Proton Polarizations in a Gas-Turbine Spin Refrigerator Using (Yb, Y) ${\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{S}{\mathrm{O}}_4\right)}_3$ · 9${\mathrm{H}}_2$O

A proton spin refrigerator using a high-speed rotating sample has been used to obtain proton polarizations as high as 35% in polycrystalline (Yb, Y) ${\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{S}{\mathrm{O}}_4\right)}_3$ · 9${\mathrm{H}}_2$O (abbreviated YES: Yb). The sample is rotated at speeds up to 1000 revolutions per second (rps) and is suspended over pumped liquid He at $\approx 1$ K in a dc transverse magnetic field of 10 to 15 kOe. Experimental measurements of steady-state proton polarizations and polarization buildup rates show that the predominant cross-relaxation mechanism of the proton spins and Yb spins in YES: Yb, at sample rotational frequencies greater than about 30 rps and for magnetic fields of about 10 kOe, is a two-spin process in which one proton spin and one Yb spin flip, conserving total Zeeman energy. The significance of this result is that the proton polarization can become as high as the electron polarization using YES: Yb in a spin refrigerator. Failure to achieve higher proton polarizations in these experiments is attributed to a rise in the sample temperature above 1 K due to nonresonant relaxation heating as the sample is rotated in the magnetic field. Analysis of the energy flow into the electron-spin system allows calculation of the crystal heating in a rotating-sample or rotating-field spin refrigerator. Experimental measurements of this heating are consistent with the theory. Optimum experimental conditions needed to achieve high proton polarizations are analyzed, taking into account both cross-relaxation probabilities and nonresonant heating. Polarizations of 40-60% are predicted for a polycrystalline YES: Yb sample rotating at 100 rps in a magnetic field of 12 kOe at temperatures of 1-1.5 K. Higher polarizations will probably require pulsed fields and lower temperatures.