The current information recording media (memories) use the electro-magnetic properties of semiconductors and magnetic materials. In principle, their information recording densities per area are now approaching the limit. Various studies are now looking to overcome this limit, and one of the most promising candidates for the breakthrough is the recoding method that uses magnetic skyrmions^*1 (hereafter referred to as skyrmion or skyrmions).

The skyrmion is a nanometer-sized particle made of magnets (spins), and it can be used as bits in next-generation ultra-high-density memory, which is the reason that its research is actively underway all over the world. However, the research on skyrmions has mostly focused on devices, materials, and current driving techniques, and only a limited effort has been spent related to the research on how to read information from skyrmions. The development of a simple high-speed reading method has been requested.

The research group led by Youtarou Takahashi (Associate Professor of Graduate School of Engineering, The University of Tokyo) focused on the magneto-optical Kerr effect (described later), which rotates the polarization plane^*2 of light reflected from a magnetized material. For the first in the world, it observed the “topological magneto-optical Kerr effect,” where a skyrmion rotates the polarization plane of light in a material with a high density of skyrmions. This observation suggests it will lead to a simple high-speed reading method that is essential for realizing the next-generation skyrmion memory. This achievement is one step forward to developing skyrmion devices incorporating laser photonics in the future (Fig. 1).

*1: Magnetic skyrmion
A structure where electron spins (magnetic moments) in a substance are arranged in a nanometer-sized swirl. It has the property of a topologically stable particle and functions as an information bit. It is desired to be used as a memory.

*2: Polarization plane
Light is a transversal wave in which electric and magnetic fields oscillate in a plane perpendicular to the direction of propagation. The polarization plane is a plane that contains the oscillating electric field and the propagation direction. This phenomenon that twists (rotates) the polarization plane is one of the magneto-optical effects.

Skyrmions are tiny nanoscale spin-swirling objects that are highly stable. They can be driven with a small current and packed into a high-density structure. These special properties attract attention as a candidate for memory, and active research is underway all over the world. While the research of skyrmion materials is being advanced, the development of a new simple method of quickly reading the information recorded by using skyrmions has been requested.

Until now, researchers have detected skyrmions electrically using the Hall effect (a phenomenon in which applying a magnetic field perpendicular to a current induces an electromotive force in a direction perpendicular to both the current and the magnetic field) that involved emergent magnetic fields (described later), which is called the topological Hall effect^*3. This method cannot break the upper limit of the reading speed, which presented a request for a faster reading method that detects optical reactions. However, no one has ever observed an optical reaction that enables reading skyrmions at an ultra-high speed, despite the fact that this research is important for both the basic science and realization of skyrmion memory.

*3: Topological Hall effect
The Hall effect is induced by an emergent magnetic field. The general Hall effect is caused by a magnetic field that bends the electron path, and an emergent magnetic field caused by skyrmions also bends electron paths to induce an effective Hall effect. This effect can be used to detect skyrmions electrically.

The research group focused on the compound Gd₂PdSi₃ that has a skyrmion lattice (Fig. 2(b)) in which skyrmions are densely arrayed, as shown in Fig. 2, to measure the magneto-optical Kerr effect.

The magneto-optical Kerr effect is a phenomenon in which, when linearly polarized light is incident on a magnetized magnetic material, its reflected light produces a polarization rotation. Together with the Faraday effect that rotates the transmitted light polarization, these effects are called the magneto-optical effects. These effects have been used as the principle of measuring the magnetic properties of substances and as the reading principle of magneto-optical devices, such as opto-isolators and magneto-optical disks.

The skyrmion produces a magnetic field from the interaction between the spin structure and electrons in a crystal. This magnetic field is not real but virtual; it is called the “emergent magnetic field,” where electrons behave as if a magnetic field exists. In general, the rotation angle of polarization is proportional to the magnitude of magnetization of a substance, while an emergent magnetic field induced by skyrmions rotates the polarization regardless of the magnitude of magnetization of the substance. This phenomenon is the topological magneto-optical Kerr effect, which is the principle of reading skyrmions.

The research group actually used Gd₂PdSi₃ to measure the magneto-optical Kerr effect over a wide frequency band of light. The results showed that the polarization plane rotates significantly when skyrmions emerge. In addition, when skyrmions are eliminated by applying a magnetic field, this polarization rotation also ceases, which shows that this effect was caused by the emergent magnetic field induced by skyrmions (Fig. 2(c)). This observation of optical reactions paves the new way for the development of the ultra-high-speed skyrmion reading method, which is essential to skyrmion memory.

This research showed that the topological magneto-optical effect could be induced in the near-infrared range. Since various laser techniques are available in the near-infrared range, combining these findings with laser photonics is expected to enable reading skyrmions more precisely.

Furthermore, in addition to the application of skyrmion devices, such as memories, this research achievement is significant in terms of basic science. While the magneto-optical Kerr effect is a magnetic field detection method widely used in the field of spintronics and other physics, this newly observed topological magneto-optical effect comes from the emergent magnetic field induced by skyrmions, which is a new magneto-optical phenomenon that does not depend on the magnitude of magnetization of a substance. Conventionally, it was thought that using heavy elements with large atomic numbers was required in order to obtain a large magneto-optical effect. However, using emergent magnetic fields provides a prospect that even relatively inexpensive materials with small atomic numbers may induce a large magneto-optical effect.