Measurements on single magnetic particles

Abstract

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 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> 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 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</inf> , the particle subsequently rotated (in the steady field) by 180°. Thus by varying the pulse field H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</inf> 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 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> and CrO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</inf> being approximately 430 Oe and 700 Oe respectively.