Dilute Cr3+ paramagnetic resonance in AlK alum and AlNH4 alum

Single-crystal Cr3+ paramagnetic-resonance spectra have been recorded at 1.3 K and similar to 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 Cr3+ 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 cm(-1). The hyperfine coupling constant \A\ for Cr-53 was measured as (1.6 +/- 0.05) X 10(-3) cm(-1). The paramagnetic spectra are dominated by the (- 1/2, 1/2) 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 (- 1/2, 1/2) resonance line is favorable for dynamic pumping of proton polarization.