Marina Spasova

Magnetic and optical tunable microspheres with a magnetite/gold nanoparticle shell

Multifunctional core-shell microspheres consisting of a polystyrene 640 nm diameter core covered with a selectable number of layers of magnetite (12 nm) and silica-coated gold (15 nm) nanoparticles have been fabricated using the layer-by-layer (LbL) technique in aqueous solution. By varying the shell thickness and layer composition the magnetic and optical properties and the diameter of the colloids can be controlled independently. Optical spectra of the core-shell colloidal microspheres show a well-resolved Au surface plasmon peak in the visible which red-shifts with the number of adsorbed Au nanoparticle layers. The magnetic moment per sphere increases nearly linearly with the number of adsorbed magnetite nanoparticle layers, and the spheres can be assembled into chains of up to 1 mm length by deposition in a magnetic field.

Room-Temperature Ferromagnetism in Antiferromagnetic Cobalt Oxide Nanooctahedra

Cobalt oxide octahedra were synthesized by thermal decomposition. Each octahedron-shaped nanoparticle consists of an antiferromagnetic CoO core enclosed by eight {111} facets interfaced to a thin (∼ 4 nm) surface layer of strained Co3O4. The nearly perfectly octahedral shaped particles with 20, 40, and 85 nm edge length show a weak room-temperature ferromagnetism that can be attributed to ferromagnetic correlations appearing due to strained lattice configurations at the CoO/Co3O4 interface.

Ferromagnetic resonance of monodisperse Co particles

Two-dimensional arrays of monodisperse nanosized Co particles are prepared on carbon and glass substrates by a magnetophoretic deposition technique from colloidal suspensions. Transmission electron microscopy (TEM) reveals a complicated cubic crystalline structure of the particles and hexagonal ordering over several micrometers squared, if the colloidal suspension is dried in magnetic fields of up to 0.8 T. Angular-dependent ferromagnetic resonance (FMR) spectra of 4-, 5-, 9-, and 12-nm-diameter particles at 297 K show that the easy axis of magnetization is in plane and that only the 12 nm particles are measured below the blocking temperature estimated to be 656 K. The resonance linewidth is on the order of 0.1 T, indicating a much larger magnetic inhomogeneity of the particles than the small geometric and size distribution (<10%) observed by TEM suggests. Characteristic differences of the FMR spectra for different substrates and deposition parameters are observed.

Stearate-Based Cu Colloids in Methanol Synthesis: Structural Changes Driven by Strong Metal-Support Interactions

Metal stearate‐stabilized Cu nanoparticles, synthesized by an efficient one‐step process, were applied in the continuous liquid‐phase synthesis of methanol. After optimizing the reduction procedure, twofold higher rates of methanol formation were found for Cu–Zn colloids, compared to the conventional ternary Cu/ZnO/Al2O3 catalyst applied as fine powder in the liquid phase. Structural changes were investigated as a function of time on stream; after reduction in H2, spherical, well‐separated 5–10 nm Cu particles stabilized by a Zn stearate shell were found. Under catalytic high‐pressure conditions Zn stearate was hydrolyzed forming ZnO. High‐resolution transmission electron microscopy revealed the presence of triangular ZnO prisms with truncated edges. Applying optimized synthesis conditions these triangularly shaped ZnO particles were found to be mostly attached to the spherical Cu particles. The catalytic results and the structural and spectroscopic characterization suggest that these ZnO particles act as a reservoir, releasing ZnOx species, which diffuse onto the Cu particles and promote the catalytic activity.

A guideline for atomistic design and understanding of ultrahard nanomagnets.

Magnetic nanoparticles are of immense current interest because of their possible use in biomedical and technological applications. Here we demonstrate that the large magnetic anisotropy of FePt nanoparticles can be significantly modified by surface design. We employ X-ray absorption spectroscopy offering an element-specific approach to magnetocrystalline anisotropy and the orbital magnetism. Experimental results on oxide-free FePt nanoparticles embedded in Al are compared with large-scale density functional theory calculations of the geometric- and spin-resolved electronic structure, which only recently have become possible on world-leading supercomputer architectures. The combination of both approaches yields a more detailed understanding that may open new ways for a microscopic design of magnetic nanoparticles and allows us to present three rules to achieve desired magnetic properties. In addition, concrete suggestions of capping materials for FePt nanoparticles are given for tailoring both magnetocrystalline anisotropy and magnetic moments.

