Mitsuru Akaki

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.

One-Third Magnetization Plateau with a Preceding Novel Phase in Volborthite

We have synthesized high-quality single crystals of volborthite, a seemingly distorted kagome antiferromagnet, and carried out high-field magnetization measurements up to 74 T and ^{51}V NMR measurements up to 30 T. An extremely wide 1/3 magnetization plateau appears above 28 T and continues over 74 T at 1.4 K, which has not been observed in previous studies using polycrystalline samples. NMR spectra reveal an incommensurate order (most likely a spin-density wave order) below 22 T and a simple spin structure in the plateau phase. Moreover, a novel intermediate phase is found between 23 and 26 T, where the magnetization varies linearly with magnetic field and the NMR spectra indicate an inhomogeneous distribution of the internal magnetic field. This sequence of phases in volborthite bears a striking similarity to those of frustrated spin chains with a ferromagnetic nearest-neighbor coupling J_{1} competing with an antiferromagnetic next-nearest-neighbor coupling J_{2}.

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.

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.

Successive Magnetic Transitions of Ca2CoSi2O7 in High Magnetic Fields

Magnetic and dielectric properties of \aa kermanite Ca

Multiferroic properties of an åkermanite Sr2CoSi2O7single crystal in high magnetic fields

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Effects of magnetic field and pressure on the antiferromagnetic and weak-ferromagnetic orders in tetrahedral spin chain system Cu3Mo2O9

CoSi

Internal magnetic field effect on magnetoelectricity in orthorhombic RMnO3 crystals

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Magnetoelectric property in 3d transition metal oxide with tetrahedral structure

O

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

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