Sean R. C. McMitchell

A Polar Corundum Oxide Displaying Weak Ferromagnetism at Room Temperature

Combining long-range magnetic order with polarity in the same structure is a prerequisite for the design of (magnetoelectric) multiferroic materials. There are now several demonstrated strategies to achieve this goal, but retaining magnetic order above room temperature remains a difficult target. Iron oxides in the +3 oxidation state have high magnetic ordering temperatures due to the size of the coupled moments. Here we prepare and characterize ScFeO3 (SFO), which under pressure and in strain-stabilized thin films adopts a polar variant of the corundum structure, one of the archetypal binary oxide structures. Polar corundum ScFeO3 has a weak ferromagnetic ground state below 356 K—this is in contrast to the purely antiferromagnetic ground state adopted by the well-studied ferroelectric BiFeO3.

Artificial construction of the layered Ruddlesden-Popper manganite La2Sr2Mn3O10 by reflection high energy electron diffraction monitored pulsed laser deposition.

Pulsed laser deposition has been used to artificially construct the n = 3 Ruddlesden–Popper structure La2Sr2Mn3O10 in epitaxial thin film form by sequentially layering La1–xSrxMnO3 and SrO unit cells aided by in situ reflection high energy electron diffraction monitoring. The interval deposition technique was used to promote two-dimensional SrO growth. X-ray diffraction and cross-sectional transmission electron microscopy indicated that the trilayer structure had been formed. A site ordering was found to differ from that expected thermodynamically, with the smaller Sr2+ predominantly on the R site due to kinetic trapping of the deposited cation sequence. A dependence of the out-of-plane lattice parameter on growth pressure was interpreted as changing the oxygen content of the films. Magnetic and transport measurements on fully oxygenated films indicated a frustrated magnetic ground state characterized as a spin glass-like magnetic phase with the glass temperature Tg ≈ 34 K. The magnetic frustration has a clear in-plane (ab) magnetic anisotropy, which is maintained up to temperatures of 150 K. Density functional theory calculations suggest competing antiferromagnetic and ferromagnetic long-range orders, which are proposed as the origin of the low-temperature glassy state.

High permittivity SrHf0.5Ti0.5O3 films grown by pulsed laser deposition

High permittivity SrHf0.5Ti0.5O3 films (k=62.8) have been deposited on (001) Nb–SrTiO3 single crystal conducting substrates by pulsed laser deposition. The SrHf0.5Ti0.5O3 films grow epitaxially with atomically smooth surfaces (root mean square roughness 4.8 A) and a c-axis orientation parallel to the substrate. The measured band gap of SrHf0.5Ti0.5O3 is 3.47 eV compared with 3.15 eV in SrTiO3. Under an applied electric field of 600 kV/cm, the leakage current density of the SrHf0.5Ti0.5O3 films is 4.63×10−4 A/cm2. These attractive dielectric properties and enhanced band gap values make SrHf0.5Ti0.5O3 a promising candidate for high-k dielectric applications in silicon-based integrated circuits.

Growth of M-type hexaferrite thin films with conical magnetic structure

Thin films of the M-type hexaferrite, BaFe10.2Sc1.8O19, have been grown on Al2O3 (00.1) substrates by pulsed laser deposition. Post-deposition annealing improves the structural quality and produces completely relaxed thin films. The post-annealed films show magnetic behavior corresponding to a conical magnetic structure, which is required to establish the magnetoelectric effect in hexaferrites. The magnetic phase diagram has been obtained from hard magnetization curves. Finite-size effects due to the restricted length scale of the magnetic helix explain differences in magnetic properties between thin films and the bulk.

