Discovery of iron sulfide in the exploration of altermagnetic materials

A research group led by Professor Shinichiro Seki of the Research Center for Advanced Science and Technology at the University of Tokyo has, for the first time in the world, discovered a material within the new class of magnetic materials known as altermagnets that enables information writing and reading at room temperature.

In this study, the group discovered that iron sulfide (FeS), a magnetic semiconductor, functions as an altermagnet at room temperature. It was further demonstrated that FeS can reliably retain information, even without an external magnetic field, and that the stored information can be read out as an electrical difference.

Conventional magnetic materials and their issues

Ferromagnetic materials used as magnetic recording elements

Behind the ability of magnets to attract iron and store information lies the phenomenon known as magnetism. Magnetism originates from the properties of electrons in matter. Electrons behave like tiny magnets, and this property is called spin. There are two spin directions: up ↑ and down ↓.

Normally, spins point in random directions, so no net magnetism is apparent. However, when an external magnetic field is applied, many spins align in the same direction. When this happens, the material becomes magnetized, and this phenomenon is called magnetization. Thus, substances that are easily magnetized are called magnetic materials.

Among magnetic materials, those where the spins align in the same direction and strong magnetization remains even after the magnetic field is removed are called ferromagnetic materials. Iron and cobalt are familiar examples and are used as information storage materials. Within ferromagnetic materials, it is possible to create states in which the spins are aligned either up or down. By mapping these two states to 0 and 1, information can be read and written.

Limitations of conventional ferromagnetic and antiferromagnetic materials

However, it is well known that the magnetic fields generated by ferromagnetic materials also influence their surroundings. Unintended magnetic fields that appear around magnetic materials are called leak fields. When the magnetic memory elements of a storage device are miniaturized and densely arranged, these leak fields can affect neighboring elements, potentially rewriting them into states that differ from the information you wanted to save. Therefore, there were limits to increasing the density of memory elements. In addition, because the way to rewrite information was by reversing spin directions, the operating speed was constrained, making it hard to further increase the information processing speed.

On the other hand, there are other magnetic materials called antiferromagnets, in which adjacent spins are always aligned in opposite directions, causing the overall magnetization of the material to cancel out to zero. This means that no externally detectable magnetic field is generated, making it difficult to physically distinguish and read out the ↑↓ and ↓↑ spin arrangements. Therefore, they are considered unsuitable for information storage.

Iron sulfide enables information read/write operations at room temperature

In recent years, the concept of altermagnets has been theoretically proposed. These materials would combine antiferromagnet-like spin arrangements with unique crystal symmetries to enable information reading and writing, even when net magnetization is zero.

In altermagnets, adjacent spins are aligned in opposite directions, just as in antiferromagnets. However, because the crystal structure is not symmetric, the two spin arrangements ↑↓ and ↓↑ are internally distinct states. As a result, even though the overall magnetization of the material is zero, differences in spin arrangement manifest as differences in how the electrons move within the crystal (Fig. 1).

In altermagnets, despite the absence of any external magnetic field, electrons show behavior as though a magnetic field were present due to quantum mechanical effects. This effect is called a fictitious field. When the direction of the fictitious field differs, the direction of the transverse voltage generated when a current passes through the material also changes. By utilizing this property, it becomes possible to distinguish between the two spin configurations, ↑↓ and ↓↑, as electrical differences rather than magnetic ones (Fig. 2).

In this study, through researching materials that can function as altermagnets, the group discovered that FeS satisfies these conditions at room temperature. FeS, much like antiferromagnets, has neighboring spins aligned in opposite directions, while also possessing the unique atomic arrangement characteristics of altermagnets, thereby matching the theoretically proposed properties of altermagnets.

Furthermore, the research group demonstrated that the two spin configurations, ↑↓ and ↓↑, can be switched by an external magnetic field and that these states are stably retained, even after the magnetic field is removed. This means that information can be reliably recorded. In addition, these two spin configurations could be clearly distinguished by the difference in Hall resistivity^*1. In other words, this experiment demonstrated that FeS can be utilized as an altermagnet at room temperature.

*1 Hall resistivity
A quantity that represents the magnitude of the voltage that appears laterally to the flow of an electrical current as it passes through a material

Applications as a next-generation information storage medium

Altermagnets, having zero net magnetization, generate no stray magnetic fields, thereby enabling higher density in magnetic memory elements. Furthermore, the response speed is reportedly over 100 times faster than that of ferromagnetic materials, so even-faster information processing applications are expected. Moreover, altermagnets of FeS are highly promising as the next generation of magnetic memory elements because they can read and write information at room temperature.

This achievement demonstrates the existence of altermagnets that function at room temperature and marks a major step toward realizing the next generation of ultra-high-density and ultra-high-speed information storage technologies.