A novel vibrating-sample magnetometer used to measure magnetic hysteresis of rock at low temperature

English, Gerald Michael (1995) A novel vibrating-sample magnetometer used to measure magnetic hysteresis of rock at low temperature. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (2MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

A vibrating-sample magnetometer was constructed, calibrated and used to measure the magnetic hysteresis of rock samples. The magnetometer was designed around a large electromagnet capable of producing fields of approximately 3600 Oe (360 mT). Between the poles of this magnet, pick-up coils and a liquid nitrogen bath surrounded a sample port assembly in which the sample could be vibrated. The sample port assembly contained a heating element that allowed the sample temperature to be varied from room temperature to 98 K (-175ﾟC). A novel method was used to vibrate the large (5 gram) rock samples. The sample was mounted off-axis on a rotating shaft in a way that kept the sample's orientation in the applied field from changing while the sample moved uniformly in a circle of 2 mm radius 29.2 times a second. This induced a 29.2 Hz sinusoidal voltage in the pick-up coils proportional to the magnetization of the sample. Cycling the magnetic field intensity allowed hysteresis loops to be automatically plotted. -- Seven magnetite-bearing Precambrian dolerite dyke samples which had likely retained their original remanence direction for over a billion years were studied. Measurements of their magnetic hysteresis as a function of low temperature were used to investigate what mechanisms allowed them to retain such stable remanence. -- The coercive force, Hc, of the samples decreased as a linear function of saturation magnetostriction on cooling. This suggests that internal stresses opposing domain wall motion are mainly responsible for the ability of most of these rocks to retain stable remanence. The one clear exception was the specimen of highest coercive force (Hc = 487 Oe = 3.9x10⁴ A/m) in which Hc hardly changed on cooling suggesting that remanence was carried by single-domain grains with shape anisotropy. -- The ratio of saturation remanence to saturation magnetization JR/Js was found to change linearly with saturation magnetostriction and with coercive force on cooling. These linear relationships and that between coercive force and saturation magnetostriction were used to infer what proportion of the magnetite present was in the form of single-domain or pseudo-single-domain grains using the method of Hodych (1990). This method could not be applied to the sample of highest coercive force because its coercive force and saturation remanence did not change on cooling. -- Reversible work, Wrev, done on descending from saturation to the remanence point in a hysteresis loop was measured as a function of low temperature. When Wrev was estimated by integrating to a constant magnetization rather than to a constant field, Wrev was found to decrease on cooling. However, this does not allow inference of what is controlling domain rotation in the samples.

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