Step-wise and continuous thermal demagnetization and theories of thermoremanence

Abstract

Summary. The difference between step-wise and continuous thermal demagnetization of rock specimens is examined with particular reference to the presence of multi-component magnetizations. The effect of the thermal dependence of the intensity of spontaneous magnetization is clearly evident in recordings of continuous demagnetization when a least-squares analysis of a two component system is performed. The differences between spontaneous magnetization, saturation magnetization and high-field magnetization are discussed and it is shown that the normalized thermal dependence of spontaneous magnetization can be determined with negligible error by measuring magnetization as a function of temperature in sufficiently hgh fields (> 0.1 T for magnetite, > 1 T for pyrrhotite). Using experimentally determined thermal dependence of high-field magnetization to correct the continuously observed intensities leads to analyses comparable to those of step-wise demagnetization data. Experiments designed to compare the temperature dependence of an apparent spontaneous magnetization, derived from the observed thermal decay of saturation remanence carried by a multidomain (MD) magnetite bearing sample, with the true spontaneous magnetization of magnetite reveal a systematic difference, with the apparent spontaneous magnetization decreasing more rapidly than the true spontaneous magnetization. This difference is minor, however, compared to the thermal decay of spontaneous magnetization and to a first-order approximation the use of the thermal decay of saturation magnetization to correct intensities should usually be adequate. Similar conclusions are supported by experiments with MD pyrrhotite bearing samples. These experiments serve to constrain models of MD remanence. The approximate proportionality of blocked remanence (TRM and saturation remanence) to spontaneous magnetization which is observed for these samples, which contain predominantly MD grains, resembles the behaviour expected for non-interacting single domain (SD) grains. Conventional MD theories assume that domain structure remains stable below the blocking temperature and attribute remanence carried by MD grains to Barkhausen discreteness of domain wall positions. However, if the domain multiplicity does not change with temperature, remanence due to Bark