C. Johansson

Magnetic properties of two-dimensional arrays of epitaxial Fe (001) submicron particles

The magnetic properties of two-dimensional arrays of Fe particles with well-defined geometry, prepared by electron lithography from epitaxial Fe (001) films of thickness of 50 nm, have been studied. Circular particles with diameters of 0.6 and 0.4 m and rectangular particles 0.9 m0.3 m and 0.7 m0.2 m, were positioned in square and rectangular lattices, respectively, with lattice constants about twice the particle dimensions. Samples were prepared with the lattices oriented along the 100 and 110 directions. Hysteresis curves were obtained in the field range 2 T at temperatures between 30 and 300 K. The hysteresis curves are characteristic for single particles with a multidomain zero-field state. The magnetization and demagnetization processes are mainly governed by the geometrical shape of the particles and their orientation with respect to the crystal directions and the applied field. This implies that the high quality in the epitaxial Fe films is preserved during the patterning process. Magnetic images, obtained by magnetic force microscopy, support the conclusions drawn from the magnetization measurements. The circular particles showed a fourfold symmetry in their magnetic image, which is compatible with the expected domain structure in a disk with cubic crystalline anisotropy

Ferromagnetism of the Me3(Fe(CN)6)2.H2O compounds, where Me=Ni and Co

We have studied the structural and magnetic properties of two compounds from the Me32+(Fe3+(CN)6-)23-.H2O family, where Me=Ni and Co. From x-ray analysis we find that the compounds crystallize in the face-centred cubic structure with the space group of F43m. Both the Fe and Me ions are coordinated octahedrally by six carbon atoms and six nitrogen atoms, respectively. The Fe ions are in a strong crystalline field, while the Me ions are in an intermediate field with cubic symmetry. From the magnetic studies a quantitative bulk ferromagnetic behaviour (a spontaneous magnetization and a hysteresis loop) has been established for both complex compounds. In addition, from the magnetization against temperature curves we obtained, for both compounds, the Curie as well as Curie-Weiss temperatures, the Curie constants and the effective moments in the paramagnetic state. From the assumed value of the spin for every cation we fitted the chi T=F(T) curves and from the field dependence of the magnetization we determined the high-field susceptibility. The magnetic properties of the compounds were analysed in the framework of the mean-field theory.

The influence of particle size and interactions on the magnetization and susceptibility of nanometre-size particles

We studied the magnetic properties of frozen magnetic liquids containing amorphous Fe1-xCx particles. By varying the particle size and concentration, we could separate single particle effects from interactions. In samples containing particles with median diameters 5.0 nm and 3.8 nm and with spontaneous saturation magnetizations 7.1 kA m-1 and 9.2 kA m-1, effects of particle interactions are observed. For 3.2 nm no interactions were observed. In samples with negligible interactions the superparamagnetic blocking temperature increases with the particle size. The effective anisotropy constant increases with decreasing particle size, implying that the anisotropy is enhanced by surface effects.

Motion of nanometer sized magnetic particles in a magnetic field gradient

Using magnetic particles with sizes in the nanometer range in biomedical magnetic separation has gained much interest recently due to their higher surface area to particle volume and lower sedimentation rates. In this paper, we report our both theoretical and experimental investigation of the motion of magnetic particles in a magnetic field gradient with particle sizes from 425 nm down to 50 nm. In the experimental measurements, we monitor the absorbance change of the sample volume as the particle concentration varies over time. We also implement a Brownian dynamics algorithm to investigate the influence of particle interactions during the separation and compare it to the experimental results for validation. The simulation agrees well with the measurements for particle sizes around 425 nm. Some discrepancies remain for smaller particle sizes, which may indicate that additional factors also influence the separation for the smaller size range. We observe that the separation process includes the formation of...

The influence of magnetic anisotropy on the magnetization of small ferromagnetic particles

The authors calculated the magnetization of small ferromagnetic particles with uniaxial anisotropy. The numerical data were compared with experimental results for a magnetic liquid consisting of small amorphous Fe0.75C0.25 particles in decalin. From the comparison an anisotropy K approximately=3*105 J m-3 was estimated.

Proactive assembly systems-realising the potential of human collaboration with automation

Manufacturing competitiveness relies on the companies ability to rapidly reconfigure their assembly systems. This paper introduces assembly system proactivity, a concept based on interrelated levels of human involvement, automation, and information handling. Increased and structured human involvement contributes to increased system ability to proactively address predicted and unpredicted events. Correct involvement of human operators will utilize the fully combined potential of human and technical capabilities, providing cost-efficient assembly system solutions. The ProAct project is developing proactive assembly system models and evaluating proactive, feature-based solutions. Focus is on realizing the potential of cost-efficient and semi-automated systems with relevant human involvement, i.e. highly skilled operators that add flexibility and functionality.

The magnetization of magnetic liquids containing amorphous Fe1−xCx particles

Abstract The magnetization of amorphous Fe 1− x C x particles in decalin was measured in magnetic fields up to 12 T at temperatures between 10 and 250 K. For particles with a diameter of 3.2 nm, the zero field cooled magnetization has a maximum at 20 K. This is interpreted as a blocking of the superparamagnetic relaxation of single particles.

Zero-field cooled magnetization of amorphous Fe1−xCx particles : field dependence of the maximum

We measured the zero-field cooled magnetization, MZPC of a frozen magnetic liquid with amorphous Fe1-xCx particles. MZFC has a maximum at Tp approximately=20 K, which is interpreted as due to blocking of superparamagnetic relaxation in single particles. The maximum shifts towards higher temperatures with increasing measuring field. The shift is explained by the nonlinear field dependence of the magnetization of particles with a size distribution. At temperatures below Tp the magnetic particles have a coercivity and remanence in good agreement with those expected for an ensemble of non-interacting uniaxial particles with a random distribution of easy axes.

Magnetic properties of magnetic liquids with iron-oxide particles — The influence of anisotropy and interactions

Abstract Magnetic liquids containing iron-oxide particles were investigated by magnetization and Mossbauer measurements. The particles were shown to be maghemite with a spontaneous saturation magnetization M s = 320 kA m −1 at 200 K and a normalized high-field susceptibility χ / M o = 5.1 × 10 −6 m kA −1 , practically independent of temperature. M s increases with decreasing temperature according to an effective Bloch law with an exponent larger than 1.5, as expected for fine magnetic particles. The model of magnetic particles with uniaxial anisotropy and the actual size distribution gives a consistent description of independent measurements of the temperature dependence of the hyperfine field and the isothermal magnetization versus field. From this an effective anisotropy constant of about 4.5 × 10 4 J m −3 is estimated for a particle with diameter 7.5 nm. The magnetic relaxation, as observed in zero-field-cooled magnetization and isothermal remanence decay, is influenced by interactions and strongly dependent on the applied magnetic field.

An Approach to Proactive Assembly Systems: -Towards competitive assembly systems

Manufacturing competitiveness is highly dependant on companies ability to rapidly reconfigure their manufacturing and assembly systems. Efforts to approach emerging and self-reconfigurable systems could be successfully complemented by efficient integration of highly flexible human operators into the system. The concept of system proactivity is introduced which is based on the interrelated levels of automation, information, and competence in the assembly system. An ongoing project to develop proactive assembly systems, ProAct, is described.