Pinning effect on the band gap modulation of crystalline BexZn1−xO alloy films grown on Al2O3(0001)

We have investigated the influence of Be concentration on the microstructure of BexZn1−xO ternary films (from x = 0 to 0.77), grown on Al2O3(0001) substrates using radio-frequency co-sputtering. With increasing Be concentration, the (0002) X-ray diffraction peak shows a systematic shift from 33.86° to 39.39°, and optical spectroscopy shows a blue-shift of the band gap from 3.24 to beyond 4.62 eV towards the deep UV regime, indicating that Be atoms are incorporated into the host ZnO lattice. During the band-gap modulation, structural fluctuations (e.g. phase separation and compositional fluctuation of Be) in the ternary films were observed along with a significant change in the mean grain size. X-ray photoelectron spectroscopy indicates higher concentrations of metallic Be states found in the film with the smaller grain size. Correlation between these two observations indicates that Be segregates to near grain boundaries. A model structure is proposed through simulation, where an increase in grain growth driving force dominates over the Be particle pinning effect. This leads to further coalescence of grains, reactivation of grain growth, and the uniform distribution of Be composition in the BexZn1−xO alloy films.

Optimal growth and thermal stability of crystalline Be0.25Zn0.75O alloy films on Al2O3 (0001)

The influence of growth temperature on the synthesis of BexZn1−xO alloy films, grown on highly-mismatched Al2O3(0001) substrates, was studied by synchrotron x-ray scattering, high-resolution transmission electron microscopy and photoluminescence measurements. A single-phase BexZn1−xO alloy with a Be concentration of x = 0.25, was obtained at the growth temperature, Tg = 400 °C, and verified by high-resolution transmission electron microscopy. It was found that high-temperature growth, Tg≥600 °C, caused phase separation, resulting in a random distribution of intermixed alloy phases. The inhomogeneity and structural fluctuations observed in the BexZn1−xO films grown at high temperatures are attributed to a variation in Be composition and mosaic distribution via atomic displacement and strain relaxation.

Epitaxial growth of cubic MnSb on GaAs AND InGaAs(111)

The cubic polymorph of the binary transition metal pnictide (TMP) MnSb, c-MnSb, has been predicted to be a robust half-metallic ferromagnetic (HMF) material with minority spin gap ≳1 eV. Here, MnSb epilayers are grown by molecular beam epitaxy (MBE) on GaAs and In0.5Ga0.5As(111) substrates and analyzed using synchrotron radiation X-ray diffraction. We find polymorphic growth of MnSb on both substrates, where c-MnSb co-exists with the ordinary niccolite n-MnSb polymorph. The grain size of the c-MnSb is of the order of tens of nanometer on both substrates and its appearance during MBE growth is independent of the very different epitaxial strain from the GaAs (3.1%) and In0.5Ga0.5As (0.31%) substrates.

Epitaxial growth and enhanced conductivity of an IT-SOFC cathode based on a complex perovskite superstructure with six distinct cation sites

Epitaxial thin films of the 10 layer cubic perovskite superstructure Ba1.7Ca2.4Y0.9Fe5O13 were grown by pulsed laser deposition, retaining the six distinct cation sites found in the bulk material. Growth on single crystal strontium titanate (STO) (0 0 1) substrates changes the observed symmetry from orthorhombic to tetragonal and orients the layer stacking direction of the superstructure normal to the substrate plane. The material is a candidate cathode for solid oxide fuel cells (SOFCs) and in the intermediate temperature (IT) region at 600 °C we measure the in-plane AC conductivity of the thin film as 30 S cm−1, significantly enhanced over 3.5 S cm−1 found for the polycrystalline form. This is assigned to reduction of the grain boundary density and alignment of the planes predicted to have the highest electronic and ionic conductivities. High resolution electron microscopy measurements demonstrate the atomic site ordering producing the superstructure and reveal defects associated with stacking faults in the ordering sequence.

Simultaneous dynamic electrical and structural measurements of functional materials

A new materials characterization system developed at the XMaS beamline, located at the European Synchrotron Radiation Facility in France, is presented. We show that this new capability allows to measure the atomic structural evolution (crystallography) of piezoelectric materials whilst simultaneously measuring the overall strain characteristics and electrical response to dynamically (ac) applied external stimuli.