Possible evidence for electromagnons in multiferroic manganites

Magnetodielectric materials are characterized by a strong coupling of magnetic and dielectric properties and in rare cases simultaneously exhibit both, magnetic and polar order. Among other multiferroics, TbMnO3 and GdMnO3 reveal a strong magneto-dielectric (ME) coupling and as a consequence fundamentally new spin excitations exist: Electro-active magnons, or electromagnons, i. e. spin waves which can be excited by ac electric fields. Here we show that these excitations appear in the phase with an incommensurate (IC) magnetic structure of the manganese spins. In external magnetic fields this IC structure can be suppressed and the electromagnons are wiped out, thereby inducing considerable changes in the index of refraction from dc up to THz frequencies. Hence, besides adding a new creature to the zoo of fundamental excitations, the refraction index can be tuned by moderate magnetic fields, which allows the design of a new generation of optical switches and optoelectronic devices.Comment: 4 Pages, 2 figure

Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4

Multiferroic materials, which reveal magnetic and electric order, are in the focus of recent solid state research. Especially the simultaneous occurrence of ferroelectricity and ferromagnetism, combined with an intimate coupling of magnetization and polarization via magneto-capacitive effects, could pave the way for a new generation of electronic devices. Here we present measurements on a simple cubic spinel with unusual properties: It shows ferromagnetic order and simultaneously relaxor ferroelectricity, i.e. a ferroelectric cluster state, reached by a smeared-out phase transition, both with sizable ordering temperatures and moments. Close to the ferromagnetic ordering temperature the magneto-capacitive coupling, characterized by a variation of the dielectric constant in an external magnetic field, reaches colossal values of nearly 500%. We attribute the relaxor properties to geometric frustration, which is well known for magnetic moments, but here is found to impede long-range order of the structural degrees of freedom.Comment: 4 pages, 3 figure

Study on Resistance Switching Properties of Na0.5Bi0.5TiO3Thin Films Using Impedance Spectroscopy

The Na0.5Bi0.5TiO3(NBT) thin films sandwiched between Au electrodes and fluorine-doped tin oxide (FTO) conducting glass were deposited using a sol–gel method. Based on electrochemical workstation measurements, reproducible resistance switching characteristics and negative differential resistances were obtained at room temperature. A local impedance spectroscopy measurement of Au/NBT was performed to reveal the interface-related electrical characteristics. The DC-bias-dependent impedance spectra suggested the occurrence of charge and mass transfer at the interface of the Au/NBT/FTO device. It was proposed that the first and the second ionization of oxygen vacancies are responsible for the conduction in the low- and high-resistance states, respectively. The experimental results showed high potential for nonvolatile memory applications in NBT thin films

Growth of single crystals in the (Na1/2Bi1/2)TiO3-(Sr1-xCax)TiO3 system by solid state crystal growth

Ceramics based on (Na1/2B1/2)TiO3 are promising candidates for actuator applications because of large strains generated by an electric field-induced phase transition. For example, the (1-x)(Na1/2Bi1/2)TiO3-xSrTiO(3) system exhibits a morphotropic phase boundary at x = 0.2-0.3, leading to high values of inverse piezoelectric constant d*(33), which can be further improved by the use of single crystals. In our previous work, single crystals of (Na1/2B1/2)TiO3-SrTiO3 and (Na1/2B1/2)TiO3-CaTiO3 were grown by the solid state crystal growth technique. Growth in the (Na1/2B1/2)TiO3-SrTiO3 system was sluggish whereas the (Na1/2B1/2)TiO3-CaTiO3 single crystals grew well. In the present work, 0.8(Na1/2Bi1/2)TiO3-0.2(Sr1-xCax)TiO3 single crystals (with x = 0.0, 0.1, 0.2, 0.3, 0.4) were produced by the solid state crystal growth technique in an attempt to improve crystal growth rate. The dependence of mean matrix grain size, single crystal growth distance, and electrical properties on the Ca concentration was investigated in detail. These investigations indicated that at x = 0.3 the matrix grain growth was suppressed and the driving force for single crystal growth was enhanced. Replacing Sr with Ca increased the shoulder temperature T-s and temperature of maximum relative permittivity T-max, causing a decrease in inverse piezoelectric properties and a change from normal to incipient ferroelectric behavior