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}.

Numerous attempts have been made to realize crossed coupling between ferroelectricity and magnetism in multiferroic materials at room temperature. BiFeO 3 is the most extensively studied multiferroic material that shows multiferroicity at temperatures significantly above room temperature. Here we present high-field experiments on high-quality mono-domain BiFeO 3 crystals reveal substantial electric polarization orthogonal to the widely recognized one along the trigonal c axis. This novel polarization appears to couple with the domains of the cycloidal spin order and, hence, can be controlled using magnetic fields. The transverse polarization shows the non-volatile memory effect at least up to 300 K.

Magnetoelectric properties are studied by a combined experimental and theoretical study of a quasi-two-dimensional material composed of square cupolas, Ba(TiO)Cu4(PO4)4. The magnetization is measured up to above the saturation field, and several anomalies are observed depending on the field directions. We propose a S=1/2 spin model with Dzyaloshinskii-Moriya interactions, which well reproduces the full magnetization curves. Elaborating the phase diagram of the model, we show that the anomalies are explained by magnetoelectric phase transitions. Our theory also accounts for the scaling of the dielectric anomaly observed in experiments. The results elucidate the crucial role of the in-plane component of Dzyaloshinskii-Moriya interactions, which is induced by the noncoplanar buckling of square cupola. We also predict a 'hidden' phase and another magnetoelectric response both of which appear in nonzero magnetic field.PACS numbers: 77.80. Fm,75.85.+t,75.30.Kz Spatial asymmetry is a source of interesting phenomena in a broad range of condensed matter physics. A well-known example is the molecular asymmetry of water H 2 O, which leads to an electric polarization in each molecule. The asymmetry is at play also in magnets: the loss of inversion symmetry activates the asymmetric interactions through the relativistic spin-orbit coupling, such as the Dzyaloshinskii-Moriya (DM) interaction [1,2]. The asymmetric interactions lead to intriguing magnetism, e.g., weak ferromagnetism in antiferromagnets and spin-spiral ordering in helimagnets. They have also attracted growing interest as an origin of the magnetoelectric (ME) effect, that is, cross correlations between dielectricity and magnetism [3,4].Recently, an interesting series of chiral antiferromagnets, A(TiO)Cu 4 (PO 4 ) 4 (A = Ba, Sr) with space group P 42 1 2, was newly synthesized [5]. The materials have a quasi-two-dimensional structure, composed of an alternating array of Cu 4 O 12 clusters, as shown in Fig. 1(a). Each Cu 4 O 12 cluster consists of four corner-sharing CuO 4 plaquettes, forming a noncoplanar buckled structure termed (irregular) square cupola. The asymmetric unit can carry ME-active magnetic multipoles [6] associated with Cu spins. Indeed, a divergent anomaly of the dielectric constant was observed at the Néel temperature (T N =9.5 K) in magnetic fields applied along the [100] and [110] directions for A = Ba [7]. Although the ME response was argued by the magnetic quadrupole associated with noncoplanar antiferromagnetic ordering, the microscopic understanding is not fully obtained. It is highly desired to clarify how the unique asymmetry arising from the square cupolas affects the magnetic and dielectric properties in this series of compounds. [010][001] In this Letter, combining experimental and theoretical studies, we clarify the microscopic mechanism of ME behavior in A(TiO)Cu 4 (PO 4 ) 4 . First, from the magnetization measurement for the compound with A = Ba up to above the saturation field, we find several anomalies depending on the field dire...

We have investigated the dielectric and magnetic properties of Eu 0.595 Y 0.405 MnO 3 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 plase 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 TbMnO 3 , in which the 4f moments of Tb 3+ ions play an important role in such a ferroelectric phase switching, the magnetic-field-induced switching between ferroelectric phases in Eu 0.595 Y 0.405 MnO 3 does not originate from the magnetic transition of the rare-earth 4f moments, but from that of the Mn 3d spins.

The magnetic and dielectric properties ofåkermanite Sr 2 CoSi 2 O 7 single crystals in high magnetic fields were investigated. We have observed finite induced electric polarization along the c axis in high fields, wherein all Co spins were forcibly aligned to the magnetic field direction. Existence of the induced polarization in the spin-polarized state accompanied with the finite slope in the magnetization curve suggests the possible role of the orbital angular momenta in the excited states as its microscopic origin. The emergence of the field-induced polarization without particular magnetic order can be regarded as the magnetoelectric effects of the second order from the symmetry point of view. A low magnetic field-driven electric polarization flip induced by a rotating field, even at room temperature, has been successfully demonstrated.

Magnetic field induced polarization and magnetoelectric effect of Ba0.8Ca0.2TiO3-Ni0.2Cu0.3Zn0.5Fe2O4 nanomultiferroic J. Appl. Phys. 113, 17C731 (2013); 10.1063/1.4795820 O 17 and N 93 b NMR investigation of magnetoelectric effect in Pb ( Fe 1 / 2 Nb 1 / 2 ) O 3

We have systematically investigated the magnetic properties of (Fe 1Ày Zn y ) 2 Mo 3 O 8 (0 y 1). The ground state of the parent compound, Fe 2 Mo 3 O 8 (y ¼ 0), is an antiferromagnetic (AFM) insulator. The substitution of magnetic Fe 2þ with nonmagnetic Zn 2þ gives rise to ferromagnetic (FM) correlation. With increasing y, the FM correlation evolves and becomes the most enhanced with y ¼ 0:5. One of the possible explanations for the origin of the FM behavior is that Zn 2þ ions prefer the octahedral oxygen coordination to the tetrahedral one. In addition, the magnetizations when y ¼ 0:125 and 0.25 show a stepwise behavior at low temperatures, implying that several magnetic phases are competing with one another.

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