Low-temperature magnetization and AC susceptibility of magnetite: effect of thermomagnetic history

Abstract

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.

Grain size dependence of low-temperature remanent magnetization in natural and synthetic magnetite: Experimental study

“…The experimental results described above are consistent with the results of prior studies of synthetic magnetites (Kosterov, 2001(Kosterov, , 2003Smirnov and Tarduno, 2002;CarterStiglitz et al, 2006), biogenic magnetite and natural magnetite-bearing samples (Brachfeld et al, 2001(Brachfeld et al, , 2002Carter-Stiglitz et al, 2002, 2006.…”

“…In contrast, for large multidomain (MD) magnetite grains, the FC SIRM has been found to be significantly lower than the ZFC SIRM (e.g., Kosterov, 2001;Brachfeld et al, 2002;Carter-Stiglitz et al, 2006). Such a difference in the R LT ratio reflects different mechanisms governing the SIRM acquisition in single-domain and multidomain magnetite below T V (Kosterov, 2003). However, the evolution of R LT ratio between the two extreme cases representing SD and MD magnetite grains remains largely unknown.…”

“…However, the evolution of R LT ratio between the two extreme cases representing SD and MD magnetite grains remains largely unknown. Unfortunately, only two data points are currently available for pseudo-single domain (PSD) magnetite, both showing nearly equal ZFC and FC SIRMs (Smirnov and Tarduno, 2002;Kosterov, 2003). Further accumulation of experimental data for PSD magnetites, therefore, may be important for better understanding the magnetization processes in monoclinic magnetite.…”

Rock-magnetic properties of single zircon crystals sampled from the Tanzawa tonalitic pluton, central Japan

“…One sample (TNZ03-08-41) showed clear transition temperature~35 K. If this transition indicates the pyrrhotite, it has remanent magnetization at room temperature and may affect paleointensity experiment. The intensity of FC remanence was larger than that of ZFC remanence, which is a characteristic behavior of SD and fine-grained PSD magnetite (Moscowitz et al 1993;Kosterov 2003;Carter-Stiglitz et al 2006), although the zircon crystals may contain magnetic minerals other than pure magnetite.…”

Low-temperature magnetic properties of andesitic rocks from Popocatepetl stratovolcano, Mexico

“…Let us consider first the magnetic susceptibility data. The χ-T curves show a rather variable shape, but neither of them resembles well-known curves for multidomain Muxworthy, 1999;Skumryev et al, 1999;Kosterov, 2003) or pseudo-single domain (Kosterov, 2003) magnetite. However, these curves can be classified into two groups according to their shape, and, furthermore, these groups overlap significantly with those identified in RT-SIRM cycling experiments (Table 1).…”