Mamoru Fukunaga

New Technique for Measuring Ferroelectric and Antiferroelectric Hysteresis Loops

A new technique, the double-wave method (DWM), is developed to refine ferroelectric D – E hysteresis loops. The method discriminates non-hysteresis components from ferroelectric D – E hysteresis loops by applying identical unipolar waves twice. It extracts only the hysteresis component automatically without any assumptions. The DWM can compensate intricate non-hysteresis components in D – E loops which traditional methods cannot. It can separate an antiferroelectric component from ferroelectric and other components by applying a bias electric field. As a typical example of a ferroelectric, D – E loops of NaNO 2 by the DWM are examined. To demonstrate the advantages of the DWM, D – E loops of multiferroic ErMn 2 O 5 are presented. Antiferroelectric-like components in D – E loops are separated and fine ferroelectric loops of ErMn 2 O 5 are observed by the DWM.

Magnetic and ferroelectric properties of multiferroic RMn2O5

The magnetic and ferroelectric properties of multiferroic RMn2O5 (R = Y, Tb, Ho, Er, Tm) are reviewed based on recent neutron diffraction and dielectric measurements. Successive phase transitions of magnetic and dielectric ordering were found to occur simultaneously in this system. The characteristic magnetic ordering of the system exhibits an incommensurate–commensurate phase transition, and again transitions to an incommensurate phase. Special attention is given to the magnetic structure in order to discuss the mechanism for the introduction of ferroelectric polarization. For all the compounds examined, the spin configuration for Mn4+ and Mn3+ ions in the commensurate magnetic phase, where spontaneous electric polarization occurs, was determined to be a transverse spiral spin structure propagating along the c-axis. By contrast, the alignment of the induced 4f moment of R3+ ions showed variation, depending on the character of each of the elements. Corresponding responses to external fields such as a magnetic field, hydrostatic pressure etc at low temperature are strongly dependent on the rare earth element present in the RMn2O5 system. The so-called colossal magnetoelectric effect in this system can be easily interpreted by the phase transition from the magnetic incommensurate and weak ferroelectric phase to the commensurate and ferroelectric phase.

Origin of colossal dielectric response of CaCu3Ti4O12 studied by using CaTiO3∕CaCu3Ti4O12∕CaTiO3 multilayer thin films

To elucidate the origin of the colossal dielectric response (CDR) of CaCu3Ti4O12 (CCTO), multilayer thin films of CCTO interposed in insulating CaTiO3 (CTO) were synthesized using a pulsed laser deposition technique. The capacitance C of CTO/CCTO/CTO films with different layer thicknesses is measured. After removing the capacitance of CTO by extrapolating C to zero CTO thickness, the real part of dielectric constant of CCTO is estimated to be 329–435, which is much smaller than the reported value for CCTO thin films. This fact indicates that the CDR of CCTO is extrinsic and originates from an internal barrier layer capacitor.

Ferroelectricity and Ferrimagnetism of Hexagonal YbFeO3 Thin Films

The coexistence of ferroelectricity and magnetic order in epitaxial hexagonal YbFeO3 (h-YbFeO3) thin films deposited on yttria stabilized zirconia substrates is examined using an impedance analyzer, a second harmonic generation (SHG) microscope, an X-ray diffractometer, a transmission electron microscope and a superconducting quantum interference device magnetometer. The results show that h-YbFeO3 films exhibit ferroelectricity below 350 K with threefold translational periodicity along the [110] direction similar to rare earth manganese oxides, and ferrimagnetism with a four-up and two-down spin configuration of Fe þ3 and Yb þ3 ions along the c-axis at low temperatures. In addition, a possibility of new ferroelectric phases with an electronic origin induced by the magnetic ordering is discussed based upon SHG and D–E hysteresis measurements.

Simultaneous Measurements of Magnetic Neutron Diffraction, Electrical Polarization and Permittivity of Multiferroic ErMn2O5

In order to determine an exact relationship between magnetic ordering and ferroelectricity in multiferroic rare-earth manganese oxides, we have performed simultaneous measurements of neutron diffraction, ferroelectric D – E hysteresis loops and the permittivity e of ErMn 2 O 5 down to 2.5 K. Special attention was paid to the area around the ferroelectric phase transition temperature. In contrast to previous expectations based on separate measurements, the peak of e was found to coincide with a magnetic incommensurate–commensurate phase transition. However, it was confirmed that electrical polarization appears at the transition to a one-dimensionally incommensurate magnetic phase, which is higher than the peak of e by 1.2 K. A previously unknown low-temperature incommensurate magnetic phase of ErMn 2 O 5 below 8 K was found. The lowest-temperature phase appears to be paraelectric according to the D – E loops, while pyroelectric measurements show that the phase exhibits a small polarization.

