PROPERTIES OF FERRITES AT LOW-TEMPERATURES (INVITED)

At cryogenic temperatures magnetic properties of ferrites change signi ficantly from their values at room temperature, which has been the mai n regime for most device applications. Recently, microwave ferrite dev ices with superconducting microstrip circuits have been demonstrated a t a temperature of 77 K with virtually no electrical conduction losses . Conventional ferrimagnetic garnet and spinel compositions, however, are not generally optimized for low temperatures and may require chemi cal redesign if the full potential of these devices is to be realized. Saturation magnetizations increase according to the Brillouin-Weiss f unction dependence that is characteristic of all ferromagnetic materia ls. Increased magnetocrystalline anisotropy and magnetostriction can h ave large effects on hysteresis loop squareness and coercive fields th at are essential for stable phase shift and efficient switching. Rare- earth impurities and other ions with short spin-lattice relaxation tim es can cause increased microwave losses. In this article, the basic ma gnetochemistry pertaining to ferrites will be examined for adaptation of ferrite technology to cryogenic environments. (C) 1997 American Ins titute of Physics.