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Innovative spintronics created by phantom particles

In 1928, the British physicist Paul Dirac devised the Dirac equation to extend quantum theory to relativity. The following year, the German mathematician Hermann Weyl found a solution for the massless Dirac particle. This particle, also called the Weil particle, has long been studied as the elementary particle solution describing neutrinos. However, experiments conducted by a research team at the Super-Kamiokande at the University of Tokyo led to the discovery that neutrinos have mass, and the Weil particle was thought to be a "phantom particle" that did not actually exist. Recently, the Weil particle was discovered in a magnetic material by a team at the University of Tokyo. The name of this magnetic material is "antiferromagnet. This is a completely different property from that of ferromagnetic materials, which we are familiar with as magnets. For example, ferromagnetic bodies have magnetization so that they emit magnetic field lines around them, while antiferromagnetic bodies have no magnetization.

Ferromagnetic bodies have been used as compasses since BC. They are also indispensable for motors and power generation using electromagnetic induction. Recently, they are also used in non-volatile memory devices that do not require standby power to prolong the battery life of smartphones. On the other hand, since antiferromagnets do not have magnetization, their existence went unnoticed for a long time, and mankind first confirmed their existence about 70 years ago. However, antiferromagnets are now attracting the keen attention of scientists worldwide as a memory material that is superior to ferromagnets. Not only is it the best for miniaturizing memory because it does not emit a leakage field around it, but its operating speed is said to be two orders of magnitude faster than that of the innovative spintronic memory created by ferromagnetic particles.

Thanks to the Weil particle discovered in the magnetic body mentioned above, this antiferromagnetic state without magnetization will be easily detectable. Weil particles have the property of macroscopically enhancing the quantum mechanical phase effect of electrons, which makes the detected signal 100 to 1000 times larger than usual. For example, the anomalous Hall effect, which has only been observed in ferromagnetic materials, has recently been found to be detectable in antiferromagnetic materials. This is thanks to the Weil particle, and is indeed a remarkable achievement after a century since the Hall effect was discovered in the latter half of the 19th century.

In order to use the antiferromagnetic body with the Weil particle, "Weil antiferromagnet," for memory, it is necessary to make it possible to completely control the binary signals of 0 and 1 that the antiferromagnetic state exhibits by means of an electric current instead of a magnetic field. This is the first time in the world that this has been achieved at room temperature. This achievement, which shows that the 0 and 1 states of antiferromagnetic states can be completely controlled by electric current, means that in the future it will be possible to control Weil particles and perform information operations in about 10 picoseconds.

In this way, concepts from the elementary particle and space fields are playing an active role in condensed matter physics, opening up a new field of condensed matter physics. Moreover, it leads to the construction of future applied technology.

Figure: Schematic diagram of an electrical writing experiment on a Weil antiferromagnet Mn3Sn device (a) Distribution of Weil particle pairs at current and their current control in an inverted device consisting of Weil antiferromagnet Mn3Sn and heavy metal W. By changing the direction of the write current, the direction of the torque created by the spin current generated in W also changes, and the antiferromagnetic order of Mn3Sn and the corresponding direction of the Weil particle pairs can be controlled. As a result, writing and reading can be performed using detection signals corresponding to the 0 and 1 information created by the Weil particles. (b) Magnetic field dependence of the Hall voltage in the W/Mn3Sn device. In the magnetic field dependence, the external magnetic field is applied perpendicular to the film surface. (c) Dependence of the Hall voltage on the write current in the W/Mn3Sn device. The vertical axis on the right shows a 100% inversion rate. (d) Magneto-optical Kerr effect microscope image of a W/Mn3Sn device during an inversion experiment, where the entire area of the device is inverted from black to gray in response to magnetic octupolar polarization by applying write current in the x direction . (i) and (ii) correspond to the observed images of the device in (i) and (ii) in Figure (c).

The results of this study were published in T. Higo et al, Nature 607, 474 (2022).

(Press release, July 21, 2022)

Published in Faculty of Science News, November 2022

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