| Abstract
| - Nanometer-size ε-Fe2O3, prepared by degradation of Y3Fe5O12 garnet in silica at 1000 °C is orthorhombic with three octahedral FeO6 and one tetrahedral FeO4. It exhibits a complex magnetic behavior resulting in a rich phase diagram (see figure) consisting of two transitions (480 and 110 K) separating three magnetic ground states, one paramagnetic and two canted-antiferromagnetics. Mössbauer spectroscopy points to a change in coordination of the tetrahedral FeO4.
- The elusive ε-Fe2O3 has been obtained as nanoparticles by vacuum heat treatment of yttrium irongarnet in a silica matrix at 300 °C followed by annealing at 1000 °C for up to 10 h in air and employingformamide as a gel modifier. Its nuclear structure is temperature independent as observed from the neutronpowder diffraction patterns and has been modeled by the published structures on analogous MM‘O3compounds. It displays complex magnetic properties that are characterized by two transitions: one at480 K from a paramagnet (P) to canted antiferromagnet (CAF1) and the second at ca. 110 K from thecanted antiferromagnet (CAF1) to another canted antiferromagnet (CAF2) that has a smaller resultantmagnetic moment (i.e., smaller canting angle). The latter transition resembles that of Morin for α-Fe2O3at 260 K. The magnetization shows unusual history dependence: it has a bifurcation below 100 K if thefield is applied at low temperatures after zero-field-cooled, whereas the bifurcation is above 150 K if thefield is applied at high temperatures. The magnetic hardness first increases slightly from 300 to 200 K,then it drastically decreases to zero at 100 K and follows a further increase down to 2 K. The coercivefield reaches an unexpected and quite exceptional 22 kOe at 200 K. There appears to be a further ill-defined metamagnetic transition below 50 K, characterized by a doubling of the measured magnetizationin 50 kOe. The AF1−AF2 transition is accompanied by sharp peaks in both the real and imaginarycomponents of the ac-susceptibility due to the hard−soft effect, and their peak maxima shift to lowertemperatures on increasing the frequency. Mössbauer spectra are characterized by a change in hyperfinefield of the tetrahedral Fe by ca. 40% around the transition, suggesting a change of geometry.
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