MAGNETIC-SUSCEPTIBILITY AND REMANENT COERCIVE FORCE IN GROWN MAGNETITE CRYSTALS FROM 0.1 MU-M TO 6 MM

Initial susceptibility is frequently used as a palaeoclimatic indicato r in sediments, but its grain size dependence is not well established. We measured initial magnetic susceptibility chi(0) in grown and natur al magnetite crystals ranging from 0.09 mu m to 6 mm in grain size. Ov er these five decades of grain diameter, the presented initial suscept ibilities are essentially independent of grain size with a mean value of 3.1 SI and a standard deviation of +/-0.4 SI. Numerical results of micromagnetic calculations for cylindrical particles in the size range 0.06 mu m < d < 0.120 mu m agree well with the experimental data. Ini tial susceptibilities of grown synthetic and natural magnetite crystal s larger than 80 mu m can be explained with demagnetizing factors N ap proximate to 1/3 and large intrinsic susceptibility (chi(i) > 200) usi ng the relation chi(0) = chi(i)(1 + N chi(i)) The observed number of m agnetic domains in magnetite grains between 50 mu m and 1000 mu m is t oo low for the required demagnetizing factor of about 0.33. In a lamel lar domain model one needs a higher number of domains than those obser ved, to obtain a demagnetizing factor of 0.33. A simple lamellar strip e domain model without closure domains is therefore not a good approxi mation for large magnetite grains. Remanent coercive force of grown ma gnetite grains shows a weak dependence on grain diameter. The remanent coercive force H-cr decreases gradually from about 35 mT to 10 mT bet ween 0.09 mu m and 6 mm. A noticeable drop in H-cr occurs at a grain s ize of about 110 mu m, which is interpreted as the transition from pse udo-single-domain to multidomain grains. The remanent coercive force o f magnetite grains is not a sensitive indicator of grain size, unlike coercive force or saturation remanent magnetization.