Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

We studied the high-field phase diagram of a chiral-lattice antiferromagnet Sr(TiO)Cu4(PO4)4 by means of ultrasound, dielectric, and magnetocaloric-effect measurements. These experimental techniques reveal two new phase transitions at high fields, which have not been resolved by previous magnetization experiments. Specifically, the c66 acoustic mode shows drastic changes with hysteresis for magnetic fields applied along the c axis, indicating a strong magnetoelastic coupling. Combined with cluster mean-field theory, we discuss the origin of these phase transitions. By considering the chiral-twist effect of Cu4O12 cupola units, which is inherent to the chiral crystal structure, the phase diagram is reasonably reproduced. The agreement between experiment and theory suggests that this material is a unique quasi-two-dimensional spin system with competing exchange interactions and chirality, leading to a rich phase diagram.

DOI： 10.1103/PhysRevB.108.054434

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

In a chiral medium, any mirror symmetries are broken, which induces unique physical properties represented by natural optical rotation. When electromagnetic waves propagate through a chiral medium placed in a magnetic field, the refractive index, or equivalently, the absorption encountered by the electromagnetic waves differs depending on whether it travels parallel or antiparallel to the magnetic field. Such a phenomenon is known as magnetochiral dichroism (MChD), which is the characteristic interplay between chirality and magnetism. Similar to chirality, the so-called ferroaxial order, an emergent ferroic state of crystalline materials, is also characterized by mirror symmetry breaking. In contrast to chiral materials, however, the mirror symmetry perpendicular to the crystalline principal axis is allowed in ferroaxial materials. In other words, chirality and thus phenomena unique to chirality can be induced by breaking the remaining mirror symmetry by applying an electric field. Here, we show electric control of chirality and resulting electric field–induced MChD (E-MChD) of the short-wavelength infrared region in a ferroaxial crystal, NiTiO3. We performed spectroscopy measurements of E-MChD by taking a difference of absorption coefficients obtained with and without electric and magnetic fields. As a result, E-MChD was observed around the excitation energy corresponding to Ni2+ d-d magnetic-dipole transitions. The result is nicely explained by adopting the theory of MChD concerning the pseudo-Stark splitting of the energy state. Ferroaxial materials therefore provide platforms to achieve electric control of chirality-related phenomena.

DOI： 10.1073/pnas.2303251120

PubMed

Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41567-022-01816-4

Other URL： https://www.nature.com/articles/s41567-022-01816-4

Publishing type：Research paper (scientific journal)  

The archetypal magnetoelectric (ME) antiferromagnet Cr2O3, which has the diagonal components of linear ME tensor, exhibits several different types of nonreciprocal optical effects: the electric field-induced (E-induced) Faraday effect, the electric field-induced magnetic circular dichroism (E-induced MCD), and the spontaneous nonreciprocal rotation of reflected light (NRR). In principle, antiferromagnetic (AFM) domains of Cr2O3 are expected to be distinguished by using these effects. However, such domain observations have never been reported mainly due to the smallness of the effects studied to date. In this study, we demonstrate that all these nonreciprocal optical effects allow the observation of AFM domains in Cr2O3. We first measured spectra of the E-induced Faraday effect and E-induced MCD, and found that relatively large effects appear in the visible light region. Then, by imaging spatial distributions of the three nonreciprocal effects via field and light-polarization modulation imaging techniques, we succeeded in visualizing AFM domains. Intriguingly, the domain patterns obtained via the bulk sensitive E-induced Faraday effect and MCD and those via the surface-sensitive NRR are identical to each other, revealing that the AFM domains in Cr2O3 are uniform in the thickness direction. The domain observation methods provided here will be widely applied to various ME antiferromagnets, not only insulators but also metals, with the diagonal components of linear ME tensor.

DOI： 10.1103/PhysRevResearch.4.043063

Publishing type：Research paper (scientific journal)  

We report dielectric, thermodynamical, acoustic, and optical properties of a chiral molecular crystal, 1,1'-bi-2-naphthol 2-dimethylsulfoxide (BINOL center dot 2DMSO). We find two successive phase transitions at T-c1 = 190 K and T-c2 = 125 K. The first transition at T-c1 is characterized by an order-disorder transition of the guest molecules DMSO along with ferroelectricity. At the second transition of T-c2, the crystal structure deforms from tetragonal to monoclinic, leading to domain formation. Low-temperature x-ray diffraction suggests that the space group changes from P4(1)2(1)2 (P4(3)2(1)2) to P4(1) (P4(3)) at T-c1, and down to P1(1)2(1) at T-c2.

