Critical Size of Particles in Superparamagnetic Magnetic Agents Keeping Stable and Superparamagnetism in Magnetic Fields

Explores the critical size of particles in superparamagnetic magnetic agent(SMA) keeping stable and superparamagnetism in magnetic fields. Based on superparamagnetic theory, the expression for keeping superparamagnetism is deduced. Based on magnetic theory, the mechanism of stability of contrast agent is analyzed, and the critical size of magnetic microparticles keeping stable in the contrast agent developed. The critical size of Fe<sub<3</sub<O<sub<4</sub< and Fe<sub<2</sub<O<sub<3</sub< particals keeping superparamagnetism is 25.0nm or 31.5nm respectively. The size decreases as the magnetic fields grow up. Meanwhile the size of microparticles decreases as the gradient magnetic fields grow up. The definite size to be chosen is the lesser one between them.