Formation of Nanoparticles of ε-Fe₂O₃ from Yttrium Iron Garnet in a Silica Matrix: An Unusually Hard Magnet with a Morin-Like Transition below 150 K

The elusive ε-Fe 2 O 3 has been obtained as nanoparticles by vacuum heat treatment of yttrium iron garnet in a silica matrix at 300 °C followed by annealing at 1000 °C for up to 10 h in air and employing formamide as a gel modifier. Its nuclear structure is temperature independent as observed from the neutron powder diffraction patterns and has been modeled by the published structures on analogous MM‘O 3 compounds. It displays complex magnetic properties that are characterized by two transitions: one at 480 K from a paramagnet (P) to canted antiferromagnet (CAF1) and the second at ca. 110 K from the canted antiferromagnet (CAF1) to another canted antiferromagnet (CAF2) that has a smaller resultant magnetic moment (i.e., smaller canting angle). The latter transition resembles that of Morin for α-Fe 2 O 3 at 260 K. The magnetization shows unusual history dependence: it has a bifurcation below 100 K if the field is applied at low temperatures after zero-field-cooled, whereas the bifurcation is above 150 K if the field 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 coercive field 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 magnetization in 50 kOe. The AF1−AF2 transition is accompanied by sharp peaks in both the real and imaginary components of the ac-susceptibility due to the hard−soft effect, and their peak maxima shift to lower temperatures on increasing the frequency. Mössbauer spectra are characterized by a change in hyperfine field of the tetrahedral Fe by ca. 40% around the transition, suggesting a change of geometry.