Mössbauer Effect: Applications to Magnetism

The Mössbauer effect, the resonant absorption of nuclear gamma rays in solids, may be used to obtain the hyperfine structure of ${\mathrm{Fe}}^{57}$ in magnetic materials. Experiments are performed by observing the absorption by stable ${\mathrm{Fe}}^{57}$ of the 14.4-kev gamma ray coming from a source which contains radioactive ${\mathrm{Fe}}^{57}$ produced by the decay of ${\mathrm{Co}}^{57}$ . The experiments are not limited to naturally iron-bearing materials; other substances can be studied, provided only, that small amounts of cobalt can be introduced into lattice sites of interest. The magnetic moments of the ground and first excited states of ${\mathrm{Fe}}^{57}$ are known and make possible direct determination of the field at the iron nucleus once the hyperfine structure has been measured. The magnetic field at iron nuclei has been determined in the ferromagnetic transition metals ($\mathrm{Fe}{\text{\hspace{0.17em}3.42×10}}^5\hspace{0.17em}\mathrm{oe}$ , $\mathrm{Co}{\text{\hspace{0.17em}3.12×10}}^5\hspace{0.17em}\mathrm{oe}$ , $\mathrm{Ni}{\text{\hspace{0.17em}2.80×10}}^5\hspace{0.17em}\mathrm{oe}$ at 0°K), but no hyperfine structure has been observed down to 4°K in the antiferromagnetic transition metals, Mn and Cr. In the case of yttrium-iron garnet the fields at the iron atoms in the two types of sites have been obtained (tetrahedral ${\text{3.9×10}}^5\hspace{0.17em}\mathrm{oe}$ , octahedral ${\text{4.7×10}}^5\hspace{0.17em}\mathrm{oe}$ ). The most complete analysis so far has been made in ${\mathrm{FeF}}_2$ where the magnetic field in the antiferromagnetic state ${\mathrm{(H}}_{\text{T\hspace{0.17em}=\hspace{0.17em}0}}{\text{\hspace{0.17em}=\hspace{0.17em}3.40×10}}^5\hspace{0.17em}\mathrm{oe})$ and the quadrupole splitting in the paramagnetic state (31.2 Mc/sec) have been obtained. Other materials under investigation are the iron oxides and some ferrites, where, for trivalent iron, fields in the vicinity of ${\text{5.0×10}}^5\hspace{0.17em}\mathrm{oe}$ have generally been found.