Session X28: Focus Session: Magnetoelectric Coupling in Multiferroic Systems

Quantitative investigation of magnetoelectric coupling in various forms of multiferroics

Magnetoelectric susceptibility (MES) is probably the most direct way of estimating the magnitude of magnetoelectric coupling in many forms of magnetoelectric and/or multiferroic materials. Historically, the MES has been measured in numerous existing magnetoelectric materials in broad field, frequency, and temperature ranges and their MES values have been tabulated [1]. With growing interest worldwide toward applications of multiferroics for novel memory and sensor devices, however, there have been ever-increasing demands to measure quantitatively the MES of multiferroic thin films. Yet, the measurements of thin film MES become challenging in spite of its large MES value because the magnetoelectric voltages, proportional to the film thickness, usually get too small to be measured reliably. Herein, we introduce a highly sensitive magnetoelectric susceptometer that can detect the charge variation down to $\sim $10$^{-17}$C in a few gauss oscillating magnetic field. Using this specific setup, we could measure the MES of multiferroic thin films or single crystals with unprecedented accuracy and sensitivity in cryogenic (down to 2 K) and magnetic field (up to 9 T) environments. In this talk, we summarize a number of key results based on this technique; (1) MES of a 300 nm BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ nanopillar structure as well as those of a 250 nm BiFeO$_{3}$ film and of a BiFeO$_{3}$ single crystal. (2) MES of (Pb,Zr)TiO$_{3}$-NiFe$_{2}$O$_{4}$ nanocomposite films, and (3) temperature- and field-dependent MES in representative multiferroic crystals/films including TbMn$_{2}$O$_{5}$ , GaFeO$_{3}$, and Cr$_{2}$O$_{3}$. In particular, we demonstrate that the MES of the film with the nanopillar structure is enhanced by approximately one order of magnitude reaching 2×10$^{-10}$ s/m at room temperature, compared with those of a pure BiFeO$_{3}$ film and a single crystal. Furthermore, based on detailed field and temperature dependent MES studies, we show that magnetoelectric coupling in TbMn$_{2}$O$_{5}$ has been mediated and amplified by the large magnetoelastic effect. \\[4pt] [1] G. A. Smolenskii and I. E. Chupis, Sov. Phys. Usp. 25, 475 (1982); F. W. Hehl \textit{et al.} Phys. Rev. A 77, 022106 (2008).   

Magnetoelectric effects induced by domain walls

We explore the possibility to observe high-temperature multiferroic behavior in thin films of ordinary ferrimagnets. In thin films magnetostatic interactions induce periodic stripe domain patterns. We show that stripe domain patterns, stabilized by magneto-dipolar interactions, have ferroelectric properties similar to those of magnetic spirals in bulk materials. We studied behavior of domain patterns and the induced electric polarization in applied magnetic and electric fields using mean field approximation and Monte Carlo simulations. We find a sharp anomaly in dielectric constant close to polarization-flip transition, induced by an external electric field. We also studied the domain walls in conical spiral multiferroics, where magnetization \textbf{M} coexists with the electric polarization \textbf{P} induced by the cyloidal spiral. The structure of these domain walls explains the conservation of \textbf{P}$\times $\textbf{M }recently observed in CoCr$_{2}$O$_{4}$ as well as the magnetic field dependence of the polarization vector \textbf{P} in ZnCr$_{2}$Se$_{4}$.   

