SUMMARY Saturation isothermal remanent magnetization (SIRM) and AC susceptibility have been measured as a function of temperature between 5 K and room temperature for one multidomain and three pseudo‐single‐domain magnetite samples after cooling in a zero (ZFC) and in a strong magnetic field (FC), and also after three partial field coolings (PFC) when a magnetic field had been turned on in 300–150, 150–80 and 80–5 K ranges, respectively. For the multidomain sample, SIRM(5 K) after ZFC is about twice as high as after FC, while the low‐field susceptibility is higher after FC. SIRM and susceptibility curves measured after PFC(300–150 K) and PFC(150–80 K) coincide with those measured, respectively, after ZFC and FC. PFC(80–5 K) curves are intermediate between the two extremes. This behaviour can be fairly well understood within the framework of a simple model, introduced back in the 1950s, which assumes that on cooling through the Verwey transition in a strong magnetic field easy magnetization axes in the low‐temperature phase are set along the [001] directions of the high‐temperature (cubic) phase closest to the field direction. If, on the other hand, the Verwey transition is passed in a zero field and a strong magnetic field is applied below the transition temperature, some of easy axes, initially set at random, can still be switched into the field direction, explaining observed SIRM and susceptibility versus temperature curves measured after PFC(80–5 K). Pseudo‐single‐domain grains show a more complex behaviour, which depends strongly on sample stoichiometry. In two samples with a relatively small non‐stoichiometry (Verwey temperatures are 122 and 110 K, respectively) SIRM(5 K) is higher by 5–7 per cent after FC. After both PFC(150–80 K) and PFC(80–5 K), SIRMs are nearly equal in magnitude to SIRM acquired after FC, but are thermally demagnetized at a different rate below the Verwey transition. Low‐field susceptibilities also show different temperature dependences below TV, dependent on a preceding thermomagnetic treatment. A strongly non‐stoichiometric sample (TV= 95 K) shows very large, over 70 per cent, difference between SIRMs after FC and ZFC, respectively, and a 35 per cent difference between susceptibilities measured under the same conditions. These results suggest that in magnetite grains several microns in size, easy magnetization axes setting on first passing the Verwey transition from above and switching of easy axes on subsequent cooling below the transition occur, qualitatively, in the same way as in grains of larger size, but switching of easy axes is considerably facilitated. The latter process is subject to further enhancement in strongly non‐stoichiometric magnetite, where it seems to be possible even in zero field, resulting in SIRM, susceptibility and magnetic hysteresis properties fairly different from more stoichiometric samples. Strongly depressed, compared with stoichiometric magnetite, magnetocrystalline anisotropy of the low‐temperature phase could be a physical mechanism for this behaviour.

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The normally magnetized zone of the Jurassic Lesotho basalts, although providing apparently quite reliable palaeofield directions Kosterov & Perrin 1996), shows anomalous behaviour when studied in vacuum using the Thellier palaeointensity method: typically the slope of the natural remanent magnetization–thermoremanent magnetization (NRM–TRM) curves is very steep at intermediate temperatures (200 to 400–460 °C). In order to elucidate the reasons for such an anomalous behaviour, six representative samples (from a total of 74 studied using this method) were subjected to a variety of analyses. These experiments indicate that the magnetic properties are dominated by pseudo‐single‐domain (PSD) magnetite grains some 1 μm in size, resulting from high‐temperature oxidation of titanomagnetite. Laboratory heatings in vacuum up to the Curie point do not change significantly the room‐temperature hysteresis characteristics or the initial susceptibility k. Similarly, the k(T ) curves in vacuum are (with a single exception) rather reproducible. Since the laboratory TRMs yield almost ideal NRM–TRM plots, the anomalous NRM–TRM plot is presumably due to some peculiarity of the natural TRM. The partial TRM (pTRM) acquisition capacity in the moderate temperature range (cooling from 200 to 20 °C) is generally very strongly reduced after heating to 270 °C, which indicates that some magnetic alteration has already occurred at these temperatures. Hysteresis measurements between room temperature and the Curie temperature Tc show that some small (less than 10 per cent) but significant irreversible changes in hysteresis characteristics also occur during heating. In particular, the coercive force Hc0 at room temperature is typically reduced after heating at a moderate temperature (175 °C) but increases after treatments at 475 °C and, more pronouncedly, at 580 °C. The saturation magnetization Js0 remains unchanged, except for a very small decrease (less than 5 per cent) occurring in some samples after the two latter treatments. These changes are most clearly seen on Hc(T )–Js(T ) bilogarithmic plots, which show that the moderate‐temperature change in coercivity can extend up to 200–250 °C. Thus hysteresis measurements as a function of temperature offer a promising tool for sample pre‐selection for Thellier experiments. Alternating‐field demagnetization and cycling of pTRMs at liquid‐nitrogen temperature suggest that the blocking mechanism is largely multidomain‐like near room temperature but becomes less so as the Curie point is approached. The main reason for the failure of the Thellier experiments is the loss of a fraction of the NRM (natural TRM) at temperatures apparently lower than the blocking temperatures in nature. It is suggested that this anomalous behaviour results from the reorganization of the domain structure of the PSD grains during heating. This transformation, which seems to be triggered by the coercivity decrease observed at very moderate temperatures, can reduce the NRM intensity without requiring any correlated pTRM acquis...

