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Electron magnetic resonance (EMR) spectroscopy was used to determine the magnetic properties of maghemite (γ-Fe2O3) nanoparticles formed within size-constraining Listeria innocua (LDps)-(DNA-binding protein from starved cells) protein cages that have an inner diameter of 5 nm. Variable-temperature X-band EMR spectra exhibited broad asymmetric resonances with a superimposed narrow peak at a gyromagnetic factor of g ≈ 2. The resonance structure, which depends on both superparamagnetic fluctuations and inhomogeneous broadening, changes dramatically as a function of temperature, and the overall linewidth becomes narrower with increasing temperature. Here, we compare two different models to simulate temperature-dependent lineshape trends. The temperature dependence for both models is derived from a Langevin behavior of the linewidth resulting from 'anisotropy melting.' The first uses either a truncated log-normal distribution of particle sizes or a bi-modal distribution and then a Landau-Liftshitz lineshape to describe the nanoparticle resonances. The essential feature of this model is that small particles have narrow linewidths and account for the g ≈ 2 feature with a constant resonance field, whereas larger particles have broad linewidths and undergo a shift in resonance field. The second model assumes uniform particles with a diameter around 4 nm and a random distribution of uniaxial anisotropy axes. This model uses a more precise calculation of the linewidth due to superparamagnetic fluctuations and a random distribution of anisotropies. Sharp features in the spectrum near g ≈ 2 are qualitatively predicted at high temperatures. Both models can account for many features of the observed spectra, although each has deficiencies. The first model leads to a nonphysical increase in magnetic moment as the temperature is increased if a log normal distribution of particles sizes is used. Introducing a bi-modal distribution of particle sizes resolves the unphysical increase in moment with temperature. The second model predicts low-temperature spectra that differ significantly from the observed spectra. The anisotropy energy density K1, determined by fitting the temperature-dependent linewidths, was ∼50 kJ/m3, which is considerably larger than that of bulk maghemite. The work presented here indicates that the magnetic properties of these size-constrained nanoparticles and more generally metal oxide nanoparticles with diameters d < 5 nm are complex and that currently existing models are not sufficient for determining their magnetic resonance signatures. [ABSTRACT FROM AUTHOR]

A biomimetic approach with ferritin as a size-constrained reaction vessel for the controlled synthesis of metal oxide nanoparticles containing both Fe and Co oxides is used for exerting synthetic control over the magnetic exchange bias behavior. It is demonstrated that the significant magnetic exchange bias exhibited by these materials and the magnetic moment can be tailored by adjusting the synthesis conditions and Fe/Co ratio.

A set of N-coupled differential equations is developed for calculating the history-dependent magnetization of an ensemble of single domain magnetic particles. It is shown that a value of 16 for N is sufficient for reasonable convergence but that a two-state model yields inaccurate results.

A method for analyzing the shape, periodicity, and position of the interference patterns in specular (theta-2theta) scattering spectra to determine roughness parameters is presented. The means and rms thickness values of the distribution control the periodicity and depth of the interference patterns in the spectra.

The surface contribution to the magneto-crystalline anisotropy energy of spherical magnetite nanoparticles are investigated. Surface atoms were found to increase the volume anisotropy energy density of the particles, and this effect increased sublinealry with particle curvature.

This work demonstrates the critical role of orbital hybridizations in the FeO6 octahedral distortion, composition-driven phase transition, and bonding covalency in multiferroic (Bi1-xSmx)FeO3 (x = 0.10-0.20) ceramics in the vicinity of the morphotropic phase boundary (MPB). Sequential composition-driven transitions from the polar rhombohedral R3c to antipolar orthorhombic Pbam and then Pnma phases were revealed as the system crosses the MPB with increasing Sm. A coexistence of ferroelectric (FE) rhombohedral R3c and antiferroelectric (AFE) PbZrO3-like orthorhombic Pbam symmetries was identified by the 1/2{000}, 1/4 {100}, 1/4{110}, 1/4{111}, and 1/4{121} superlattice diffractions at x = 0.12-0.16. In addition to R3c and Pbam space groups, the nonpolar SmFeO3-like orthorhombic Pnma space group becomes the predominant phase at x = 0.20 confirmed by the 1/2{100} superlattice diffractions. The Fe L3-edge and oxygen Kedge synchrotron X-ray absorptions indicate that the O 2p-Fe 3d and O 2p-Bi 6s/6p orbital hybridizations were decreased as the system approaches the MPB. [ABSTRACT FROM AUTHOR]

Micro-Raman spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy ( TEM), oxygen vacancies, synchrotron X-ray absorption spectroscopy, magnetizations, optical band gaps, and photovoltaic ( PV) effects have been studied in (Bi1− xLa x)FeO3 ( BFO100 xL) ceramics for x = 0.0, 0.05, 0.10, and 0.15. XRD, Raman spectra, and TEM confirm a rhombohedral R3 c symmetry with the tilted FeO6 oxygen octahedra in all compounds. The low-frequency Raman vibrations become broader and shift toward higher frequency as La3+ increases. Fe K-edge synchrotron X-ray absorptions reveal that Fe3+ valence and Fe-O-Fe bond angle are not modified by the La3+ substitution. All compounds exhibit a linear antiferromagnetic feature. Optical transmission reveals band gaps in the range of 2.22-2.24 eV. The heterostructures of indium tin oxide ( ITO) film/(Bi1− xLa x)FeO3 ceramics/Au film show a p- n junction-like I- V characteristic behavior. The maximal PV power conversion efficiency can reach 0.19% in ITO/ BFO15L/Au under illumination of λ = 405 nm. A junction-like theoretical model can reasonably describe open-circuit voltage and short-circuit current as a function of illumination intensity. [ABSTRACT FROM AUTHOR]