Chemical and magnetic properties of rapidly cooled metastable ferri-ilmenite solid solutions - IV

The fine structure of self-reversed thermoremanent magnetization

Peter Robinson, S. A. McEnroe, K. Fabian, R. J. Harrison, C. I. Thomas, H. Mukai

Research output: Contribution to journalArticle

6 Citations (Scopus)

Abstract

Magnetic experiments, a Monte Carlo simulation and transmission electron microscopy observations combine to confirm variable chemical phase separation during quench and annealing of metastable ferri-ilmenite compositions, caused by inhomogeneous Fe-Ti ordering and anti-ordering. Separation begins near interfaces between growing ordered and anti-ordered domains, the latter becoming progressively enriched in ilmenite component, moving the Tiimpoverished hematite component into Fe-enriched diffusion waves near the interfaces. Even when disordered regions are eliminated, Fe-enriched waves persist and enlarge on anti-phase boundaries between growing and shrinking ordered and anti-ordered domains.Magnetic results and conceptual models show that magnetic ordering with falling T initiates in the Fe-enriched wave crests. Although representing only a tiny fraction of material, identified at highest Ts on a field-cooling curve, they control the 'pre-destiny' of progressive magnetization at lower T. They can provide a positive magnetic moment in a minority of ordered ferrimagnetic material, which, by exchange coupling, then creates a self-reversed negative moment in the remaining majority. Four Ts or T ranges are recognized on typical field-cooling curves: TPD is the T range of 'pre-destination'; TC is the predominant Curie T where major positive magnetization increases sharply; TMAX is where magnetization reaches a positive maximum, beyond which it is outweighed by self-reversed magnetization and TZM is the T where total magnetization passes zero. Disposition of these Ts on cooling curves indicate the fine structure of self-reversed thermoremanent magnetization. These results confirm much earlier suspicions that the 'x-phase' responsible for self-reversed magnetization resides in Fe-enriched phase boundaries.

Original languageEnglish
Pages (from-to)1375-1396
Number of pages22
JournalGeophysical Journal International
Volume196
Issue number3
DOIs
Publication statusPublished - Mar 2014
Externally publishedYes

Keywords

  • Magnetic mineralogy and petrology
  • Microstructures
  • Phase transitions
  • Rock and mineral magnetism

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