Mitsumasa Nakajima

Annealing Temperature Dependences of Ferroelectric and Magnetic Properties in Polycrystalline Co-Substituted BiFeO3 Films

Multiferroic Co-substituted BiFeO3 films were fabricated by chemical solution deposition method followed by post deposition annealing at various temperatures. The substitution of cobalt of B-sites for iron in BiFeO3 was promoted at relatively high temperatures. The B-site substitution by cobalt promoted increases in saturation magnetization and spontaneous magnetization. By substitution, leakage current density was suppressed in a high-electric-field region, and ferroelectric hysteresis (P–E) loops became measurable even at room temperature. The optimal annealing temperature for the coexistence of a high remanent polarization and a high remanent magnetization was 923 K having a high B-site substitution ratio of cobalt.

Spontaneous polarization estimation from the soft mode in strain-free epitaxial polar axis-oriented Pb(Zr,Ti)O3 thick films with tetragonal symmetry

The ferroelectric property, crystal structure, and A1(1TO) soft mode of almost strain-free epitaxial polar axis-oriented tetragonal lead zirconate titanate thick films grown on CaF2 substrates were investigated by changing the Zr/(Zr+Ti) ratio and the temperature. The square of spontaneous polarization (Ps), Ps2, monotonically increased as tetragonal distortion, (c/a)−1, increased where a and c are the lattice parameters of the films and as the square of frequency in A1(1TO), ω2[A1(1TO)], increased. Additionally, the relationships of Ps2, (c/a−1), and ω2[A1(1TO)] were experimentally obtained, which confirmed the Ps value obtained from (c/a−1) and ω[A1(1TO)] without direct polarization measurements.

Composition control and thickness dependence of {100}-oriented epitaxial BiCoO3–BiFeO3 films grown by metalorganic chemical vapor deposition

xBiCoO3–(1−x)BiFeO3 films were deposited by metalorganic chemical vapor deposition. Although the film composition changed with deposition temperature, the composition could be adjusted by varying the input source gas composition at 700 °C. Moreover, adjusting the deposition time could change 0.16BiCoO3–0.84BiFeO3 film thickness. The crystal symmetry changed from rhombohedral to tetragonal as the film thickness decreased for 0.16BiCoO3–0.84BiFeO3 films grown on both (100)SrTiO3 and (100)cSrRuO3∥(100)SrTiO3 substrates, implying that the x value of the crystal symmetry boundaries between the tetragonal and rhombohedral structures changes with film thickness.

In situ Raman spectroscopy for characterization of the domain contributions to electrical and piezoelectric responses in Pb(Zr,Ti)O3 films

We employed in situ Raman spectroscopy under electric field for (100)/(001)-oriented tetragonal Pb(Ti0.61Zr0.39)O3 films with (001)-volume fraction (VC) of 35%. The increase in VC was revealed above 200 kV/cm, which resulted in the larger remanent polarization. In addition, the application of high enough field also brings a feature, i.e., large reversible change in VC with/without electric field that can quantitatively explain the enhanced piezoelectric response. These demonstrate the usefulness of in situ Raman observation to probe the domain contributions to the electrical and piezoelectric responses.

Impact of pulse poling on static and dynamic ferroelastic-domain contributions in tetragonal Pb(Ti, Zr)O3 films determined by in-situ x–ray diffraction analysis

The effects of bipolar pulse poling on the ferroelastic domain structure and their contribution to the electrical and piezoelectric properties of Pb(Ti0.7Zr0.3)O3 films are investigated. Micro x-ray diffraction measurements clearly show that the volume fraction of the c-domain increases irreversibly as the poling field is increased, leading to changes in the remanent polarization, dielectric constant, and piezoelectric coefficient. Theoretical estimations well explain the changes of remanent polarization and dielectric constant, but the increase in piezoelectric coefficient is much larger than the theoretical estimation. In-situ x-ray diffraction analysis under an electric field reveals that this disagreement is due to the unexpected activation of the ferroelastic domain wall motion. Our results provide new insight into the poling effect on the electric and piezoelectric properties of ferroelectric films.

