Saturation Effects in Paramagnetic Resonance

Abstract

Saturation effects have been observed in the paramagnetic resonance absorption of 3.2-cm microwaves by diluted samples of iron ammonium alum and chromium potassium alum at 2 to 4\ifmmode^\circ\else\textdegree\fi{}K. For all samples the spin-lattice relaxation time ${T}_{1}$ was found to vary inversely with the temperature, in agreement with theoretical expectation that the direct rather than the "Raman" process for the spin-lattice quantum transfer predominates at helium temperatures. Diluted samples of iron ammonium alum with iron to aluminum atom ratios of 1:18, 1:59, and 1:80 showed ${T}_{1}$ to be essentially independent of dilution and yielded $T{T}_{1}=5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ sec \ifmmode^\circ\else\textdegree\fi{}K for the $m=\frac{1}{2}\ensuremath{\rightarrow}\ensuremath{-}\frac{1}{2}$ transition. The low-field side peak in the iron alum spectrum for the (100) orientation, comprised of two unresolved transitions, yielded an effective value of $T{T}_{1}=1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ sec \ifmmode^\circ\else\textdegree\fi{}K. The $m=\frac{1}{2}\ensuremath{\rightarrow}\ensuremath{-}\frac{1}{2}$ transition for diluted chromium alum with a chromium to aluminum atom ratio of 1:10 yielded $T{T}_{1}=3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ sec \ifmmode^\circ\else\textdegree\fi{}K in agreement with the calculated value. In all samples the spin-spin relaxation time was found to be temperature independent and of the order of ${10}^{\ensuremath{-}9}$ sec. The results for ${T}_{1}$ are at variance with those obtained with low-frequency paramagnetic relaxation experiments, which show a faster temperature dependence than ${T}^{\ensuremath{-}1}$ and an increase in ${T}_{1}$ with dilution.