Jung H. Park

Co-metal clustering as the origin of ferromagnetism in Co-doped ZnO thin films

The origin of ferromagnetism in ZnO-based systems was investigated using Co-doped ZnO thin films as prototypical examples of II–VI-based diluted magnetic semiconductors. In spite of the atomic-scale dissolution of Co ions in wurtzite ZnO, both the magnetization-temperature curve and the magnetization-field curve demonstrated that Zn1−xCoxO thin films were paramagnetic for x⩽0.12. On the other hand, Zn1−xCoxO films with x greater than 0.12 were characterized by the Co-metal clustering and apparently showed room-temperature ferromagnetism. The discrepancy between the zero-field cooling and the field cooling curves further indicates that Co-doped ZnO films (for x>0.12) are superparamagnetic and the observed ferromagnetism originates from the nanometer-sized Co clusters.

Magnetoelectric coupling of [00l]-oriented Pb(Zr0.4Ti0.6)O3–Ni0.8Zn0.2Fe2O4 multilayered thin films

Multilayered thin films consisting of alternatively stacking Pb(Zr0.4Ti0.6)O3 (PZT) and Ni0.8Zn0.2Fe2O4 (NZFO) layers were fabricated to exploit a strain-mediated coupling of piezoelectricity and magnetostriction. The 450-nm-thick PZT/NZFO multilayer fabricated by pulsed laser deposition showed magnetodielectric effects upon applying a static magnetic field. The magnetoelectric (ME) susceptibility values estimated using these magnetodielectric responses were in the range of 15–30mV∕cmOe at a zero magnetic-field strength and were comparable to those obtained using a more commonly employed “dynamic” ME method.

Magnetoelectric coupling susceptibility from magnetodielectric effect

A multiferroic material with a pronounced degree of the magnetoelectric (ME) coupling at room temperature is of great technological importance. A widely used method of indirectly assessing the ME coupling is based on the magnetodielectric (MD) effect, which accounts for the variation in dielectric permittivity under an applied bias magnetic field. At the present stage, however, there is no systematic guideline on evaluating the ME coupling susceptibility from the MD effect. Here we propose a simple theoretical method of estimating the ME coupling susceptibility using the MD data. This alternative approach is applied to an epitaxially grown La-doped BiFeO3 thin film as an illustrative example.

Strain-mediated magnetoelectric coupling in BaTiO3-Co nanocomposite thin films

The authors present a metal-ceramic type composite film in which Co particles with the mean size of 70nm are uniformly dispersed in an insulating ferroelectric BaTiO3 (BTO) matrix. Transmission electron microscopy images revealed that the Co particle was tightly surrounded with the BTO matrix in atomic scales. The magnetostrictive strain-induced change in the relative dielectric permittivity was demonstrated by observing pronounced magnetodielectric effects under a static bias magnetic field. The present BTO-Co composite thin film also exhibited large dynamic magnetoelectric responses (160–170mV∕cmOe at 50kHz) at room temperature by virtue of a tight bonding between the two constituting piezoelectric and magnetostrictive phases.

Polarized Raman scattering of highly [111]-oriented Pb(Zr,Ti)O3 thin films in the rhombohedral-phase field

Highly [111]-oriented rhombohedral lead zirconate titanate [Pb(Zr1−xTix)O3;PZT] thin films were prepared on Pt(111)∕Ti∕SiO2∕Si substrates by suitably combining pulsed laser deposition with sol-gel coating. Two distinctive types of the backscattering geometry were employed for the polarized Raman scattering study: (i) normal backscattering in which the propagation direction of relevant phonons is parallel to the principal [111] polar axis of the rhombohedral 3m(C3v) point group for observing E(TO) phonons, and (ii) side-view backscattering in which the phonon-propagation direction is normal to the [111] direction for isolating A1(TO) phonons. Room-temperature Raman spectra of the [111]-oriented PZT60∕40 film (with Zr∕Ti=60∕40) having the high-temperature rhombohedral form were examined and compared with those of the [111]-oriented low-temperature rhombohedral PZT90∕10 film which is characterized by the tilting of oxygen octahedra about the principal [111] direction. Similarities and differences in the obse...

Subpeak structure of the lowest-frequency E-symmetry transverse optical phonon in Ba-doped PbTiO3 single crystals

Polarized Raman scattering study of Ba-doped PbTiO3 single crystals indicated that, for a limited temperature range between 650 and 750 K, the line shape of the E(1TO) “soft” mode could not be described by a single damped harmonic oscillator but was adequately fitted by two independent oscillators. The line-shape fitting of the E(1TO) spectrum further indicated a gradual intensity inversion between the two subpeaks with increasing temperature. For temperatures below the crossover point, the subpeak with a higher mode frequency dominated the line shape. Contrary to this, the line shape was predominantly described by the subpeak with a lower frequency for temperatures above the crossover point. The subpeak structure becomes progressively evident with increased content of barium (Ba) dopant. We examined possible origins of the observed subpeak structure with the crossover in the integrated intensity and attributed this anomalous behavior to localized defects caused by the Ba impurity.