Session M6: Focus Session: Magnetic Oxide Thin Films and Heterostructures: Ferroelectric Effects

Anisotropic conductance at improper ferroelectric domain walls

In ferroelectric oxides natural interfaces spontaneously arise in form of domain walls. Just like artificially constructed interfaces these domain walls are a rich source for fascinating physics resulting from their low symmetry, geometric confinement, electrostatics, and strain. Enhanced electronic transport properties are for instance reported to emerge at domain walls in various ferroelectrics such as BaTiO$_{\mathrm{3}}$, BiFeO$_{\mathrm{3}}$, LiNbO$_{\mathrm{3}}$, or Pb(Zr$_{\mathrm{0.2}}$T$_{\mathrm{i0.8}})$O$_{\mathrm{3}}$. In my talk I will demonstrate and discuss additional degrees of freedom that arise at domain walls in so-called improper ferroelectrics -- systems in which the domain formation is determined by a primary order parameter other than the polarization. Due to the secondary nature of the polarization rather unusual domain wall configurations are stabilized leading to novel functionalities. Here, I will present two examples: Geometrically driven ferroelectric domain walls with anisotropic conductance properties and their magnetic analogue, i.e. hybrid domain walls in magnetically induced ferroelectrics. Results gained by cathode-lens microscopy, scanning-probe microscopy, and nonlinear optics will be shown providing insight to the domain wall physics on nano- to mesoscopic length scales.   

Polaronic nature of competing interfacial ferromagnetic/antiferromagnetic order in a La0.7Ca0.3MnO3/BiFeO3 heterostructure

We reveal the polaronic behavior associated with reduced interfacial ferromagnetic order in a La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/BiFeO$_{3}$ (LCMO/BFO) heterostructure, which is likely the origin of tunable magnetotransport upon switching the ferroelectric polarity in LCMO/BFO. This is discovered through the difference in dynamical spectral weight transfer between LCMO and LCMO/BFO at low temperatures. This polaronic feature in LCMO/BFO decreases in relatively high magnetic fields due to the increased spin alignment, while no discernible change is found in the LCMO film at low temperatures. These results thus shed new light on the intrinsic mechanisms governing magnetoelectric coupling in this heterostructure and potentially offer a new route to enhancing multiferroic functionality.   

Element-Specific Depth Profile of Magnetism and Stoichiometry at the La$_{0.67}$Sr$_{0.33}$MnO$_3$/BiFeO$_3$ Interface

Depth-sensitive magnetic, structural and chemical characterization is important in the understanding and optimization of novel physical phenomena emerging at interfaces of transition metal oxide heterostructures. In this work we have investigated an epitaxial bi-layers of ferromagnetic La$_{0.33}$Sr$_{0.67}$MnO$_3$ (LSMO) / multiferroic BiFeO$_3$. Polarised Neutron Reflectivity measurements conducted at OPAL, Australia; Chalk River, Canada; and FRM-II, Munich provided the absolute magnetic moment at the interface and X-ray Resonant Magnetic Reflectivity measurements performed at BESSY-II, Berlin provided element specific magnetic information. Our measurements indicate a region of depleted magnetization extending into the LSMO at the interface. Additional resonant X-ray reflection measurements indicate a corresponding region with an altered Mn- and O-content as origin of the reduction of the magnetic moment. This will help to systematically tune the interface stoichiometry to achieve a desired property.   

Examination of exchange fields at LaSrMnO$_3$/BiFeO$_3$ interfaces

The complex oxide materials are providing a vast playground of interesting material properties that couples spin, orbital, and charge degrees of freedom. We examine the presence of significant magnetization within the antiferromagnetic layer of BiFeO$_3$ (BFO) between ferromagnetic (FM) LaSrMnO$_3$ layers. Using a classical exchange field to account for orbital reconstruction and possible inter-layer mixing, we quantify the energy scale for the interface exchange based from polarized neutron reflectivity measurements. Furthermore, we estimate the critical layer thickness in which the magnetization will be reduced to zero (or close to zero).   

