New directions in low temperature nuclear orientation

The basis of the low temperature nuclear orientation technique is summarized. The present limitations on accessible temperatures and the orders of magnitude of hyperfine interactions in the metallic systems currently studied are discussed briefly. The broad applicability to many elements and the high sensitivity of this singles counting method are emphasized. Specific recent developments are discussed in more detail. The use of a dilution refrigerator to cool to ≈ 10mK nuclei of isotopes far from stability ‘on-line’, after production in an accelerator and electromagnetic selection, is a major extension of the method. The minimum half-life is now limited by the nuclear spinlattice relaxation time, typically of order 10–100 s at 10 mK. Aspects of these experiments are considered and recent results given for Cs and I isotopes. Secondly, the extension of the related technique of nuclear magnetic resonance or oriented nuclei (NMR/ON) to antiferromagnetic insulators is described. A new cooling mechanism involving nuclear-magnon coupling gives access to much lower temperatures than previously reached in these systems. Recent precision work on MnCl₂, 4H₂O is discussed, along with its possible extension to nuclei of lanthanide elements. Finally, the use of nuclear orientation to study ordering below 10 mK of enhanced nuclear moment systems is briefly surveyed, with HoVO₄ as the test case. NMR/ON experiments at high pressure are proposed.