Exploring the Potential of New Magnetic Materials.

Tetsuya Sato (Professor, Department of Applied Physics and Physico-Informatics)

It is well known that magnetic materials (substances with magnetic properties) are used in a wide range of applications, such as storage media like hard disks and motors for electric vehicles. Magnetism originates from the fact that each atom and electron acts as a microscopic magnet. Among magnetic materials, besides ferromagnets (materials that are attracted to magnets) in which these microscopic magnets align in parallel, there are also antiferromagnets, where they align in an antiparallel fashion, and spin glasses, where they are oriented in random directions (Figure 1). Despite the existence of materials exhibiting such diverse magnetic properties, applications of magnetic materials have traditionally focused on improving the functionality of well-understood ferromagnets, which has somewhat limited the range of materials utilized.

In the Sato Laboratory, we are exploring the potential of new magnetic materials by reconsidering the fundamental principles of magnetism. We are researching methods to artificially transform non-magnetic materials into ferromagnets and investigating the potential applications of magnetic materials other than ferromagnets.

Figure 1. The arrangement of microscopic magnets in atoms for various magnetic materials. In a ferromagnet (a), the microscopic magnets all point in one direction. In an antiferromagnet (b), they point in alternating opposite directions. In a spin glass (c), they point in random directions.

Do you know how many substances composed of a single element exhibit ferromagnetism at room temperature? Excluding gadolinium, a metal that loses its ferromagnetism around room temperature, there are only three, as shown in red in Figure 2(a). That's quite a small number, isn't it? It seems natural, then, to think about finding ways to increase the number of ferromagnetic substances. One method we devised to turn non-magnetic materials into ferromagnets was to reduce them to the nanoscale. In 2003, the Sato Laboratory was the first to discover that by reducing palladium (Pd), a non-magnetic metal, to the nanoscale, ferromagnetism emerges on the particle surface (Figure 3). This is caused by a change in the energy state of electrons on the material's surface, which makes it more difficult for electrons within the metal to move. Following this discovery, various other materials that exhibit ferromagnetism at the nanoscale were found around the world (Figure 2(b)). Furthermore, by carefully considering this phenomenon, we can see that new ferromagnets can be created by applying an electric field or irradiating the material with light. We are currently proposing various methods to create new ferromagnets, aiming to further expand the range of substances that exhibit ferromagnetism.

Figure 2. Elements known to be ferromagnetic at room temperature (a) and elements in which ferromagnetism was discovered at the nanoscale after 2003 (b).

Figure 3. A transmission electron microscope image of Pd nanoparticles with a size of about 10 nm. The particles have a polyhedral structure, and it is believed that ferromagnetism appears on their square faces [the (100) plane].

We are also focusing on the fascinating memory phenomena exhibited by spin glasses, a type of magnetic material in which microscopic magnets are oriented in random directions (Figure 4). Spin glasses exhibit human-like memory, such as being able to store multiple types of information in a single region (multi-level memory) and recalling information when given a small cue (associative memory). This is related to the process where the complex interplay of forces between microscopic magnets causes their orientations to slowly approach a stable state. Elucidating this phenomenon is one of the major challenges in statistical physics. If this phenomenon can be successfully harnessed, it may be possible to create advanced memory devices that function just like human memory.

Figure 4. In a spin glass, the microscopic magnets point in random directions, but each of their diverse arrangement patterns can be considered a single piece of information. These arrangement patterns are related to multi-level memory and associative memory.

As introduced above, our research has been driven by a desire to pioneer a new world of materials by focusing on their potential (as magnetic materials) from a perspective different from conventional viewpoints, based on quantum mechanics and statistical physics. We are fascinated by the various forms that materials occasionally reveal. Believing that the expansion of this captivating world of materials will further enrich our lives, we intend to continue our research in the future.