Gyromagnetism and the remanence acquired by a rotating rock in an alternating field

Abstract

Although alternating field (a.f.) demagnetisation is routinely used in palaeomagnetism it has relatively recently been recognised that rotation of rock samples in the presence of an a.f. may introduce unwanted components of magnetisation. Doell and Cox¹ observed such magnetisations in 1967 but the phenomenon was not attributed to the effect of rotation until Wilson and Loniax² performed experiments about a single rotation axis and demonstrated the remarkable result that when an alternating field was applied at right angles to the axis of slow rotation, a remanent magnetisation was left antiparallel to the rotation vector after the a.f. had been smoothly reduced to zero. This magnetisation has been termed rotational remanent magnetisation (RRM). One of the explanations considered in attempts to account for the effect is that it is gyroscopic in origin, but order of magnitude calculations based on the rotation rate of the rock have indicated that such effects are insignificantly small^2,3, especially as RRM is detectable at rotation rates as low as 0.02 revolutions per second (r.p.s.). It is reported here that if appropriate assumptions are made regarding the magnetic behaviour of the rock, gyroscopic effects may be large enough to explain RRM.