Session V30: Focus Session: Multiferroic Properties of Oxide Films

Understanding Magnetism in Multiferroics

Multiferroics are interesting materials not only because of their exciting order parameters, but for the potential for parameter coupling. In order to understand the magnetoelectric coupling, the individual order parameters must first be understood. BiFeO3 (BFO), a room temperature ferroelectric and an antiferromagnet, is an excellent model system for understanding the coupling between ferroelectricity and magnetism. A combination of in-plane and out-of-plane piezoresponse force microscopy (PFM) allows 3D mapping of the ferroelectric polarization directions in micron-sized regions of the films. The magnetic order of BFO was obtained by using x-ray linear dichroism images using a photoelectron emission microscope (PEEM). When compared with our dichroism models, angle and temperature dependent absorption measurements allow decoupling and direction determination of the two order parameters, ferroelectric and magnetic, contributing to the photoemission signal. These studies reveal a strain-driven reduction in magnetic symmetry in thin films, leading to the formation of a preferred magnetic axis as opposed to the observed easy plane for bulk films. This reduction along with the previous proof of FE-AFM coupling allows electrical control of its magnetic axis. This electrical BFO control has a strong effect on ferromagnets even at room temperature.   

Infrared and Raman spectroscopy of the magneto-electric couping in BiFeO$_3$

We measured the phonon spectra of BiFeO$_3$ single crystals utilizing infrared spectroscopy and Raman scattering. The data was taken from 4 K to 300 K using a fine temperature step. Small accidents observed in the temperature dependence of phonon frequencies, in particular the lowest $E$ mode, have corresponding features in the electromagnon response. High temperature data, up to 1200 K, on ceramics (infrared) or single crystals (Raman) also show phonon frequency renormalization at the N\'eel temperature. Our results reinforce a picture where the ferroelastic character of BiFeO$_3$ plays an important role.   

Ferroelectric Size-effect on BiFeO$_{3}$ Films in Ultra-high Vacuum

The ferroelectric size effect is a highly pursued and controversial topic encompassing the scaling of polar distortion, domain structure and switchable polarization. We have studied epitaxial BiFeO$_{3}$ films using ultrahigh vacuum piezoresponse force microscopy. The films in vacuum develop out-of-plane polarization domains which, based on their lateral dimensions, drastically violate the Kittel law. From the analysis of the piezoresponse amplitude, we have established that the presence of a topographic island induces in-plane rotation of the polarization vector by 90$^{o}$. These findings are analyzed using a first-principles based model Hamiltonian approach. We have also obtained stable and reproducible piezoresponse hysteresis loops on the 2 nm films. Experiments conducted at the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, sponsored by the Division of Scientific User Facilities, U.S. Department of Energy.   

Electrical conduction at domain walls in multiferroic BiFeO3

We report the observation of room temperature electronic conductivity at ferroelectric domain walls in BiFeO3. The origin and nature of the observed conductivity is probed using a combination of conductive atomic force microscopy, high resolution transmission electron microscopy and first-principles density functional computations. We show that a structurally driven change in both the electrostatic potential and local electronic structure (i.e., a decrease in band gap) at the domain wall leads to the observed electrical conductivity. We estimate the conductivity in the wall to be several orders of magnitude higher than for the bulk material. Additionally we demonstrate the potential for device applications of such conducting nanoscale features.   

Ferroelastic domain formation in epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ thin films

Epitaxial La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) thin films are known to form domains to reduce substrate-induced strain. For instance, on cubic SrTiO$_{3}$ (100), LSMO thin films can have up to four rhombohedrally-strained variants. These individual strain states can distort the MnO$_{6}$ octahedra and lead to unique electrical and magnetic properties. We have used synchrotron x-ray diffuse scattering to probe the in- and out-of-plane domain structure of a 5 nm LSMO film grown on SrTiO$_{3}$ (100). Satellites are present near integer order and half-order peaks that result from octahedral tilting in the coherent LSMO film. By analyzing the amplitude, position, and anisotropy of the satellite peaks for multiple half-order peaks, we obtain a robust measure of the dimensions of the domains, the ordering of the variants, and the strain state of each variant. Implications of the domain structure on magnetic properties will be discussed.   

