Exploiting Symmetry Mismatch to Control Magnetism in a Ferroelastic Heterostructure.

In the bulk, LaCoO_{3} (LCO) is a paramagnet, yet the low-temperature ferromagnetism (FM) is observed in tensile strained thin films, and its origin remains unresolved. Here, we quantitatively measured the distribution of atomic density and magnetization in LCO films by polarized neutron reflectometry (PNR) and found that the LCO layers near the heterointerfaces exhibit a reduced magnetization but an enhanced atomic density, whereas the film's interior (i.e.

X-ray Magnetic Circular Dichroism Spectroscopy Applied to Nitrogenase and Related Models: Experimental Evidence for a Spin-Coupled Molybdenum(III) Center.

Nitrogenase enzymes catalyze the reduction of atmospheric dinitrogen to ammonia utilizing a Mo-7Fe-9S-C active site, the so-called FeMoco cluster. FeMoco and an analogous small-molecule (Et N)[(Tp)MoFe S Cl ] cubane have both been proposed to contain unusual spin-coupled Mo sites with an S(Mo)=1/2 non-Hund configuration at the Mo atom. Herein, we present Fe and Mo L -edge X-ray magnetic circular dichroism (XMCD) spectroscopy of the (Et N)[(Tp)MoFe S Cl ] cubane and Fe L -edge XMCD spectroscopy of the MoFe protein (containing both FeMoco and the 8Fe-7S P-cluster active sites).

Nanoscale ferroelastic twins formed in strained LaCoO films.

The coexistence and coupling of ferroelasticity and magnetic ordering in a single material offers a great opportunity to realize novel devices with multiple tuning knobs. Complex oxides are a particularly promising class of materials to find multiferroic interactions due to their rich phase diagrams, and are sensitive to external perturbations. Still, there are very few examples of these systems.

Tuning the Néel Temperature of Hexagonal Ferrites by Structural Distortion.

To tune the magnetic properties of hexagonal ferrites, a family of magnetoelectric multiferroic materials, by atomic-scale structural engineering, we studied the effect of structural distortion on the magnetic ordering temperature (T_{N}) in these materials. Using the symmetry analysis, we show that unlike most antiferromagnetic rare-earth transition-metal perovskites, a larger structural distortion leads to a higher T_{N} in hexagonal ferrites and manganites, because the K_{3} structural distortion induces the three-dimensional magnetic ordering, which is forbidden in the undistorted structure by symmetry. We also revealed a near-linear relation between T_{N} and the tolerance factor and a power-law relation between T_{N} and the K_{3} distortion amplitude.

Control of Epitaxial BaFeAs Atomic Configurations with Substrate Surface Terminations.

Atomic layer controlled growth of epitaxial thin films of unconventional superconductors opens the opportunity to discover novel high temperature superconductors. For instance, the interfacial atomic configurations may play an important role in superconducting behavior of monolayer FeSe on SrTiO and other Fe-based superconducting thin films. Here, we demonstrate a selective control of the atomic configurations in Co-doped BaFeAs epitaxial thin films and its strong influence on superconducting transition temperatures by manipulating surface termination of (001) SrTiO substrates.

Phase Coexistence and Kinetic Arrest in the Magnetostructural Transition of the Ordered Alloy FeRh.

In materials where two or more ordering degrees of freedom are closely matched in their free energies, coupling between them, or multiferroic behavior can occur. These phenomena can produce a very rich phase behavior, as well as emergent phases that offer useful properties and opportunities to reveal novel phenomena in phase transitions. The ordered alloy FeRh undergoes an antiferromagnetic to ferromagnetic phase transition at ~375 K, which illustrates the interplay between structural and magnetic order mediated by a delicate energy balance between two configurations.

Fluorination of epitaxial oxides: synthesis of perovskite oxyfluoride thin films.

While the synthesis of ABO3 perovskite films has enabled new strategies to control the functionality of this material class, the chemistries that have been realized in thin film form constitute only a fraction of those accessible to bulk chemists. Here, we report the synthesis of oxyfluoride films, where the incorporation of F may provide a new means to tune physical properties in thin films by modifying electronic structure. Fluorination is achieved by spin coating a poly(vinylidene fluoride) (PVDF) solution onto oxygen-deficient films.

Room-temperature multiferroic hexagonal LuFeO3 films.

The crystal and magnetic structures of single-crystalline hexagonal LuFeO(3) films have been studied using x-ray, electron, and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K.

Spin polarization measurement of homogeneously doped Fe(1-x)Co(x)Si nanowires by Andreev reflection spectroscopy.

We report a general method for determining the spin polarization from nanowire materials using Andreev reflection spectroscopy implemented with a Nb superconducting contact and common electron-beam lithography device fabrication techniques. This method was applied to magnetic semiconducting Fe(1-x)Co(x)Si alloy nanowires with x̅ = 0.23, and the average spin polarization extracted from 6 nanowire devices is 28 ± 7% with a highest observed value of 35%.

Element-specific magnetometry of EuS nanocrystals.

A soft x-ray absorption and x-ray magnetic circular dichroism (XMCD) study of the ferromagnetism in solution-grown EuS nanocrystals (NCs) is reported. The absorption edges of Eu M(5) and M(4), S K, O K, and P K were probed to determine elementally specific contributions to the magnetism of EuS NCs. Differential spin absorption was observed by XMCD at the Eu M(5,4) edges confirming the presence of a magnetic moment on the Eu(2+) 4f shell.