Response of the Mössbauer spectrum of paramagneticFe3+inAl2O3to nuclear dipole fields

Abstract

Experimental $^{57}\mathrm{Fe}$ M\"ossbauer (ME) spectra of ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$(${\mathrm{Fe}}^{3+}$) single crystals are presented. Measurements were made down to 60 mK and in applied magnetic fields from 0 to 100 G At 60 mK the ME spectra of the ${S}_{z}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}$ ionic ground state was isolated, and its response to weak (less than 100 G) applied magnetic fields was examined. A qualitative discussion of the anisotropic response to magnetic fields is given. In zero-applied magnetic field the spectrum is strongly influenced by nuclear dipole fields from surrounding $^{27}\mathrm{Al}$ nuclei. From a theoretical reconstruction of the zero-applied-field spectrum the average magnitude of the $^{27}\mathrm{Al}$ nuclear dipole field is determined to be 4.5 \ifmmode\pm\else\textpm\fi{} 1.5 G. Good agreement is obtained with a theoretical calculation which involves the identical lattice sum which occurs in the Van Vleck formula for the second moment of the nuclear dipole broadening of an NMR line. The connection between the dipole field observed in the ME experiment and the second-moment calculation of the broadening of an NMR line is discussed. Also considered are possible relaxation mechanisms. The spin-lattice relaxation rates are estimated using experimental data for the lattice thermal conductivity and application of the fluctuation-dissipation theorem to the reversible heat flow. It is concluded that at 60 mK relaxation effects are not observed in the ME spectra and also that broadening due to ${\mathrm{Fe}}^{3+}$-${\mathrm{Fe}}^{3+}$ interaction is negligible compared to the nuclear dipole broadening, except for a broad single-line contribution in the center of the spectrum.