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Injection of pure spin current using a nonlocal geometry is a promising method for controlling magnetization in spintronic devices from the viewpoints of increasing freedom in device structure and avoiding problems related to charge current. Here, we report an experimental demonstration of magnetization switching of a perpendicular magnetic nanodot induced by vertical injection of pure spin current from a spin polarizer with perpendicular magnetization. In comparison with direct spin injection, the current amplitude required for magnetization switching is of the same order and shows smaller asymmetry between parallel-to-antiparallel and antiparallel-to-parallel switching. Simulation of spin accumulation reveals that, in the case of nonlocal spin injection, the spin torque is symmetric between the parallel and antiparallel configuration because current flows through only the spin polarizer, not the magnetic nanodot. This characteristic of nonlocal spin injection is the origin of the smaller asymmetry of the switching current and can be advantageous in spintronic applications.

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.

The design concept of microwave-assisted magnetic recording (MAMR) using the flux control (FC) effect has been proposed as a technology for hard disk drives (HDDs). In this type of MAMR, the magnetization of an in-gap device (FC device) is reversed against the gap field by spin-transfer torque, enhancing the amplitude and gradient of the recording field. In this paper, we study the magnetization dynamics of an FC device fabricated in the write gap of an HDD write head. The operation of the FC device is analyzed by measuring the temporal resistance change in the sub-nanosecond region. Reversal of the FC device becomes faster as the bias current is increased and can be completed by 0.5 ns after the transition of the write current. The experimental results are reproduced by micromagnetic simulations using a head model, confirming that the simulations correctly describe the magnetization dynamics of the actual device. The simulations show that the recording field gain by the FC device appears with little delay after the rise of the recording field and that the FC device operates effectively even at a fast write rate of approximately 3 Gbit/s. Furthermore, we demonstrate the effectiveness of boosting the bias current, which can realize both fast and reliable operation of the FC device. These results indicate that the FC device operates as designed and that MAMR using the FC effect is promising for extending the recording density of HDDs.

A spin-torque oscillator (STO) is a microwave oscillator based on a magnetoresistive (MR) element [1], [2], and has been investigated for applications such as information storages, communications, and computing units. For these applications, large output signal and stable oscillation are necessary. So far, it has been reported that the large output signal can be obtained by out-of-plane precession (OPP) of free layer in an STO based on a magnetic tunnel junction (MTJ) and having an in-plane pinned layer [3]–[6]. This is because in OPP change of angle between the magnetizations of free and pinned layers is large and the relative angle change is converted into the output signal through the large MR effect of the MTJ. To further improve the output power and the stability of the OPP, we have recently fabricated an STO based on an in-plane MTJ with a synthetic ferrimagnet (SyF) free layer (SyF-STO). We have chosen the SyF free layer because improvement of the output power and the stability has been reported for in-plane oscillation [7]–[9]. We have investigated whether the OPP is possible in the SyF-STO. As a result, by the OPP of the free layer we have obtained large output power of the order of $1 \mu \mathrm {W}$ in perpendicular field [10]. (In Ref. [10] we reported the maximum output power of $3.7 \mu \mathrm {W}$. This value includes an error in estimation, and the correct value is $0.93 \mu \mathrm {W}$.) In this study, to clarify the reason for the large output power obtained in the SyF-STO, we compare the SyF-STO with an STO with a single free layer (SFL-STO) by micromagnetic simulation. Figure 1 shows schematics of these STOs. The SyF-STO has two free layers which are antiferromagnetically coupled (AFC) by the interlayer coupling. The pinned layers are also AFC. An external field $H_{z}$ and a current $I$ are applied in the perpendicular direction. The spin-transfer torque acts on the free layer 2 and the pinned layer 1. To the bottom pinned layer, a $- x -$direction magnetic field is applied, modeling an exchange bias field. In the simulation, we use the parameters corresponding to the sample in the experiment. The parameters for the SFL-STO are the same as the SyF-STO except that the SFL-STO has a single free layer. Figure 2(a) shows oscillation powers of each STO as a function of $H_{z}$ for $I=8.5$ mA, which are obtained from the $y -$components of the spatially-averaged magnetizations. The oscillation power is estimated by time average of square of the $y -$component of normalized magnetization. For $H_{z}\ge 5$ kOe, both STOs exhibit the large oscillation powers due to the OPP. In the SyF-STO, the magnetizations of the two free layers are synchronized and their in-plane components are in opposite directions. The oscillation power is larger for the SyF-STO. In the parameter range of the OPP, the oscillation frequencies of both STOs show essentially the same dependence on $H_{z}$, that is, the oscillation frequencies increase, showing a step. By investigating the waveforms of the magnetizations, we find that this step originates from coupled magnetization oscillation of the free and pinned layers due to magnetic dipolar field and the spintransfer torque. This dependence of the oscillation freqeuncy on $H_{z}$ well reproduces that observed in the experiment [10]. Such steps of oscillation frequency have been reported for SFL-STOs with OPP [1], [7], [11]. To clarify the reason of the difference of the oscillation power, we compare amplitudes of the spatially-averaged magnetizations of the free layers. Figure 2(b) shows time averages of the amplitudes. Since the magnetization is normalized, the amplitude is closer to 1 when the magnetization is spatially uniform. It is found that the amplitude for the SyF-STO is kept near 0.9 for the OPP, while that for the SFL-STO decreases to about 0.75 at around $H_{z}=5$ kOe. This difference means that the free layer magnetization of the SyF-STO is uniform compared with the SFL-STO. Inset in Fig. 2(b) shows magnetization configurations of the free layers of the SyF- and SFL- STOs for $H_{z}=5$ kOe. The $y -$component is shown by the color scale at a moment when the in-plane components of the spatially-averaged magnetizations are in the $x -$direction. In the SyF-STO, although local fluctuations can be seen, their magnitudes are relatively small. On the other hand, in the SFL-STO, the free layer is divided into two regions in which the in-plane components of the magnetizations are almost in opposite directions. This non-uniform magnetization pattern reduces stray field outside the free layer, and stabilizes magnetic field distribution. From this result we think that the almost uniform magnetization precession in the SyF-STO is because the SyF free layer reduces the stray field and thus the magnetic field distribution can be stabilized even by the uniform magnetizations. The uniform OPP of free layer magnetizations leads to the larger output power, which is a possible explanation for the large output power observed in the experiment [10]. Finally, in Fig. 2(c) we compare threshold current for the OPP, and find that the threshold current is smaller for the SyF free layer. It can be thought that this lower threshold current enables the OPP by a bias voltage lower than a voltage at which the dielectric breakdown of the MTJ occurs.

