Polarization of protons by the spin-refrigerator mechanism in yttrium ethyl sulfate

Proton polarizations in excess of 50% have been achieved in single crystals of Y(C2H5SO4)3x9H2O doped with about 0.01 at.% Yb. The protons are polarized by the spin-refrigerator process, in which the crystals are rotated in a magnetic field at low temperature. Typical operating parameters are F/sub r/ = 40--90 revolutions/sec, H = 9--11 kOe, and T = 0.6 K. Measurements of the maximum polarization and of polarization-buildup curves are presented for a variety of operating conditions. These results are found to be in reasonably good agreement with predictions obtained from model calculations which simulate the polarization mechanism at the atomic level. The calculations show that multiple--spin-flip processes are important, and that these processes produce a significant reduction in the maximum attainable proton polarization. Measurements of the proton spin-relaxation time are also reported. Relaxation times in excess of 100 h were observed for magnetic fields of less than 1 kOe