Internal Fields at Low Temperatures in CoPd Alloys

The hyperfine splitting of the 14.4-kev gamma kev ray line in ${\mathrm{Fe}}^{57}$ has been measured for a series of sources, each containing ${\mathrm{Co}}^{57}$ activity doped into a host lattice of CoPd. Although Pd itself is not ferromagnetic, the alloys with Co are all ferromagnetic, with Curie temperatures ranging from 1404°K for pure Co down to 130°K for a 3% Co alloy (the most dilute which we have studied). The internal field associated with the hyperfine splitting is a function of temperature for a given alloy; however, at temperatures small compared to the Curie temperature, we find that each source shows very nearly the same internal field, namely -308 kgauss. The relationship of this behavior to current theories of the internal field in Fe and to the nature of ferromagnetism in CoPd is briefly discussed.