Microwave Ferrites for Cryogenic Applications

Abstract
Recent advances in microwave ferrite device technology have seen the introduction of superconductivity that virtually eliminates insertion losses due to electrical conduction in microstrip circuits. The conventional ferrimagnetic spinel and garnet compositions, however, are not generally optimized for temperatures in the vicinity of 77 K and may require chemical redesign in order to realize the full potential of these devices. For microwave transmission, absorption losses may be reduced by a natural lengthening of the spin-lattice relaxation time and the suppression of hopping electron activity at low temperatures. However, these properties could be degraded by fast-relaxing impurities that broaden ferrimagnetic resonance lines. At low temperatures, saturation magnetizations increase according to the Brillouin-Weiss function behavior that is characteristic of most magnetic materials. Increased magnetocrystalline anisotropy energies will produce greater coercive fields that will lead directly to higher hysteresis loop switching energies. Other parameters that are influenced adversely by reduced temperatures are the magnetostriction constants, which can cause significant deterioration in the stability of the remanence ratios of hysteresis loops. To modify room temperature ferrites for devices that use high-T_c superconductors, magnetochemistry must be applied to obtain the desired parameter values, which in turn must be chosen through design tradeoffs and are set by the performance requirements of particular device applications.