Beyond Néel’s theories: thermal demagnetization of narrow-band partial thermoremanent magnetizations

Abstract

Partial thermoremanent magnetization (pTRM) was imparted over a narrow temperature interval, T=370–350°C, to a suite of crushed and annealed natural magnetite samples, ranging in grain size from ≈1 to 125–150 μm and in domain state from small pseudo-single-domain (PSD) to multidomain (MD). In this way, effectively a single blocking temperature, T B, of pTRM was activated. Stepwise thermal demagnetization did not erase the pTRMs sharply at T B, as for single-domain (SD) grains. Demagnetization began well below 350°C and continued above 370°C, with a median unblocking temperature, T UB, close to 360°C. The largest grains deviated most from SD behavior. Their pTRM demagnetized over the entire interval from room temperature to the Curie point, in accordance with predictions for MD grains. In terms of the unblocking temperature distribution f( T UB) or slope d M/d T of the thermal demagnetization curve, SD grains have a sharp spectrum, T UB= T B; MD grains were observed to have a broad, roughly symmetrical spectrum centered on T B; and intermediate size grains in the PSD range had non-Gaussian spectra, combining a central peak near T B with broad tails above and below T B. In this respect, PSD grains display a superposition of SD and MD remanences, not a blend between the two. Practical implications of these observations are that Thellier’s law of reciprocity ( T UB= T B) will be increasingly violated as grain size increases in the PSD range. The low- T UB part of f( T UB) produces anomalously large demagnetization of NRM in low-temperature heating steps of Thellier-type paleointensity determinations and a sagging shape of the Arai plot. The high- T UB part of f( T UB) results in undemagnetized remanence at and above T B in thermal demagnetization. Among pre-treatments designed to make remanence more SD-like in subsequent thermal cleaning, alternating field (AF) pre-cleaning sharpened f( T UB) more effectively than low-temperature demagnetization for the 20 μm sample.