David B. Lambrick

Studies of the double surfactant layer stabilization of water-based magnetic fluids

A comparison of the ability of C6C18 carboxylic acids to stabilize aqueous magnetite fluids is made. Micelle formation prevented complete dispersion of magnetite by stearic and myristic acids, but was overcome by introducing the acid in several portions. Improvements in fluid preparation are reported, achieved by perchloric acid treatment of surfactant-stabilized magnetite produced in basic solution, followed by redispersion of the particles in dilute base. The quantity of surfactant in the primary layer has been determined for C10C18 acids, and the particle surface area occupied per molecule of surfactant found to range between ca. 21 and 38 A2. For decanoic and myristic acids the groups pack efficiently and form a “condensed” film over the surface of the particles. Ammonium and monomethylammonium salts of the same acids were used in attempts to form the secondary layers. Ammonium salts produced dispersion in all cases except Sarkosyl-“O,” whereas MeNH3+ salts of oleic and myristic acids were successful. Quantitative data on the relative amounts of surfactant in each layer are presented and discussed.

“Stripped” magnetic particles. Applications of the double surfactant layer principle in the preparation of water-based magnetic fluids

Abstract The double surfactant layer principle for colloidal stability is applied to the design of magnetite particles having primary and secondary surfactant layers composed of different fatty acids. Concentrated aqueous dispersions, containing up to 32% by weight Fe3O4, are prepared, which are stable to dilution (60 fold). Specific magnetizations of up to 28 J T−1 kg−1 (28 emu/g) are recorded. Particles are initially stripped of surfactant which is surplus to the primary layer, in a treatment prior to the addition of the secondary surfactant layer. The particles having solely a primary surfactant layer, disperse readily in nonpolar solvents to give fluids containing up to 33% by weight Fe3O4. Differences occur for some fatty acids between the use of NH+4 and CH3NH+3 salts for the formation of the secondary surfactant layer, and are ascribed to the ready decomposition of NH4 salts at 90°C. Oleic acid and Sarkosyl-“O,” having unsaturation in their hydrocarbon chains, are found to be suitable for the primary surfactant layer when straight chain fatty acids are used as the second surfactant, but not for use as the secondary surfactant layer. Redispersion of double surfactant layer coated particles, separated from aqueous media at pH

Proteins and carbohydrates as alternative surfactants for the preparation of stable magnetic fluids

A one-stage preparation of stable aqueous magnetic fluids is reported, whereby colloidal Fe/sub 3/O/sub 4/ particles are dispersed using naturally occurring polymers and their derivatives (e.g. gelatine, polygalacturonic acid, carboxymethyl-cellulose and succinylated gelatine) as surfactant materials. Low-toxicity materials have been used to permit possible medical use of the fluids. Using a variety of surfactant concentrations at the time of particle formation, control of particle size has been achieved, and particles as small as 3.0 nm in diameter obtained. Stable fluids with up to 6% Fe/sub 3/O/sub 4/ content can be produced, having magnetizations of approximately 2 JT/sup -1/ Kg/sup -1/ (20 Gauss). Fluids diluted 30-fold remain stable colloids. >

Preparation and properties of Ni-Fe magnetic fluids

Abstract A heteronuclear organometallic precursor containing Ni and Fe atoms in the ratio 2:1 has been used to prepare hydrocarbon based magnetic fluids. The structure of the resulting magnetic particles has been found by electron diffraction to be that of the ordered Ni3Fe intermetallic. Electron micrographs reveal particle diameters of 7–8 nm. Magnetic analysis indicates diameters of 3–4 nm and anisotropies of ≈105Jm-3.

Preparation and properties of metallic iron ferrofluids

A stable hydrocarbon based metallic iron ferrofluid has been produced by the decomposition of enna-carbonyldi-iron. After five months only about 4% of the iron had oxidised. From magnetic and electron microscopy studies a median particle diameter of less than 50 angstrom has been found. The distribution of particle sizes is relatively narrow.

Magnetisation study of the magnetic phase diagram in MnSi

Abstract Magnetisation measurements have been made on a single crystal of MnSi in the temperature range 10–40 K using a vibrating sample magnetometer with the field applied in the [001] and [111] directions. The focus of attention is on the strong field dependence and anisotropic nature of the magnetic susceptibility in the vicinity of the magnetic ordering temperature.

Measurement of the anisotropy constant in Ni-Fe and Fe/sub 3/O/sub 4/ magnetic fluids

The uniaxial anisotropy constant of two magnetic field systems has been measured using magnetization and torque magnetometry. A metallic, Ni-Fe system was frozen at 77 K to obtain torque curves while an Fe/sub 3/O/sub 4/ system was set in a polyester resin at room temperature in the presence of aligning magnetic fields. The induced texture gave rise to torque measurements which yielded values for the anisotropy constant of 2.9*10/sup 5/ Jm/sup -3/ for Ni-Fe and 0.58*10/sup 5/ Jm/sup -3/ for magnetite. The magnetization measurements resulted in lower values of 1.15*10/sup 5/ Jm/sup -3/ for Ni-Fe and 0.47*10/sup 5/ Jm/sup -3/ for Fe/sub 3/O/sub 4/. Even for the lowest value obtained, however, a particle elongation of 1.5 would be required for shape to be responsible for the anisotropy. No such elongations were observed in the electron microscope suggesting other factors are present. >

An automated micrograph image size analyser

An automated electro-optic image size analyser capable of rapid data collection is described. The device is especially suited to the determination of colloidal particle size distributions and grain or magnetic domain size analysis from electron or optical micrographs. The interfacing of the analyser, which is described, requires two input/output lines and a logic zero to an 8-bit parallel port of a microcomputer as well as two ADC channels. The apparatus is thus not fundamentally specific to any single type of microcomputer.

Parametric modelling of magnetic fine particle systems: the role of single domain particle size distributions and magnetic anisotropy

We demonstrate the effects and role of anisotropy and particle size distributions upon the values of constitutive parameters determined from experimental M(H) loops of single domain nanoparticle systems. We employ experimental and magnetic material parameters as inputs to a numerical programme run from a graphical user interface (GUI) to permit modelling and interpretation of experimental data and assessment of the stability of the constitutive parameters so derived and characterisation of the departures observed from pure Langevin behaviour.

On magnetic properties of DyNi5−xAlx (x=0, 1, 1.5, 2, 2.5, 3) intermetallics

The bulk magnetic properties of the DyNi5-xAlx (x=0, 1, 1.5, 2, 2.5 and 3) intermetallic compounds, have been investigated up to 25 K. It has been found that substitution of Ni by Al decreases the Curie temperature for those compounds with the CaCu5 type of crystal structure (x >2) but when the YCo3Ga2 structure type is adopted (x>2), the Curie temperature increases once more at x=2.5. For all alloys, the paramagnetic effective Bohr magneton numbers, peff were found to be close to the free ion value of Dy3+. A complex low temperature magnetic behaviour with a possibility of up to three ordered magnetic states was observed for the alloys with x>2.