High density Gd-substituted yttrium iron garnets by coprecipitation

Gadolinium-substituted yttrium iron garnets are ferrite materials of primary importance in microwave engineering. Stoichiometric powders of nominal composition Y2.6Gd0.4Fe5O12 (i.e. Fe/(Y + Gd) = 1.67) were prepared by reverse strike coprecipitation of metal nitrates. In order to investigate the influence of composition on phase formation, non-stoichiometric powders were also synthesised. On the basis of DTA/TGA analysis, dried coprecipitates were calcined between 600 °C and 1200 °C and then characterised by ICP, XRD and HT-XRD. Amorphous coprecipitates crystallise around 700 °C in cubic garnet phase along with small amounts of YFeO3 and/or α-Fe2O3. Only iron-rich garnets, either pure or Gd-substituted, calcined at 1200 °C or above display a single-phase cubic garnet. According to thermal dilatometry results, calcined powders were sintered in air up to 1470 °C. The microstructure of sintered ceramics is made up of fine grains, the average size ranging between 3 μm and 13 μm. Density of sintered bodies ranged from 88% to 98%. Ferromagnetic resonance linewidth (ΔH−3 dB) ranged between 4352.9 A m−1 and 4392.7 A m−1, depending on composition and microstructure.