Evidence for direct order-disorder transition inNi73Fe27

Using Mössbauer-effect spectroscopy, we followed the order-disorder transition in ${\mathrm{Ni}}_{73}$ ${\mathrm{Fe}}_{27}$ "at temperature." We present direct experimental evidence that at this composition no two-phase region exists, in accordance with the Gibbs theorem, but contrary to recent theoretical and experimental work. In addition, the time dependence of the hyperfine field during ordering is studied after different heat treatments.