arXiv subjects

Daisuke Morikawa

Publications and source records attributed to Daisuke Morikawa.

7 recordsLinked to original sources

arXiv2026

Direct Imaging of Temperature Evolution of Polar Nanoregions and Chemically Ordered Regions in PMN Relaxor: Evidence for Polar Phase Percolation

Polar nanoregions (PNRs) are central to understanding the exceptional dielectric and piezoelectric properties of relaxor ferroelectrics and are key to advancing dielectrics for high-energy storage. However, direct real-space imaging of their formation and evolution remains a major challenge in condensed matter physics. Here, we report the real-space mappings of both PNRs and chemically ordered regions (CORs) in the prototypical relaxor Pb(Mg1/3Nb2/3)O3 and their temperature dependence using convergent-beam electron diffraction (CBED) combined with four-dimensional scanning transmission electron microscopy (4D-STEM). The results reveal that CORs, with sizes of 2-5 nm, remain static with temperature and act to suppress PNR growth. In contrast, PNRs evolve from isolated 2-5 nm regions at room temperature to interconnected structures ~10 nm in size at low temperatures, indicative of a percolation transition. These observations support the random-field model, in which PNRs emerge from a paraelectric matrix and their growth and collective interactions are constrained by random local fields associated with CORs.

cond-mat.mtrl-sci↗

arXiv2023

Unusual nanoscale coexistence of polar-nonpolar domains underlying oxygen storage properties in Ho(Mn, Ti)O$_{3+\delta}$

Hexagonal manganese oxides RMnO$_3$ show intriguing topological ferroelectric-domain walls with variable conductivity, leading to domain wall engineering. Despite the numerous experimental studies on the polar nanoscale structures, controlling ferroelectric domains has not been sufficiently investigated. Here, we reveal the unprecedented coexistence of polar-nonpolar nanoscale domains that can be formed by substituting Ti ions in HoMnO$_3$. Unusual polar nanoscale domains are embedded in nonpolar domains with different crystallographic symmetry. This polar-nonpolar coexisting structure is naturally assembled by adjusting the lattice length during a solid-state reaction process. Furthermore, the comprehensive study reveals that the reversible microstructural change with a nonpolar-polar transition is strongly correlated with the oxygen storage properties in Ho(Mn, Ti)O$_{3+\delta}$. The present results provide important insight into the nanoscale polar-nonpolar domain coexistence in functional rare-earth manganese oxides, RMnO$_3$.

cond-mat.mtrl-sci↗

arXiv2022

Topological melting of the metastable skyrmion lattice in the chiral magnet Co$_9$Zn$_9$Mn$_2$

In a $β$-Mn-type chiral magnet Co$_9$Zn$_9$Mn$_2$, we demonstrate that the magnetic field-driven collapse of a room temperature metastable topological skyrmion lattice passes through a regime described by a partial topological charge inversion. Using Lorentz transmission electron microscopy, the magnetization distribution was observed directly as the magnetic field was swept antiparallel to the original skyrmion core magnetization, i.e. negative magnetic fields. Due to the topological stability of skyrmions, a direct transition of the metastable skyrmion lattice to the equilibrium helical state is avoided for increasingly negative fields. Instead, the metastable skyrmion lattice gradually transforms into giant magnetic bubbles separated by $2π$ domain walls. Eventually these large structures give way to form a near-homogeneously magnetized medium that unexpectedly hosts a low density of isolated skyrmions with inverted core magnetization, and thus a total topological charge of reduced size and opposite sign compared with the initial state. A similar phenomenon has been observed previously in systems hosting ordered lattices of magnetic bubbles stabilized by the dipolar interaction and called "topological melting". With support from numerical calculations, we argue that the observed regime of partial topological charge inversion has its origin in the topological protection of the starting metastable skyrmion state.

cond-mat.mes-hall↗

arXiv2022

Coexisting Z-type charge and bond order in metallic NaRu$_2$O$_4$

How particular bonds form in quantum materials has been a long-standing puzzle. Two key concepts dealing with charge degrees of freedom are dimerization (forming metal-metal bonds) and charge ordering (CO). Since the 1930s, these two concepts have been frequently invoked to explain numerous exciting quantum materials, typically insulators. Here we report dimerization and CO within the dimers coexisting in metallic NaRu$_2$O$_4$. By combining high-resolution x-ray diffraction studies and theoretical calculations, we demonstrate that this unique phenomenon occurs through a new type of bonding, which we call Z-type ordering. The low-temperature superstructure has strong dimerization in legs of zigzag ladders, with short dimers in legs connected by short zigzag bonds, forming Z-shape clusters: simultaneously, site-centered charge ordering also appears. Our results demonstrate the yet unknown flexibility of quantum materials with the intricate interplay among orbital, charge, and lattice degrees of freedom.

cond-mat.str-el↗

arXiv2021

Magnetism of Kitaev spin-liquid candidate material RuBr$_3$

The ruthenium halide $α$-RuCl$_{3}$ is a promising candidate for a Kitaev spin liquid. However, the microscopic model describing $α$-RuCl$_{3}$ is still debated partly because of a lack of analogue materials for $α$-RuCl$_{3}$, which prevents tracking of electronic properties as functions of controlled interaction parameters. Here, we report a successful synthesis of RuBr$_{3}$. The material RuBr$_{3}$~possesses BiI$_3$-type structure (space group: $R\overline{3}$) where Ru$^{3+}$ form an ideal honeycomb lattice. Although RuBr$_{3}$ has a negative Weiss temperature, it undergoes a zigzag antiferromagnetic transition at $T_\mathrm{N}=34$ K, as does $α$-RuCl$_{3}$. Our analyses indicate that the Kitaev and non-Kitaev interactions can be modified in ruthenium trihalides by changing the ligand sites, which provides a new platform for exploring Kitaev spin liquids.

cond-mat.str-el↗

arXiv2020

Structural-transition-driven antiferromagnetic to spin-glass transition in Cd-Mg-Tb 1/1 approximants

The magnetic susceptibility of the 1/1 approximants to icosahedral quasicrystals in a series of Cd85-xMgxTb15 (x = 5, 10, 15, 20) alloys was investigated in detail. The occurrence of antiferromagnetic to spin-glass-like transition was noticed by increasing Mg. Transmission electron microscopy analysis evidenced a correlation between the magnetic transition and suppression of the monoclinic superlattice ordering with respect to the orientation of the Cd4 tetrahedron at T > 100 K. The possible origins of this phenomenon were discussed in detail. The occurrence of the antiferromagnetic to spin-glass -like magnetic transition is associated with the combination of chemical disorder due to a randomized substitution of Cd with Mg and the orientational disorder of the Cd4 tetrahedra.

cond-mat.mtrl-sci↗

arXiv2019

Metamagnetic transitions and magnetoelectric responses in a chiral polar helimagnet Ni$_2$InSbO$_6$

Magnetic-field effect on the magnetic and electric properties in a chiral polar ordered corundum Ni$_2$InSbO$_6$ has been investigated. Single-crystal soft x-ray and neutron diffraction measurements confirm long-wavelength magnetic modulation. The modulation direction tends to align along the magnetic field applied perpendicular to the polar axis, suggesting that the nearly proper-screw type helicoid should be formed below 77\,K. The application of a high magnetic field causes a metamagnetic transition. In a magnetic field applied perpendicular to the polar axis, a helix-to-canted antiferromagnetic transition takes place through the intermediate soliton lattice type state. On the other hand, a magnetic field applied along the polar axis induces a first-order metamagnetic transition. These metamagnetic transitions accompany a change in the electric polarization along the polar axis.

cond-mat.mtrl-sci↗