Parity-odd magnetoelectric multipoles such as magnetic quadrupoles and toroidal dipoles contribute to various symmetry-dependent magnetic phenomena and formation of exotic ordered phases. However, the observation of domain structures emerging due to symmetry breaking caused by these multipoles is a severe challenge because of their antiferromagnetic nature without net magnetization. Here, we report the discovery of nonreciprocal linear dichroism for visible light (~4% at 1.8 eV) in a magnetic quadrupole ordered phase of antiferromagnetic Pb(TiO)Cu 4 (PO 4) 4 , which enables the identification of magnetic quadrupole domains of opposite signs. Symmetry considerations indicate that nonreciprocal linear dichroism is induced by the optical magnetoelectric effect, i.e., the linear magnetoelectric effect for electromagnetic waves. Using the nonreciprocal linear dichroism, we successfully visualize spatial distributions of quadrupole domains and their isothermal electric-field switching by means of a transmission-type polarized light microscope. The present work exemplifies that the optical magnetoelectric effect efficiently visualizes magnetoelectric multipole domains responding to external perturbations.

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Magnetoelectric multiferroic materials can exhibit a variety of functional properties such as electric field control of magnetization and nonreciprocal electromagnetic responses. Such a magnetoelectric response may be further enriched by the combination of the magnetoelectric order and a peculiar crystallographic order, such as crystal chirality. Recently, it was reported that a chiral‐lattice magnet Pb(TiO)Cu4(PO4)4 showing a magnetoelectric quadrupole order in its ground state exhibits anomalous chirality‐induced tilt of magnetization vector with respect to an applied magnetic field. In this progress report, additional results that advance the understanding of chirality‐induced tilt of magnetization vector are presented. It is found that chirality‐induced tilt of the magnetization vector also exists in a magnetic‐field‐induced ferroelectric (FI‐FE) phase of this compound that is stabilized in magnetic fields higher than 16 tesla. The resulting transverse component of the magnetization can be switched with an applied electric field through a polarization reversal. The analysis indicates that this transverse component is as large as ≈0.014 μB per f.u, suggesting that the tilting angle of the magnetization in the FI‐FE phase is much larger than that in the low‐field phase. Also, as another research progress, electric field control of nonreciprocal directional dichroism in the FI‐FE phase is demonstrated.

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