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 ∼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±0.003. The zero-field Stark splitting $2D$ was measured for AlK alum (Al:Cr=435:1), and equals 0.011 ${\mathrm{cm}}^{-1\mathrm{}}$. The hyperfine coupling constant $\mathrm{}\left|A\right|\mathrm{}$ for ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}$ was measured as (1.6±0.05)×${10}^{-3\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The paramagnetic spectra are dominated by the (-½, ½) 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 (-½, ½) resonance line is favorable for dynamic pumping of proton polarization.