Large electric polarization in high pressure synthesized orthorhombic manganites $R$MnO$_{3}$ (R=Ho,Tm,Yb and Lu) by using the double-wave PE loop measurements

The magnitude of electric polarization via the conventional pyroelectric current and/or PE loop measurements often is ambiguous due to resistive components of the sample. To avoid this, a new technique called the double-wave method has been recently developed [1], in which only hysteretic PE components can be measured. Using this technique, we have measured the ferroelectric polarization of the orthorhombic $R$MnO$_{3}$ (R=Ho, Tm, Yb, and Lu) synthesized under high pressure. Large remnant polarization P$_{r}$ up to 920 $\mu $C/m$^{2}$ is observed at 10 K for LuMnO$_{3}$. Furthermore, the P$_{r}$ vs. temperature data from the PE loop has shown consistency with that measured through the pyroelectric current measurements, supporting a theoretical prediction of large polarization in the $E$-type spin structure in this system [2]. We also discuss the influence of thermal histories on the ferroelectric domain dynamics and possible internal bias field effects originating from oxygen vacancies in $R$MnO$_{3}$. [1] M. Fukunaga, \textit{et al.} J. Phys. Soc. Jpn. \textbf{77}, 064706 (2008). [2] I. A. Sergienko,\textit{ et al}. Phys. Rev. Lett., \textbf{97, }227204 (2006)   

Chemical engineering of the critical magnetic field for switching ferroelectricity in multiferroic hexaferrites

Multiferroics wherein the magnetic and ferroelectric order parameters coexist with their large cross-coupling effects have promising application potentials for multifunctional devices. To realize various technical exploitations, not only the capability of switching ferroelectricity with low magnetic field but also the tunability of the critical magnetic field ($B_{c})$ for the switching is essential. Herein, we report our discovery of a novel chemical route to engineer $B_{c}$ in a low field regime in the multiferroic hexaferrite system and discuss its mechanism.   

Ferroelectric and magnetic properties of multiferroic BaCoF$_4$ thin films

Thin films of BaCoF$_4$ have been successfully fabricated by molecular beam epitaxy and e beam technique on sapphire substrate (110), with a buffer layer of palladium grown using sputtering dc in argon atmosphere. Here we investigated the structural, morphological and ferroelectric properties were analyzed by means of various characterization techniques. The x- ray patterns showed that the films were oriented, but RHEED showed that the films were polycrystalline in the plane. AFM images showed a relatively granular surface. Measurements of the dielectric polarization showed that the films were ferroelectric at room temperature. The effect of magnetic fields on the ferroelectric properties at cryogenic temperatures will be described.   

All thin film magnetoelectric magnetic field sensors.

We have fabricated prototype ac magnetic field sensors operating at room temperature based on all thin film ME devices showing strong magnetoelectric (ME) coupling. The ME layers consist of a sol-gel derived Pb(Zr$_{0.52}$Ti$_{0.48})$TiO$_{3}$ (PZT) film and a dc magnetron sputter deposited magnetostrictive Fe$_{70}$Ga$_{30}$ (FeGa) film. The bilayer structures are prepared on micromachined Si wafers, and the laser cutting technique is used to release and isolate the cantilevers for optimization of the sensor performance. The PZT layer and the FeGa layer couple via the piezoelectric d$_{31}$ mode and the corresponding ME coupling coefficient is as high as $\approx $ 2 V/(Oe cm) for a lateral dimension of 1 mm$^{2}$ device at the mechanical resonant frequency of 333 Hz of a Si cantilever. The soft magnetic FeGa film requires dc bias magnetic field of around 90 Oe to operate the thin film ME device. The coupling between the PZT and the FeGa films is remarkably improved by depositing a 40 nm thick Pt intermediate layer. The clamping effect on the ME coupling is dramatically reduced by back-etching the Si cantilever down to 35 $\mu $m thick. The present work indicates presence of robust ME coupling in microfabricated multilayer thin film ME devices.   

Magnetic anisotropy modulation of magnetite in Fe$_3$O$_4$/BaTiO$_3$(100) epitaxial structures

Temperature dependent magnetometry and transport measurements on epitaxial Fe3O4 films grown on BaTiO$_3$(100) single crystals by molecular beam epitaxy show a series of discontinuities that are attributed to changes in the magnetic anisotropy induced by strain in the different crystal phases of BaTiO$_3$. High resolution x-ray diffraction measurements show that the magnetite film is under tensile strain at room temperature, which is ascribed to the lattice expansion of BaTiO$_3$ at the cubic to tetragonal transition, indicating that the magnetite film is relaxed at the growth temperature. From the magnetization versus temperature curves, the variation in the magnetic anisotropy is determined and compared with numerical estimates for the magnetoelastic anisotropies. In particular, the tensile strain in the Fe$_3$O$_4$ films is shown to give rise to a strong perpendicular magnetic anisotropy, as observed experimentally. These results demonstrate the possibility of using the piezoelectric response of BaTiO$_3$ to modulate the magnetic anisotropy of magnetite films.   

