Particle size distribution and magnetic structure of colloidal ferrite particles

A colloidal suspension of very fine magnetic particles is called a ferromagnetic liquid, or "ferrofluid". The colloidal particles are extremely small ({dollar}approx{dollar}10 nm, or 100 angstrom) and are found to be very different in their magnetic properties from the bulk material. The size of such particles, defined by images observed by electron microscopy of a sample and called the physical size, usually ranges from 50 to 300 angstrom in diameter. Magnetic domain theory predicts that if the size of a particle is smaller than the critical value it will likely become single domain particle. The size of such a domain is called the magnetic moment size, or simply 'magnetic size', which is defined as the cube root of magnetic moment of a particle divided by 4{dollar}pi{dollar}/3 times the saturated magnetization of the bulk material (for a spherical particle).; The first goal in this investigation is to determine the physical size and the magnetic size of particles in given samples. The second goal is to answer the question: if the magnetic size of a particle is smaller than the critical value, is the particle necessarily a single domain particle? If the particle magnetic size is smaller than its physical size, what is the magnetization distribution within the particle and the 'magnetic structure' of the particle?; The sample of colloidal particles used in this investigation is a water based Fe{dollar}sb3{dollar}O{dollar}sb4{dollar} ferrofluid designated EMG805 made by Ferrofluidics Corp. In order to determine the dependence of various quantities on particle size, the sample was fractionationed using chromatographic techniques. The original sample was also diluted into different dilutions, which were used to obtain information about the effects of interparticle interactions.; A colloid of CoFe{dollar}sb2{dollar}O{dollar}sb4{dollar} suspended in kerosene obtained from G. E. Corp. was also investigated. Fractionation was not performed as kerosene is not compatible with the available gel. Its magnetic, physical and crystallite size distributions were also determined.; Experiments employed electron microscopy, small angle neutron scattering with and without polarized neutrons, magnetization measurement and X-ray powder diffraction. It has been found that the independent domain model of colloidal particles should be replaced by a mixture model. (Abstract shortened with permission of author.)...