Uncovering the nature of emergent magnetic monopoles

Magnetic monopoles are elementary particles with isolated magnetic charges in three dimensions. In other words, they behave as isolated north or south poles of a magnet.  Magnetic monopoles have attracted continuous research interest since physicist Paul Dirac’s first proposal in 1931. However, real magnetic monopoles have not yet been observed and their existence remains an open question. On the other hand, scientists have discovered quasi-particles that mathematically behave as magnetic monopoles in condensed matter systems, resulting in interesting phenomena.

Recently, researchers discovered that a material called manganese germanide (MnGe) has a unique periodic structure, formed by special magnetic configurations called hedgehogs and antihedgehogs, which is called a magnetic hedgehog lattice. In these special configurations, the magnetic moments point radially outward (hedgehog) or inward (antihedgehog), resembling the spines of a hedgehog. These hedgehogs and antihedgehogs act like magnetic monopoles and antimonopoles, serving as sources or sinks of emergent magnetic fields. MnGe exhibits what is known as a triple-Q hedgehog lattice. However, recent experiments have shown that the substitution of Ge with Si (MnSi_1-xGe_x) transforms the arrangement into the quadruple-Q hedgehog lattice (4Q-HL). This new arrangement, also found in the perovskite ferrite SrFeO₃, provides a promising avenue for studying and controlling the properties of hedgehog lattices. Moreover, these magnetic monopoles can also induce electric fields through moving following Maxwell’s laws of electromagnetism. To understand the resulting new physical phenomena, it is essential to study the inherent excitations of hedgehog lattices.

In a recent study, Professor Masahito Mochizuki and Ph.D. course student Rintaro Eto, both from the Department of Applied Physics at Waseda University, theoretically studied the collective excitation modes of 4Q-HLs in MnSi_1-xGe_x and SrFeO_3­. “Our research clarified the unknown dynamical nature of emergent magnetic monopoles in magnetic materials for the first time. This can inspire future experiments on hedgehog-hosting materials with applications in electronic devices and for bridging particle physics and condensed-matter physics,” says Mochizuki. Their study was published in the journal Physical Review Letters on 31 May 2024.

Utilizing the three-dimensional Kondo-lattice model, the researchers reproduced the two distinct 4Q-HLs found in MnSi_1-xGe_x and SrFeO₃ and analyzed their dynamical properties. They discovered that the 4Q-HLs have collective excitation modes associated with the oscillation of Dirac strings. A Dirac string is a theoretical concept in quantum mechanics which describes a string that connects a magnetic monopole and a magnetic antimonopole, in this case, a hedgehog and an antihedgehog. The researchers found that the number of these excitation modes depends on the number and configuration of Dirac strings, offering a way to experimentally determine the spatial configuration of hedgehogs and antihedgehogs and their unique topology in real magnets such as MnSi_1-xGe_x and SrFeO₃. This finding offers insights into the dynamics of hedgehog lattices in other magnets as well. Moreover, the finding enables us to switch on and off the excitation modes through controlling the presence or absence of the Dirac strings with external magnetic field.

Explaining the significance of their results, Eto remarks, “The collective spin excitation modes revealed in the study are elementary excitations that directly reflect the presence (or absence) of emergent magnetic monopoles. Thus, our findings will be a fundamental guideline for studying more detailed dynamical nature of emergent monopoles in magnetic materials in the future. Moreover, they might become the building blocks of novel field-switchable spintronic devices such as nano-sized power generators, light-voltage converters, and light/microwave filters based on emergent electromagnetism.”

These discoveries have the potential to open new research avenues in fundamental physics and lead to the development of new technologies involving emergent magnetic monopoles in magnets.

The paper has been published, and its information is available from,

URL: https://link.aps.org/doi/10.1103/PhysRevLett.132.226705

DOI: 10.1103/PhysRevLett.132.226705

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Reference

Authors: Rintaro Eto¹ and Masahito Mochizuki¹

Title of original paper: Theory of Collective Excitations in the Quadruple-Q Magnetic Hedgehog Lattices

Journal: Physical Review Letters

DOI: https://doi.org/10.1103/PhysRevLett.132.226705

Affiliations:

¹Department of Applied Physics, Waseda University, Japan

About Waseda University

Located in the heart of Tokyo, Waseda University is a leading private research university that has long been dedicated to academic excellence, innovative research, and civic engagement at both the local and global levels since 1882. The University has produced many changemakers in its history, including nine prime ministers and many leaders in business, science and technology, literature, sports, and film. Waseda has strong collaborations with overseas research institutions and is committed to advancing cutting-edge research and developing leaders who can contribute to the resolution of complex, global social issues. The University has set a target of achieving a zero-carbon campus by 2032, in line with the Sustainable Development Goals (SDGs) adopted by the United Nations in 2015. 

To learn more about Waseda University, visit https://www.waseda.jp/top/en  

About Professor Masahito Mochizuki

Masahito Mochizuki is currently a Professor at the Department of Applied Physics at Waseda University, Japan. He obtained his M.S. and Ph.D. from the University of Tokyo in 2000 and 2003, respectively. He also leads his own research group at Waseda University, which focuses on theoretical research on the condensed-matter physics. He has over 100 publications with over 4,000 citations. He is also the recipient of the 2022 JPSJ Outstanding Referee Award from The Physical Society of Japan and the 2019 Waseda University Research Award. His research interests include magnetoelectric phenomena of multiferroics, topological magnetism, spintronics, multiorbital superconductivity, orbital physics in Mott insulators, and negative thermal expansion phenomena.

About Rintaro Eto

Rintaro Eto has been a Ph.D. course student in the Department of Applied Physics at the Graduate School of Science and Engineering at Waseda University in Tokyo since 2023. His research interests include topological magnetic textures, many-body theory for magnetic excitations, and nontrivial magnon band topology.