DOI： 10.7566/JPSJ.91.064702

Publishing type：Research paper (scientific journal)  

We report high-resolution single-crystal inelastic neutron scattering measurements on the spin-1/2 antiferromagnet Ba(TiO)Cu-4(PO4)(4). This material is formed from layers of four-site "cupola" structures, oriented alternately upwards and downwards, which constitute a rather special realization of two-dimensional (2D) square-lattice magnetism. The strong Dzyaloshinskii-Moriya (DM) interaction within each cupola, or plaquette, unit has a geometry largely unexplored among the numerous studies of magnetic properties in 2D Heisenberg models with spin and spatial anisotropies. We have measured the magnetic excitations at zero field and in fields up to 5 T, finding a complex mode structure with multiple characteristic features that allow us to extract all the relevant magnetic interactions by modeling within the linear spin-wave approximation. We demonstrate that Ba(TiO)Cu-4(PO4)(4) is a checkerboard system with almost equal intra-and interplaquette couplings, in which the intraplaquette DM interaction is instrumental both in enforcing robust magnetic order and in opening a large gap at the Brillouin-zone center. We place our observations in the perspective of generalized phase diagrams for spin-1/2 square-lattice models and materials, where exploring anisotropies and frustration as routes to quantum disorder remains a frontier research problem.

DOI： 10.1103/PhysRevB.105.214406

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)Cu-4(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 approximate to 0.014 mu(B) 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.

DOI： 10.1002/aelm.202200167

Publishing type：Research paper (scientific journal)  

Switching crystallographic chirality is nontrivial because there is no simple conjugate field to chirality. Here we demonstrate chirality switching in an inorganic crystalline material by manipulating the boundaries of chiral domains with laser irradiation. Our study material is Ba(TiO)Cu4(PO4)4, exhibiting a chiral structure at room temperature and a chiral-achiral phase transition at 710 °C. By irradiation of a laser beam with a wavelength at which Ba(TiO)Cu4(PO4)4exhibits strong optical absorption, local heating is induced. This leads to reconstructions of chiral domain boundaries, revealed by optical rotation measurements. In the reconstruction process, energetically unstable domain boundaries tend to be minimized, affecting resultant domain patterns. On the basis of this feature, we successfully manipulate chiral domain patterns by scanning the laser beam on the sample surface. Our findings provide a unique approach to controlling chirality in inorganic crystalline materials.

DOI： 10.1021/acs.jpclett.2c00606

PubMed

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

Conventional magnetic memories rely on bistable magnetic states, such as the up and down magnetization states in ferromagnets. Increasing the number of stable magnetic states in each cell, preferably composed of antiferromagnets without stray fields, promises to achieve higher-capacity memories. Thus far, such multi-stable antiferromagnetic states have been extensively studied in conducting systems. Here, we report on a striking optical response in the magnetoelectric collinear antiferromagnet Bi2CuO4, which is an insulating version of the representative spintronic material, CuMnAs, with four stable Neel vector orientations. We find that, due to a magnetoelectric effect in a visible range, which is enhanced by a peculiar local environment of Cu ions, absorption coefficient takes three discrete values depending on an angle between the propagation vector of light and the Neel vector-a phenomenon that we term antiferromagnetic trichroism. Furthermore, using this antiferromagnetic trichroism, we successfully visualize field-driven reversal and rotation of the Neel vector.Antiferromagnets have great promise for use in spin-based electronics; however, detecting the Neel vector is challenging due to the lack of a net magnetization. Here, Kimura et al demonstrate an intriguing optical response, where the optical absorption depends on the angle of the Neel vector.

