It is well known that serious discrepancies exist between Curie temperature and cell parameter data for synthetic titanomaghemites from different investigations. Intrinsic differences between the initial unoxidized titanomagnetites have been suggested as possible sources for these discrepancies and include the following: (1) variations in the degree of high temperature non-stoichiometry; and (2) variations in the initial cation distributions. Experimental and theoretical results indicate, however, that (1) and (2) cannot account for all the scatter in the data. A suite of TM60 samples with different initial degrees of non-stoichiometry were synthesized at 1288°C. The TM60 samples were subsequently oxidized in air at low temperatures (≤ 300°C) to produce single phase titanomaghemites. Curie temperatures and cell parameters for the titanomaghemites, after identical heat-treatments at low temperatures, were practically identical regardless of the initial degree of non-stoichiometry of the starting TM60 composition. In addition, Curie temperatures as a function of oxidation (z) for TM60 were determined theoretically using the molecular field model and two different initial cation distributions (Akimoto and Néel-Chevallier). The results indicate that variations in the initial cation distribution produce only small differences in calculated Curie temperatures for any value of z (20°-30° C maximum) and that these differences become smaller with increasing z. These calculations suggest, but do not prove, that regardless of the possibility of a temperature dependent cation distribution in titanomagnetites, variations in the initial cation distribution cannot explain the inconsistencies in the published magnetic data. Systematic differences in the methods of determination of the oxidation parameter, z, may contribute the largest source of scatter in the published data, at least for Ti-poor titanomagnetites (TMφ-TM40). Furthermore, measurement errors and systematic differences associated with use of varying techniques for determining Curie temperatures and cell parameters could contribute additional scatter. Recent evidence, using Mössbauer spectroscopy and neutron diffraction, indicates that Ti-clustering and sub-microscopic chemical inhomogeneities, produced by short-range chemical ordering, may be prevalent in sintered Ti-rich titanomagnetites. Sub-microscopic chemical inhomogeneities could produce variations in magnetic properties within the same nominal composition in both titanomagnetite and titanomaghemite depending on the size, compositions, and variations in compositions of these fine scale chemical inhomogeneities; this cause alone could account for the quantitative differences observed in the published data. Finally, it is strongly recommended that standard procedures for determining Curie temperatures, cell parameters, and most importantly, oxidation parameters be adopted and used consistently in future investigations.
Bibliographical noteFunding Information:
i thank a number of people who generously contributed their talents to this project. The TM6O samples were synthesized at Temple University and I thank Gene Ulmer for his assistance. Will Maze (Princeton) assisted with the microprobe analysis, Chuck Lawson (USGS) performed the MOssbauer analysis at the Johnson Space Center, and Jessica Posey-Dowty and Maria Borcsik (both at Princeton) painstakingly did the wet chemical analysis. The magnetic hysteresis measurements were obtained at the University of Minnesota, where I spent the spring of 1985. Discussions there with Subir Banerjee, Susan Halgedahl and Guy Smith helped formulate some of my ideas for this manuscript. I benefited also from discussions with Rob Hargraves, Chuck Lawson, Alexandra Navrotsky, and B.J. Wanamaker. A travel grant from AGU made it possible for me to attend the IAGA scientific assembly. This research was supported in part by NSF grants EAR 8115520 (Princeton) and OCE-8214652 (Minnesota).
Copyright 2014 Elsevier B.V., All rights reserved.