Introduction

All magnetizations are produced by an applied magnetic field, but certain magnetization processes, for example isothermal remanent magnetization (IRM) and alternating-field (AF) demagnetization, are field-driven in a more restricted sense. They are produced isothermally, usually at or near T0, over a time scale of at most a few minutes. They result, in other words, from the sole influence of an applied field. Even this definition is not entirely accurate: τ0 is so much less than ordinary measurement times that thermal fluctuations play some role in practically all field-induced processes. For weak applied fields and small V, Hq or Hf are > H0, and thermal excitation plays a major role (§8.7, 9.4.2).

Isothermal remanences do not carry useful paleomagnetic information. An IRM produced by the geomagnetic field is easily reset by later weak fields of similar magnitude: it lacks paleomagnetic stability. Only isothermal remanences due to strong fields, for instance saturation isothermal remanence (SIRM) and anhysteretic remanence (ARM) produced by the combination of a steady field and a strong but decaying AF, have the requisite stability, and these occur in nature only when outcrops have been struck by lightning and remagnetized. However, the stepwise acquisition or removal of magnetizations in the laboratory is used as a means of erasing NRM's of low stability (AF demagnetization) and of determining the composition and domain structure of mineral magnetic carriers (hysteresis, IRM acquisition, ‘DC demagnetization’, Preisach analysis).