DOI： 10.1038/s41467-022-28215-w

Publishing type：Research paper (scientific journal)  

The so-called ferroaxial transition characterized by a rotational structural distortion that breaks a mirror symmetry has gained growing interest in terms of a new class of ferroic state in crystalline materials. RbFe(MoO4)2, which belongs to glaserite-type compounds, X(;1)Y(;2)[M(TO4)2], is one of the most representative materials showing a ferroaxial transition, i.e., ferroaxial materials. Considering a variety of glaserite-type compounds, we expect that they provide a good arena for ferroaxial materials. In this work, we explored new ferroaxial materials by formula-based screening using a regular expression search and the symmetry detection algorithm. As a result, we found that a glaserite-type compound, K2Zr(PO4)2, is one of the promising candidates for ferroaxial materials. Experimentally, we demonstrate that K2Zr(PO4)2 shows a ferroaxial transition at about 700 K, which is well explained by ab initio phonon calculations. The ferroaxial nature of K2Zr(PO4)2 is further confirmed by the observation of its domain structures using a linear electrogyration effect, i.e., optical rotation in proportion to an applied electric field. Our work provides an effective approach to exploring ferroaxial materials.

DOI： 10.1021/acs.chemmater.2c03540

Publishing type：Research paper (scientific journal)  

DOI： 10.1103/physrevresearch.3.l042043

Other URL： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevResearch.3.L042043/fulltext

Publishing type：Research paper (scientific journal)  

The ferroaxial order, which is characterized by a rotational structural distortion in a crystal, has been recently proposed as one of ferroic orders. Though the domain formation is a characteristic feature in ferroic materials, there has been little study done concerning that for the ferroaxial order. Here, we investigate ferroaxial domains that are formed through a ferroaxial transition in two representative ferroaxial materials, NiTiO3 and RbFe(MoO4)2. We spatially resolve their domain structures using an optical method based on electric-field-induced optical rotation, that is, electrogyration (EG). In NiTiO3, multidomains are constructed when crystals undergo a ferroaxial transition and the domain size depends on the cooling rate around the transition temperature. Furthermore, the ferroaxial domain structure obtained by the EG measurement is well matched with that by scanning x-ray diffraction. RbFe(MoO4)2 also exhibits multidomain states in which domain patterns are different each time a crystal undergoes a ferroaxial transition. In addition, the temperature dependence of the EG signal well obeys that of the order parameter of a first-order phase transition. These results ensure the effectiveness of the EG effect to elucidate the nature of ferroaxial order.

DOI： 10.1103/PhysRevMaterials.5.124409

Authorship：Corresponding author   Publishing type：Research paper (scientific journal)  

We study nonreciprocal directional dichroism (NDD) of magnetoelectric Pb(TiO)Cu4(PO4)4 in magnetic fields up to 49 T via magneto-optical spectroscopy in the (near-)visible range. Measuring the optical absorption coefficient (α) for counter-propagating light beams, we find that the extracted nonreciprocal components show a strong light polarization dependence in a magnetic-field-induced ferroelectric phase (16 to 45 T at 2 K). The ratio of nonreciprocal to reciprocal components of α is relatively large, exceeding 13% at a photon energy of 1.4 eV for a specific light polarization. The magnetic-field dependence of the NDD closely resembles to that of a theoretically calculated antiferromagnetic order parameter. This suggests that NDD provides a unique approach to detecting an antiferromagnetic order parameter.

DOI： 10.7566/jpsj.90.123701

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

In materials showing a linear magnetoelectric (ME) effect, unconventional functionalities can be anticipated such as electric control of magnetism and nonreciprocal optical responses. Thus, the search for new linear ME materials is of interest in materials science. Here, using a recently proposed design principle of linear ME materials, which is based on the combination of local structural asymmetry and collinear antiferromagnetism, we demonstrate that an anion-deficient fluorite derivative, Mn3Ta2O8, is a new linear ME material. This is evidenced by the onset of magnetic-fieldinduced electric polarization in its collinear antiferromagnetic phase below T-N = 24 K. Furthermore, we also find an antiferroelectric-like phase transition at T-S = 55 K, which is attributable to an off-center displacement of magnetic Mn2+ ions. The present study shows that Mn3Ta2O8 is a rare material that exhibits both ME and antiferroelectric-like transitions. Thus, Mn3Ta2O8 may provide an opportunity to investigate the physics associated with complicated interactions between magnetic (spin) and electric dipole degrees of freedom.

