This effort, based on collaboration with the Instituto Nacional de Técnica Aeroespacial (INTA) in Madrid, Spain, examines the role of nanostructure on the temperature-dependent remanent magnetic state of spinodally decomposed two phase mixtures in the Fe2TiO4-Fe3O4 pseudo-binary titanomagnetite system. The pseudo-binary Fe2TiO4-Fe3O4 system is an interesting system for using magnetic measurements to probe the kinetics of phase transformations. This oxide system has a miscibility gap with spinodal decomposition. The two phases appearing in the decomposition are a strongly magnetic magnetite and a more weakly magnetic Ti-rich spinel. Many starting compounds in a homogeneous metastable solid solution are non-magnetic at temperatures where the decomposition kinetics can be monitored in reasonable experimental times. The magnetite formed by the decomposition reaction is magnetic at these temperatures and its magnetization is a measure of the volume fraction transformed. Time-dependent magnetization measurements are used to monitor the kinetics of spinodal decomposition for compositions within the spinodes and nucleation and growth kinetics for compositions outside of the spinodes. The investigators at Carnegie Mellon University (CMU) have developed synthesis routes for compounds in the pseudobinary Fe2TiO4-Fe3O4 system that allow to more accurately define the asymmetric miscibility gap in this system. The fine microstructure resulting from spinodal decomposition and exchange anisotropy mechanisms for coupling may explain a large slowly decaying remanent state for these minerals on Mars. The non-saturating behavior of Ti-rich spinels is hypothesized as arising from to non-collinear spins. The coupling of the spins in the ferrimagnetic magnetite in modulated spinodal structures is of interest. Certain compounds are of further interest because they have magnetic transitions that are within the day to night temperature swing on Mars and can therefore be detected with miniaturized magnetic sensors.

The research also sheds light on the role of these minerals on terrestrial and extraterrestrial magnetic field anomalies and uses magnetic measurements as a probe of the kinetics of decomposition. The titanomagnetites offer a rich magnetic system to explore the role of fine microstructure on magnetic properties. They are important minerals in basalts and a commonly occurring mineral on the moon and Mars. Since both the moon and Mars lack an intense global magnetic field, magnetic mapping is even more powerful on these two bodies than on Earth. On Earth magnetic surveys are complicated by the presence of the main field, which makes measurements of crustal anomalies challenging and difficult to discern. The global magnetic mapping together with the study of the minerals of the crust and surface and their remanent state can give clues to the geomagnetic evolution of a planet. Miniaturized magnetic sensors are further developed at INTA, with support from the Spanish Ministry of Science and Innovation, based on magnetic films developed at CMU that allow the remanent magnetic state of extraterrestrial minerals to be studied within the natural day to night temperature swing on the Martian surface. These sensors are also made available for characterization of biomagnetic systems on Earth. The proposed research involves two shared Ph.D students and develops mechanisms for undergraduate student exchanges. Students will also attend an INTA summer school on "Mars and its Enigmas" held at INTA. A second "Small Magnetic Sensors and Sensor Materials" satellite meeting to the Magnetism and Magnetic Materials (MMM) Conference will be proposed to disseminate results.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Lynnette D. Madsen
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Carnegie-Mellon University
United States
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