Maj Hanson

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.

The frequency dependence of the complex susceptibility of magnetic liquids

Abstract The frequency dependence of the magnetic susceptibility of kerosene or oil-based magnetic liquids has been measured. The volume fractions of particles range from 0.028 to 0.192. The liquids exhibit a broad maximum in the imaginary part at frequencies of the order of 10 MHz. There are no discernible effects of particle interactions in the values of the initial low-frequency susceptibility, which are practicaly constant up to 10 5 Hz, or in the height of the maximum. There are particle interactions manifesting themselves in the details of the shape of the susceptibility curves in the frequency region approaching the maximum. The observed dissipative behaviour is interpreted as a relaxation process in which single particles with a distribution of sizes change their magnetization direction by Neel rotation.

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.

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.

Arrays of epitaxial Co submicron particles: Critical size for single-domain formation and multidomain structures

Arrays of Co particles with well-defined geometry and lateral sizes in the range 0.1–0.5 μm were prepared by electron beam lithography and studied by magnetization measurements and magnetic force microscopy. The starting materials were epitaxial Co films, with in-plane magnetic anisotropy, having thicknesses in the range 10–50 nm. The particles change from multidomain behavior to that of stable single domains in zero field, as the lateral dimension decreases within these ranges. For particles of circular shape, i.e., without in-plane shape anisotropy, the critical size for forming single domains was found to be about 200 nm in lateral extension and 20 nm thickness. Elliptical particles prepared with 20 nm thickness and lateral dimensions 150 by 450 nm were found to be stable single domains, of good uniformity, in zero field. The switching of these particles from one magnetization direction along the long axis to the other, requires an applied field BS≈(100±20)mT. A comparison with other published results ...

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.

Self-organized MnAs quantum dots formed during annealing of GaMnAs under arsenic capping

Formation of MnAs quantum dots in a regular ring-like distribution has been found on molecular beam epitaxy grown (GaMn)As(100) surfaces after low-temperature annealing under As capping. The appearance of the dots depends on the thickness and Mn concentration in the (GaMn)As layer. With 5 at. % substitutional Mn the quantum dots showed up for layers thicker than 100 nm. For thinner layers the surfaces of the annealed samples are smooth and well ordered with 1×2 surface reconstruction, just as for as-grown (GaMn)As. The annealed surfaces are Mn rich, and are well suited for continued epitaxial growth.