Some Applications of Spin Precession Methods to Problems in Materials Science
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SOME APPLICATIONS OF SPIN PRECESSION METHODS TO PROBLEMS IN MATERIALS SCIENCE
E. N. KAUFMANN Bell Laboratories, Murray Hill, New Jersey 07974 USA
ABSTRACT Nuclear and electron resonance and the Mdssbauer effect are techniques which observe the interaction of moments with fields directly in the energy domain. An energy splitting, however, also implies the precession of the moment in the field. When a means exists to determine the orientation of the moment then the precession can be observed in the time domain. The direction of radiation emitted in a nuclear, muonic or atomic decay is correlated to the direction of corresponding moments (spins) and can thus act as a detector of spin precession. In the language of nuclear, muonic and atomic physics, these methods are called perturbed angular correlations (PAC), muonic spin rotation, and quantum beats, respectively. Below, these methods will be illustrated by displaying some examples of the application of perturbed angular correlations to a variety of materials systems.
INTRODUCTION The basis of all spin precession methods [11 is the detection of nuclear radiation emitted by a decaying nucleus or in the case of ItSR a decaying muon whose spin is not randomly oriented with respect to the laboratory frame. A classical analogy can be drawn to the radiation pattern generated by a simple antenna. Antennae of various shapes (dipole, quadrupole, etc.) will radiate in a nonisotropic fashion. The nonisotropic radiation pattern is, naturally, fixed to the orientation of the antenna itself. In a similar fashion the radiation pattern from a decaying nucleus whose spin is not isotropically oriented will be anisotropic and will be oriented in space according to the orientation of the nuclear spin. The radiation direction is, of course, in the quantum case coupled to the nuclear spin because of the conservation of angular momentum. Spin precession methods are therefore based on the production in the first instance of a nonisotropic distribution of directions of nuclear spins followed by the detection of emitted radiation. An anisotropiz spin distribution can be achieved in several ways. For example, a nucleus which is cooled to temperatures of a few milliKelvins in the presence of an external field (either an electric field gradient or magnetic field) will minimize its interaction energy with the field by orienting its moment (and therefore its spin) with respect to the field. This is known as the nuclear orientation method [11. The anisotropy of subsequent decay radiation indicates the degree to which the nucleus was oriented and therefore the strength of the interaction, i.e. a product of moment and field. This is not a spin precession method but is an energy-based method. It is the energy splitting of the nuclear sublevels by the field which causes the orientation to occur. Thus, although the anisotropic nuclear radiation is used as a detection device this method properly belongs in the category of other energy methods, such as nuclear resonance or the Mossbauer effect. Nu
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