A special multilayer sample Si/[Mo/Si]₄₅/⁵⁷Fe/Nb has been prepared for the depth selective investigations of the hyperfine fields in thin iron layer at low temperatures above and below the superconducting transition in the top Nb layer (T_c ∼ 8 K) by means of the nuclear resonant reflectivity with standing waves. The periodic multilayer [Mo/Si]₄₅ below the iron layer in our sample was used as "a standing wave generator". A weak magnetic hyperfine splitting in the ⁵⁷Fe layer was detected just at low temperature. A slight variation of the nuclear resonant reflectivity time spectra measured above and below T_c was observed. At first it was supposed that this change of the spectrum shape is caused by the spatial modulation of ferromagnetic domains in the ⁵⁷Fe layer caused by a proximity effect. A closer analysis, however, reveals that the spectrum variations are due to just the changes of the relative weights of the magnetic and paramagnetic phases in ⁵⁷Fe layer.