Préparation de nanocomposites magnétiques de grenat/SiO2 et sphinelle/SiO2 par la méthode sol-gel et caractérisation

STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

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

The elusive ε-Fe2O3 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‘O3 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-Fe2O3 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