Magnetic structure observed in an applied field in reentrant ${\mathrm{Au}}_{0.81}$${\mathrm{Fe}}_{0.19}$ and ${\mathrm{Ni}}_{0.81}$${\mathrm{Mn}}_{0.19}$ single crystals

Spin correlations have been studied in two reentrant single crystals, ${\mathrm{Au}}_{0.81}$ ${\mathrm{Fe}}_{0.19}$ and ${\mathrm{Ni}}_{0.81}$ ${\mathrm{Mn}}_{0.19}$, by small-angle neutron scattering at low temperature (11 K) and in applied fields in the range of 1.5 to 8 kOe. They reveal the existence of a magnetic structure, in all q directions, which can be attributed mainly to transverse spin components. These results, which are insensitive to the crystal orientation, corroborate previous observations in polycrystalline ${\mathrm{Ni}}_{0.78}$ ${\mathrm{Mn}}_{0.22}$ and amorphous (${\mathrm{Fe}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$ ${)}_{75}$ ${\mathrm{P}}_{16}$ ${\mathrm{B}}_6$ ${\mathrm{Al}}_3$ systems. The overall similarities found in these three systems, which have very different nuclear structure or a different kind of chemical short-range order, is very remarkable. Measurements of the elastic scattering contribution carried out in the 11 to 200 K temperature range at constant field on ${\mathrm{Ni}}_{0.81}$ ${\mathrm{Mn}}_{0.19}$ reveal that the structure disappears well below $T_c$ at a temperature which could be assigned to the canting transition of mean-field theory.