Spin-phonon coupling and ferroelectricity in magnetoelectric gallium ferrite

Gallium ferrite (GaFeO$_{3}$ or GFO) is a low temperature ferrimagnet and room temperature piezoelectric wherein the magnetic transition temperature (T$_{\mathrm{C}})$ could be tailored to room temperature and above by tuning the stoichiometry and processing conditions. Such tunability of the magnetic transition temperature renders GFO a unique perspective in the research of multiferroics to potentially demonstrate room temperature magnetoelectric effect attractive for futuristic digital memory applications. Recent studies in several transition metal oxides highlight the importance of spin-phonon coupling in designing novel multiferroics by means of strain induced phase transition. In the present work, we have systematically studied the evolution of phonons in good quality samples of GFO across the T$_{\mathrm{C}}$ using Raman spectroscopy. Using the phonon softening behavior and nearest neighbor spin-spin correlation function below T$_{\mathrm{C}}$ we estimated spin-phonon coupling strength in the magnetically ordered state. In the process, we also show, for the first time, the presence of a spin glass phase in GFO where the spin-glass transition has a signature of abrupt change in spin-phonon coupling strength. Though GFO is piezoelectric and crystallizes in polar Pc2$_{1}$n symmetry, its ferroelectric nature remained controversial probably due to the large leakage current in the bulk material. To address this issue, we deposited epitaxial thin film on single crystalline yttria stabilized zirconia (YSZ) substrate using indium tin oxide (ITO) as a bottom conducting layer. We demonstrate clear evidence of room temperature ferroelectricity in the thin films from the 180$^{\mathrm{o}}$ phase shift of the piezoresponse upon switching the electric field. Further, suppression of dielectric anomaly in presence of an external magnetic field clearly reveals a pronounced magneto-dielectric coupling across the magnetic transition temperature. In addition, using first principles calculations we elucidate that Fe ions are not only responsible for ferrimagnetism as observed earlier, but give rise to the observed ferroelectricity also, making GFO an unique single phase multiferroic.   

Intrinsic magnetic properties of multiferroic h-LuFeO3

The discovery of multiferroic materials with large magnetoelectrical couplings would lead to significant advancements in many technologies. Hexagonal LuFeO$_{\mathrm{3}}$ ($h$-LuFeO$_{\mathrm{3}})$ is a multiferroic that has recently been reported to be multiferroic at room temperature; in this work, we grow 200 nm thick $h$-LuFeO$_{\mathrm{3}}$ thin films to determine its intrinsic magnetic properties. We first deposit $h$-LuFeO$_{\mathrm{3}}$ in a composition-spread geometry, creating samples that range from iron rich to lutetium rich. We use x-ray diffraction, atomic force microscopy, scanning transmission electron microscopy, and SQUID magnetometry to determine the region of the sample that is nearest to perfect stoichiometry. After identifying this region, we grow an additional sample with a rotating sample stage to ensure uniform composition throughout the sample. We determine the magnetic properties to be quite different from previously reported findings, most notably with a higher $T_{\mathrm{N}}=$147 K. Our findings show that it is easy for $h$-LuFeO$_{\mathrm{3}}$ to incorporate defects and impurities phases, leading to degraded magnetic properties compared to the stoichiometric phase.   

Low-temperature structure transition in hexagonal LuFeO3

The structural change of h-LuFeO$_3$ films at low temperature has been studied using x-ray diffraction and x-ray absorption experiments. The results are analyzed using the displacements of three phonon modes that are related to the P6$_3$/mmc to P6$_3$cm structural transition. The data indicate that the in-plane motion of the Fe and apex oxygen are responsible for the observed anomaly in both x-ray absorption and diffraction experiments. This subtle structural transition may be an origin of the low temperature magnetic phase transition at $T_R$=130 K.   

Neutron Investigations of Multiferroic LuFe1-xMnxO3

While many new multiferroic materials have surfaced, only BiFeO$_{3}$ has been shown to evince coupling of both order parameters at room temperature. Materials in which the application of an electric field can directly switch the magnetization by 180 degrees have also been elusive. New theoretical predictions suggest that this will be possible in hexagonal LuFeO$_{3}$. Recent measurements of LuFeO$_{3}$ are promising. Bulk LuFeO$_{3}$ crystallizes in the Pbnm space group. However, it can be stabilized in the P6$_{3}$cm space group in thin films. Films are found to be ferroelectric at room temperature with a remanent polarization of 6.5 $\frac{\mu C}{cm^{2}}$along the c-axis and is of a respectable magnitude, evincing long range magnetic order with spins in the plane forming the familiar 120 degree structure. At lower temperatures, it was found that the moments begin to cant. Theoretical predictions suggest that this canted moment can be switched with an electric field. Unfortunately, this canting occurs at 130 K. While the recent work in films is exciting, it is important to understand what is intrinsic to the material. Recently, we have been able to stabilize ceramic samples of LuFeO$_{3}$ in the hexagonal form. During this talk we will discuss the magnetic structure of this compound in the bulk. We will also discuss our inelastic neutron scattering results.   

