Low temperature magnetic cores and a preamp for low impedance cryogenic sources

The permeabilities at 10 KHz of a number of commercial magnetic materials have been measured at 293, 77, and 4.2 K. Supermalloy and Metglas 2826 (a metallic glass) were measured from 100 Hz to 5 MHz at these temperatures, and the complex permeability vs frequency of Matglas at 4.2 K is presented. These measurements show that Metglas has very little temperature variation in its permeability, but possesses slightly less permeability at 4.2 K than Supermalloy, at all frequencies, in spite of annealing and careful handling. Using Metglas, a matching transformer was fabricated to couple low impedance, cryogenic sources to a low noise JFET. This resulted in a 5-dB noise figure from 20 to 60 KHz for a 10-Ω source at 4.2 K.