D. Akahoshi

Local crystal structure analysis with 10-pm accuracy using scanning transmission electron microscopy

We demonstrate local crystal structure analysis based on annular dark-field (ADF) imaging in scanning transmission electron microscopy (STEM). Using a stabilized STEM instrument and customized software, we first realize high accuracy of elemental discrimination and atom-position determination with a 10-pm-order accuracy, which can reveal major cation displacements associated with a variety of material properties, e.g. ferroelectricity and colossal magnetoresistivity. A-site ordered/disordered perovskite manganites Tb(0.5)Ba(0.5)MnO(3) are analysed; A-site ordering and a Mn-site displacement of 12 pm are detected in each specific atomic column. This method can be applied to practical and advanced materials, e.g. strongly correlated electron materials.

Gigantic magnetoelectric effect caused by magnetic-field-induced canted antiferromagnetic-paramagnetic transition in quasi-two-dimensional Ca2CoSi2O7 crystal

We have investigated the magnetic and dielectric properties of Ca2CoSi2O7 crystal. The dielectricity and magnetism of Ca2CoSi2O7 are strongly coupled below a canted antiferromagnetic transition temperature (TN). Magnetic fields induce electric polarization below TN. Interestingly, the magnetic-field-induced electric polarization is detected even without poling electric fields. Below TN, a canted antiferromagnetic-paramagnetic transition is induced by magnetic fields. The large magnetocapacitance is observed around TN. The origin of the large magnetocapacitance is due to the magnetic-field-induced the canted antiferromagnetic-paramagnetic transition.

Magnetic-field-induced switching between ferroelectric phases in orthorhombic-distortion-controlled RMnO3

We have investigated the dielectric and magnetic properties of Eu0.595Y0.405MnO3 without the presence of the 4f magnetic moments of the rare-earth ions and have found two ferroelectric phases with polarization along the a and c axes in a zero magnetic field. A magnetic-field-induced switching from one to the other ferroelectric phase took place in which the direction of ferroelectric polarization changed from the a axis to the c axis by the application of magnetic fields parallel to the a axis. In contrast to the case of TbMnO3, in which the 4f moments of Tb3+ ions play an important role in such a ferroelectric phase switching, the magnetic-field-induced switching between ferroelectric phases in Eu0.595Y0.405MnO3 does not originate from the magnetic transition of the rare-earth 4f moments but from that of the Mn 3d spins.

Comparative study of ordered and disordered Y1−xSrxCoO3−δ

We have succeeded in preparing A-site ordered and disordered Y1∕4Sr3∕4CoO3−δ with various oxygen deficiencies δ, and have made a comparative study of their structural and physical properties. In the A-site ordered structure, oxygen vacancies order, and δ=0.34 sample shows a weak ferromagnetic transition beyond 300K. On the other hand, in the A-site disordered structure, no oxygen-vacancy ordering is observed, and δ=0.16 sample shows a ferromagnetic metallic transition around 150K. A-site disordering destroys the orderings of oxygen vacancies and orbitals, leading to the strong modification of the electronic phases.

Development of high-speed polarizing imaging system for operation in high pulsed magnetic field

A high-speed polarizing microscope system combined with a 37 T pulse magnet has been developed. This system was applied to successfully visualize the field-induced collapse of charge-orbital ordering in a layered manganite La(1/2)Sr(3/2)MnO(4). Quantitative analyses of the obtained polarizing microscope images provided clear evidence of this transition in contrast to rather moderate changes in magnetization and magnetoresistance. The ability of this system to carry out quantitative analysis was further tested through the observation of Faraday rotation in a Tb(3)Ga(5)O(12) crystal. The Verdet constant determined from the polarizing images is in reasonable agreement with that in literature. Local intensity analyses of the images indicate that we can investigate magneto-optical signals within an accuracy of 0.85% in an area of 9.6 x 9.6 microm(2).

