The Magnetization of Ferromagnetic Colloids

Abstract

The magnetization curves of colloidal magnetite (${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$) and colloidal siderac ($\ensuremath{\gamma}$-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$) have been determined by a null method for fields from 0 to 400 gauss. The measured curves have been found to agree with a theoretical curve based on the following assumptions: (1) Each colloid particle is a permanent magnet. This assumption is in accord with a prediction of the theory of ferromagnetism that small ferromagnetic particles be single magnetic domains. (2) There exists a distribution in the size of the particle moments similar to the weight distributions which have been found for other colloids. (3) The particles behave as the molecules of a classical paramagnetic gas. On this basis the average particle moment is found to be ${5.2(10)}^{\ensuremath{-}16}$ for the magnetite sol and ${7.4(10)}^{\ensuremath{-}16}$ for the siderac sol. Several other quantities related to the colloids are briefly discussed.