Kazuhisa Okano

Magnetic tunnel junction device with perpendicular magnetization films for high-density magnetic random access memory

We present here a magnetic tunnel junction device using perpendicular magnetization films designed for magnetic random access memory (MRAM). In order to achieve high-density MRAM, magnetic tunnel junction devices with a small area of low aspect ratio (length/width) is required. However, all MRAMs reported so far consist of in-plane magnetization films, which require an aspect ratio of 2 or more in order to reduce magnetization curling at the edge. Meanwhile, a perpendicular magnetic tunnel junction (pMTJ) can achieve an aspect ratio=1 because the low saturation magnetization does not cause magnetization curling. Magnetic-force microscope shows that stable and uniform magnetization states were observed in 0.3 μm×0.3 μm perpendicular magnetization film fabricated by focused-ion beam. In contrast, in-plane magnetization films clearly show the presence of magnetization vortices at 0.5 μm×0.5 μm, which show the impossibility of information storage. The PMTJ shows a magnetoresistive (MR) ratio larger than 50% w...

Human magnetoencephalogram measurements using newly developed compact module of high-sensitivity atomic magnetometer

In the field of biomagnetic measurements, optically pumped atomic magnetometers (OPAMs) are expected to be alternative sensors to magnetometers based on superconducting quantum interference devices (SQUIDs). In addition, miniaturized OPAMs are required for practical use. To address this issue, we developed a compact module of a high-sensitivity OPAM with a pump–probe arrangement and potassium used as the sensing atom. Because the noise spectrum density of the OPAM reached 21 fTrms/Hz1/2 at 10 Hz, we attempted to use it to measure human magnetocardiograms (MEGs). Compared with the results obtained with SQUID-based magnetometers, we could successfully observe distinct features of event-related desynchronization in the 8–13 Hz band associated with eye opening. The results demonstrate the feasibility of using the OPAM module for neuromagnetic field measurements.

Development of high-sensitivity portable optically pumped atomic magnetometer with orthogonal pump and probe laser beams

Optically pumped atomic magnetometers (OPAMs) have been demonstrated to show sensitivities better than the order of a few femtoteslas. In this study, we developed a portable potassium OPAM module using an electrically heated oven and orthogonal pump and probe beams coupled from polarization-maintaining optical fibers. The OPAM module shared a volume as small as 700 cm3, and its footprint was as small as 60 × 60 mm2; it operated with a single-channel sensitivity of 14 fTrms/Hz1/2 at 100 Hz when the 2-cm cubic potassium cell was heated to 180 °C. In our module, the optical beam path was folded along the magnetic field to be measured. The development of a magnetic sensor device comprising an array of OPAM modules will be an important step toward realizing a biomagnetic imaging system based on OPAMs. The orthogonal two-beam OPAMs will be beneficial for realizing a flexible placement of the array.

A plateau in the sensitivity of a compact optically pumped atomic magnetometer

In a compact optically pumped atomic magnetometer (OPAM), there is a plateau in the sensitivity where the dependence of the sensitivity on pumping power is small compared with that predicted by the uniform polarization model. The mechanism that generates this plateau was explained by numerical analysis. The distribution of spin polarization in the alkali metal cell of an OPAM was modeled using the Bloch equation incorporating a diffusion term and an equation for the attenuation of the pump beam. The model was well-fitted to the experimental results for a module with a cubic cell with 20 mm sides and pump and probe beams with 8 mm diameter. On the plateau, strong magnetic response was generated at the regions that were not illuminated directly by the intense pump beam, while at the same time spin polarization as large as 0.5 was maintained due to diffusion of the spin-polarized atoms. Thus, the sensitivity of the magnetometer monitored with a probe beam decreases only slightly with increasing pump beam int...