Magnetic and dielectric properties of A2CoSi2O7 (A=Ca, Sr, Ba) crystals

We have investigated the magnetic and dielectric properties of A2CoSi2O7 (A=Ca, Sr, and Ba) crystals with a two-dimensional network of CoO4 and SiO4 tetrahedra connected with each other through the corners. In Ca2CoSi2O7, a weak ferromagnetic transition occurs at 5.7 K, where the dielectric constant parallel to the c axis shows a concomitant anomaly. The large magnetocapacitance effect is observed below 5.7 K; Δ(H)/(0) = [(H) – (0)]/(0) reaches 13 % at 5.1 K. These results indicate a strong coupling between the magnetism and dielectricity in Ca2CoSi2O7. Sr2CoSi2O7 shows a similar magnetoelectric behavior to that of Ca2CoSi2O7. In contrast, in Ba2CoSi2O7, which has the different arrangement of SiO4 and CoO4 tetrahedra from that of Ca2CoSi2O7, the magnetocapacitance is hardly observed. The key for the magnetocapacitance effect of A2CoSi2O7 lies in the quasi-two-dimensional crystal structure.

Nanostructural evidence at the phase boundary of A-and C-type antiferromagnetic phases in Nd1-xSrxMnO3 crystals

A weak ferromagnetic phase appears around the phase boundary between the A-type and C-type antiferromagnetic phases in overdoped Nd1−xSrxMnO3 crystals (0.6≤x≤0.7). We investigated the crystal structure and the magnetic domain structure around the phase boundary, by powder synchrotron x-ray diffractometry and transmission electron microscopy. The phase boundary exists between x = 0.62 in the phase separation region where the A-type and C-type antiferromagnetic phases coexist and x = 0.625 in the C-type antiferromagnetic phase. By transmission electron microscopy, the orbital-disordered nanodomains in the C-type antiferromagnetic matrix phase were only observed in the vicinity of the phase boundary in the C-type antiferromagnetic phase. This is a nanoscale inhomogeneity but is not a conventional phase separation for manganites. We also successfully observed magnetic domain structures in the weak ferromagnetic phase by Lorentz electron microscopy. Moreover, we found that the ferromagnetic correlation becomes a short-range correlation when the orbital-disordered nanodomains appear. We consider that the orbital-disordered nanodomains interfere with the long-range ferromagnetic correlation.

Internal magnetic field effect on magnetoelectricity in orthorhombic RMnO3 crystals

Abstract We have investigated the role of the 4 f moment on the magnetoelectric (ME) effect of orthorhombic R MnO 3 ( R = rare earth ions ). In order to clarify the role of the 4 f moment, we prepared three samples: ( Eu , Y ) MnO 3 without the 4 f moment, TbMnO 3 with the anisotropic 4 f moment, and ( Gd , Y ) MnO 3 with the isotropic 4 f moment. The ferroelectric behaviors of these samples are different from each other in a zero magnetic field. ( Eu , Y ) MnO 3 and ( Gd , Y ) MnO 3 show the ferroelectric polarization along the a axis in the ground state, while TbMnO 3 shows it along the c axis. Such difference may arise from the influence of the anisotropic Tb 3 + 4 f moment. The direction of the ferroelectric polarization of R MnO 3 is determined by the internal magnetic field arising from the 4 f moment.

The Effect of f–d Magnetic Coupling in Multiferroic RMnO3 Crystals

We have established detailed magnetoelectric phase diagrams of (Eu 0.595 Y 0.405 ) 1- x Tb x MnO 3 (0≤ x ≤1) and (Eu,Y) 1- x Gd x MnO 3 (0≤ x ≤0.69), whose average ionic radii of R -site ( R : rare...

Persistent and reversible phase control in GdMnO3 near the phase boundary

We have investigated temperature and magnetic-field dependence of dielectric properties in the orthorhombic GdMnO3 single crystal which is located near the phase boundary between the ferroelectric/spiral-antiferromagnetic phase and the paraelectric/A-type-antiferromagnetic one. In this compound, strong phase competition between these two phases results in a unique phase diagram with large temperature and magnetic-field hystereses. Based on the phase diagram, we have successfully demonstrated the persistent and reversible phase switching between them by application of magnetic fields.