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Optimization of materials exhibiting high-temperature superconductivity for producing controllable nano-devices is crucial for industrial applications. Herein we report a comprehensive study of the diffusion process between YBa2Cu3O7−δ (YBCO) and iron particles. Fe diffusion into the YBCO matrix could be fundamental for multilayer systems with YBCO/Fe-alloy interfaces. We have found that the orthorhombic YBCO structure adopts to 3 wt% Fe, while for higher Fe content, a formation of BaFeO3−δ and iron oxides was observed. Complementary measurements confirmed the strong superconductivity suppression in YBCO-Fe materials containing more than 7 wt% Fe. The YBCO with diffused Fe material retain the unit cell orthorhombicity (max. 3 wt% Fe), and their superconducting properties follow the principle of critical scaling with different exponents (γ). The critical current density (Jc), pinning fields (HP) exhibit γ = 1, the first critical field (Hc1) shows γ = 1/2, and critical temperature (Tc) demonstrates γ = 7/4.

Using density functional theory, we study the lattice dynamical properties of magnetite (Fe$_3$O$_4$) in the high-temperature cubic and low-temperature monoclinic phases. The calculated phonon dispersion curves and phonon density of states are compared with the available experimental data obtained by inelastic neutron, inelastic x-ray, and nuclear inelastic scattering. We find a very good agreement between the theoretical and experimental results for the monoclinic $Cc$ structure revealing the strong coupling between charge-orbital (trimeron) order and specific phonon modes. For the cubic phase, clear discrepancies arise which, remarkably, can be understood assuming that the strong trimeron-phonon coupling can be extended above the Verwey transition, with lattice dynamics influenced by the short-range trimeron order instead of the average cubic structure. Our results establish the validity of trimerons (and trimeron-phonon coupling) in explaining the physics of magnetite much beyond their original formulation., Comment: 11 pages, 9 figures, 1 table

When a magnetic field is applied to a single crystal of magnetite at $124\phantom{\rule{0.16em}{0ex}}\mathrm{K}gTg50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the monoclinic ${c}_{M}$ axis, which is the easy magnetization axis, switches to one of the ⟨100⟩ cubic directions, nearest to the direction of the magnetic field, and the phenomenon known as an axis switching (AS) occurs. A global symmetry probe, resonant x-ray scattering, and a local probe, M\"ossbauer spectroscopy, are used to better understand the mechanism of axis switching. The behavior of three subsystems ordered below the Verwey transition temperature ${T}_{V}$, i.e., lattice distortion, an orbital, and charge orderings, was observed via resonant x-ray scattering as a function of an external magnetic field. This was preceded by calculation of selected peak intensities using the fdmnes code. The M\"ossbauer spectroscopy studies confirmed that the magnetic field triggers electronic rearrangements and atomic displacements. The structure observed after the process of axis switching is very similar to the one obtained after cooling below ${T}_{V}$ with the magnetic field applied along one of the initial ⟨100⟩ cubic directions and distinct from the cooling in the absence of a magnetic field. From all the experimental observations of the phenomenon done so far, it is clear that AS starts from the fluctuations between octahedral iron orbitals that ultimately lead to the Verwey transition, but also to the higher-temperature trimeron dynamics. Therefore, further observation of the axis switching may be a key point to the understanding of a majority of strongly correlated electronic behavior in magnetite as well as in other transition metal oxides.

A simple and cost effective technique for obtaining highly stable carbon coated nickel nanostructures at relatively low reaction temperatures is reported. Thermogravimetric analysis was carried out to optimize the annealing temperatures. The powder X-ray diffraction pattern reveals peaks corresponding to face centered cubic nickel. High resolution transmission electron micrograph shows the formation of nickel nanostructures with ∼5 nm thick carbon coating. This is confirmed by Raman spectroscopy. The nickel/carbon core/shell nanostructures exhibited a shelf life of more than a year, with high thermal stability and excellent magnetic properties. This synthesis route provides scope for large scale production of nickel/carbon nanostructures.

Investigations of the dynamic process of the appearance of the Verwey transition in magnetite are performed by simultaneous measurements of magnetic susceptibility χ AC and the intensity of the ( 1 1 1 2 2 ) superstructure peak, using the coherent X-ray diffraction (CXD) technique. The results suggest that while the volume of a new low-temperature monoclinic phase increases gradually across the transition, it is initially in the form of grains too small to form the well-developed superlattice reflection. Finally, at the end of the transition, those small grains coalesce into micrometer size structural domains, reflected in ( 1 1 1 2 2 ) superlattice peak.

