Strain-Driven Nanoscale Phase Competition near the Antipolar–Nonpolar Phase Boundary in Bi_0.7La_0.3FeO₃ Thin Films

Complex-oxide materials tuned to be near phase boundaries via chemistry/composition, temperature, pressure, etc. are known to exhibit large susceptibilities. Here, we observe a strain-driven nanoscale phase competition in epitaxially constrained Bi_0.7La_0.3FeO₃ thin films near the antipolar–nonpolar phase boundary and explore the evolution of the structural, dielectric, (anti)­ferroelectric, and magnetic properties with strain. We find that compressive and tensile strains can stabilize an antipolar PbZrO₃-like <i>Pbam</i> phase and a nonpolar <i>Pnma</i> orthorhombic phase, respectively. Heterostructures grown with little to no strain exhibit a self-assembled nanoscale mixture of the two orthorhombic phases, wherein the relative fraction of each phase can be modified with film thickness. Subsequent investigation of the dielectric and (anti)­ferroelectric properties reveals an electric-field-driven phase transformation from the nonpolar phase to the antipolar phase. X-ray linear dichroism reveals that the antiferromagnetic-spin axes can be effectively modified by the strain-induced phase transition. This evolution of antiferromagnetic-spin axes can be leveraged in exchange coupling between the antiferromagnetic Bi_0.7La_0.3FeO₃ and a ferromagnetic Co_0.9Fe_0.1 layer to tune the ferromagnetic easy axis of the Co_0.9Fe_0.1. These results demonstrate that besides chemical alloying, epitaxial strain is an alternative and effective way to modify subtle phase relations and tune physical properties in rare earth-alloyed BiFeO₃. Furthermore, the observation of antiferroelectric-antiferromagnetic properties in the <i>Pbam</i> Bi_0.7La_0.3FeO₃ phase could be of significant scientific interest and great potential in magnetoelectric devices because of its dual antiferroic nature

Microbial Reduction of Arsenic-Doped Schwertmannite by <i>Geobacter sulfurreducens</i>

The fate of As­(V) during microbial reduction by <i>Geobacter sulfurreducens</i> of Fe­(III) in synthetic arsenic-bearing schwertmannites has been investigated. During incubation at pH7, the rate of biological Fe­(III) reduction increased with increasing initial arsenic concentration. From schwertmannites with a relatively low arsenic content (<0.3 wt %), only magnetite was formed as a result of dissimilatory iron reduction. However, bioreduction of schwertmannites with higher initial arsenic concentrations (>0.79 wt %) resulted in the formation of goethite. At no stage during the bioreduction process did the concentration of arsenic in solution exceed 120 μgL¹, even for a schwertmannite with an initial arsenic content of 4.13 wt %. This suggests that the majority of the arsenic is retained in the biominerals or by sorption at the surfaces of newly formed nanoparticles.Subtle differences in the As <i>K</i>-edge XANES spectra obtained from biotransformation products are clearly related to the initial arsenic content of the schwertmannite starting materials. For products obtained from schwertmannites with higher initial As concentrations, one dominant population of As­(V) species bonded to only two Fe atoms was evident. By contrast, schwertmannites with relatively low arsenic concentrations gave biotransformation products in which two distinctly different populations of As­(V) persisted. The first is the dominant population described above, the second is a minority population characterized by As­(V) bonded to four Fe atoms. Both XAS and XMCD evidence suggest that the latter form of arsenic is that taken into the tetrahedral sites of the magnetite.We conclude that the majority population of As­(V) is sorbed to the surface of the biotransformation products, whereas the minority population comprises As­(V) incorporated into the tetrahedral sites of the biomagnetite. This suggests that microbial reduction of highly bioavailable As­(V)-bearing Fe­(III) mineral does not necessarily result in the mobilization of the arsenic

A One-Pot Synthesis of Monodispersed Iron Cobalt Oxide and Iron Manganese Oxide Nanoparticles from Bimetallic Pivalate Clusters

Monodispersed iron cobalt oxide (Fe₂CoO₄) and iron manganese oxide (Mn_0.43Fe_2.57O₄) nanoparticles have been synthesized using bimetallic pivalate clusters of [Fe₂CoO­(O₂C^tBu)₆(HO₂C^tBu)₃] (<b>1</b>), Co₄Fe₂O₂(O₂C^tBu)₁₀(MeCN)₂] (<b>2</b>), and [Fe₂MnO­(O₂C^tBu)₆(HO₂C^tBu)₃] (<b>3</b>) respectively as single source precursors. The precursors were thermolyzed in a mixture of oleylamine and oleic acid with either diphenyl ether or benzyl ether as solvent at their respective boiling points of 260 or 300 °C. The effect of reaction time, temperature and precursor concentration (0.25 or 0.50 mmol) on the stoichiometry, phases or morphology of the nanoparticles were studied. TEM showed that highly monodispersed spherical nanoparticles of Fe₂CoO₄ (3.6 ± 0.2 nm) and Mn_0.43Fe_2.57O₄ (3.5 ± 0.2 nm) were obtained from 0.50 mmol of <b>1</b> or <b>3</b>, respectively at 260 °C. The decomposition of the precursors at 0.25 mmol and 300 °C revealed that larger iron cobalt oxide or iron manganese oxide nanoparticles were obtained from <b>1</b> and <b>3</b>, respectively, whereas the opposite was observed for iron cobalt oxide from <b>2</b> as smaller nanoparticles appeared. The reaction time was investigated for the three precursors at 0.25 mmol by withdrawing aliquots at 5 min, 15 min, 30 min, 1 h, and 2 h. The results obtained showed that aliquots withdrawn at reaction times of less than 1 h contain traces of iron oxide, whereas only pure cubic iron cobalt oxide or iron manganese oxide was obtained after 1 h. Magnetic measurements revealed that all the nanoparticles are superparamagnetic at room temperature with high saturation magnetization values. XMCD confirmed that in iron cobalt oxide nanoparticles, most of the Co²⁺ cations are in the octahedral site. There is also evidence in the magnetic measurements for considerable hysteresis (>1T) observed at 5 K. EPMA analysis and ICP-OES measurements performed on iron cobalt oxide nanoparticles obtained from [Fe₂CoO­(O₂C^tBu)₆(HO₂C^tBu)₃] <b>(1)</b> revealed that stoichiometric Fe₂CoO₄ was obtained only for 0.50 mmol precursor concentration. All the nanoparticles were characterized by powder X-ray diffraction (p-XRD), transmission electron microscopy (TEM), inductively coupled plasma-optical emission spectroscopy (ICP-OES), electron probe microanalysis (EPMA), X-ray magnetic circular dichroism (XMCD), and superconducting quantum interference device (SQUID) magnetometry

