Hallmarks of maghemitization in low-temperature remanence cycling of partially oxidized magnetite nanoparticles

In environmental, soil, and sediment magnetism, it is important to be able to estimate the degree of oxidation of magnetite grains. We report a new method for finding the oxidation parameter z semiquantitatively from cooling-warming cycles of room temperature remanences. We measured magnetization M continuously for stoichiometric and partially oxidized magnetites with average grain sizes of 37 and 220 nm during zero-field cycling of 2.5 T saturation isothermal remanent magnetization (SIRM) from 300 K to 20 K and back to 300 K. Oxidized magnetites were obtained by heating stoichiometric magnetite in air at 100 degrees C, 150 degrees C, and 200 degrees C. In other experiments, SIRM was given at 10 K, and M was monitored during zero-field warming to 300 K. In the oxidized magnetites, SIRM at first increases in cooling from 300 K and then decreases in approaching the Verwey transition. The hump-like form is even more pronounced in the warming curves above T-V. For maghemite, the fully oxidized end member, we found reversible cooling-warming curves with no Verwey transition. In partially oxidized grains, consisting of a maghemite surface layer and a largely unoxidized core, a Verwey transition is resolvable up to high degrees of oxidation. Hallmarks of maghemitization include (1) a smeared-out Verwey transition shifted to lower temperatures when warming 20 K SIRM, (2) a shifted and broadened transition region in both cooling and warming of 300 K SIRM, and (3) humped cooling and warming curves of 300 K SIRM between 300 K and T-V. Property 3 has excellent diagnostic value. It results from the combination of a slowly increasing M of maghemite and the rapid and nonlinear decrease in M of magnetite during cooling and is seen even for the slight initial oxidation of the reduced 37 nm magnetite. Certain properties, such as the change in M in warming from 20 K to T-V and the change in initial and final M values in a complete cooling-warming cycle, are roughly proportional to the oxidation parameter z. However, the proportionality factors also depend on grain size d, which would have to be known independently in order to estimate z.