LOW-TEMPERATURE BEHAVIOR OF SINGLE-DOMAIN THROUGH MULTIDOMAIN MAGNETITE

We have investigated the low-temperature behavior of a suite of 'grown ' synthetic and natural magnetites that span single-domain (SD) and mu ltidomain (MD) behavior. Synthetic samples had been grown in the labor atory either in an aqueous medium or in glass. Natural samples include d SD magnetites occurring in plagioclase and truly MD magnetites in th e form. of large octahedra. In all experiments a sample was first give n a saturation remanence at room temperature; next, moment was measure d continuously during cooling and warming between 230 K and 60 K. Simi lar to results reported earlier by other workers, magnetic memory is l arge in SD samples, whereas truly MD samples are almost completely dem agnetized by cycling between room temperature and 60 K. Pseudo-single- domain samples exhibit behavior that is intermediate with respect to t hat of the SD and truly MD states. When data from this study are combi ned with data obtained by Hartstra [10] from sized, natural magnetites , it is found that the percentage of total remanence that survives cyc ling between room temperature and 60 K decreases linearly with the log arithm of grain size and, thus, with increasing number of domains. Thi s relation suggests that memory can provide a reasonable estimate of g rain size in those magnetite-bearing rocks for which these samples pro vide good analogues. Remarkably, some of the large natural octahedra p rovide a magnified view of MD response to low temperatures and thus re veal two surprising and intriguing types of behavior. First, below app roximately 180 K these octahedra demagnetize through a series of large Barkhausen jumps. Second, near 117 K these same octahedra exhibit a ' wild zone', where magnetic moment executes large, random excursions. W e interpret these two phenomena as direct evidence for the unpinning a nd irreversible displacement of domain walls in response to the drop i n coercivity and, possibly, the broadening of domain walls as temperat ures drop toward the isotropic point. One implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carr ied by MD grains, even without passage through the isotropic point of magnetite.