| Popis: |
Paramagnetic resonance (p.r.) at low temperatures has been investigated in a number of substances in the solid state. Paramagnetic ions in the solid state are subjected to strong crystalline electric fields which profoundly modify the magnetic properties of the free ion. The orbital momentum of the free ion is completely or partially "quenched", leaving the spin momentum unaffected, except to a small extent due to spin-orbit coupling. The ground state consists of a number of levels, generally singlets or doublets. The separations of these levels may be varied by the application of an external magnetic field. If two levels have energies W1 and W2 and an external r.f. magnetic field, of frequency v such that hv = W1 - W2, is applied, transitions can be excited between the levels, within the limitations of certain selection rules. As the population of the two levels depends upon the Boltzman distribution there is a nett absorption of energy from the r.f. field when transitions occur. If this energy is lost to the crystal lattice quickly enough it is not reradiated. In practice a fixed frequency is chosen and the separation of the energy levels is adjusted by applying an external magnetic field so that the above condition is satisfied. By studying the fields at which the absorption occurs the position of the energy levels in zero field can be inferred. The energies of the levels may be expressed concisely in the form of a "spin hamiltonlan" which involves a number of parameters which have to be found experimentally. |
