Investigation of ferrimagnetic materials at low temperatures and some applications in low-noise receivers

The microwave absorption of polycrystalline ferrimagnetic garnets has been measured at room temperature, at 77°K and at 4.2°K. Undoped and aluminium doped ytirium iron garnets (YIG, YIG-Al), mixed gadolinium-yttrium iron garnets (Gd-YIG), and calcium-vanadium-bismuth garnet (CVB) have been investigated. At a fixed frequency of 4.2 Gc/s the low-field losses and the resonance loss of disk-shaped samples were displayed as a function of the magnetic field using a magic T bridge. In all measurements on disks an unsymmetrical shape of the resonance curve or additional absorption peaks were observed. At room temperature a series of magnetostatic modes became excited ; at low temperatures anisotropy effects are believed to cause the disturbed line shape. For undoped YIG the absorption curve was similar, but it was broader at low temperatures. CVB turned out to have the smallest low-field losses of all materials investigated. These measurements have been performed in order to test and compare material properties essential in cryogenic temperature ferrimagnetic devices. The performance of resonance isolators for ruby traveling-wave masers using undoped YIG and of Y-junction circulators operating at cryogenic temperatures is reported. CVB has been found to be especially well fitted for below resonance circulators because of its small low-field losses.