Experimental evidence for lamellar magnetism in hemo-ilmenite by polarized neutron scattering

Abstract

Large local anomalies in the Earth's magnetic field have been observed in Norway, Sweden, and Canada. These anomalies have been attributed to the unusual magnetic properties of naturally occurring hemo-ilmenite, consisting of a paramagnetic ilmenite host ($\ensuremath{\alpha}$-Fe${}_{2}$O${}_{3}$-bearing FeTiO${}_{3}$) with exsolution lamellae ($\ensuremath{\approx}$$3\phantom{\rule{0.222222em}{0ex}}\ensuremath{\mu}$m thick) of canted antiferromagnetic hematite (FeTiO${}_{3}$-bearing $\ensuremath{\alpha}$-Fe${}_{2}$O${}_{3}$) and the mutual exsolutions of the same phases on the micron to nanometer scale. The origin of stable natural remanent magnetization (NRM) in this system has been proposed to be uncompensated magnetic moments in the contact layers between the exsolution lamellae. This lamellar magnetism hypothesis is tested here by using polarized neutron diffraction to measure the orientation of hematite spins as a function of an applied magnetic field in a natural single crystal of hemo-ilmenite from South Rogaland, Norway. Polarized neutron diffraction clearly shows that the ilmenite spins do not contribute to the NRM and that hematite spins account for the full magnetization at ambient temperature. Hematite sublattice spins are shown to adopt an average angle of 56${}^{\ensuremath{\circ}}$ with respect to a saturating magnetic field, which is intermediate between the angle of 90${}^{\ensuremath{\circ}}$ predicted for a pure canted moment and the angle of 0${}^{\ensuremath{\circ}}$ predicted for a pure lamellar moment. The observed NRM is consistent with the vector sum of lamellar magnetism and canted antiferromagnetic contributions. The relative importance of the two contributions varies with the length scale of the microstructure, with the lamellar contribution increasing when exsolution occurs predominantly at the nanometer rather than the micrometer scale.