MAGNETIC PROPERTIES AND COLLECTIVE EXCITATIONS OF SYSTEMS WITH SINGLET CRYSTAL-FIELD GROUND STATE

We discuss the magnetic behavior of rare-earth systems where the crystal-field-only ground state of the rare-earth ion is a singlet. For such materials there is a threshold value for the ratio of exchange to crystal-field interaction necessary for magnetic ordering even at zero temperature. The magnetization process for singlet ground-state systems is first discussed from the effective field point of view. We then treat the theory of the collective excitations and the relationship of their behavior to the magnetic ordering process. A discussion is given of the present experimental situation and of a variety of promising possibilities for future experiments to increase understanding of such systems. The techniques discussed are neutron diffraction and scattering, susceptibility, high-field magnetization, nuclear magnetic resonance, specific heat, and paramagnetic resonance experiments. The discussion of the experimental situation emphasizes the role of the rare-earth - Group V compounds. The system TbxY1-xSb illustrates the behavior obtained when, by alloying, one increases the exchange beyond the critical value necessary for magnetic ordering. © 1969 The American Institute of Physics.