Four sets of NMR signals, two each, from ⁵⁵Mn and ⁷⁵As nuclei have been observed. The temperature dependences of ⁵⁵Mn resonances have been studied from 77 to 311 K and that of ⁷⁵As, from 77 K to about 250 K. The results show that there is a phase transition at T₁ ≈ 220 K. This transition may be due to introduction of a local spontaneous distortion in the region of the domain walls in the lattice, resulting in lowering of symmetry at low temperatures. Another possibility is the canting of spins which would lower the magnetic group symmetry. The observed resonances have been assigned to arise from the nuclei at the edge and the centre of the domain walls at temperatures T >T₁ and from two types of wall edges with inequivalent orientation of atomic spins at T < T₁. The isotropic hyperfine field at 0 K obtained by extrapolating the resonance frequencies are 227 and 285.1 kOe at ⁵⁵As nuclei, respectively. The anisotropy in the hyperfine field is nearly zero at ⁵⁵Mn nuclei and about 5.8 kOe at ⁷⁵As nuclei at 0 K.