Charles M. Brooks

Atomically engineered ferroic layers yield a room-temperature magnetoelectric multiferroic

Materials that exhibit simultaneous order in their electric and magnetic ground states hold promise for use in next-generation memory devices in which electric fields control magnetism. Such materials are exceedingly rare, however, owing to competing requirements for displacive ferroelectricity and magnetism. Despite the recent identification of several new multiferroic materials and magnetoelectric coupling mechanisms, known single-phase multiferroics remain limited by antiferromagnetic or weak ferromagnetic alignments, by a lack of coupling between the order parameters, or by having properties that emerge only well below room temperature, precluding device applications. Here we present a methodology for constructing single-phase multiferroic materials in which ferroelectricity and strong magnetic ordering are coupled near room temperature. Starting with hexagonal LuFeO3—the geometric ferroelectric with the greatest known planar rumpling—we introduce individual monolayers of FeO during growth to construct formula-unit-thick syntactic layers of ferrimagnetic LuFe2O4 (refs 17, 18) within the LuFeO3 matrix, that is, (LuFeO3)m/(LuFe2O4)1 superlattices. The severe rumpling imposed by the neighbouring LuFeO3 drives the ferrimagnetic LuFe2O4 into a simultaneously ferroelectric state, while also reducing the LuFe2O4 spin frustration. This increases the magnetic transition temperature substantially—from 240 kelvin for LuFe2O4 (ref. 18) to 281 kelvin for (LuFeO3)9/(LuFe2O4)1. Moreover, the ferroelectric order couples to the ferrimagnetism, enabling direct electric-field control of magnetism at 200 kelvin. Our results demonstrate a design methodology for creating higher-temperature magnetoelectric multiferroics by exploiting a combination of geometric frustration, lattice distortions and epitaxial engineering.

Thermal conductivity as a metric for the crystalline quality of SrTiO3 epitaxial layers

Measurements of thermal conductivity Λ by time-domain thermoreflectance in the temperature range 100<T<300 K are used to characterize the crystalline quality of epitaxial layers of a prototypical oxide, SrTiO3. Twenty samples from five institutions using two growth techniques, molecular beam epitaxy and pulsed laser deposition (PLD), were analyzed. Optimized growth conditions produce layers with Λ comparable to bulk single crystals. Many PLD layers, particularly those that use ceramics as the target material, show surprisingly low Λ. For homoepitaxial layers, the decrease in Λ created by point defects correlates well with the expansion of the lattice parameter in the direction normal to the surface.

Atomic-resolution chemical imaging of oxygen local bonding environments by electron energy loss spectroscopy

Electron energy loss spectroscopy on an aberration-corrected scanning transmission electron microscope was used to map the elemental composition and bonding in a thin film of the multiferroic LuFe2O4 with atomic resolution. A two-dimensional analysis of the fine structure of the O-K edge yielded distinct signals for the two inequivalent oxygen sites in the crystal. Comparison to an ab-initio simulation showed that these two components can be interpreted in terms of the differing hybridization of the O p orbitals to the Lu and Fe d orbitals, thus producing an atomic-resolution map of the local oxygen bonding environment.

Quasiparticle mass enhancement and temperature dependence of the electronic structure of ferromagnetic SrRuO3 thin films.

We report high-resolution angle-resolved photoemission studies of epitaxial thin films of the correlated 4d transition metal oxide ferromagnet SrRuO(3). The Fermi surface in the ferromagnetic state consists of well-defined Landau quasiparticles exhibiting strong coupling to low-energy bosonic modes which contributes to the large effective masses observed by transport and thermodynamic measurements. Upon warming the material through its Curie temperature, we observe a substantial decrease in quasiparticle coherence but negligible changes in the ferromagnetic exchange splitting, suggesting that local moments play an important role in the ferromagnetism in SrRuO(3).

The adsorption-controlled growth of LuFe2O4 by molecular-beam epitaxy

We report the growth of single-phase (0001)-oriented epitaxial films of the purported electronically driven multiferroic, LuFe2O4, on (111) MgAl2O4, (111) MgO, and (0001) 6H-SiC substrates. Film stoichiometry was regulated using an adsorption-controlled growth process by depositing LuFe2O4 in an iron-rich environment at pressures and temperatures where excess iron desorbs from the film surface during growth. Scanning transmission electron microscopy reveals reaction-free film-substrate interfaces. The magnetization increases rapidly below 240 K, consistent with the paramagnetic-to-ferrimagnetic phase transition of bulk LuFe2O4. In addition to the ∼0.35 eV indirect band gap, optical spectroscopy reveals a 3.4 eV direct band gap at the gamma point.