DOI： 10.1021/acs.inorgchem.1c02461

Publishing type：Research paper (scientific journal)  

Circular dichroism observed by resonant X-ray diffraction at the CuL3 edge was investigated in a chiral antiferromagnet, Pb(TiO)Cu4(PO4)4, which shows magnetic quadrupole order (TN=7K). At temperatures above TN, space-group-forbidden reflection 100 is observed due to the anisotropic tensor of susceptibility (ATS) scattering. With decreasing temperature, the reflection intensity shows substantial circular dichroism below TN. We found one-to-one correspondence between the sign of the circular dichroism and that of the sample's crystallographic chirality. In addition, the reflection intensity depends on the product of poling magnetic and electric fields. The circular dichroism observed in this study is interpreted as the interference between the ATS and the magnetic scatterings. This finding shows that the interference scattering probes both the chirality and the order parameter of the magnetic quadrupole order in this chiral antiferromagnet.

DOI： 10.1103/physrevb.103.174409

Other URL： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.103.174409/fulltext

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

<title>Abstract</title>Chiral-lattice magnets can exhibit a variety of physical phenomena when time-reversal symmetry is broken by their magnetism. For example, nonreciprocal responses of (quasi)particles have been widely observed in chiral-lattice magnets with macroscopic magnetization. Meanwhile, time-reversal symmetry can also be broken in antiferromagnets without magnetization. Here we report an unconventional chirality-magnetism coupling in a chiral-lattice antiferromagnet Pb(TiO)Cu₄(PO₄)₄ whose time-reversal symmetry is broken by an ordering of magnetic quadrupoles. Our experiments demonstrate that a sign of magnetic quadrupoles is controllable by a magnetic field only, which is generally impossible in consideration of the symmetry of magnetic quadrupoles. Furthermore, we find that the sign of magnetic quadrupoles stabilized by applying a magnetic field is reversed by a switching of the chirality. Our theoretical calculations and phenomenological approach reveal that this unusual coupling between the chirality and magnetic quadrupoles is mediated by the previously-unrecognized magnetic octupoles that emerge due to the chirality.

DOI： 10.1038/s41535-021-00355-0

Other URL： http://www.nature.com/articles/s41535-021-00355-0

Authorship：Corresponding author   Publishing type：Research paper (scientific journal)  

Ferrochiral transition, i.e., a transition involving an emergence of chirality, provides an unique opportunity to achieve a nonvolatile reversible control of chirality with external fields. However, materials showing pure ferrochiral transitions, which are accompanied by no other types of ferroic transition, are exceedingly rare. In this study, we propose that a pure ferrochiral transition is achieved by a combination of antipolar and antiferroaxial orderings of structural units, and substantiate this proposal through a study of the chiral compound Ba(TiO)Cu4(PO4)4. Single crystal X-ray diffraction measurements have revealed that this material undergoes a second order ferrochiral transition whose order parameter is described by an antiferroaxial (staggered) rotation of antipolar structural units, thus demonstrating our proposal. Furthermore, by measuring spatial distributions of optical rotation, we successfully visualized a temperature evolution of ferrochiral domains across the transition temperature and demonstrated the relationship between chirality and optical rotation. This work provides a guide to find a pure ferrochiral transition, thus providing an opportunity to achieve a ferroic control of chirality.

DOI： 10.1021/jacs.1c00391

PubMed

Publishing type：Research paper (scientific journal)  

This article was originally published online on 6 October 2020 with an error in Fig. 6. Figure 6 appears correctly below. All online and printed versions of the article were corrected on 8 October 2020. AIP Publishing apologizes for these errors. (Figure Presented).

DOI： 10.1063/5.0033708

PubMed

DOI： 10.11316/jpsgaiyo.74.2.0_1972

CiNii Article

Presentation type：Oral presentation (general)  

Venue：上智大学四谷キャンパス  

Presentation type：Oral presentation (general)  

Venue：上智大学四谷キャンパス  

Presentation type：Oral presentation (invited, special)  

Venue：九州工業大学　戸畑キャンパス  

Presentation type：Poster presentation  

Venue：さわやかちば県民プラザ(千葉県柏市)  

Presentation type：Poster presentation  

Venue：千葉県柏市  

Presentation type：Oral presentation (general)  

Venue：京都工芸繊維大学松ヶ崎キャンパス  

Presentation type：Oral presentation (general)  

Venue：京都工芸繊維大学　松ヶ崎キャンパス  

Presentation type：Poster presentation  

Venue：近畿大学東大阪キャンパス  

Presentation type：Poster presentation  

Venue：近畿大学