Magnetic properties of arrays of interacting Co nanocrystals

Monodisperse Co FCC nanocrystals with 12 nm diameter were self-assembled into regular quasi-two-dimensional triangular periodic arrays on carbon substrates from a toluene-based colloidal suspension. At 300 K the regular arrays show a collective magnetic behaviour due to dipolar coupling. A remanent magnetization with an easy axis in the film-plane and an uniaxial in-plane anisotropy field of 0.037 T were determined by SQUID magnetometry and angular dependent ferromagnetic resonance.

Magnetic characterization of iron nanocubes

Nearly perfect single crystalline Fe core-shell nanocubes with (100) facets and 13.6 nm edge length were prepared by wet-chemical methods. While the core is metallic, the shell is composed of either Fe3O4 or γ-Fe2O3. The cubes were deposited onto GaAs substrates with monolayer coverage as proved by scanning electron microscopy. Oxygen and hydrogen plasmas were used to remove the ligand system and the oxide shell. Both types of samples were investigated by ferromagnetic resonance. While the g-factor (g=2.09) and crystalline anisotropy (K4=4.8×104 J/m3) of the pure iron cubes show up with bulk values, the saturation magnetization is reduced to (M(5K)=(1.2±0.12)×106 A/m) 70% of bulk value and the effective damping parameter (α=0.03) is increased by one order of magnitude with respect to bulk Fe.

Self-assembly and magnetism in core-shell microspheres

We report on the fabrication and characterization of magnetic composite colloids with a defined shape, composition and multilayer shell thickness. They consist of a core of a polystyrene microsphere (640 nm diameter) coated with consecutive shells of Fe3O4 nanoparticles (12 nm diameter), polyelectrolytes and Au nanoparticles (15 nm). The homogeneity of the coating was confirmed by transmission electron microscopy. Composite core-shell microspheres were self-assembled into periodically ordered chains up to 2 mm in length by magnetophoretic deposition. The self-organization was visualized by optical and atomic force microscopy. Magnetic properties were determined by angular dependent ferromagnetic resonance (FMR) and superconducting quantum interference device magnetometry between 5 and 300 K. The FMR reveals long-range magnetic order at 300 K due to dipolar coupling and an easy axis in plane along the chains. We find a magnetic moment that is reduced in comparison with the magnetite bulk value.

Splenic red pulp macrophages are intrinsically superparamagnetic and contaminate magnetic cell isolates

A main function of splenic red pulp macrophages is the degradation of damaged or aged erythrocytes. Here we show that these macrophages accumulate ferrimagnetic iron oxides that render them intrinsically superparamagnetic. Consequently, these cells routinely contaminate splenic cell isolates obtained with the use of MCS, a technique that has been widely used in immunological research for decades. These contaminations can profoundly alter experimental results. In mice deficient for the transcription factor SpiC, which lack red pulp macrophages, liver Kupffer cells take over the task of erythrocyte degradation and become superparamagnetic. We describe a simple additional magnetic separation step that avoids this problem and substantially improves purity of magnetic cell isolates from the spleen.

Ultrasmall particle detection using a submicron Hall sensor

We demonstrate detection of a single FePt nanoparticle (diameter 150 nm, moment ∼107 μB) using an ultrasensitive InSb Hall sensor with the bar lateral width of 600 nm. The white noise of a typical nanodevice, SV1/2≈28 nV/√Hz, is limited only by two-terminal resistance of the voltage leads which results in a minimum field sensitivity of the device Bmin=0.87 μT/√Hz. To detect a single FePt bead, we employed a phase-sensitive method based on measuring the ac susceptibility change in a bead when exposed to a switched dc magnetic field. Such nano-Hall devices, enabling detection of potentially even smaller moments, are of considerable significance both for nanomagnetic metrology and high sensitivity biological and environmental detectors.