Complex domain structure in relaxed PbTiO3 thick films grown on(100)cSrRuO3//(100)SrTiO3 substrates

Domain structures of epitaxial PbTiO3 films grown on (100)cSrRuO3//(100)SrTiO3 substrates by metalorganic chemical vapor deposition were investigated by x-ray diffraction (XRD) and piezoresponse force microscopy (PFM) techniques. It was found that with increasing film thickness, the domain structure changed from simple (001) polarization orientation to a complicated mixture of (001) and (100) orientations. PFM mappings showed that in the thicker films (∼1100 nm), the zigzag (001)/(100) domain boundaries made an angle of approximately 87° instead of 90° typically observed in (001)/(100) domain patterns in thinner (<300 nm) films. Full-relaxed tilting angle θ1 + θ2 + θ3 = 3.4° obtained from cross-sectional profile analysis of topological step-terrace structure was in good agreement with 3.4° and 3.6° angle values obtained from XRD measurements and theoretical prediction, respectively.

Enhancement of piezoelectric response in (100)/(001) oriented tetragonal Pb(Zr, Ti)O3 films by controlling tetragonality and volume fraction of the (001) orientation

(100)/(001)-oriented 2 μm-thick Pb(Ti, Zr)O3, films with the Zr/(Zr + Ti) ratio of 0.39 and 0.50 were prepared by metalorganic chemical vapor deposition. The volume fraction of (001)-oriented domain, Vc, was controlled by selecting the kinds of substrates with different thermal expansion coefficient. The effective piezoelectric constant increased up to 310 pm/V with decreasing the Vc and tetragonality (lattice parameter ratio of c axis to a axis) determined by the decrease of the Zr/(Zr + Ti) ratio. Observed large piezoelectric response was considered to be mainly contributed by the extrinsic effect, such as the 90o domain switching. These experimental inputs suggest the new concept for enhancement of the piezoelectric properties.

Single crystal-like selection rules for unipolar-axis oriented tetragonal Pb(Zr,Ti)O3 thick epitaxial films

We investigated the polarized Raman spectra of a strain-free, unipolar-axis oriented tetragonal Pb(Zr,Ti)O3 thick epitaxial film. We evaluated the single crystal-like selection rules of the A1- and E-symmetry components, and found an anomalous behavior in the angular dependence of the A1(1TO)-mode intensity similar to that observed in high-Tc superconductor single crystals. Raman tensor analyses of the A1(1TO) mode revealed complex phases may exist between two independent Raman-tensor components even in the single 180° domain state.

Effects of Substrate Clamping on Electrical Properties of Polycrystalline Piezoelectric Films

To clarify the effects of substrate clamping on electrical properties of polycrystalline piezoelectric films, microstructures, crystalline structures, and electrical properties of aerosol deposition (AD)-processed Pb(Zr,Ti)O3 (PZT) films were evaluated as a function of firing temperature and compared with those of AD-processed bulk specimens formed from the same starting powder. It was found that there are two different effects of substrate clamping on electrical properties in polycrystalline AD-processed PZT films. 90° domain wall motion is severely limited and makes little contribution to dielectric and piezoelectric responses due to an in-plane compressive stress state, resulting in significantly lower dielectric constant and field-induced strain. Furthermore, the c-domain distribution is increased by irreversible 90° domain wall motion from the a-domain to the c-domain due to an in-plane compressive stress during cooling through the phase transition temperature, resulting in higher remanent polarization.

Piezoelectric Properties of {100}-Oriented Epitaxial BiCoO3–BiFeO3 Films Measured Using Synchrotron X-ray Diffraction

Electric polarization and the strain of {100}-oriented epitaxial BiFeO3 and BiFeO3–BiCoO3 films grown on (100)cSrRuO3 ∥ (100)SrTiO3 substrates were simultaneously measured using synchrotron X-ray diffraction. The apparent piezoelectric coefficient d33,obs values of BiFeO3 and BiFeO3–BiCoO3 films were 28 and 63 pm/V, while the observed electrostrictive coefficient Q11,obs values of the films calculated from electric polarization and strain were 1.4 ×10-2 and 3.6 ×10-2 m4/C2, respectively. These results implied that the films become soft by the addition of BiCoO3 into BiFeO3.