Ruslan Prozorov

Plastic Vortex Creep in YBa2Cu3O7-x Crystals.

Local magnetic relaxation measurements in YBa2Cu3O72x crystals show evidence for plastic vortex creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the “fishtail” magnetization. In this range the activation energy Upl, which decreases with field, drops below the elastic (collective) creep activation energy, Uel, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa 2Cu3O72x. [S0031-9007(96)00878-2]

Sonochemical preparation of amorphous nickel

A method for the preparation of amorphous Ni powder with particle size of about 10 nm is reported. The sonolysis was performed on neat Ni(CO)4 and on its solution in decane. The nickel nanoparticles were subjected to scanning and transmission electron microscopy, differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction and SQUID magnetization measurements. These measurements confirm the amorphous state of the particles.

SYNTHESIS OF PURE AMORPHOUS FE2O3

A method for the preparation of pure amorphous Fe 2 O 3 powder with particle size of 25 nm is reported in this article. Pure amorphous Fe 2 O 3 can be simply synthesized by the sonication of neat Fe(CO) 5 or its solution in decalin under an air atmosphere. The Fe 2 O 3 nanoparticles are converted to crystalline Fe 3 O 4 nanoparticles when heated to 420 °C under vacuum or when heated to the same temperature under a nitrogen atmosphere. The crystalline Fe 3 O 4 nanoparticles were characterized by x-ray diffraction and M¨ossbauer spectroscopy. The Fe 2 O 3 amorphous nanoparticles were examined by Transmission Electron Micrography (TEM), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and Quantum Design SQUID magnetization measurements. The magnetization of pure amorphous Fe 2 O 3 at room temperature is very low (

Plastic Vortex Creep in YBa{sub {bold 2}}Cu{sub {bold 3}}O{sub {bold 7{minus}}}{ital x} Crystals

Local magnetic relaxation measurements in YBa2Cu3O72x crystals show evidence for plastic vortex creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the “fishtail” magnetization. In this range the activation energy Upl, which decreases with field, drops below the elastic (collective) creep activation energy, Uel, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa 2Cu3O72x. [S0031-9007(96)00878-2]

Magnetic irreversibility and the Verwey transition in nanocrystalline bacterial magnetite

The magnetic properties of biologically-produced magnetite nanocrystals biomineralized by four different magnetotactic bacteria were compared to those of synthetic magnetite nanocrystals and large, high quality single crystals. The magnetic feature at the Verwey temperature, TV , was clearly seen in all nanocrystals, although its sharpness depended on the shape of individual nanoparticles and whether or not the particles were arranged in magnetosome chains. The transition was broader in the individual superparamagnetic nanoparticles for which TB TV and the Verwey transition is sharply defined. No correlation between particle size and TV was found. Furthermore, measurements of M (H, T, time) suggest that magnetosome chains behave as long magnetic dipoles where the local magnetic field is directed along the chain. This result confirms that time-logarithmic magnetic relaxation is due to the collective (dipolar) nature of the barrier for magnetic moment reorientation.

A Sharp Peak of the Zero-Temperature Penetration Depth at Optimal Composition in BaFe2(As1–xPx)2

A Spike Inside the Dome The transition temperature Tc of iron-based superconductors has a dome-shaped dependence on chemical doping, and the superconductivity that develops underneath may obscure a potential quantum critical point (QCP) residing at absolute zero. With the aim of detecting signatures of this quantum criticality, Hashimoto et al. (p 1554; see the Perspective by Sachdev) measured the penetration depth of the pnictide series BaFe2(As1−xPx)2 as a function of x. A sharp peak right around the point where Tc has a maximum (x = 0.30) was observed, implying that the superfluid density diminishes sharply where one would expect it to be the most robust. This unusual finding is interpreted as a sign of a QCP at x = 0.30. A quantum critical point may be lurking inside the superconducting dome of a pnictide series. In a superconductor, the ratio of the carrier density, n, to its effective mass, m*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λL. In two-dimensional systems, this ratio n/m* (~1/λL2) determines the effective Fermi temperature, TF. We report a sharp peak in the x-dependence of λL at zero temperature in clean samples of BaFe2(As1–xPx)2 at the optimum composition x = 0.30, where the superconducting transition temperature Tc reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of Tc/TF at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality.

Self-assembly and biphasic iron-binding characteristics of Mms6, a bacterial protein that promotes the formation of superparamagnetic magnetite nanoparticles of uniform size and shape

Highly ordered mineralized structures created by living organisms are often hierarchical in structure with fundamental structural elements at nanometer scales. Proteins have been found responsible for forming many of these structures, but the mechanisms by which these biomineralization proteins function are generally poorly understood. To better understand its role in biomineralization, the magnetotactic bacterial protein, Mms6, which promotes the formation in vitro of superparamagnetic magnetite nanoparticles of uniform size and shape, was studied for its structure and function. Mms6 is shown to have two phases of iron binding: one high affinity and stoichiometric and the other low affinity, high capacity, and cooperative with respect to iron. The protein is amphipathic with a hydrophobic N-terminal domain and hydrophilic C-terminal domain. It self-assembles to form a micelle, with most particles consisting of 20-40 monomers, with the hydrophilic C-termini exposed on the outside. Studies of proteins with mutated C-terminal domains show that the C-terminal domain contributes to the stability of this multisubunit particle and binds iron by a mechanism that is sensitive to the arrangement of carboxyl/hydroxyl groups in this domain.

Magnetic irreversibility and relaxation in assembly of ferromagnetic nanoparticles

Measurements of the magnetic irreversibility line and time-logarithmic decay of the magnetization are described for three

Unconventional London Penetration Depth in Single-Crystal Ba(Fe0.93Co0.07)2As2 Superconductors

{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}

Cobalt Ferrite Nanocrystals: Out-Performing Magnetotactic Bacteria

samples composed of regular amorphous, acicular amorphous, and crystalline nanoparticles. The relaxation rate is the largest and the irreversibility temperature is the lowest for the regular amorphous nanoparticles. The crystalline material exhibits the lowest relaxation rate and the largest irreversibility temperature. We develop a phenomenological model to explain the details of the experimental results. The main new aspect of the model is the dependence of the barrier for magnetic relaxation on the instantaneous magnetization and therefore on time. The time-dependent barrier yields a natural explanation for the time-logarithmic decay of the magnetization. Interactions between particles as well as shape and crystalline magnetic anisotropies define an energy scale that controls the magnetic irreversibility. Introducing this energy scale yields a self-consistent explanation of the experimental data.