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[1] An unpredicted effect of the Verwey transition in magnetite is that a field-cooled (FC) remanent magnetization can be less intense than a zero-field cooled (ZFC) isothermal remanence. The effect, only documented in a handful of multidomain (MD) samples, is thought to be unique to MD material. Data for new MD samples all show an elevation of ZFC over FC remanences. Current theory suggests that the FC easy axis bias alone produces the effect. We measured hysteresis loops after three cooling pretreatments; the results are inconsistent with the aforementioned theory. They are, however, consistent with a previous hypothesis which cites the absence of transformational twins in FC samples as an important factor. Our initial low-temperature domain observations in FC and ZFC magnetite further support this theory. We also present data for MD titanomagnetites (x = 0.16, 0.35). These samples also show elevated ZFC remanences below a critical temperature (T crit ). The titanomagnetites' frequency dependence of susceptibility around T crit , the suppression of the amplitude dependence of susceptibility below T crit , and Mössbauer data suggest that the change in magnetic anisotropy at T crit is related to a suppression of B site electron hopping at low temperature, at least on the timescale of the magnetic measurements. Given our remanence data, field cooling must affect the orientation of the new low-temperature magnetic easy axis. We appeal to the same process as we did for magnetite to explain the elevation of ZFC moments, noting that the exact nature of the transition across T crit is not completely understood.Citation: Carter-Stiglitz, B., B. Moskowitz, P. Solheid, T. S. Berquó, M. Jackson, and A. Kosterov (2006), Low-temperature magnetic behavior of multidomain titanomagnetites: TM0, TM16, and TM35,

Summary Hysteresis loops have been measured as a function of temperature between 10 K and room temperature for two samples of pseudo‐single‐domain magnetite. One sample, with mean grain size of 2–3 μm, displays just surface oxidation, the bulk of the material remaining relatively stoichiometric with a Verwey transition temperature (TV) of 112 K. In contrast, another sample, which contains much finer grains (∼0.15 μm), having been exposed to air for about 20 yr, shows evidence of oxidation affecting the whole grain volume. This process has apparently formed two‐phase grains in which the core is composed of non‐stoichiometric magnetite with a TV of 95 K, and a superficial layer of probably pure maghemite. In accordance with previous studies, for both samples hysteresis properties below the Verwey transition depend critically on the mode of cooling through the transition. The difference in sample stoichiometry affects the temperature dependence of the hysteresis properties over the whole temperature range studied. Below the Verwey transition, the (relatively) stoichiometric sample shows a behaviour fairly close to that reported previously for a sample of similarly sized magnetite with a TV of 118 K. Common features include a rapid decrease of theMrs/Ms ratio with increasing temperature after cooling in a strong magnetic field (FC), compared with the near constancy of this parameter after zero‐field cooling (ZFC); (2) equally rapid decrease of the coercive force with increasing temperature after ZFC; (3) a small but significant difference between the temperature dependences of the coercive force after ZFC for a demagnetized versus a magnetized starting magnetic state. In the non‐stoichiometric sample some of these features are also observed; however, it also shows a distinctive behaviour of the Mrs/Ms ratio after ZFC, which reaches a maximum at 30–35 K. Above the Verwey transition, the stoichiometric sample shows the behaviour typical of magnetite, i.e. an increase in the Mrs/Ms ratio and the coercive force, which starts approximately 10 K above the TV and extends up to 200–210 K. In contrast, in the non‐stoichiometric sample both parameters just slowly decrease between 110 K and room temperature.

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x 10 Am . This dispersion is attributed mainly to inaccurate estimates of the ancient field rather than to secular variation. Although the number of cooling units for which paleointensity estimates could be obtained is limited, the results nevertheless indicate that the early Cretaceous VDM was significantly lower than that for the recent field but greater than has been reported for this portion of the Mesozoic Dipole Low period.