A lattice of microtraps for ultracold atoms based on patterned magnetic films

We have realized a two dimensional permanent magnetic lattice of Ioffe-Pritchard microtraps for ultracold atoms. The lattice is formed by a single 300 nm magnetized layer of FePt, patterned using optical lithography. Our magnetic lattice consists of more than 15000 tightly confining microtraps with a density of 1250 traps/mm$^2$. Simple analytical approximations for the magnetic fields produced by the lattice are used to derive relevant trap parameters. We load ultracold atoms into at least 30 lattice sites at a distance of approximately 10 $\mu$m from the film surface. The present result is an important first step towards quantum information processing with neutral atoms in magnetic lattice potentials.Comment: 7 pages, 7 figure

Fabrication of magnetic atom chips based on FePt

We describe the design and fabrication of novel all-magnetic atom chips for use in ultracold atom trapping. The considerations leading to the choice of nanocrystalline exchange coupled FePt as best material are discussed. Using stray field calculations, we designed patterns that function as magnetic atom traps. These patterns were realized by spark erosion of FePt foil and e-beam lithography of FePt film. A mirror magneto-optical trap (MMOT) was obtained using the stray field of the foil chip.Comment: 5 pages, 5 figure

Pseudogap-less high Tc_{c} superconductivity in BaCox_{x}Fe2−x_{2-x}As2_{2}

The pseudogap state is one of the peculiarities of the cuprate high temperature superconductors. Here we investigate its presence in BaCo$_{x}$Fe$_{2-x}$As$_{2}$, a member of the pnictide family, with temperature dependent scanning tunneling spectroscopy. We observe that for under, optimally and overdoped systems the gap in the tunneling spectra always closes at the bulk T$_{c}$, ruling out the presence of a pseudogap state. For the underdoped case we observe superconducting gaps over large fields of view, setting a lower limit of tens of nanometers on the length scale of possible phase separated regions.Comment: 5 pages, 3 figure

Learning form Nature to improve the heat generation of iron-oxide nanoparticles for magnetic hyperthermia applications.

The performance of magnetic nanoparticles is intimately entwined with their structure, mean size and magnetic anisotropy. Besides, ensembles offer a unique way of engineering the magnetic response by modifying the strength of the dipolar interactions between particles. Here we report on an experimental and theoretical analysis of magnetic hyperthermia, a rapidly developing technique in medical research and oncology. Experimentally, we demonstrate that single-domain cubic iron oxide particles resembling bacterial magnetosomes have superior magnetic heating efficiency compared to spherical particles of similar sizes. Monte Carlo simulations at the atomic level corroborate the larger anisotropy of the cubic particles in comparison with the spherical ones, thus evidencing the beneficial role of surface anisotropy in the improved heating power. Moreover we establish a quantitative link between the particle assembling, the interactions and the heating properties. This knowledge opens new perspectives for improved hyperthermia, an alternative to conventional cancer therapies

Pair Correlations in a Bidisperse Ferrofluid in an External Magnetic Field:Theory and Computer Simulations

The pair distribution function g(r) for a ferrofluid modeled by a bidisperse system of dipolar hard spheres is calculated. The influence of an external uniform magnetic field and polydispersity on g(r) and the related structure factor is studied. The calculation is performed by diagrammatic expansion methods within the thermodynamic perturbation theory in terms of the particle number density and the interparticle dipole–dipole interaction strength. Analytical expressions are provided for the pair distribution function to within the first order in number density and the second order in dipole–dipole interaction strength. The constructed theory is compared with the results of computer (Monte Carlo) simulations to determine the range of its validity. The scattering structure factor is determined using the Fourier transform of the pair correlation func-tion g(r) – 1. The influence of the granulometric composition and magnetic field strength on the height and position of the first peak of the structure factor that is most amenable to an experimental study is analyzed. The data obtained can serve as a basis for interpreting the experimental small[1]angle neutron scattering results and determining the regularities in the behavior of the structure factor, its dependence on the fractional com-position of a ferrofluid, interparticle correlations, and external magnetic field. © Pleiades Publishing, Inc., 2014