Development of All Oxide Exchange Bias Systems

Multiferroic materials exhibit multiply states of order which are often coupled. Bizmuth Ferrite (BFO$_{3})$ is a room temperature antiferromagnetic, ferroelectric materials, where electrical control of magnetism and vice versa has been established. Combining BFO$_{3}$ with ferromagnetic oxides such as Magnetite (Fe$_{3}$O$_{4})$ or Lanthanum Strontium Manganate (L$_{.7}$S$_{.3}$MO) could yield interesting system with electrically controllable exchange bias. We have used a novel deposition tool employing two pulsed electron beam sources (PEBS) to deposit epitaxial layers of BFO$_{3}$, LSMO, and Fe$_{3}$O$_{4}$ onto STO, LAO, and MgO substrates. We are in the process of making bilayers of these materials and examining the quality and influence of the oxide interface on the development and system control of the exchange bias.   

Growth and characterization of multiferroic BiMnO$_{3}$ thin films

BiMnO$_{3}$ is a rare single phase, multiferroic compound which displays both ferromagnetic and ferroelectric properties. However, it is complicated to grow thin films of BiMnO$_{3 }$due to the volatility of bismuth and substrate induced strain. We have grown thin films of BiMnO$_{3}$ on SrTiO$_{3}$ (100) substrates using pulsed laser deposition. These films have a ferromagnetic $T_{C}$ of about 95 K and electric polarization vs. electric field curves have confirmed their ferroelectric properties. The structure and chemical composition of these thin films have been characterized using x-ray diffraction, atomic force microscopy, scanning electron microscopy, and Auger electron spectroscopy. We will present evidence of the sensitivity of the multiferroic properties of BiMnO$_{3}$ thin films to the growth conditions and substrate induced strain.   

Multiferroicity in half-doped manganites

Using a joint approach of density functional theory and model calculations, we focus on unconventional physical mechanisms leading to multiferroicity in a prototypical half-doped manganite, La$_{0.5}$Ca$_{0.5}$MnO$_3$. We focus on the strong competition between two exotic charge-orbital-spin ordered states. These are, (1) the charge and orbital ordered zig-zag spin state, also known as the CE-state, and (2) a spin-dimer Zener-polaron state. Both these states respect the inversion symmetry of the lattice and hence can not be ferroelectric. We identify a single variational parameter in terms of the coherent rotation of spin-dimers which interpolates between these two ordered states. It is shown that the true groundstate could be intermediate between these two and can break the inversion symmetry of the lattice. Using DFT calculations we show that the groundstate is indeed ferroelectric with a polarization up to few $\mu C/cm^2$.   

Element specific magnetic moments of Ni and Mn in multiferroic Bi$_2$NiMnO$_6$ film grown on SrTiO$_3$ substrate

X-ray magnetic circular dichroism (XMCD) and x-ray absorption spectroscopy (XAS) at Ni L$_{2,3}$ edges and at Mn L$_{2,3}$ edges have been performed at 4.5 K and higher temperatures in a multiferroic thin film grown on SrTiO$_3$(001) substrate. These spectra show that Ni is in a divalent state and Mn is in a tetravalent state. The total magnetic moment of Mn is found to be about 2.8 $\mu_B$. The total magnetic moment at the Ni site is strongly reduced from the 2.0 $\mu_B$ expected for divalent Ni. We have also detected a small orbital magnetic moment at both Mn and Ni sites. We suggest that the weaker crystal fields at the Ni and Mn sites in the thin film give rise to an orbital moment. These results will be compared with the predictions of local spin density calculations. Supported by US Dept. of Energy.   

