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Superconductors are materials that offer no resistance to electrical current flowing through them. Combining their study with spintronics, which deals with the intrinsic spin of electrons and their use in electronics, has paved the way for the new field of superconducting spintronics. Of particular importance is the study of spin-triplet Cooper pairs, which consist of groups of electrons that move together as a single unit. This behavior is pivotal for generating what are known as supercurrents. Developing intricate logic and memory circuits that utilize this unique behavior demands answers to two critical questions: how to efficiently generate spin-triplet Cooper pairs and how to precisely control their behavior.
In response to these challenges, researchers have engineered spin-triplet superconducting valves capable of switching superconducting currents on and off as per the need. These valves require only a modest external magnetic field to actively manipulate the behavior of spin-triplet Cooper pairs. However, they are fabricated from ferromagnetic Josephson junctions (JJs) consisting of a thin layer of the non-superconducting material sandwiched between superconductors. They require complex and delicate engineering to prevent interference from stray magnetic fields.
To make the fabrication of spin-triplet superconducting valves easier, a team of researchers led by Assistant Professor Kun-Rok Jeon from the Department of Physics at Chung-Ang University, Korea, has now developed an antiferromagnetic analogue of the spin-triplet supercurrent spin valves. While ferromagnetic materials are magnetically attracted, antiferromagnetic materials effectively cancel out magnetic fields, displaying no magnetic attraction. The team employed the manganese-germanium compound Mn3Ge, a chiral antiferromagnetic substance, to craft the antiferromagnetic spin-triplet JJs, which hold significant promise for the development of superconducting spintronic circuits. Their paper was made available online on March 30, 2023, and was published in Volume 18 of the journal Nature Nanotechnology on July 2023.
Emphasizing the importance of this study, Dr. Jeon says: "Our proof-of-concept demonstration that spin-polarized triplet supercurrents can be turned on and off via the magnetic field-modulated Berry curvature in a single chiral antiferromagnet Mn3Ge offers a new paradigm that will interest scientists from across the fields of condensed matter physics and nanotechnology."
Fundamentally, the control of the spin-triplet Cooper pairs in antiferromagnetic materials hinges on the Berry curvature, a fundamental property of materials which indicates how the energy levels of electrons respond to an external magnetic field. The researchers modified the Berry curvature of Mn3Ge to produce "fictitious magnetic fields," enabling precise control of supercurrents with minimal energy input.
They theoretically verified the observed supercurrent behavior of the antiferromagnetic spin valve and leveraged it to fabricate a superconducting quantum interference device (SQUID), a sensitive magnetometer used to measure extremely subtle magnetic fields. These devices are used in a wide range of fields, including medical imaging, geophysics, and materials characterization. The functionality of the SQUID demonstrates the potential of antiferromagnetic spin valves in the realm of superconducting spintronics.
The present findings not only enhance our understanding of the role of Berry curvature in singlet-to-triplet pair conversion but also inspire future theoretical investigations into the intricate interplay between Berry curvature and spin-triplet pairing. Dr. Jeon highlights the broader implications of their work: "This study can advance the field of superconducting spintronics and potentially lead to a new generation of green supercomputers with much less operation energy compared with that of today's semiconductor technology."
