R.C. Woodward

Deformation of a hydrophobic ferrofluid droplet suspended in a viscous medium under uniform magnetic fields

The effect of applied magnetic fields on the deformation of a biocompatible hydrophobic ferrofluid drop suspended in a viscous medium is investigated numerically and compared with experimental data. A numerical formulation for the time-dependent simulation of magnetohydrodynamics of two immiscible non-conducting fluids is used with a volume-of-fluid scheme for fully deformable interfaces. Analytical formulae for ellipsoidal drops and near-spheroidal drops are reviewed and developed for code validation. At low magnetic fields, both the experimental and numerical results follow the asymptotic small deformation theory. The value of interfacial tension is deduced from an optimal fit of a numerically simulated shape with the experimentally obtained drop shape, and appears to be a constant for low applied magnetic fields. At high magnetic fields, on the other hand, experimental measurements deviate from numerical results if a constant interfacial tension is implemented. The difference can be represented as a dependence of apparent interfacial tension on the magnetic field. This idea is investigated computationally by varying the interfacial tension as a function of the applied magnetic field and by comparing the drop shapes with experimental data until a perfect match is found. This estimation method provides a consistent correlation for the variation in interfacial tension at high magnetic fields. A conclusion section provides a discussion of physical effects which may influence the microstructure and contribute to the reported observations.

The use of MFM for investigating domain structures in modern permanent magnet materials

Abstract Magnetic force microscopy occupies a special niche in the array of techniques which are currently available for imaging magnetic structures of high energy permanent magnet materials, yielding high resolution data under ambient conditions on bulk samples. The high anisotropies of permanent magnet samples mean that to a good approximation the tip stray field does not modify the magnetic state of the sample. However strong stray fields, of the order of 1xa0Tesla, may be experienced by a tip in close proximity to the sample. These stray fields are known to perturb the magnetic state of the tip considerably. As a result, contrast may be generated between neighboring domains, or at domain boundary walls or a combination of both, depending on the degree of perturbation of the tip. Examples demonstrating this variation with a series of batch-fabricated tips with various magnetic coatings ranging in coercivity from ∼1400 Oe to Oe are presented. The origin of the different types of observed contrast is discussed, and it is shown that very low coercivity tips coated with Fe nanoparticles embedded in a SiO 2 matrix provide the most readily understood contrast, and have an additional benefit in that they are `self-focusing. That is, the active magnetic volume scales with the characteristic dimension of the sample domain structure. Image interpretation is also discussed in the context of specific MFM images from samples of neodymium iron boron (sintered, melt-quenched and nanocomposite) and barium ferrite.

Stability of Polydimethylsiloxane-Magnetite Nanoparticle Dispersions Against Flocculation: Interparticle Interactions of Polydisperse Materials

The colloidal stability of dispersions comprised of magnetite nanoparticles coated with polydimethylsiloxane (PDMS) oligomers was investigated theoretically and experimentally. Particle-particle interaction potentials in a theta solvent and in a good solvent for the PDMS were predicted by calculating van der Waals, electrostatic, steric, and magnetic forces as functions of interparticle separation distances. A variety of nanoparticle sizes and size distributions were considered. Calculations of the interparticle potential in dilute suspensions indicated that flocculation was likely for the largest 1% of the population of particles. Finally, the rheology of these complexes over time in the absence of a solvent was measured to probe their stabilities against flocculation as neat fluids. An increase in viscosity was observed upon aging, suggesting that some agglomeration occurs with time. However, the effects of aging could be removed by exposing the sample to high shear, indicating that the magnetic fluids were not irreversibly flocculated.

Magnetic force microscopy images of high-coercivity permanent magnets

Abstract The stray magnetic field distributions at the surfaces of isotropic and anisotropic NdFeB-type magnets have been imaged by magnetic force microscopy. Polished surfaces of bulk samples produced by melt-quenching, hot-rolling, and sintering were examined using two techniques which provided complementary information on the structure of the domains at the surface. For the first technique a high-coercivity magnetic tip was used, and for the second, a soft magnetic tip was used. The structures which have been observed are compared with those reported by other workers using techniques of Lorentz electron microscopy and optical Kerr microscopy and generally appear to be in good agreement. Preliminary analysis of the images collected from the hot-rolled material suggests the presence of circular ‘bubble’ domains, alongside the more regularly observed stripe domains. These appear to be reverse domains, involving only a small amount of material, which have arisen at the exposed surface to minimize the magnetostatic energy at the surface. Imaging of the die-upset material proved to be somewhat more difficult than for the other materials due to strong stray fields at the surface that affected the tips magnetic moment during imaging. This problem was largely overcome by the use of special high-coercivity tips. The results nonetheless are in accordance with recently published Kerr micrographs for the same material.

