the layer-selective operations. Regarding media for three-dimensional recording, when layers of a perpendicular magnetic material are vertically stacked, dipolar interaction between multiple recording layers arises and is expected to cause problems, such as degradation of thermal stability and switching field distribution. To solve these problems, we propose the use of an antiferromagnetically coupled structure consisting of hard and soft magnetic layers. Because the stray fields from these two layers cancel each other, antiferromagnetically coupled media can reduce the dipolar interaction.

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.

Effects of power fluctuation on fast magnetic field detection using a spin-torque oscillator

We study the effects of power fluctuation on a high-data-transfer-rate read head with a spin-torque oscillator using a nonlinear oscillator model. By numerically solving the model under random sequences of applied pulsed magnetic fields (corresponding to stray fields from data bits), the bit-error rate is estimated. For a large damping rate of power, the bit errors are caused primarily by phase fluctuation that is enhanced by amplitude-phase coupling. In contrast, for a small damping rate of power, the bit errors are caused primarily by power fluctuation and are independent of amplitude-phase coupling.

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].

A theoretical study of thermally activated magnetization switching under microwave assistance

Microwave-assisted magnetization switching (MAS) is attracting much attention because of its applications in future magnetic recording techniques such as microwave-assisted magnetic recording and three-dimensional magnetic recording. For the implementation of these applications, it is indispensable to elucidate large-amplitude magnetization excitation induced by a microwave magnetic field, which accounts for switching field reduction in MAS. In addition, for understanding thermally activated switching at finite temperature, it is necessary to estimate barrier height under microwave assistance. In this work, the paths and barrier height of magnetization switching under microwave assistance were theoretically investigated. A single domain magnet having a uniaxial easy axis along the z direction with an effective anisotropy field was considered. The switching in a static magnetic field in the -z direction Hz and a microwave field Hrf rotating counterclockwise in the x-y plane at a frequency of ωrf were also studied.