Researchers reveal the dynamic nature of emergent magnetic monopoles in real magnets for the first time

Magnetic monopoles are elementary particles with magnetic charges isolated in three dimensions. In other words, they behave like isolated north or south poles of a magnet. Magnetic monopoles have attracted continued research interest since physicist Paul Dirac’s first proposal in 1931. However, true magnetic monopoles have yet to be observed, and their existence remains an open question. On the other hand, scientists have discovered quasi-particles that mathematically behave like 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 anti-hedgehogs, which is called a magnetic hedgehog lattice.

In these particular configurations, the magnetic moments are directed radially outward (hedgehog) or inward (anti-hedgehog), resembling the spines of a hedgehog. These urchins and antiurchins act as 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 replacing 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, offers a promising avenue for studying and controlling the properties of hedgehog gratings. Moreover, these magnetic monopoles can also induce electric fields by moving following Maxwell’s laws of electromagnetism. To understand the new physical phenomena that result, it is essential to study the inherent excitations of hedgehog gratings.

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-HL in MnSi_1-xGe_x and SrFeO₃. Their study was published in the journal Physical review papers on May 31, 2024.

“Our research elucidated the unknown dynamical nature of emergent magnetic monopoles in magnetic materials for the first time. This may inspire future experiments on hedgehog-bearing materials with applications in electronic devices and for bound particle physics and condensed matter physics, ” says Mochizuki.

Using the three-dimensional Kondo lattice model, the researchers reproduced two distinct 4Q-HLs found in MnSi_1-xGe_x and SrFeO₃ and analyzed their dynamic properties. They found that 4Q-HLs have collective excitation modes associated with the oscillation of Dirac strings.

A Dirac string is a theoretical concept in quantum mechanics that describes a string connecting 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 arrays, providing 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 discovery provides insights into the dynamics of hedgehog lattices in other magnets as well. Furthermore, the finding enables us to turn on and off the excitation modes by controlling the presence or absence of Dirac strings with an external magnetic field.

Explaining the significance of their results, Eto said: “The collective spin excitation modes detected in the study are elementary excitations that directly reflect the presence (or absence) of emergent magnetic monopoles. Thus, our findings will be a fundamental guide for the study more detailed dynamic nature of emergent monopoles in magnetic materials in the future.

“Furthermore, they can 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 avenues of research in fundamental physics and lead to the development of new technologies involving emergent magnetic monopoles in magnets.