The mechanism of self-reversal of thermoremanence in natural hemoilmenite crystals: new experimental data and model

New magnetic and mineralogical findings on self-reversing hemoilmenite, (Fe 2-y TiyO3) grains from Pinatubo lavas (1991 eruption) provide important clu es regarding the acquisition process of reverse thermoremanent magnetizatio n (rTRM) in this solid solution series. Magnetic force microscopy indicates the presence of multidomain magnetic structures in coexisting strongly and weakly magnetic crystallographic regions having compositions of y congruen t to 0.54 and 0.53, respectively. Continuous thermal demagnetization of nat ural and laboratory TRM carried out on both whole rock samples and single h emoilmenite crystals shows that the magnitude of a normal TRM (nTRM) compon ent, observed at temperatures above the Curie point of the self-reversing p hase, is much too large to be carried by a phase that is entirely cation-di sordered. Consistent with this observation are findings using transmission electron microscopy (TEM) which, in contrast to that what is commonly assum ed, reveals the weakly magnetic regions to be magnetically heterogeneous. S pecifically, these regions are found to contain tiny (20-40 nm) domains tha t are cation-ordered and evidently ferrimagnetic dispersed within the catio n-disordered, presumably spin-canted antiferromagnetic matrix. Given these findings, we argue that the so-called nTRM-carrying x-phase is itself parti ally cation-ordered, and, thus, ferrimagnetic, as postulated first by Ishik awa and Syono (J. Phys. Soc., Jpn. 17 (1962) 714). We propose a "nanophase" self-reversal model for the ilmenite-hematite soli d solution series in which the rTRM and nTRM components are carried by the cores and margins, respectively, of the tiny, partially cation-ordered nano -sized domains observed by TEM. Due to the partial cation order, both the c ore and the margin of each domain are expected to behave in a ferrimagnetic fashion at temperatures below their respective Curie points. However, give n the kinetics of the ordering process, their cation distributions need be antiphase, which causes their magnetic moments to be oppositely aligned. Si nce it is most reasonable to consider each margin to be slightly more Fe-ri ch than the inside core, upon cooling, the margins acquire a magnetic reman ence first (a nTRM). Then, upon further cooling, given that the intralayer and interlayer nearest-neighbor superexchange interactions are ferromagneti c and antiferromagnetic, respectively, the net magnetic moment of the core material need be oppositely aligned (producing a rTRM). The nano-sized regions would indeed behave in a superparamagnetic (SP) fash ion if magnetically uncoupled to adjacent material; however, the spins in t he margins (the x-phase) must be locked through superexchange to those of t he surrounding disordered matrix, which we also claim to be locally enriche d in iron. If so, then the magnetization of the x-phase can be both highly- coercive and thermally stable, as observed experimentally. Upon stepwise th ermal demagnetization, the self-reversed remanence measured at room tempera ture is not destroyed until the unblocking temperature of the disordered Fe -enriched aureole (approximately 410 degreesC) is reached. Mineralogical co nsiderations and magnetic evidence from previous works suggest that this mo del is generally valid for self-reversed dacitic pumice, in particular the Mt. Haruna dacite and the 1985 Nevado del Ruiz dacitic andesite. (C) 2001 P ublished by Elsevier Science B.V.