Dependence of Nuclear PolarizedFe57Mössbauer Spectra on the Magnitude and Sign of the Hyperfine Field

The Mössbauer spectra of ${\mathrm{Fe}}^{57}$, with the ${\mathrm{Co}}^{57}$ source in iron and palladium matrices, respectively, have been measured in a ${\mathrm{He}}^3$/${\mathrm{He}}^4$ dilution refrigerator. Both the magnitude and sign of the effective field at ${\mathrm{Co}}^{57}$ can be determined from nuclear polarization effects in the spectra, without recourse to an external magnetic field. Experimental data are also presented on the Kapitza thermal boundary resistance which, together with radioactive self-heating, prevents the source from cooling below 0.15°K.