Magnetic field microprobe in superconductors carrying a current

A new technique to probe local magnetic fields inside superconductors utilizing the Mössbauer Effect (ME) is reported. The ME nucleus responds to the local magnetic field resulting in a modified hyperfine ME spectrum from which the local field and its direction may be deduced. Concentrations of a few ppm of the radioactive probe nucleus are sufficient; the allowable depth of the probe depends on the attenuation of the ME γ ray. For⁵⁷Fe (⁵⁷Co parent) in superconductors the field acts directly on the nuclear moment and the resulting sensitivity is only a few kG. For a thin probe consisting of⁵⁷Co in Pd the response is enhanced by the "giant" localized moment providing a sensitivity at 1 K of better than 100 G. Results of such a probe placed at various locations across a commercial Nb3Sn tape carrying a current are reported and compared with calculations of the field profile using several models. For thin tape geometries the usual models prove to be inadequate.