Thermal demagnetization of VRM and pTRM of single domain magnetite: No evidence for anomalously high unblocking temperatures

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Abstract
Stepwise thermal demagnetization of viscous remanent magnetization (VRM) and partial thermoremanent magnetization (pTRM) of single‐domain magnetite with mean grain size 37 nm reveals only minor deviations from the predictions of Néel theory. Three different initial states were used, following (1) alternating field demagnetization to 100 mT, (2) thermal demagnetization to 610°C, and (3) modified thermal demagnetization, consisting of zero‐field heating to 610°C followed by zero‐field cooling to the VRM or pTRM acquisition or blocking temperature, TB. VRMs were produced by applying a 0.1 mT field for 3.5 hr, starting from initial states 2, 3 and 1, in that order, at TB=283°C and from states 3, 2 and 1 at TB=404°C. In a final experiment, pTRM was produced by applying 0.1 mT during cooling from TB1=414°C to TB2=404°C, starting from state 3. In all but one case, VRM began to unblock significantly (90% of initial intensity) ≈15°C below TB, dropped to ≈50% at TB, and then tailed off, dropping below 10% ≈40°C above TB=283°C or ≈25°C above TB=404°C. Partial TRM behaved in the same way, except that demagnetization was essentially complete by TB1=414°C. The maximum unblocking temperatures (TUB) observed are exactly as predicted by the theory of Pullaiah et al. (1975). Although very long acquisition times cannot be tested, we find no evidence for anomalously high TUBs of VRM in single‐domain grains. In previously reported cases of rocks with unexpectedly high TUBs, the VRM was probably carried by multidomain grains.