Synthesis and Analytical Centrifugation of Magnetic Model Colloids

This thesis is a study of the preparation and thermodynamic properties of magnetic colloids. First, two types of magnetic model colloids are investigated: composite colloids and single-domain nanoparticles. Thermodynamics of magnetic colloids is studied using analytical centrifugation, including a specially adapted centrifuge for measuring heavy and strongly light absorbing colloids. Magnetic composite colloids can be prepared from thermodynamically stable Pickering emulsions of 3-methacryloxypropyl trimethoxysilane (TPM) oil in water stabilized by magnetite or cobalt ferrite nanoparticles. The emulsion droplet size can be increased by the relative volume of oil, the amount of salt, and the evolution of the droplets over time. This evolution over time is due to a gradual transfer of the interfacial nanoparticles to the oil phase, which can be utilized to controllably transfer aqueous nanoparticles to the TPM oil phase by the grafting of TPM onto the surface of the nanoparticles. Magnetic nanoparticles can be prepared by various methods, which yield particles that either have good crystallinity, contain twinning defects, or have a high density of dislocations. These crystal lattice defects can have a detrimental influence on the magnetic properties of the particles. It is shown that a low geometric size polydispersity of magnetic nanoparticles does not guarantee low polydispersity of the magnetic dipole moments. To study the thermodynamics of magnetic colloids by analytical centrifugation, a LUMiFuge stability analyzer is used for the first time to measure the osmotic equation of state of concentrated dispersions of magnetic nanoparticles. The LUMiFuge is equipped with homebuilt measurement cells with glass capillaries with an internal thickness of only 50 micrometers that allow measurement of concentrated, strongly light absorbing colloidal dispersions. For sufficiently small colloids also an analytical ultracentrifuge (AUC) can be used to determine the osmotic equation of state. Homebuilt AUC centerpieces with optical path lengths as low as 50 micrometers have been realized and the results are compared with the equations of state from the LUMiFuge. Sterically stabilized magnetic iron oxide nanoparticles with a diameter of 13 nm dispersed in an apolar solvent are used as an experimental realization of the dipolar hard spheres known from theory and computer simulations. The experimental osmotic pressures of the magnetite fluids are significantly below Van 't Hoff's law due to magnetic interparticle attractions. The osmotic second virial coefficient obtained from the experimental osmotic pressures corresponds to a dipolar coupling constant in the range of 2.0-2.4. An isotropic, Van der Waals-like phase separation, if any, is expected at a value of the dipolar coupling constant of 2.5, close to our experimentally obtained value, yet no indications for such a phase transition are found. The dipolar coupling parameter obtained by analytical centrifugation is in reasonable agreement with the coupling parameter calculated from independent magnetic measurements of the nanoparticles. This agreement clearly confirms that analytical centrifugation is an accurate method to quantify interactions between colloidal particles

Sedimentation equilibria of ferrofluids: I. Analytical centrifugation in ultrathin glass capillaries

Analytical centrifugation is used for the first time to measure sedimentation equilibrium concentration profiles of a ferrofluid, a concentrated colloidal dispersion of strongly absorbing magnetic nanoparticles. To keep the optical absorbance from becoming too strong, the optical path length is restricted to 50 μm by placing the dispersion in a flat glass capillary. The concentration profile is kept from becoming too steep, despite the relatively high buoyant mass of the nanoparticles, by making novel use of a low-velocity analytical centrifuge that was not designed to measure equilibrium profiles. The experimental approach is validated by comparison with profiles obtained using an analytical ultracentrifuge. At concentrations of a few hundred grams per liter, the osmotic pressures calculated from the equilibrium profiles are lower than expected for hard spheres or non-interacting particles, due to magnetic dipolar interactions. By following the presented experimental approach, it will now also be possible to characterize the interparticle interactions of other strongly absorbing colloidal particles not studied before by analytical centrifugatio