Titan o' the Magnetites - Greig Paterson - 2021 MagIC Workshop (1/19/21)
Channel: Magnetics Information Consortium (MagIC)
Date: 20/01/2021

" Titan o' the Magnetites" by Greig Paterson, Department of Earth, Ocean and Ecological Sciences, University of Liverpool

Talk presented at the virtual 2021 MagIC Workshop (https://earthref.org/events/MAGIC/2021/) - Rock and Paleomagnetism through Time and Space. Jan 19th-21st, 2021.

Abstract:
Magnetite is the most well-known magnetic mineral found in geological material and, by virtue of extensive study over the last several decades, it forms the cornerstone of paleo-, rock, and environmental magnetic studies. Over the last three decades, magnetite has been micromagnetically modelled to understand a wide range of properties and behavior, from its critical grains size, the influence of shape and interaction fields, configurational anisotropy, and including detail simulations of hysteresis and first order reversal curves. The most recent micromagnetic studies of magnetite indicate that single vortex (SV) magnetizations are extremely stable, but the transition between Néelian single domain (SD) to vortex states results in highly unstable magnetizations.

Magnetite, however, is only one end-member of the titanomagnetite solid solution series (Fe(3-x)TixO4; known as TMx, where x is the percentage of Ti) and pure magnetite (x ≤ 5) accounts for only around one third of magnetic carriers in natural igneous materials, with titanomagnetites and titanomaghemites constituting the majority of carriers. 
 
Figure caption: Domain state for equidimensional cuboctahedrons of titanomagnetite as a function of composition and grain size. Grain size is given in equivalent spherical diameter (ESD). SP: superparamagnetic; SD: singe domain; SV: single vortex.

Here we present the first systematic micromagnetic analysis of the titanomagnetite (TM) series spanning magnetite (TM00) to TM60.  Exploring the critical grain for TM we find an increasing trend in the minimum and maximum grain size capable of supporting a uniform single domain (SD) state with increasing titanium concentration. The nature of this volumetric increase is sufficient to offset the decrease in saturation moment with Ti substitution, such that uniformly magnetized SD titanomagnetites are capable of carrying a stronger moment than SD magnetite and may contribute more to a specimen’s bulk magnetization if both compositions are present.  We further reveal that some high-titanium TMs may not have a stable uniform SD state, instead transition from a superparamagnetic state to a stable single vortex state. Our analyses to-date reveal important trends in grain stability and highlight the necessity of fully understanding the role of the titanomagnetite series in paleomagnetic recorders.