 |
| Titel |
Ordering, nanostructure and high-field magnetization of quenched and annealed metastable ilmenite-hematite solid solutions |
| VerfasserIn |
Karl Fabian, Christopher I. Thomas, Suzanne A. McEnroe, Peter Robinson, Hiroki Mukai |
| Konferenz |
EGU General Assembly 2013
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250079841
|
|
|
|
|
|
| Zusammenfassung |
| The ilmenite-hematite solid solution series xFeTiO3-(1 - x)Fe2O3 can generate extremely
unusual magnetic properties in natural rocks and has been investigated for more than fifty
years. Both, ilmenite (FeTiO3) and hematite (Fe2O3) are antiferromagnetic, but intermediate
compositions are either antiferromagnetic or ferrimagnetic, depending on their chemical
order. Within a single sample, nano-scale variations in local composition x and ordering state
Q depend on minute details of the cooling and annealing history, and have large effects on the
magnetic properties, which include self-reversal of thermoremanent magnetization and large
exchange bias. We present a systematic study of magnetic properties of samples in the
composition range of 0.6 -¤ x -¤ 0.7 with differing nanostructure and consequently differing
magnetic properties. Using high-field measurements up to 7Â T, together with TEM
images and theoretical models we classify nanostructure formation in terms of x,
Q, and characteristic size d. These characteristics are then linked to the magnetic
properties. The sample characterization relies on average mean-field models of Ms(T).
To implement the varying Fe and Ti densities, and the distribution of Fe ions in
the variably ordered solid solutions, the models either use statistical interactions
between sites, whereby they effectively average over all possible configurations, or
they describe specific random configurations. Statistical mean field models are
successful in predicting the Curie temperatures TC and Ms(T) curves of the Ilmx solid
solutions. The results depend on the interaction coefficients, which either had been
determined by neutron diffraction measurements (Samuelson and Shirane, 1979),
by Monte-Carlo model fits (Harrison, 2006), or by density-functional theoretic
calculations (Nabi et al. 2010). Hysteresis branches have been measured for a wide
variety of samples at different temperatures 40Â K, 100Â K and 300Â K. None of
them saturate at 7Â T, the strongest field available to us so far. Some of the samples
show the beginnings of a pseudo-metamagnetic transition at the upper limits of the
measurements. In previous models this is explained by anti-phase boundaries and exchange
coupling between ordered and disordered regions with differing sizes and hence
differing responses to an external field. These effects will be studied further up to 60Â T
using a European high-field laboratory within the EuroMagNET II/EMFL scheme. |
|
|
|
|
|
|