Nanotechnology Supporting Tera-scale Information Storage

In modern society, those who control information are the victors. The unsung hero supporting this information society is storage technology, the "warehouse of information." This technology, which inexpensively stores vast amounts of terabyte-scale information that may be used at any time, without corruption, and allows for quick access when needed, may not be glamorous, but it is indispensable to modern society. Unfortunately, no single storage device yet exists that can perform this role ideally, but storage is currently realized through system configurations centered around hard disk drives (HDDs).

In an HDD, small permanent magnets are created on a magnetic disk, and digital information of 1s and 0s is recorded as the N and S poles of these magnets. Even now, the size of these magnets is on the order of several tens by several hundreds of nanometers, and efforts are underway to make them even smaller. The difficulty in storage lies not only in shrinking the recording magnets but also in accurately reading the information within a short time frame of a few nanoseconds. Therefore, we are advancing the research and development of sensors that can convert the weak magnetic fields from these tiny magnets into electrical signals with high sensitivity.

When two magnetic plates, such as cobalt or iron, are separated by an insulator by a small distance of a few nanometers and a voltage is applied, a small current flows due to the tunnel effect. The magnitude of this current changes depending on the spin of the electrons within the magnetic plates. Spin is a quantum mechanical quantity that describes the state of an electron, and this phenomenon is called the spin magnetoresistive effect. If we can make the flowing current change significantly when the electron spin in one of the magnetic plates changes slightly due to a minute magnetic field, we can create a highly sensitive magnetic sensor capable of detecting even smaller recorded information. In other words, this allows for the storage of a larger amount of information.

The reason for separating the two magnetic plates is to prevent their magnetic states from directly influencing each other. In practice, a thin insulating film, a non-magnetic metal film, or a combination of both is used. By selecting various materials and states, we aim to enhance the spin magnetoresistive effect.

[Note] Tera: a prefix for units representing one trillion (10^12). Nano: a prefix for units representing one billionth (10^-9).