A large gyromagnetic effect in greigite

Abstract

Rotational and anhysteretic remanent magnetization (RRM and ARM) acquisition curves of two sediment samples containing greigite have been measured up to peak fields of 80 mT at rotation frequencies between 5 and 95 revolutions per second (rps). At 95 rps the ARM (70 uT direct field, applied antiparallel to the RRM) increased almost linearly with peak field but the RRM increased approximately exponentially. The effective field (Bg), defined in this case as 70 x RRM/ARM, was about 1100 uT for the two samples after the application of an alternating field (AF) of 80 mT peak. Bg is approximately 10 times higher than previously observed for magnetite of size 1 um. Although greigite is the dominant ferrimagnetic mineral present in these samples, other studies (Snowball 1997a), have shown that low concentrations of detrital multidomain magnetite are also present, so that the high value of Bg must be regarded as a lower limit. Unlike a magnetite sample that was used for comparison, both greigite samples had a negative RRM at all rotation frequencies below 50 rps but, like magnetite, there was an increase in RRM when it changed from negative to positive at 50 rps. In addition, unlike magnetite, the ARM was not constant but approximately halved as the RRM became strong and positive above 50 rps (ARM antiparallel to RRM). Thus there appeared to be an interaction between the ARM and the RRM. Further investigation of this interaction by applying weak direct fields parallel and then antiparallel to the RRM (80 mT peak AF, 95 rps rotation rate) showed that the ARM was linear with direct field but was indeed always smaller when the direct field producing it was antiparallel to the RRM. At present it is not clear why gyromagnetic remanences are so strong in greigite. Such high values of Bg have never been observed before except in the very special case of a self-reversing lithium chromium ferrite near its moment compensation temperature. These high values of Bg might enable RRM to be used as an indicator for greigite if they turn out to be unique to this mineral.