Induced-moment spin-glass system: Nonstoichiometric PrP

Stoichiometric praesodymium phosphide, PrP, has a crystalline electric field (CEF) singlet ground state and exhibits temperature-independent Van Vleck susceptibility at low temperatures. PrP does not show magnetic ordering down to 1 K. In contrast, phosphorous-deficient samples ($\mathrm{Pr}{\mathrm{P}}_y$) show temperature-dependent susceptibilities at low temperatures, indicating an induced magnetic moment on the Pr sites. Selected samples of $\mathrm{Pr}{\mathrm{P}}_y$ for $0.95>~y>~0.85$ show spin-glass behavior. Measurements of the magnetic properties (some of which were performed in high hydrostatic pressures to 8 kbar) include: (1) high-field magnetization versus field to 60 kOe, (2) low-field magnetization versus temperature for $1.4<~T<~20$ K, and (3) ac susceptibility versus temperature over a very wide frequency range, $0.003<~f<~{10}^4$ Hz. The ac susceptibility measurements for Pr${\mathrm{P}}_{0.95}$ show that for $f<\mathrm{}1\mathrm{}$ Hz, the spin-glass transition temperature, $T_f\simeq 9$ K, is independent of $f$, and with hydrostatic pressure $T_f$ increases as $\frac{d{T}_f}{\mathrm{dP}}=+0.12\mathrm{}$ K/kbar. Time-dependent remanent effects are observed for $T<~T_f$. No specific-heat anomaly is discernible near $T_f$. Pressure-dependent magnetization measurements to 8 kbar show that the magnetic moments and/or interactions between moments increase with decreasing lattice constant, consistent with pressure studies on stoichiometric PrP. The magnetic properties of non-stoichiometric $\mathrm{Pr}{\mathrm{P}}_y$ depend strongly on lattice constant and method of preparation. Susceptibility data on Pr${\mathrm{S}}_{0.85}$ and ${\mathrm{Pr}}_{0.1}$ ${\mathrm{La}}_{0.9}$ ${\mathrm{P}}_{0.85}$, which do not show spin-glass behavior, illustrate the influence of anion vacancies on the magnetic properties. The CEF level scheme is drastically altered by the symmetry changes produced by P vacancies, but it is believed that increased conduction-electron concentration plays an important role in inducing the Pr moments in nonstoichiometric PrP compounds.