Georgy M. Tsoi

Fabrication and characterization of Co1−xFex alloy nanowires

Co1−xFex alloy nanowires with 40 nm diam and x=0–1.0 were fabricated by electrodeposition in nanopores of alumina templates. The crystalline structure of the nanowires is concentration dependent and shows a transition from the cobalt hexagonal-closed-packed structure (hcp) to a face-centered-cubic structure (fcc) in the concentration range 0<x<0.1. For an iron content greater than 15 at % the structure becomes body-centered-cubic (bcc). The nanowires with an aspect ratio of about 8 show a highly anisotropic magnetization with the easy axis along the length of the wire. The squareness of the hysteresis loops is between 72% and 93% for magnetic fields parallel to the wires while the squareness is less than 13% for fields perpendicular to the wires. The coercivity along the easy axis reaches a maximum of 2150 Oe for x=0.55 and is almost concentration independent for 0.5<x<0.6.

Magnetic properties of nanosized iron oxide particles precipitated in alginate hydrogels

Nanoparticles of γ-Fe2O3 (size 2–3nm) were precipitated in alginate hydrogels by cross-linking sodium alginate with Fe ions in a methanol-water solution. The zero-field-cooled and field-cooled magnetization measurements between 5 and 350K and the hysteresis in the M vs H relation below the blocking temperature indicate superparamagnetic behavior. The temperature dependence of the coercive field is not consistent with the T1∕2 behavior predicted by Neel and Brown for the noninteracting particles. The average diameter of the nanoparticles determined from the magnetic data is consistently larger than the corresponding particle size determined by x-ray diffraction, perhaps due to interparticle magnetic interactions.

Memory effects and magnetic interactions in a γ‐Fe2O3 nanoparticle system

The low-temperature dynamics of a magnetic nanoparticle system (γ‐Fe2O3—alginate nanocomposite with average particle size around 4nm) have been studied by superconducting quantum interference device measurements. Using different temperature and field protocols, memory phenomena in the dc magnetization and magnetic relaxation have been observed at temperatures below its blocking temperature TB=37K. However, aging experiments show an absence of any waiting time dependence in the magnetization relaxation. These observations indicate that the dynamics of this nanoparticle system are governed by a wide distribution of particle relaxation times which arise from the distribution of particle sizes and weak interparticle interactions.

Modeling of Hysteresis and Magnetization Curves for Hexagonally Ordered Electrodeposited Nanowires

A computational model has been developed to investigate how the magnetostatic interactions affect the hysteresis and magnetization curves for hexagonal arrays of magnetic nanowires. The magnetization coupling between nanowires arises from the stray fields produced by the other nanowires composing the array such that the field at each nanowire is the sum of the external field and the interaction field with the other nanowires. Using only two adjustable parameters: the interaction between nearest neighbors and the width of the Gaussian distribution in switching fields centered around the measured coercivity, simulations are compared with the experimentally measured hysteresis and magnetization curves for electrodeposited Co0.45Fe0.55 alloy nanowires with diameters from 12 to 48 nm. Excellent agreement is found for all nanowire systems except for the largest diameter arrays where deviations from the Gaussian distribution of switching fields need to be considered.

Zero magnetization states in electrodeposited Co0.45Fe0.55 nanowire arrays

Co0.45Fe0.55 alloy nanowires with 12 to 35 nm diameter and 12 μm length were fabricated by electrodeposition in porous anodic alumina templates. The initial magnetization curves reveal that the zero magnetization state is not unique and is determined by the field history (ac demagnetization process) leading to the zero average moment state. For ac demagnetization processes with the field applied parallel to the nanowire axis, the subsequent magnetization curves suggest that an individual nanowire behaves as a single domain with neighboring nanowires being antiparallel to each other in the zero magnetization state. However, for a demagnetization process with the field applied perpendicular to the nanowires, a different zero magnetization state is created in which the individual nanowires consist of multidomains having opposite axial orientations. These results are consistent with the asymmetric (symmetric) behavior found in the minor hysteresis loops measured after perpendicular (parallel) ac demagnetizati...

Quasiparticle scattering in superconductors

We compare results of high-resolution magnetic flux (susceptibility) measurements in very weak magnetic field, performed of three classes of superconductors. They reveal astonishing details of the transition to the superconducting state. Although Pb behaves also on this scale according to BCS predictions, La is more reminiscent of HTS materials, which exhibit anomalous features. We suggest that known structure peculiarities are due to a strong electron-lattice instability and lead to a resonance electron back scattering.