We show that the Raman scattering technique can give complete structural information for one-dimensional systems, such as carbon nanotubes. Resonant confocal micro-Raman spectroscopy of an (n,m) individual single-wall nanotube makes it possible to assign its chirality uniquely by measuring one radial breathing mode frequency omega(RBM) and using the theory of resonant transitions. A unique chirality assignment can be made for both metallic and semiconducting nanotubes of diameter d(t), using the parameters gamma(0) = 2.9 eV and omega(RBM) = 248/d(t). For example, the strong RBM intensity observed at 156 cm(-1) for 785 nm laser excitation is assigned to the (13,10) metallic chiral nanotube on a Si/SiO2 surface.

Structural (n,m) determination of isolated single wall carbon nanotubes by resonant Raman scattering

The discovery of the spin-torque effect has made magnetic nanodevices realistic candidates for active elements of memory devices and applications. Magnetoresistive effects allow the read-out of increasingly small magnetic bits, and the spin torque provides an efficient tool to manipulate - precisely, rapidly and at low energy cost - the magnetic state, which is in turn the central information medium of spintronic devices. By keeping the same magnetic stack, but by tuning a device's shape and bias conditions, the spin torque can be engineered to build a variety of advanced magnetic nanodevices. Here we show that by assembling these nanodevices as building blocks with different functionalities, novel types of computing architecture can be envisaged. We focus in particular on recent concepts such as magnonics and spintronic neural networks.

Spin-torque building blocks

Magnetic nanostructures are being developed for use in many aspects of our daily life, spanning areas such as data storage, sensing and biomedicine. Whereas patterned nanomagnets are traditionally two-dimensional planar structures, recent work is expanding nanomagnetism into three dimensions; a move triggered by the advance of unconventional synthesis methods and the discovery of new magnetic effects. In three-dimensional nanomagnets more complex magnetic configurations become possible, many with unprecedented properties. Here we review the creation of these structures and their implications for the emergence of new physics, the development of instrumentation and computational methods, and exploitation in numerous applications.

/pdf/three-dimensional-nanomagnetism-2bkngvttqg.pdf

Three-dimensional nanomagnetism

Stripline (SL), vector network analyzer (VNA), and pulsed inductive microwave magnetometer (PIMM) techniques were used to measure the ferromagnetic resonance (FMR) linewidth for a series of Permalloy films with thicknesses of 50 and 100nm. The SL-FMR measurements were made for fixed frequencies from 1.5to5.5GHz. The VNA-FMR and PIMM measurements were made for fixed in-plane fields from 1.6to8kA∕m (20–100Oe). The results provide a confirmation, lacking until now, that the linewidths measured by these three methods are consistent and compatible. In the field format, the linewidths are a linear function of frequency, with a slope that corresponds to a nominal Landau-Lifshitz phenomenological damping parameter α value of 0.007 and zero frequency intercepts in the 160–320A∕m (2–4Oe) range. In the frequency format, the corresponding linewidth versus frequency response shows a weak upward curvature at the lowest measurement frequencies and a leveling off at high frequencies.

/pdf/ferromagnetic-resonance-linewidth-in-metallic-thin-films-4nmku8h46u.pdf

Ferromagnetic resonance linewidth in metallic thin films: Comparison of measurement methods | NIST

Real-time observations were made of the shape change from pyramids to domes during the growth of germanium-silicon islands on silicon (001). Small islands are pyramidal in shape, whereas larger islands are dome-shaped. During growth, the transition from pyramids to domes occurs through a series of asymmetric transition states with increasing numbers of highly inclined facets. Postgrowth annealing of pyramids results in a similar shape change process. The transition shapes are temperature dependent and transform reversibly to the final dome shape during cooling. These results are consistent with an anomalous coarsening model for island growth.

Transition states between pyramids and domes during Ge/Si island growth

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.

Numerical Simulation on Temporal Response of 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.

https://iopscience.iop.org/article/10.1143/APEX.4.013003/pdf

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

Reservoir computing (a framework for machine learning) implemented in physical systems has attracted much attention for real-time computing in $e.g.$ time-series modeling and prediction, for which a single spin-torque oscillator (STO) has been proposed. The performance of a single STO will be limited, though. To solve this issue, the authors study reservoir computing on an $a\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}y$ of STOs, showing numerically that the system's performance can improve with more STOs, and can become remarkably better than for a standard neural-network model. Interestingly, performance is best near a boundary between synchronized and disordered states, suggesting an enhancement at the edge of chaos.

Reservoir Computing on Spin-Torque Oscillator Array

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.

http://iopscience.iop.org/article/10.1088/0957-4484/25/24/245501/pdf

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

According to an embodiment, a magnetic recording apparatus includes following elements. The spin torque oscillator generates a first oscillating magnetic field. The recording medium unit includes one or more recording medium layers which are stacked, each of the one or more recording medium layers including a recording medium and spin-wave lines each of which generates a second oscillating magnetic field. The write magnetic field source generates a write magnetic field. The controller is configured to control the spin torque oscillator, the spin-wave lines, and the write magnetic field source to simultaneously apply the write magnetic field, and the first and second oscillating magnetic fields to target medium magnetization in the recording medium.

Hirofumi Suto

Papers

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

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

Reservoir Computing on Spin-Torque Oscillator Array

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

Magnetic recording apparatus