Mössbauer Study of Hyperfine Fields and Isomer Shifts in the Fe-Rh Alloys

Hyperfine fields and isomer shifts of ${\mathrm{Fe}}^{57}$ in the Fe-Rh system have been investigated by the Mössbauer technique in the composition range between 0 and 52 at.% Rh. When the Rh concentration exceeds 20%, chemical ordering of CsCl type develops and two distinct hyperfine fields are observed, corresponding to Fe on Fe sites (FeI) and Fe on Rh sites (FeII). For example, $H_i\left(\mathrm{I}\right)=277\mathrm{}$ kOe and $H_i\left(\mathrm{II}\right)=384\mathrm{}$ kOe in an alloy containing 48% Rh. Neutron diffraction study of this alloy gave ${\mu }_{\mathrm{FeI}}=3.2\mathrm{}$ ${\mathrm{\mu }}_{\mathit{B}}$, but no direct measurement is available for ${\mu }_{\mathrm{FeII}}$. It is suggested that FeII, which is surrounded by 8 Fe nearest neighbors, may have a smaller magnetic moment than FeI despite its higher hyperfine field. The alloy with 50% Rh transforms, on cooling, from ferromagnetic to antiferromagnetic at 65°C. At this phase transition the hyperfine field shows a discontinuous decrease of 14 kOe.