The failure of thermal demagnetization could be attributed to irreversible thermal physio-chemical alterations upon heating (e.g., Wasilewski 1969 Heller et al. It is often found that remanent direction of some specimens become seriously distorted at high temperatures, but the ChRM can still be obtained by alternating field demagnetization using sister specimens (e.g., Holm and Verosub 1988). Thermal demagnetization is one of the standard demagnetization methods, and thus thermal demagnetization furnaces (or thermal demagnetizers) have been widely equipped in nearly all paleomagnetic laboratories.Īlthough widely used, the experimental success rate of thermal demagnetization can be relatively low especially for weakly magnetic samples such as sedimentary rocks. This treatment will be conducted repeatedly in a stepwise manner till the ChRM is isolated (Thellier 1966 Collinson 1975). During thermal demagnetization, specimens are heated to a pre-selected temperature, held for a period of time (e.g., 10–30 min), and then cooled down to the room temperature in “zero” magnetic field environment (Collinson 1983). Therefore, to define the stable characteristic remanent magnetization (ChRM) of geological interests, the secondary remanent magnetization needs be removed using sequential demagnetization techniques (e.g., the stepwise progressive thermal or alternating field demagnetization) (Irving et al. NRM of geological materials usually consists of multiple components. Paleomagnetism investigates changes in the geomagnetic field through geologic time by the measurement of natural remanent magnetizations (NRM) recorded by magnetic minerals in natural materials. Thermal demagnetization experiments demonstrate that the new demagnetizer can yield low noise results even for weakly magnetic samples. Simulation and practical measurements show that the heating current magnetic field can be greatly reduced by using the new technology. Here, we designed a new structure of heating wire named “straight core solenoid” to develop a new demagnetization furnace with ultra-low magnetic field noise. However, magnetic field noises, including the residual magnetic fields of the construction material and the induced fields caused by the alternating current (AC) in the heating element are always present, which can contaminate the paleomagnetic results at the elevated temperatures or especially for the magnetically weak samples. An ideal thermal demagnetizer should maintain “zero” magnetic field in the sample chamber during thermal treatments. Thermal demagnetization furnaces are widely used paleomagnetic facilities for progressive removal of naturally acquired magnetic remanence or the imparting of well-controlled laboratory magnetization.
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