Tc(VII) and Cr(VI) Interaction with Naturally Reduced Ferruginous Smectite from a Redox Transition Zone

Fe­(II)-rich clay minerals found in subsurface redox transition zones (RTZs) can serve as important sources of electron equivalents limiting the transport of redox-active contaminants. While most laboratory reactivity studies are based on reduced model clays, the reactivity of naturally reduced field samples remains poorly explored. Characterization of the clay size fraction of a fine-grained unit from the RTZ interface at the Hanford site, Washington, including mineralogy, crystal chemistry, and Fe­(II)/(III) content, indicates that ferruginous montmorillonite is the dominant mineralogical component. Oxic and anoxic fractions differ significantly in Fe­(II) natural content, but Fe_TOTAL remains constant, demonstrating no Fe loss during its reduction–oxidation cyclings. At native pH of 8.6, the anoxic fraction, despite its significant Fe­(II), ∼23% of Fe_TOTAL, exhibits minimal reactivity with TcO₄^– and CrO₄^2– and much slower reaction kinetics than those measured in studies with biologically/chemically reduced model clays. Reduction capacity is enhanced by added/sorbed Fe­(II) (if Fe­(II)_SORBED > 8% clay Fe­(II)_LABILE); however, the kinetics of this conceptually surface-mediated reaction remain sluggish. Surface-sensitive Fe L-edge X-ray absorption spectroscopy shows that Fe­(II)_SORBED and the resulting reducing equivalents are not available in the outermost few nanometers of clay surfaces. Slow kinetics thus appear related to diffusion-limited access to electron equivalents retained within the clay mineral structure

Ultralow Damping in Nanometer-Thick Epitaxial Spinel Ferrite Thin Films

Pure spin currents, unaccompanied by dissipative charge flow, are essential for realizing energy-efficient nanomagnetic information and communications devices. Thin-film magnetic insulators have been identified as promising materials for spin-current technology because they are thought to exhibit lower damping compared with their metallic counterparts. However, insulating behavior is not a sufficient requirement for low damping, as evidenced by the very limited options for low-damping insulators. Here, we demonstrate a new class of nanometer-thick ultralow-damping insulating thin films based on design criteria that minimize orbital angular momentum and structural disorder. Specifically, we show ultralow damping in <20 nm thick spinel-structure magnesium aluminum ferrite (MAFO), in which magnetization arises from Fe³⁺ ions with zero orbital angular momentum. These epitaxial MAFO thin films exhibit a Gilbert damping parameter of ∼0.0015 and negligible inhomogeneous linewidth broadening, resulting in narrow half width at half-maximum linewidths of ∼0.6 mT around 10 GHz. Our findings offer an attractive thin-film platform for enabling integrated insulating spintronics

Strain-Engineered Oxygen Vacancies in CaMnO₃ Thin Films

We demonstrate a novel pathway to control and stabilize oxygen vacancies in complex transition-metal oxide thin films. Using atomic layer-by-layer pulsed laser deposition (PLD) from two separate targets, we synthesize high-quality single-crystalline CaMnO₃ films with systematically varying oxygen vacancy defect formation energies as controlled by coherent tensile strain. The systematic increase of the oxygen vacancy content in CaMnO₃ as a function of applied in-plane strain is observed and confirmed experimentally using high-resolution soft X-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive hard X-ray photoemission spectroscopy (HAXPES). The relevant defect states in the densities of states are identified and the vacancy content in the films quantified using the combination of first-principles theory and core–hole multiplet calculations with holistic fitting. Our findings open up a promising avenue for designing and controlling new ionically active properties and functionalities of complex transition-metal oxides via strain-induced oxygen-vacancy formation and ordering

Magnetic Mesocrystal-Assisted Magnetoresistance in Manganite

Mesocrystal, a new class of crystals as compared to conventional and well-known single crystals and polycrystalline systems, has captured significant attention in the past decade. Recent studies have been focused on the advance of synthesis mechanisms as well as the potential on device applications. In order to create further opportunities upon functional mesocrystals, we fabricated a self-assembled nanocomposite composed of magnetic CoFe₂O₄ mesocrystal in Sr-doped manganites. This combination exhibits intriguing structural and magnetic tunabilities. Furthermore, the antiferromagnetic coupling of the mesocrystal and matrix has induced an additional magnetic perturbation to spin-polarized electrons, resulting in a significantly enhanced magnetoresistance in the nanocomposite. Our work demonstrates a new thought toward the enhancement of intrinsic functionalities assisted by mesocrystals and advanced design of novel mesocrystal-embedded nanocomposites