Classification and Interpretation of the Polarization of Multiferroic RMn2O5

We have collected and compared temperature-dependent polarization data of multiferroic R Mn 2 O 5 for 11 kinds of rare earths ( R ) from the literature and our own measurements. Although the reported data are inconsistent, it was found that the maximum polarization values for R =Y, Dy, Ho, Er, and Tm are almost equal above 30 K except for the phase transition temperature where the polarization disappears. The findings suggest a standard polarization value for R Mn 2 O 5 of 100–120 nC/cm 2 at 30 K in a commensurate magnetic (CM) phase. We discuss the nature and issues of the measured polarization of R Mn 2 O 5 from the data and our detailed measurement results for YMn 2 O 5 . Temperature-dependent ferroelectric hysteresis loops of YMn 2 O 5 by the double-wave method reveal that an antiferroelectric-like high-temperature incommensurate magnetic (ICM) phase and a ferroelectric low-temperature ICM phase, while the intermediate CM phase is ferrielectric.

Switching processes and formation of the stable artificial domain structure in ferroelectric LaBGeO5

The processes of polarization switching in LaBGeO 5 (LBGO) crystals are studied in details for the triangle and pulse electric field in the wide temperature range from room temperature (RT) to Tc = 530°C. It is revealed that the temperature dependence of a coercive field (f = 0.1 Hz) has an exponential form and this field reaches a value 150 kV/cm at the RT. The temperature evolution of the switching time and the form of the switching current are obtained in the mode of the successive solitary pulses and in the steady conditions. The features of “non-classical” behavior are noted (the dependence of switching parameters on the prehistory of samples, frequency dependence of the switching time). It is shown that the stable artificial domain structure at RT with the periodicity of 200 μm can be obtained by applying about 300–350 V/mm near 300°C. The stable domain structure was contrasted by means of SHG microscope probing.

Compact and Simple Apparatus for Measuring Direct Piezoelectricity

A new instrument for measuring piezoelectric coefficients has been developed. The principle of this instrument is simple, namely, applying a uniaxial stress to a sample and measuring the induced charge. The stress applied to the sample is measured as an electric signal. The main part of the instrument is shaped like a small probe with an electrode at the tip for measuring the charge, and piezoelectric coefficients can be measured easily and quickly by pressing the probe on samples by hand. The piezoelectric coefficient d33 of ferroelectric Ba1-xLaxTi1-xCrxO3 ceramics were measured using the probe, and it is revealed that the compound shows a maximum d33 of 70 pC/N when x is approximately 0.014–0.018.

Structure and Dielectric Properties of Ba 1 − x La x Ti 1 − x Cr x O 3 Ceramics

In order to develop useful lead-free ferroelectric materials based on BaTiO 3, Ba 1 m x La x Ti 1 m x Cr x O 3 ceramics were synthesized and their dielectric properties were studied. The crystal structure of Ba 1 m x La x Ti 1 m x Cr x O 3 changes in the sequence of tetragonal, cubic, rhombohedral and orthorhombic as x increases. Curie temperature T C decreases largely as x increases. The samples except for x = 0.003 show broad peak of high dielectric constant with small dielectric loss. The frequency dispersion like in relaxors is observed below room temperature with x = 0.08.

Structural and Dielectric Properties of Nickel-Doped and Lanthanum-Chromium-Doped Barium Titanate Ceramics.

Nickel-doped BaTiO3 (BaTi1-yNiyO3-δ: BTN) and lanthanum-chromium double-doped BaTiO3 (Ba1-xLaxTi1-xCrxO3: BLTC) ceramics were synthesized and their symmetry and dielectric properties were studied. Samples were sintered at 1400°C in air from stoichiometric amounts of BaCO3 and other metal oxides. X-ray diffraction data were obtained at 20°C. Dielectric properties were measured in the temperature range of 78 K to 453 K. With the increase of dopant ratio y, Curie temperature TC of BTN decreases, but when y exceeds about 0.01, a hexagonal phase appears and the dielectric constant becomes smaller. On the other hand, in the entire range of x, BLTC shows a perovskite structure and the compounds with larger x exhibit conductive rhombohedral or orthorhombic phases. In the range of x smaller than 0.1, BLTC shows a diffuse phase transition and a high dielectric constant with small tan δ. The relaxor-like frequency dispersion was observed in BLTC at low temperatures.