The influence of the external magnetic field B ( B ≤ 4 T) on resistivity in magnetite single crystal was studied at few temperatures both below and above the Verwey transition temperature T V , and in two 〈1 0 0〉 type cubic directions. We have succeeded to confirm our predictions that the magnetic axis switching affects electronic transport. It was also found that the transverse resistivity ( B ⊥ current direction) exceeds longitudinal one, but only for T > T V , with no obvious systematics below the transition temperature. This may indicate that atomic disorder, inherent and poorly defined in a material with structural domains, is the primary factor that governs transport properties below the transition temperature. Finally, and concomitant with the last statement, only very small magnetic field dependence in magnetite below T V may be inferred from our data.

Studies of the Verwey transition in zinc doped magnetite (Fe 3− x Zn x O 4 , x = 0, 0.0097, 0.011, 0.036) by simultaneous ac magnetic susceptibility ( χ ) and electric resistance measurements are presented. The temperature hysteresis of the Verwey transition increases from 0.03 K for stoichiometric magnetite to 0.5 K for Zn doped samples, despite the continuous character of the transition. The results of introductory observations of structural changes are also presented.

The results of AC magnetic susceptibility measurements of zinc ferrite (Fe3−xZnxO4, x = 0, 0.0072, 0.0174, 0.049) single crystalline samples under pressure up to 1.2 GPa in the temperature range close to the Verwey transition temperature (TV) are reported. The isotropy point TIP increases with pressure for each x but the disappearance of primary anisotropy constant at TIP has no effect on Mossbauer spectrum at ambient pressure. Finally, Mossbauer effect data in the vicinity of the Verwey transition does not support the recent claim of the inverse to normal spinel crossover at the transition.

Nuclear inelastic-scattering studies were performed to infer temperature evolution of iron atom dynamics in magnetite samples exhibiting Verwey transition of first- and second-order (type-I and type-II materials). The possible difference in this evolution could rationalize the distinct properties of these classes of materials observed in heat capacity and diffuse scattering below the Verwey transition temperature ${T}_{V}$ and could explain the change in transition order triggered by a minute (below 0.3$%$) altering of the iron sublattice. Although we have found the apparent stiffening of the phonon iron spectrum in the low-temperature phase, at the same time, we have shown that these spectra are rather similar for type-I and type-II materials, rendering the lattice vibration-based explanation of the distinct behavior of heat capacities very improbable. The calculation of phonon spectra, aimed at tracing the origin of various features in the phonon density of states (DOS), has shown that the local Coulomb interaction $U$ may have a large effect on phonon DOS. However, the change in the $U$ parameter cannot explain the difference in heat-capacity results for both classes of materials. Thus, an additional factor that differentiates these materials and possibly is responsible for the discontinuous character of the Verwey transition in stoichiometric magnetite still must be found.

The influence of the external magnetic field on magnetic and crystallographic structure of magnetite was studied. We have confirmed, by means of vibrating sample magnetometer measurements, that external magnetic field can switch the easy magnetic axis to the new crystallographic direction. We have also proved by the direct observation of the crystalline structure that this phenomenon is strictly related to the crystallographic structure relaxation.

The easy magnetic axis switching in magnetite is investigated. Magnetization data confirmed activation character of the process with activation energy of the same order as that of the Verwey transition, suggesting common origin. On the other hand this activation energy rises with pressure (up to 1.2GPa), unlike TV. The axis switching is clearly reflected in field dependence of resistivity and the direct structural data showed that it is simultaneous with the reorganization of structure. Thus, control of the structure can be possible with the application of magnetic field, as in shape memory materials. Finally, NMR showed that all, possibly decoupled entities: lattice distortion and charge and orbital orderings, change simultaneously while the axis switching occurs.

Phenomenon related to low symmetry phase of magnetite Fe3O4 below the Verwey transition is the switching of magnetic easy axis by external magnetic field connected with a structural transition. Results of nuclear magnetic resonance (NMR) studies of axis switching are presented, preceded by careful characterization by magnetic measurements. We detect changes in the F57e NMR spectra that evidence the structural transition interrelated with the axis switching. We also observe the switching process in time and analyze the dynamics of the phenomenon. Phenomenological approach according to Kolmogorov–Johnson–Mehl–Avrami is employed to find a link between the experimental data from magnetization measurements and NMR, showing that the effect observed by these two methods has the same origin. Additionally, NMR identifies separately the unswitched and switched phase during the switching process.