A comparison of methods for the measurement of the particle-size distribution of magnetic nanoparticles

Recently, interest in magnetic particles, particularly in the nanometre-size range, has increased significantly. The main driving forces behind this interest are both the development of improved synthesis techniques and an increase in the number of potential applications for suitable magnetic nanoparticles. A critical factor of interest in both the synthesis and the development of applications is the particle-size distribution. In this paper, we investigate three common techniques for determining the particle-size distribution of magnetic nanoparticles (electron microscopy, magnetic measurements and small-angle neutron scattering). We compare the distributions determined by each technique for two standard samples and discuss their advantages, disadvantages and limitations.

Reversible magnetization behavior in Sm2(Co, Fe, Cu, Zr)17

The magnetization of Sm2(Co,u200aFe,u200aCu,u200aZr)17 is determined by the motion of domain boundary walls that are subject to pinning. Measurements have been made of the relationship between the reversible magnetization Mrev and the irreversible magnetization Mirr components in Sm2(Co,u200aFe,u200aCu,u200aZr)17. The results are consistent with a model in which rapid nucleation of domain walls occurs at the beginning of magnetization reversal followed by gradual unpinning of the domain walls. For Sm2(Co,u200aFe,u200aCu,u200aZr)17 the parameter η, defined as η=(dMrev/dMirr)H is similar in form to previously observed variations of η in sintered NdFeB but very different from η variations due to Stoner–Wohlfarth coherent rotation of magnetization vectors.

Evolution of magnetic microstructure in high-coercivity permanent magnets imaged with magnetic force microscopy

Magnetic force microscopy (MFM) has been shown to give high-resolution imaging of magnetic domain structures in a variety of high-coercivity permanent magnets [Folks et al., J. Magn. Magn. Mater. (in press)]. We show that this technique can be extended by the application of external fields during imaging, thus allowing direct observation of submicron microstructural evolution as a function of field. Electromagnets mounted on the MFM supplied fields up to 7 kOe laterally and 3 kOe vertically. In sintered materials, submicron processes such as depinning of domain walls at grain boundaries, domain fragmentation, and hysteresis were observed. MFM tips having very low coercivity highlighted domain walls, whereas higher-coercivity tips suffered unpredictable rotation of their magnetic moment due to both the sample and applied fields, leading to images which are difficult to interpret. For imaging of the finer-grained melt-quenched magnets, however, relatively high-coercivity tips were superior. These results sh...

Magnetic properties of novel resin-bonded exchange coupled rare-earth magnets

Abstract The magnetic properties of compaction-molded resin-bonded magnets which exhibit exchange spring behaviour have been measured. The starting compositions of the alloys were Nd4.5Fe73B18.5Co3Ga1 and Nd3.5Dy1Fe73B18.5Co3Ga1 and the preparation method consisted of melt-quenching followed by annealing for a short period. The magnets have remanent magnetisation approximately 70% of saturation magnetisation. The variation of the magnetic properties with temperature in the range 250–320 K has been investigated and magnetic viscosity measurements have been made. These data are compared with those from a conventional resin-bonded NdFeB-type permanent magnet. It has been found that the exchange coupled materials have smaller variations in magnetic properties over the temperature range investigated, and they exhibit less time dependence of magnetisation, compared with the conventional material.

Experimental determination of Lévy flight distributions of the energy barriers in spin glasses

Zero-field magnetic viscosity measurements at low temperatures were made on a series of Pd40Ni40−xFexP20 (x=10–20) samples in order to determine the apparent magnetic-moment-weighted energy barrier distributions for these spin-glass alloys. The distributions observed are best described by a stretched exponential in the form of a truncated Levy flight distribution. This form suggests a hierarchical landscape of apparent energy barriers arising from interactions between randomly oriented magnetic clusters within the material. The degree of stretching of the exponential form of the energy barrier distributions is found to increase with decreasing iron concentration.

Preparation and Characterization of Cerium Substituted Bismuth Dysprosium Iron Garnets for Magneto-Optic Applications

We report on cerium substituted bismuth dysprosium iron garnet of composition Bi1.3Ce0.3Dy1Fe5O12 prepared by biased target ion beam deposition on a fused quartz substrate and postdeposition crystallization through rapid thermal annealing in air. The resulting thin film garnet achieved promising magnetic and magneto-optic (MO) properties on a low cost amorphous substrate and with reduced thermal budget toward potential MO applications and semiconductor processing for films of similar composition.