Origin of room-temperature multiferroism in hexagonal LuFeO$_3$

Combined theoretical and experimental studies are carried out, focusing on the exchange interactions and their couplings with the structural instabilities in hexagonal LuFeO$_3$ (hLFO). We apply an extended Kugel-Khomskii model based on maximally localized Wannier functions generated from band structure calculations. The model clearly shows that the single occupied $d_{z^2}$ orbital in hLFO greatly increases the exchange coupling compared to that of hexagonal LuMnO$_3$ in which $d_{z^2}$ is empty. The interlayer exchange interaction is the key to the spin reorientation (SR) and weak ferromagnetic moment observed below 130K. Our calculations show that SR is strongly coupled to the $K_1$ phonon mode and only weakly dependent on $K_3$ and $\Gamma_2^{-}$ phonons. It indicates that the atomic displacements along positive direction of $K_1$ mode is responsible for the spin reorientation. This scenario is confirmed by our X-ray diffraction and X-ray absorption experiments. In the end, we propose that $T_{\rm SR}$ can be adjusted to be room temperature by structural competition between $K_1$ and $\Gamma_2^-$ modes in hLFO or by interface engineering.   

Interfacial magnetic properties of La$_{0.7}$Sr$_{0.3}$MnO$_3$ / BaTiO$_3$ bilayers

Interfaces between the ferromagnetic and ferroelectric oxides may host artificial multiferroic phases with a strong magnetoelectric coupling, which can potentially be utilized for energy-efficient spintronics. Key for potential applications is that the magnetization of the ferromagnet is preserved at the interface, which is not always the case in complex oxide systems. In this work, we have explored the interfacial magnetic properties of ferromagnetic La0.7Sr0.3MnO3 and ferroelectric BaTiO$_3$ bilayers. The samples studied consist 10 nm La$_{0.7}$Sr$_{0.3}$MnO$_3$/ $t$ BaTiO$_3$ (LSMO/BTO) and $t$ BaTiO$_3$ (LSMO/BTO)/10 nm La$_{0.7}$Sr$_{0.3}$MnO$_3$ bilayers grown on SrTiO$_3$ substrates, with $t$ = 1.2, 2.4 and 4.8 nm. Results from X-ray resonant magnetic scattering, X-ray magnetic circular dichroism, and x-ray and polarized neutron reflectometry are combined to provide insights into how the interfacial magnetization of La$_{0.7}$Sr$_{0.3}$MnO$_3$ is influenced by the presence of the adjacent BaTiO$_3$. We find a modified interfacial magnetization in the ferromagnetic manganite layer that is dependent on the thickness and relative position of the ferroelectric layer.   

Concurrency control of the multiferroic transition in tetragonal-like BiFeO$_{3}$

The highly-elongated tetragonal-like BiFeO$_{3}$ (BFO) shows the concurrent transition of antiferromagnetic and ferroelectric order close to room temperature [1]. Despite the \textit{concurrency} indicating strong spin-lattice coupling effect, electric switching of the magnetic state has not been demonstrated so far. In this talk, we will introduce our efforts controlling the multiferroic transition temperature by means of A-site chemical substitution. Structural, ferroelectric, and magnetic states with varying chemical substitution ratio and temperature were systematically investigated through x-ray reciprocal space maps, capacitance measurement, and soft x-ray absorption spectroscopy. Landau phenomenological theory was employed to understand the behavior of multiple order parameters in a proposed phase diagram. Finally, we will discuss a new pathway to the electric switching of the magnetic state. \\[4pt] [1] K.-T. Ko \textit{et al.}, Concurrent transition of ferroelectric and magnetic ordering near room temperature. \textit{Nature Communications} \textbf{2}, 567 (2011).