Abstract
The relationship of the composition to the dielectric loss tanδ and dielectric constant ε′ of the polycrystalline garnet Y3Al1.25FexO12 has been quantitatively determined. Single-phase dense garnet which has a tanδ<0.0002 can be prepared for values of x between 3.73 and 3.78. A high yield process, based on these data, to synthesize polycrystalline garnet with dielectric loss <0.0002 (X-band) and dielectric constant controlled within 1% is discussed briefly. The room-temperature saturation moment of garnets in the system Y3Fe5−xAlxO12 can be changed by a short heat treatment. The effect of the heat treatment of the 25°C 4πMs increases as the aluminum content of the garnet increases. Heat treatment can change the 4πMs of Y3Fe3.75Al1.25O12 by as much as 40%. In the heat-treatment range 1000° to 1250°C the data obtained by quenching small samples of Y3Fe3.75Al1.25O12 after a half-hour equilibration at temperature are thermodynamically reversible and reflect changes in the Al3+ and Fe3+ ion distribution between tetrahedral and octahedral sites. Heat treatment from 1250° to 1500°C causes tanδ at 25°C to increase continuously from 0.0001 to 0.001. The lattice defects formed by heat treatment above 1250°C decrease the activation energy required to cause a change in the aluminum and iron ion distribution between tetrahedral and octahedral sites.
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