Demagnetization consists of applying a decaying alternating magnetic field to a sample. In the absence of external direct magnetic fields and significant distortion in the applied AF, the sample will be "cleaned" of any remanent magnetization of coercivity less than the peak intensity of the applied AF. This cleaning is the result of randomizing the mobile magnetic domains along the axis of the applied field.

Because it is decaying, the amplitude of each half-cycle of the applied AF is smaller than its predecessor. With each half-cycle, the domains whose coercivities are less than the applied field align themselves with the field. During each half-cycle of the AF, a small percentage of these mobile domains will have coercivity greater than the following half-cycle and will therefore become fixed in direction. In this way, equal numbers of domains will be magnetized in the positive and negative directions oriented along the axis of demagnetization, resulting in a net zero remanent field on the sample.

Description

The MOLSPIN shielded demagnetizer enables demagnetization of specimens in peak fields up to 1500 Oersteds (150 mT). The unit has a two axis tumbler and can accommodate cylindrical samples up to 1" long and 1" diameter. The coil assembly and sample are housed inside a triple walled mu-metal bucket shield. Total power consumption is only 700 watts.

ARM Attachment

There is also an ARM attachment. This consists of the same coil as the pARM device that is placed within the demagnetizing coil and which can produce a constant field of up to one Oersted.

Application

AF demagnetization is often effective in removing secondary NRM and isolating characteristic NRM (ChRM) in rocks with titanomagnetite as the dominant ferromagnetic mineral. In such rocks, secondary NRM is dominantly carried by multidomain (MD) grains, whereas ChRM is retained by single-domain (SD) grains. MD grains have coercivity (hc) dominantly <20 mT (200 Oe), whereas SD grains have higher hc. AF demagnetization thus can remove a secondary NRM carried by the low-hc grains and leave the ChRM unaffected. AF demagnetization is a convenient technique because of speed and ease of operation and is thus preferred over other techniques when it can be shown to be effective.