First-order symmetry of weak-field partial thermoremanence in multi-domain ferromagnetic grains. 1. Experimental evidence and physical implications

Abstract

We present experimental evidence that partial thermal demagnetisation and remagnetisation treatments produce effects to the measured thermoremanent magnetisation (TRM) of an assemblage of multidomain (MD) ferromagnetic grains that are symmetrical to first order. Just as for assemblages of single domain (SD) grains, measured TRM is affected by a partial thermomagnetic treatment to an extent dependent only on the difference of the magnetic field vectors between that used to impart the TRM and that used in the partial treatment and is insensitive to the absolute field intensity. The only difference between MD and SD TRM is that the effect of a partial treatment in the former partly comprises a non-reciprocal component which cannot easily be erased. The results suggest a new paradigm for MD TRM whereby a full TRM is regarded as a background state rather than a string of partial TRMs and thermal demagnetisation treatments may be considered as overprinting the TRM rather than erasing it. The symmetrical behaviour can be explained through a simple kinematic model incorporating a temperature-dependent domain structure. This model, although greatly oversimplified, also exhibits numerous other observed traits of MD TRM behaviour including the additivity of pTRMs, the exponentially decreasing progressive effects of iterated treatments, and the concave-up Arai plots produced by a Thellier palaeointensity experiment. We argue that the conventional domain wall-pinning process is only dominant in highly stressed grains and may be responsible for a preference for MD grains to be demagnetised and consequently for a loss of symmetry which is barely discernable in this study.