Measurements on single magnetic particles

A method is described for measuring the properties of single acicular magnetic particles, of about 1 μm in length. A dilute suspension was made of, for example, particles of γFe2O2in a viscous liquid, and an individual particle viewed with the microscope. The particle, initially at remanence, was aligned by a steady field of a few Oersted. With this field still present, an opposing pulse field was then applied. If this was larger than the remanent coercive force, HR, the particle subsequently rotated (in the steady field) by 180°. Thus by varying the pulse field HRwas determined. An extension of the method permitted the determination of the remanent loop, and examples are shown for ∼ 1 μm long particles of γFe2O3and CrO2, HRbeing approximately 430 Oe and 700 Oe respectively.