OPTIMIZATION OF A SQUID FLUX TRANSFORMER COUPLED TO FERROMAGNETIC CORES FOR REGISTERING THE MAGNETIC-MOMENT OF A 2-DIMENSIONAL SAMPLE

A nonconductive ferromagnetic core is inserted into the pickup coil of a superconducting flux transformer which is matched to a 19 MHz rf-su perconducting quantum interference device (SQUID). We demonstrate that the flux sensitivity of the SQUID is enhanced by a factor of 2-2.7 wi th respect to a comparable air core flux transformer for signals up to 80 kHz. The equivalent flux noise of 2 x 10(-3) PHI0/square-root Hz i s of the same magnitude as for a conventional transformer and associat ed With external fluctuations from vibrations and from the superconduc ting shields. We take as a source a very thin spin sheet like that whi ch is formed by spin-polarized electrons excited in thin semiconductin g epitaxial layers in a photomagnetization experiment. Under experimen tal conditions (changes of the irradiated area, absorption depth, beam deflections) one expects distinct distributions of spins in the sheet and corresponding variations of the flux depicted by the flux transfo rmer. To detect the magnetic moment of a certain number (>10(10)) of s pins the proportionality between the detected flux PHI and the excited total magnetic moment m of the spins is checked. We compare various c onfigurations of spin sheets of distinct size and locations beginning with an air core flux transformer. The flux threading the pickup loop is then more efficiently coupled by employing a ferromagnetic shell-sh aped core with an axial hole (for the entrance of the light beam in ph otomagnetization studies). Thus flux line patterns are simulated for v arious sizes of cores, different permeabilities, and different air sli ts (where the sample is located). Due to additional degrees of freedom of design parameters a simultaneous increase of sensitivity is achiev ed together with a response PHI is-proportional-to m. Simulations and measurements are compared. The ultimate resolution of the magnetic mom ent is DELTAm=10(-13) A m2/square-root Hz.