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INSPEC Accession Number: 15298193 We have designed and fabricated a microelectromechanical device, based on the alternating field gradient concept, to measure surface magnetic field gradient on planets. Its sensitivity is 4 10 -4 T/m, which is appropriate for magnetite outcrops and areas with rocks formed at different stages recording geomagnetic field reversals. We present the results obtained with three different prototypes. The work of J. L. Mesa was supported by INTA training Grant TS09/12. This work has been done in the frame of PRI-PIBUS-2011-1150 and 1182 grants of the subprogram of international and bilateral projects of the Spanish Ministry of Science and Innovation. The work at CMU is supported by the National Science Foundation through Materials World Network under Grant DMR1106943. Peerreview

Titanomagnetites (TMs) [pseudobinary (1 - x)Fe 3 O 4 · xFe 2 TiO 4 oxides, where, e.g., TM75 corresponds to x-0.75] are magnetic minerals with a spinel structure. Ulvospinel (Fe 2 TiO 4 ) and TMs were made using a solid-state synthesis technique, and transmission electron microscopy (TEM) characterization and measurement of the TM samples were performed to further confirm the homogeneity of the samples and cation site occupancies. The Mossbauer spectra in the TMs exhibited an anomalous composition dependence at x=0.95, when fitted with two instead of three quadrupole split doublets. This result was reinforced by the obtained dependence of the recoilless fraction on the composition, which exhibited an extremum at x=0.95.

Ulvospinel (Fe2TiO4) and titanomagnetites (TM) (pseudo-binary (1-x)Fe3O4xFe2TiO4 system, where, e. g. TM95 corresponds to x = 0.95) were made using a solid-state synthesis technique. For single phase titanomagnetites (x>0.75), our measured lattice constants extends the previous work of Wechesler to Ti-rich solid solutions, and extrapolates to a value of 8.531 A for single phase ulvospinel, agreeing closely to the literature value of 8.534 A. The lattice constants as a function of composition (x>0.75) were fit to a straight line with the equation y = 0.09x + 8.441. Grain sizes of single phase TM pellets of as-quenched and annealed samples were determined, showing that the grain size of the annealed samples grew noticeably after 100 hrs in Ar flow at 450 °C. For TM75, the grains grew from 11.7 μm to 15.4 μm. The ratios of grain sizes vary over the composition range, from 4% to 31%, suggesting coarsening driven by diffusional growth. In the samples that initially contained wustite as a second phase, there are grains with a lath microstructure with well-defined angles, indicating preferred orientations. Small amounts of Feor Ti-cation rich phases were observed to decorate the grain boundaries after long annealing. TiO2 formed on grain boundaries of TM75 annealed sample.