| Summary: | A practical method of nuclear spin cooling in the mK region, based on dramatic reduction in the spin-lattice relaxation time T1 at the antiferromagnetic-spin flop and spin flopparamagnetic phase transitions, is described. Far lower temperatures (down to 13 mK) than have been obtained to date (∼ 90 mK) are achieved in reasonable times, greatly increasing the sensitivity of NMRON. Following resonance, active isotope spins can be rapidly reset (in times ∼ 1 second) to thermal equilibrium. For54Mn in MnCl2.4H2O, hyperfine parameters A=-203.75 MHz, P=0.155 MHz and the exchange/ crystal field parameter combination Bex + 4|D|/gμB=1.355 T have been determined from good agreement between perturbation theory to second order and detailed NMRON results in applied fields parallel to the easy axis in the range 0-0.7 T. Splitting of observed lines in zero applied field (- 3.05 MHz) is mainly due to the pseudo-quadrupole (second order magnetic) interaction. Small third order effects (∼0.05 MHz) have been resolved. © 1983 J.C. Baltzer Scientific Publishing Company.
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