Induced Moment Spin Glass System: Non-Stoichiometric Praseodymium Phosphide.

The magnetic properties of the spin glass system, non-stoichiometric praseodymium phosphide (PrP(,y), y(, )<(, )1) are investigated. PrP(,y) is determined to be a new type of spin glass in which the absence of one type of atom (phosphorous) leads to an induced moment on the praseodymium ion. The PrP(,y) system was studied with up to 15% vacancies of phosphorous atoms. The results of magnetization and susceptibility measurements, both ac and dc, are presented. These were performed in fields up to 6 Tesla and with applied hydrostatic pressures up to 10 kbar. The spin glass features of PrP(,y) were further investigated using a new low frequency ac technique; the frequency dependence of the freezing temperature, T(,f), being measured from audio frequencies down to 10('-3) Hz. For the first time, T(,f) was found to remain frequency independent at sufficiently low frequencies. The results of neutron scattering measurements on the system are also presented. These latter results include a comparison of the crystalline electric field (CEF) level structure in stoichiometric and non-stoichiometric samples. The magnetization and neutron scattering results, taken together, show that the spin glass behavior of PrP(,y) is due to the presence of low lying CEF levels. These levels arise when vacancies are created and the cubic symmetry of PrP(,1.00) is broken, and coupled with the increased conduction electron concentration in PrP(,y) they facilitate the formation of "induced moments" on some Pr sites. These are located randomly throughout the crystal; thus an induced moment spin glass state is achieved. A series of model calculations are presented which satisfactorily explain the new levels observed in PrP(,y). These calculations strongly suggest that the second order term in the CEF Hamiltonian is heavily shielded. Finally, the variation of T(,f) with applied pressure has been measured and analyzed using a theory of D. Sherrington.