DiluteCr3+Paramagnetic Resonance in AlK Alum and AlNH4Alum

Abstract

Single-crystal ${\mathrm{Cr}}^{3+}$ paramagnetic-resonance spectra have been recorded at 1.3 K and \ensuremath{\sim}19 kG for four sulfate alum crystals, in which the Al:Cr ratio varied from 215:1 to 590:1. The $g$ value of the ${\mathrm{Cr}}^{3+}$ ions is independent of crystal orientation, and equals 1.977\ifmmode\pm\else\textpm\fi{}0.003. The zero-field Stark splitting $2D$ was measured for AlK alum (Al:Cr=435:1), and equals 0.011 ${\mathrm{cm}}^{\ensuremath{-}1}$. The hyperfine coupling constant $|A|$ for $^{53}\mathrm{Cr}$ was measured as (1.6\ifmmode\pm\else\textpm\fi{}0.05)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The paramagnetic spectra are dominated by the (-\textonehalf{}, \textonehalf{}) transition, whose linewidth varied from 7 to 22 G. Spin-lattice relaxation is not strongly phonon bottlenecked, and the relaxation time varied from 2 to 3 msec at 0.95 K. With {111} planes perpendicular to the magnetic field, the (-\textonehalf{}, \textonehalf{}) resonance line is favorable for dynamic pumping of proton polarization.