DYNAMIC NUCLEAR ORIENTATION AND SPIN-LATTICE RELAXATION OF Co$sup 5$$sup 9$
A study was made of dynamic nuclear orientation of (stable) Co/sup 59/ in double nitrates. Both Jeffries and Overhauser processes were investigated in an effort to understand the nature of the various spin-lattice relaxation processes within a resolved hyperfine structure which can lead to nuclear orientation. These processes are reviewed for the case of an effective spin 1/ 2ion with resolved hyperfine structure, and the effects of competing hyperfine relaxation are discussed. Since orientation experiments with radioactive isotopes generally do not give the nuclear substate populations directly, a double frequency microwave method was used in which these populations can be measured directly. Populations were measured with a paramagnetic resonance spectrometer (operated at low power) which determined the relative intensities of the hyperfine lines before and after orientation was produced with a separate microwave pump system. The ratio of hyperfine relaxation rates for DELTA m == O and DELTA m = plus or minus 2 processes was determined from measurements of the Overhauser effect, while relative speeds of the allowed electronic and the hyperfine relaxation rates could be determined from the saturation effects which accompanied Jeffries orientation. The direct electronic relaxation rates ( DELTA m /sub S/ = plus or minus 1, DELTA m/sub I/ = O) were measured by the pulsed microwave method. The measurements were made at 1.2 deg K, with cobalt fractional concentrations of from 0.5 to 0.005%. A phenomenological model for the results is presented, in which spin-lattice relaxation is described by a fluctuating g- and A-tensor, both of which are random time functions. Relaxation rates are given in terms of spectral densities of the various components of the tensors. This model fits the data well, and the resulting spectral densities can be related to the phonon spectrum in the crystal through a suitable model for the spin-phonon interaction. The values obtained in this case imply that low- symmetry distortions of the cobalt complex predominate, while purely trigonal distortions are far less probable. The complexity of the observed relaxation shows that it will generally be impossible to interpret the results of a dynamic orientation experiment unless the various relaxation processes involved are previously measured experimentally. In particular, the assumption of a dominant direct relaxation and vanishing DELTA m = plus or minus 2 hyperfine relaxation cannot be relied upon. (Dissertation Abstr.)
- Research Organization:
- Originating Research Org. not identified
- NSA Number:
- NSA-17-037890
- OSTI ID:
- 4658744
- Resource Relation:
- Other Information: Thesis. Orig. Receipt Date: 31-DEC-63
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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