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 γFe
2
O
2
in 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, H
R
, the particle subsequently rotated (in the steady field) by 180°. Thus by varying the pulse field H
R
was determined. An extension of the method permitted the determination of the remanent loop, and examples are shown for ∼ 1 μm long particles of γFe
2
O
3
and CrO
2
, H
R
being approximately 430 Oe and 700 Oe respectively.