Hyperfine-field measurements in a dilute Cr-Mn alloy using the perturbed-angular-correlation technique

The hyperfine-field distribution on ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$ probe nuclei diluted in a ${\mathrm{Cr}}_{1-x}{\mathrm{Mn}}_x$ alloy ($x\sim 0.8$ at.%) has been measured over the temperature range 80-500 K using the $\gamma -\gamma$ perturbed-angular-correlation technique. The transition from the commensurate phase to the incommensurate phase associated with the sudden change from a Dirac-field distribution to an Overhauser distribution has been clearly observed at $T\simeq 137$ K. Also, the average hyperfine field was found to increase sharply at the incommensurate-commensurate transition. The maximum hyperfine field on ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$ probe in Cr-Mn alloy for example at 143 K is 63.3(1.0) kOe which is considerably larger than the value of 52.6(1.0) kOe observed for the same probe in a pure-chromium matrix. Magnetic perturbations arising from Mn impurities are found to be of considerably long range and their effects are approximately estimated from the small distribution of hyperfine fields observed in the commensurate phase. The commensurate-paramagnetic transition at $T_{\mathrm{N}\mathrm{é}\mathrm{el}}=460\mathrm{}$ K does not seem to be of second order but rather an inhomogeneous transition extending over a temperature of ∼ 50 K and is characterized by the gradual growing of paramagnetic regions as the temperature increases through the Néel temperature. The average hyperfine field gradually decreases as we approach the Néel transition temperature and remains constant at about 20 kOe but the number of probe atoms experiencing no magnetic hyperfine field increases gradually through the Néel transition temperature.