The DORF effect: Magnetization ripple in particulate media

In 1966, Pearce [1] introduced without analysis the DORF Effect (DisOrientation by Reverse Field) in liquid dispersions of acicular magnetic particles. Bate and Dunn [2] modified orientation magnets to circumvent the effect and produce recording tapes with improved orientation. This paper describes studies of the DORF Effect and analyzes the behavior of particle groups in liquid media when a reverse field is applied. Magnetization reversal occurs by particle group rotation through a symmetric fanning mechanism which shares features with the magnetization ripple phenomenon [3,4] in continuous metal films. The critical field for particle disorientation follows a dipole-dipole interaction relation like that discussed by Jacobs and Bean [5]. For gamma ferric oxide, critical fields are distributed over the range from 20 to 100 Oe. Rotating sample magnetometer data show the DORF Effect is a destruction of uniaxial anisotropy without creation of biaxial or other anisotropies. Hysteresis work in liquid samples is almost totally due to friction between moving particles.