Enhanced power output from a magnetically coupled piezoelectric cantilever

Piezoelectric cantilevers have been widely studied for energy scavenging applications, but suffer from poor output power outside of a narrow frequency range near the cantilever resonance. Here, we demonstrate how power output can be enhanced by applying a simple passive external force. A symmetrical and repulsive magnetic force is applied to a piezoelectric cantilever beam to compensate the cantilever spring force. The raised and compensated spring potential introduced by the magnetic coupling is found to broaden the frequency response without altering the resonant frequency or introducing damping at resonance. Furthermore, the modified cantilever responds chaotically outside of the resonant frequency, causing increased voltage output across a large spectral region. The total voltage output across the spectrum increases between 31{\%} and 87{\%}. Model calculations support these results.   

Quantitative determination of the enhanced magnetoelectric

With growing interest worldwide toward applications of multiferroic materials for novel memory and magnetic sensor devices, there have been numerous efforts to synthesize multiferroic thin films with large magnetoelectric coupling. Yet, quantitative information on the magnetoelectric susceptibility (MES) of the film is still lacking because it is difficult to measure a reduced magnetoelectric signal due to a tiny thickness. In the present work, we have determined quantitative MES for a 300 nm BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ nanostructure, 250 nm BiFeO$_{3}$ film, and BiFeO$_{3}$ single crystal with our highly sensitive magnetoelectric susceptometer operating in cryogenic (down to 2 K) and high magnetic field, $H$, (up to 9 T) environments. We find that the MES of the BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ nanostructure shows a typical anti-symmetric shape with DC magnetic field up to 340 K, as expected in the magnetoelectric coupling mediated by strain between piezoelectric and magnetostrictive materials. At room temperature, the transverse MES of the nanostructure shows a maximum of 2 $\times 10^{-10}$ s/m at low $H$ = 6 kOe. Our results also demonstrate that the MES value of the nanopillar structured film is enhanced by approximately one order of magnitude than that of pure 250 nm BiFeO$_{3}$ film and BiFeO$_{3}$ single crystal.   

Epitaxial complex oxide tunnel barriers

Tunnel junctions with complex oxide thin film barriers are of interest for studies of the critical thickness of ferroelectricity, of phonon modes in ultrathin films and of traps by inelastic tunneling spectroscopy. We show that high-quality epitaxial SrTiO$_{3}$ and BaTiO$_{3}$ tunnel barriers can be grown on Pt bottom electrodes. Coherent, epitaxial Pt films with roughness of less than a unit cell were grown on (001) SrTiO$_{3}$ to serve as bottom electrodes for epitaxial SrTiO$_{3}$ and BaTiO$_{3}$ tunnel barriers. All interfaces were atomically abrupt as confirmed by atomic resolution Z-contrast imaging. The IV characteristics were non-linear, demonstrating good insulating properties. For the SrTiO$_{3}$ barriers and voltage sweeps up to $\pm$ 0.5 V, the measured tunnel current was independent of the sweep direction. At low biases, dynamic conductance curves showed a symmetrical parabolic shape around the origin in both resistance states. At high bias, deviation from the ideal tunnel behavior was observed. A large increase of the tunnel conductance occurred above a minimum positive bias. A dramatic decrease of tunnel conductance occurred for a large negative bias, indicating bipolar switching. We show the contributions to the resistive switching. Phonon modes and traps are determined using inelastic tunneling spectroscopy with both paraelectric and ferroelectric tunnel barriers.