Geologic and archaeological samples are usually partially demagnetized in a series of incremental steps to determine their coercivity and/or blocking-temperature spectra. Viscous remanences are more readily removed than either thermal or chemical remanence, so it is often possible to isolate the original (primary) magnetization acquired when the samples were first formed. This is known as alternating magnetic field demagnetization or thermal ‘cleaning’. Chemical demagnetization can also be used on permeable sedimentary samples in which the cement is usually most readily removed by acid washing, thereby preferentially removing the chemical remanence associated with the cement and isolating the detrital remanence acquired during deposition. Direct magnetic fields can be applied to reduce the observed remanence to zero, this field corresponding to the effective mean coercitivity of the total remanence. New developments include the use of tuned microwaves to demagnetize magnetic minerals without much heating and chemical change.
Subjects: Earth Sciences and Geography.