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

Antiferromagnetically coupled (AFC) magnetic bilayer is a candidate media structure for high-density magnetic recording. Because the stray fields from the two magnetic layers of the AFC bilayer cancel each other out, switching field distribution originating from the stray fields from the adjacent data bits can be suppressed. Furthermore, in microwave-assisted magnetic recording (MAMR), which utilizes ferromagnetic resonance (FMR) excitation in a microwave field to reverse a high-anisotropy magnetic material, AFC media can suppress the distribution in FMR frequency originating from the stray fields and improve MAMR performance. In this study, we fabricate an AFC magnetic bilayer consisting of two Co/Pt multilayers with perpendicular magnetization. We use anomalous-Hall-effect-FMR in combination with a circularly polarized microwave field and carry out layer-selective analysis of FMR excitation of the two magnetic layers. We then investigate the switching behavior of an AFC bilayer nanodot in a microwave magnetic field. The switching field decreases with increasing microwave field frequency and increases abruptly at the critical frequency, and a large switching field reduction by applying a microwave field is demonstrated. This switching behavior is similar to that of a single-layer perpendicular magnetic nanodot, showing that the AFC structure does not hinder the microwave assist effect.

We introduce a method to read the data stored in a three-dimensional (3D) magnetic recording medium comprising plural storage layers. The readout is realized by selecting the storage layer with the ferromagnetic resonance frequency, and detecting the magnetization orientation with the ferromagnetic resonance absorption. This concept is experimentally confirmed with magnetic media comprising NiFe and CoFe layers. The feasibility of applying this method to a realistic 3D magnetic recording medium is discussed by calculating the absorption spectra of several storage layers with different perpendicular magnetic anisotropy constants.

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...

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.[1-4] 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.

Energy barrier height for magnetization switching is theoretically studied for a system with uniaxial anisotropy in a circularly polarized microwave magnetic field. A formulation of the Landau-Lifshitz-Gilbert equation in a rotating frame introduces an effective energy that includes the effects of both the microwave field and static field. This allows the effective-energy profiles to rigorously describe the switching paths and corresponding barrier height, which govern thermally activated magnetization switching under microwave assistance. We show that fixed points and limit cycles in the rotating frame lead to various switching paths and that under certain conditions, switching becomes a two-step process with an intermediate state.

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.

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.

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.

We used a magnetic tunnel junction (MTJ) to study the microwave-assisted switching (MAS) of a perpendicular magnetic nanodot. Because the sample structure enables the direct observation of fast magnetization dynamics through MTJ signals, the switching behavior is examined in relation with the induced magnetization excitation. By utilizing the MTJ signals, we observe several ferromagnetic resonance (FMR) modes and identify the lowest FMR mode as the cause of switching field reduction. The switching field decreases as the microwave frequency increases, and this frequency–magnetic field relationship coincides with that of the lowest FMR mode near a critical frequency. We also investigate the size dependence of MAS.

Synchronized magnetization motion among the several magnetic layers composing a spin-torque oscillator (STO) is considered an effective way to improve spectral purity. To utilize this scheme in a MgO-based STO, we have fabricated a dual free-layer STO composed of a CoFeB free layer (FL), a MgO barrier layer, and a CoFe/Ru/CoFeB synthetic ferrimagnet free layer (SyF). Unlike conventional MgO-based STOs, this structure does not have an antiferromagnetic layer that pins the SyF, leading to a large-amplitude oscillation of magnetization in the SyF. The dual free-layer STO exhibits coherent microwave emissions with power spectrum density beyond 800 nW/GHz and narrow spectral linewidth below 5 MHz (Q-factor ≈ 2000). Macrospin simulations confirm that the stable oscillations originate from the synchronized magnetization motion of the FL and the SyF through dynamical dipolar coupling.

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.

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.

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.

Amplitude fluctuations in an output signal of a tunnel-type spin-torque oscillator are examined by a time-domain measurement using a real-time oscilloscope (40 GS/s). Unsteady intermittent oscillation just above the threshold changes gradually to steady oscillation with increasing current. The amplitude fluctuation in the steady oscillation state is well described by a recent nonlinear theory, which is essentially based on a macrospin model under the thermal fluctuation, indicating that the fluctuation is attributed to the thermal magnetization fluctuation rather than to (nonthermal) mode instabilities.