Takehiro Yamaoka

Current-induced resonance and mass determination of a single magnetic domain wall.

A magnetic domain wall (DW) is a spatially localized change of magnetization configuration in a magnet. This topological object has been predicted to behave at low energy as a composite particle with finite mass. This particle will couple directly with electric currents as well as magnetic fields, and its manipulation using electric currents is of particular interest with regard to the development of high-density magnetic memories. The DW mass sets the ultimate operation speed of these devices, but has yet to be determined experimentally. Here we report the direct observation of the dynamics of a single DW in a ferromagnetic nanowire, which demonstrates that such a topological particle has a very small but finite mass of 6.6 × 10-23 kg. This measurement was realized by preparing a tunable DW potential in the nanowire, and detecting the resonance motion of the DW induced by an oscillating current. The resonance also allows low-current operation, which is crucial in device applications; a DW displacement of 10 µm was induced by a current density of 1010 A m-2.

Electric spectroscopy of vortex states and dynamics in magnetic disks

Spin-polarized radio frequency (RF) currents and RF-Oersted fields resonantly excite a magnetic vortex core confined in a micron-scale soft magnetic disk. In this study, we measured the rectifying voltage spectra caused by the anisotropic magnetoresistance oscillation due to the gyration of the vortex with different polarity and chirality. The measured spectra are presented such that we can determine the vortex properties and strength of the spin torques and Oersted field accurately and directly through analytical calculation.

Applications of high-resolution MFM system with low-moment probe in a vacuum

Magnetic force microscopy (MFM) is very useful for observing magnetic domain structures. However, due to stray fields from an MFM probe, observations of small magnetic domain structures are limited. The authors have developed a high-resolution MFM system that utilizes a low-moment probe and a quality (Q)-controlled prove driver, which allows high-quality measurement in a vacuum without disturbing domain structures. Using this system, a resolution finer than 20 nm was achieved. In this paper, the advantages of this MFM are demonstrated using a Permalloy honeycomb nanonetwork and a Permalloy semicircular loop.

Critical Dimension Measurement Using New Scanning Mode and Aligned Carbon Nanotube Scanning Probe Microscope Tip

We have developed a new scanning mode and an aligned carbon nanotube tip for atomic force microscopy (AFM) for measuring the critical dimension of deep structures. The aligned carbon nanotube (A-CNT) was assembled in the scanning electron microscope (SEM) chamber. The diameter of the tip is uniformly around 20 nm and the tip attachment angle is within ±1.5° to the sample normal. The aspect ratio (length/diameter) of the tip is greater than 30. The new scanning mode is composed of two functions, namely transporting the tip along the steep trench structure and detecting the sample surface. This mode can faithfully trace the steep side wall using a flexible CNT tip without damaging the tip. The critical dimension (CD) measurements of the shallow trench isolation (STI) were performed using the newly developed scanning mode and the A-CNT tip.

Domain structures and magnetic ice-order in NiFe nano-network with honeycomb structure

The magnetic domain configurations and the magnetization processes in a permalloy wire-based honeycomb nano-network have been investigated by means of magnetic-force microscopy and magnetoresistance measurement. The magnetic structure is mainly governed by the magnetic interaction among the magnetic pole on the vertices, being similar to the so-called “ice-rule.” The magnetization vector in a wire behaves coherently. The present results seem to give a direct analogy between the honeycomb network and an Ising system on a kagome lattice. The ice-rule type interaction, however, disappears with reducing magnetic energy at the vertices.

Current manipulation of a vortex confined in a micron-sized Fe19Ni81 disk

By measuring a rectifying planer Hall effect, we have manipulated a vortex core trapped in a single layered Fe19Ni81 disk dependent upon the magnitude of a dc current simultaneously applied with an rf current and a magnetic field. The observed behavior is attributed to a single vortex translational mode. The resonance frequency of the translational mode is found to be almost proportional to the magnitude of the dc current and to be governed by the shape of the energy potential well defined by the disk shape.

Electrical detection of vortex states in a ferromagnetic disk using the rectifying effect

A magnetic vortex core confined in a micron-scale magnetic disk is resonantly excited by both spin-polarized rf current and rf field. We found that rectifying voltage spectra caused by the resonance of vortex core are dependent not only on the core polarity, but also the chirality. These experimental results can be explained by analytically calculating the anisotropic magnetoresistance effect induced by the motion of the vortex core.

Instrumentation of the High-Vacuum Atomic Force Microscope

The high-vacuum atomic force microscope (HV-AFM) is a useful tool that is used in various sample environments and achieve good Z axis sensitivity in a noncontact mode operation because the cantilever is free from hydrodynamic damping. The instrument is designed with the aims of minimizing the measuring time, simplifying the operation and achieving high performance. A pumping pressure of about 10 -4 Pa is obtainable within 15 min. To achieve easy operation, the optical lever deflection detection head is placed outside the vacuum chamber. The vibration isolation is achieved using an elastic damper and a magnetic-floating-type turbomolecular pump. This vibration isolation method works well enough to obtain the atomic-resolution image of a highly oriented pyrolytic graphite (HOPG) sample. We checked the sensitivity of the instrument using the noncontact mode of operation of a magnetic force microscope (MFM). The Z axis sensitivity in vacuum operation was compared with that in ambient operation. The quality factor increased by more than 25 times and the Z axis sensitivity increased by more than 10 times in vacuum operation (10 -4 Pa).

Magnetic Structure of Y-shaped Permalloy Arrays Fabricated Using Damascene Technique

In this paper, we report on the observed magnetic spin structure and the micromagnetic simulation of Y-shaped permalloy (Ni80Fe20) arrays. The arrays were fabricated using the damascene technique, with electron-beam lithography. For a widely separated linear arrangement, magnetic poles are observed on the ends of two of three arms of the Y-shaped array and multidomains on the remaining arm. For a closely separated honeycomb arrangement and a pair of antisymmetrical dots (termed mirror dots), regularly aligned magnetic poles are observed. It is suggested that the honeycomb array or mirror dots have a strong magnetostatic interaction. The calculated spin distributions approximately correspond to the magnetic force microscopy (MFM) images, in good agreement with the experimental results.

Asymmetric field variation of magnetoresistance in Permalloy honeycomb nanonetwork

The magnetic properties of two-dimensional network comprising a Permalloy wire-based honeycomb structure were investigated by magnetic force microscopy and magnetoresistance measurement. These results indicate that the magnetization of the wire behaves homogenously like a binary bit and that the magnetic interaction at the vertex governs this magnetization. This allows us to achieve a magnetoelectronic device, based on the magnetic interaction among the wires.