Singlet ground-state fluctuations in praseodymium observed by muon spin relaxation in PrP andPrP0.9

Abstract

Muon spin relaxation (µSR) in the singlet ground-state compounds PrP andPrP0.9 reveals the unusual situation of a Lorentzian local field distributionwith fast-fluctuation-limit strong-collision dynamics, a case that does notshow motional narrowing. Contrary to publications by others, wherePrP0.9 was asserted to have vacancy-induced spin-glass freezing, no spin-glass freezing is seen inPrP0.9 or PrP down to . This was confirmed by magnetization measurements on these same samples. In bothcompounds, the muon spin relaxation rate does increase as temperature decreases,demonstrating increasing strength of the paramagnetic response. A Monte Carlomodel of fluctuations of Pr ions out of their crystalline-electric-field singlet groundstates into their magnetic excited states (and back down again) produces thestrong-collision-dynamic Lorentzian relaxation functions observed at each individualtemperature but not the observed temperature dependence. This model contains noexchange interaction, and so predicts decreasing paramagnetic response as the temperaturedecreases, contrary to the temperature dependence observed. Comparison of thesimulations to the data suggests that the exchange interaction is causing the system toapproach magnetic freezing (by mode softening), but fails to complete the process.