Paramagnetic resonance spectra of some ferric alums, and the nuclear magnetic moment of ⁵⁷ Fe

The paramagnetic resonance spectra of ferric rubidium sulphate and ferric potassium selenate alums (diluted with the corresponding aluminium alums) have been analyzed in detail. It is found that the splitting of the $^{6}$ S state of the ferric ion can be explained in terms of a spin Hamiltonian containing trigonal terms of the second and fourth degree, and a cubic term. The measurements show also that the axes of the cubic crystalline field are rotated through a small angle from those of the crystal unit cell, as expected from X-ray analysis of the structure (Lipson 1935). No hyperfine structure due to $^{57}$Fe could be resolved in a sample containing 40% abundance of this isotope. From analysis of the line shape, an upper limit of 0$\cdot $0006 cm$^{-1}$ could be placed on the overall width of any such structure, corresponding to a nuclear moment of 0$\cdot $05 n.m.