Development of a 0.5 T magnetic‐core alternating‐field demagnetizer

Abstract

AbstractWe have constructed an alternating‐field (AF) demagnetizer with a magnetic core in a passively air‐cooled coil that can routinely operate at fields up to 0.5 T, almost 3 times higher than we could attain before in our commercial instrument. The field is powered by a commercial 1 kW power amplifier and is transverse to the bore, uniform to ±2% over a 25 mm paleomagnetic sample, and compatible with our existing sample handler for automated demagnetization and measurement. Even harmonics are ≤1 ppm of the fundamental and so generate negligible anhysteretic remanence. The much higher peak alternating field, 2 and 5 times that commonly available in air‐core solenoidal and Helmhotz coil configurations, respectively, enables successful AF demagnetization of many samples that could not be completely demagnetized with commercially available equipment. This capability is especially useful for high‐coercivity sedimentary and igneous rocks and extraterrestrial materials that contain magnetic minerals that alter during thermal demagnetization. In addition to the benefits, this instrument brings to our own research, a much broader potential impact is that it could replace the transverse coils of most automated AF demagnetization systems in use today, whether for discrete or continuous U‐channel measurements, which are commonly limited to peak fields of ∼100 mT. Manual and tumbling demagnetizers would benefit as well by the ∼2 times increase in maximum field over those that can be attained by commercial solenoidal coils. Furthermore, we expect that it and similarly designed magnetic‐core instruments will be capable of attaining even higher fields, of order 1 T.