Field Modulated Intrinsic Positive Exchange Bias in Novel Ferrite Ru$_{0.25}$Cr$_{0.75}$O$_{2}$ near the Compensation Point

In some ferrimagnetic materials systems a compensation point is observed where the opposing sublattice magnetizations are equal and opposite resulting in a zero net magnetization. The resulting magnetization decreases below zero at temperatures below T$_{c}$ and then increase to zero at T$_{c}$. We observe this type of unusual ferrimagnetic behavior in the Ru$_{0.25}$Cr$_{0.75}$O$_{2}$ system. In addition, near the compensation point we observe positive exchange bias that can be modulated using an external applied magnetic field. Possible mechanisms will be discussed.   

Polarization Coupling in Ferroelectric Multilayers as a Function of Interface Charge Concentration

Intriguing properties of multilayered and graded ferroelectrics follow from the electrostatic and electromechanical interactions. The strength of the interlayer coupling depends on the concentration of interfacial defects with short-range local electrostatic fields. Defects may locally relax polarization differences and thus reduce the commensurate bound charge concentration at the interlayer interfaces. In this talk, we develop a theoretical analysis based on non-linear thermodynamics coupled with basic electrostatic relations to understand the role of charge compensation at the interlayer interfaces. The results show multilayered ferroelectrics with systematic variations in the composition may display a colossal dielectric response depending upon the interlayer electrostatic interactions. It is expected that other properties such as the pyroelectric and piezoelectric response will yield concomitant increases through the dielectric permittivity.   

Magnetoelectric effects in SrRuO$_{3}$/BaTiO$_{3}$ heterostructures: A First Principles Study

Ferroelectric materials in combination with ferromagnets have emerged as structures in which strong magnetoelectric coupling may exist originating from unconventional physical mechanisms. The use of oxides such as SrRuO$_{3}$ as a metal electrode has been found to be of fundamental importance for the realization of ferroelectric films with critical thicknesses down to three unit cells. Here we present a study of SrRuO$_{3}$/BaTiO$_{3}$ heterostructures within the framework of density functional theory. This heterostructure is interesting since SrRuO$_{3}$ is a weak ferromagnetic oxide metal and hence, when used as an electrode on BaTiO$_{3}$, presents the possibility of coupling between electric and magnetic order parameters. In particular we study the magnetoelectric (ME) effect at the interface of SrRuO$_{3}$/BaTiO$_{3}$ by treating SrRuO$_{3}$ as spin polarized metal. We find that magnetic properties at the interface are affected as the ferroelectric polarization in the BaTiO$_{3}$ is reversed. We discuss the origins of ME effect and compare them with previously proposed ME coupling mechanisms in Fe/BaTiO$_{3}$, Fe$_{3}$O$_{4}$/BaTiO$_{3, }$and SrRuO$_{3}$/SrTiO$_{3}$ heterostructures$^{1, 2}$. $^{1}$Niranjan et al., Phys. Rev. B, \textbf{78}, 140405 (2008); $^{2}$Rondinelli et al., Nat. Nanotechnology, \textbf{3,} 46, (2008)   

Surface Magnetoelectric Effects from First Principles.

A magnetoelectric effect allows affecting magnetic properties of materials by applying electric fields which may be interesting for potential technological applications such as electrically controlled magnetic data storage. We use density functional calculations to reveal and elucidate magnetoelectric effects due to an electric field applied to ferromagnetic metal surfaces.$^{1}$ We find that the surface magnetoelectric effect originates from spin-dependent screening of the electric field and leads to notable changes in the surface magnetization and the surface magnetocrystalline anisotropy. If the ferromagnet is a half-metal the screening charge is formed entirely by a single conducting spin channel which leads to the surface magnetoelectric coefficient being the universal constant \textit{$\mu $}$_{B}$/\textit{ec}$^{2} \quad \approx $ 6.44$\times $10$^{-14}$ \textit{Gcm}$^{2}$/$V$. This is in an excellent agreement with our first-principles calculation result for the half-metal CrO$_{2}$. These results are of considerable interest in the area of electrically-controlled magnetism and magnetoelectric phenomena. 1. C.-G. Duan et al., \textit{Phys. Rev. Lett.} 101, 137201 (2008).