Low-temperature magnetic hysteresis properties of partially oxidized magnetite

Abstract

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.

Grain sizes of susceptibility and anhysteretic remanent magnetization carriers in Chinese loess/paleosol sequences

“…In contrast, the coarser magnetic grains with a wide grain‐size distribution (100–300 nm) in the less altered loess units are independent of pedogenesis, and probably originate from aeolian sources or are produced by two‐phase low‐temperature oxidation. For detailed information about the low‐temperature oxidation, refer to Cui et al [1994] and Kosterov [2002]. With increasing pedogenesis the SP/SD grains gradually outweigh these initial coarser‐grained particles in loess, resulting in a gradual increase of Δχ/χ ARM with increasing Δχ.…”

A New Tool for Separating the Magnetic Mineralogy of Complex Mineral Assemblages from Low Temperature Magnetic Behavior

“…Component 1 is a synthetic magnetite powder [Wright Industries Inc. (W3006)] commercialized as multi-domain ∼2-3 µm particles. W3006 magnetite has been used in several previous studies (Dunlop, 2002;Kosterov, 2002;Liu et al, 2002;Smirnov and Tarduno, 2002;Yu et al, 2002;Till et al, 2010). Component 2 is a hematite powder with approximate particle size of 10-40 nm based on TEM images synthesized from ferrihydrite following Schwertmann and Cornell (1991) by David Burleson in the FIGURE 1 | The charging and discharging of the supraconducting magnet differs whether the target magnetic field is approached using the no-overshoot or oscillation modes.…”

Data report: magnetic property studies of sediments and rocks from IODP Expedition 316

“…The study of low-temperature phase transition in magnetic minerals has been increasingly utilized in rock magnetism (Banerjee, 1992;Moskowitz et al, 1998;Özdemir et al, 1993Cui et al, 1994;Kosterov, 2001aKosterov, , 2001bKosterov, , 2002Smirnov and Tarduno, 2002). Low-temperature measurements were made on representative samples to help characterize magnetic minerals and understand their rock magnetic properties.…”