Characteristics of First Order Shock Induced Magnetic Transitions in Iron and Discrimination from TRM

Abstract

The probable magnetic history of iron and iron-nickel can be specified if the material in question can be resolved using reflected light microscopy. Microstructures imparted by shock transitions are distinct from those imparted by a thermal transition-always. These results are briefly summarized. Fine particle iron (<5, Å) can exist in face centered cubic (fcc) and body centered cubic (bcc) states at ambient temperature. When ferromagnetic bcc iron is shocked above 130kb (the pressure transition-PT) a first order magnetic phase change-to antiferromagnetic phase (hexagonal close packed)-results in a demagnetization. Reversal to ferromagnetic bcc remagnetizes the specimen if an external field is present. Iron can also exist in a metastable fcc state at ambient temperature in the form of spherical precipitates elastically constrained in an fcc copper matrix. Plastic deformation associated with shock impact at levels up to 50kb, which ensures sufficient plastic strain, but negligible shock heating, transforms the antiferromagnetic fcc iron to ferromagnetic bcc iron. These samples are being used to calibrate the shock and thermal mechanisms of magnetization in fine particle iron (size range for present data ~250Å-~1, 200Å). Experiments in axial fields demonstrates that the transformation takes place on a millisecond time scale within the designated field, as the sense of the shock induced axial vector is the same as the field sense. In fields as low as 10r the axial component is oriented in the same sense as the shock normal. The amount of fcc iron transformed depends on both precipitate size and the peak pressures. The observed deformation induced anisotropy is dependent on particle size and shock level, but the 400-600Å (B3, A3) set of samples shows the maximum degree of anisotropy at all levels of shock. Different sizes of precipitates appear to support different degrees of magnetic hardening. Anisotropy in magnetic hysteresis, using hysteresis ratios RI (ratio of saturation remanence to saturation magnetization) and RH (ratio of remanent coercive force to coercive force) indicates a shock induced anisotropy in remanence. The ratio of remanence after the shock to the saturation remanence is always less than the ratio of TRM to SIRM, but the shock induced remanence is more stable to demagnetization. © 1977, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.