Testing metgals cores....

3M Tattle Tape B1 performs best, just one metglas strip, about 1 inch long, dynamic range over +-0.5Gauss (Earths Field) is 199Khz. Difficult to get a hold of, but libraries are sources.

Sensormatic EM Tags perform reasonable well and would be a second choice. 2 strips are needed, dynamic range over +-0.5Gauss is 99.3KHz, with 1 strip, 41KHz, and with 1 strip the same size as B1, 22KHz.