OPAL-RING
Yoshinobu NAKATANI Laboratory

Magnetization Structure in Magnets

Our laboratory develops simulation techniques, primarily for use in micromagnetics research, including research on hard disks, magnetoresistive random access memory (MRAM), and magneto-optical disks.
Based on detailed observations of the internal structure of magnets, they can be regarded as being composed from units having magnetic moment with N and S poles like the magnetic moment of atoms, and in even smaller units, the magnetic moment of electrons. The magnetic moments are not aligned uniformly throughout a magnet, resulting in the formation of a variety of magnetization structures.

Micromagnetics

Micromagnetics is a field of physics that studies the magnetization structures created by atomic magnetic moments present within a magnet and the dynamic behavior of magnets. While theoretical discussions and experimental observations of magnetization structures within a magnet have been underway since the beginning of the 20th century, theoretical studies have only been able to deal with simplified models. A large gap exists between theory and observation.
One way to bridge this gap is numerical simulation. However, the associated calculations require extensive computation time, and simulations exploring problems involving magnetic behavior on the scales of interest in experiments remained unfeasible for a long time. Then, in the 1980s, fine-structured magnetic bodies were developed for magnetic recording media, the Bloch line memory concept was introduced, and the size of simulation targets became quite small. Around the same time, third-generation supercomputers began running at various research institutes, greatly facilitating the massive calculations involved. This marked the real start of efforts in the field of micromagnetics to analyze magnetization structures through simulations.
In recent years, advances in microfabrication techniques have led to the production of nanosized magnets, accompanied by proposals for devices that would incorporate these magnets. Conventionally, magnetic moment manipulation had involved applying a magnetic field (for example, magnetic fields generated by electric currents). More recently, research results concerning theory and experimental results have been published on a method allowing direct manipulation of magnetic moments by electric currents (spin current). The pace of research in this area is accelerating.

A Coherent Workflow, from Tools, Ideas, Simulation, to Publication

Our laboratory began developing simulation techniques in the 1980s. Since then, we’ve undertaken collaborative research and development with other universities and with private sector entities on Bloch line memory, magnetic recording devices, magneto-optical recording devices, and other such devices.
Starting in 2006, we embarked on a joint project with NEC, Fujitsu Laboratories, and Kyoto University to develop MRAM. While laboratories now often use commercially-available tools for research, we build our own analysis tools. This allows us to perform analysis beyond the capabilities of the commercial tools currently available. Our laboratory is one of the few in the world that tackles the entire gamut of research-from the initial stages of conceptualization to simulation to the publication of results.

Applications in Industry and Research and Development

One example of an application of our results to industry is the development of the reproducing head and recording medium for magnetic recording devices (HDDs). The reproducing heads first proposed in the 1990s (e.g., MR, GMR, TMR) used thin magnetic plates as sensors. In these heads, the signals reproduced are transmitted as changes in the magnetization structure of the sensor attributable to the magnetic field of the recording medium. This requires analysis of the magnetization structure of the sensor body.
Our micromagnetics simulation technique has become an essential tool in development these heads. The size of the magnetic particles comprising recording media have become so small that the particles are now subject to thermal fluctuations, an aspect that can generally be ignored in macroscopic materials. Since these thermal fluctuations cause changes in the recorded magnetization structures, considerable research and development is now underway to develop magnetic media structures resistant to the effects of thermal fluctuations. Here, too, our simulation techniques have been adopted as an analytical tool.

The MRAM Project

A recent proposal involves MRAM, a technology that promises data information even after power is switched off, write speeds comparable to SRAM (static RAM), density comparable to DRAM (dynamic RAM), and with no limits on write/erase cycles. Given the short start-up times and low energy consumption associated with this technology, the interest in commercialized products is intense.
This MRAM project is already underway. To date, the main theme of research has been its drive method via a magnetic field. Studies have now begun on a drive method based on spin currents, which would allow higher-density designs.
Considerable room remains for MRAM modifications with respect to the shape of the magnets and recording methods. Some proposals involve techniques radically different from previously studied methods. Simulation has become an essential tool for analysis in this area as well.

Practical Applications of MRAM

At our laboratory, we first propose a calculation model, and then evaluate the model with respect to calculation criteria and accuracy. Finally, the proposed model is used to make a comprehensive analysis of the problem from both the physical and engineering perspective to achieve the research goal.
Based on the results of our research, we hope to devise technologies that result in viable MRAM.