Tuning thermal conductivity in homoepitaxial SrTiO3 films via defects

We demonstrate the ability to tune the thermal conductivity of homoepitaxial SrTiO3 films deposited by reactive molecular-beam epitaxy by varying growth temperature, oxidation environment, and cation stoichiometry. Both point defects and planar defects decrease the longitudinal thermal conductivity (k33), with the greatest decrease in films of the same composition observed for films containing planar defects oriented perpendicular to the direction of heat flow. The longitudinal thermal conductivity can be modified by as much as 80%—from 11.5 W m−1K−1 for stoichiometric homoepitaxial SrTiO3 to 2 W m−1K−1 for strontium-rich homoepitaxial Sr1+δTiOx films—by incorporating (SrO)2 Ruddlesden-Popper planar defects.

Intrinsic magnetic properties of hexagonal LuFeO3 and the effects of nonstoichiometry

We used oxide molecular-beam epitaxy in a composition-spread geometry to deposit hexagonal LuFeO3 (h-LuFeO3) thin films with a monotonic variation in the Lu/Fe cation ratio, creating a mosaic of samples that ranged from iron rich to lutetium rich. We characterized the effects of composition variation with x-ray diffraction, atomic force microscopy, scanning transmission electron microscopy, and superconducting quantum interference device magnetometry. After identifying growth conditions leading to stoichiometric film growth, an additional sample was grown with a rotating sample stage. From this stoichiometric sample, we determined stoichiometric h-LuFeO3 to have a T N = 147 K and M s = 0.018 μ B/Fe.

Direct band gaps in multiferroic h-LuFeO3

We measured the optical properties of epitaxial thin films of the metastable hexagonal polymorph of LuFeO3 by absorption spectroscopy, magnetic circular dichroism, and photoconductivity. Comparison with complementary electronic structure calculations reveals a 1.1 eV direct gap involving hybridized Fe 3dz2+O 2pz→Fe d excitations at the Γ and A points, with a higher energy direct gap at 2.0 eV. Both charge gaps nicely overlap the solar spectrum.

Imaging Local Polarization and Domain Boundaries with Picometer-Precision Scanning Transmission Electron Microscopy

Megan E. Holtz, Julia A. Mundy, Celesta S. Chang, Jarrett A. Moyer, Charles M. Brooks, Hena Das, Alejandro F. Rebola, Robert Hovden, Elliot Padgett, Craig J. Fennie, Peter Schiffer, Dennis Meier, Darrell G. Schlom, David A. Muller 1. School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA 2. Department of Physics and Materials Research Institute, University of Illinois at Urbana-Champaign, Urbana, IL, USA 3. Department of Materials Science and Engineering, Cornell University, Ithaca, NY, USA 4. Department of Materials, ETH Zürich, CH-8093 Zürich, Switzerland 5. Kavli Institute at Cornell for Nanoscale Science, Ithaca, NY, USA

Imaging Local Polarization and Domain Boundaries in Multiferroic (LuFeO3)m/(LuFe2O4)n Superlattices

Materials that couple strong ferroelectric and ferromagnetic order hold tremendous promise for nextgeneration memory devices. However, many so-called multiferroics have properties that are either weak, emerge well below room temperature, and/or lack coupling between the electric and magnetic domains, stymieing technological applications. The atomic-scale design of new multiferroics, usually realized as heterostructures or interface phases, requires a local probe of physical properties and structure inside the material. This atomic-scale feedback on ferroelectric polarization and domain structure has helped lead us to a new strong ferrimagnet-ferroelectric with the highest known simultaneous transition temperatures. These (LuFeO3)m(LuFe2O4)n superlattices (Fig 1) are constructed by integrating the ferroelectric, antiferromagnetic LuFeO3 and paraelectric, ferrimagnetic LuFe2O4. The hexagonal LuFeO3 is an improper ferroelectric where the Lu-O buckles into a polar structure [1]. We quantify the polar structure—manifest as a displacement of the lutetium atoms—with atomic precision for different superlattice layerings. Our ferroelectric domain measurements show large polarization and regular domain walls correlate with improved magnetic moment and critical temperature, TC.