Yoshinori Tokura

It was recently realized that topological spin textures do not merely have mathematical beauty but can also give rise to unique functionalities of magnetic materials. An example is the skyrmion—a nano-sized bundle of noncoplanar spins—that by virtue of its nontrivial topology acts as a flux of magnetic field on spin-polarized electrons. Lorentz transmission electron microscopy recently emerged as a powerful tool for direct visualization of skyrmions in noncentrosymmetric helimagnets. Topologically, skyrmions are equivalent to magnetic bubbles (cylindrical domains) in ferromagnetic thin films, which were extensively explored in the 1970s for data storage applications. In this study we use Lorentz microscopy to image magnetic domain patterns in the prototypical magnetic oxide–M-type hexaferrite with a hint of scandium. Surprisingly, we find that the magnetic bubbles and stripes in the hexaferrite have a much more complex structure than the skyrmions and spirals in helimagnets, which we associate with the new degree of freedom—helicity (or vector spin chirality) describing the direction of spin rotation across the domain walls. We observe numerous random reversals of helicity in the stripe domain state. Random helicity of cylindrical domain walls coexists with the positional order of magnetic bubbles in a triangular lattice. Most unexpectedly, we observe regular helicity reversals inside skyrmions with an unusual multiple-ring structure.

Multiferroics with coexistent ferroelectric and magnetic orders can provide an interesting laboratory to test unprecedented magnetoelectric (ME) responses and their possible applications. One such example is the dynamical and/or resonant coupling between magnetic and electric dipoles in a solid. As examples of such dynamical ME effects, (i) the multiferroic domain wall dynamics and (ii) the electric dipole active magnetic responses are discussed with an overview of recent experimental observations.

Electromagnetic field (EMF) is the most fundamental field in condensed-matter physics. Interaction between electrons, electron-ion interaction, and ion-ion interaction are all of the electromagnetic origin, while the other 3 fundamental forces, i.e., gravitational force, weak and strong interactions are irrelevant in the energy/length scales of condensed-matter physics. Also the physical properties of condensed-matters such as transport, optical, magnetic and dielectric properties, are almost described as their electromagnetic responses. In addition to this EMF in the low energy sector, it often happens that the gauge fields appear as the emergent phenomenon due to the projection of the electronic wavefunctions onto the curved manifold of the Hilbert sub-space. These emergent electromagnetic fields (EEMF's) play important roles in many places in condensed-matter physics including the quantum Hall effect, strongly correlated electrons, and also in non-interacting electron systems. In this article, we describe its fundamental idea and some of the applications recently studied.

Cross correlation between magnetism and electricity in a solid can host magnetoelectric effects, such as magnetic (electric) induction of polarization (magnetization). A key to attain the gigantic magnetoelectric response is to find the efficient magnetism–electricity coupling mechanisms. Among those, recently the emergence of spontaneous (ferroelectric) polarization in the insulating helimagnet or spiral‐spin structure was unraveled, as mediated by the spin‐exchange and spin–orbit interactions. The sign of the polarization depends on the helicity (spin rotation sense), while the polarization direction itself depends on further details of the mechanism and the underlying lattice symmetry. Here, we describe some prototypical examples of the spiral‐spin multiferroics, which enable some unconventional magnetoelectric control such as the magnetic‐field‐induced change of the polarization direction and magnitude as well as the electric‐field‐induced change of the spin helicity and magnetic domain.

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Conventional electromagnets are made from coils of wire, but the search is on for materials that become magnets simply by applying an electrical current. Compounds with unusual spin arrangements offer many possibilities.