R. Levin

Relaxation and exchange in a nuclear cooling agent: NMR and EPR in single crystal PrNi5☆☆☆

Abstract NMR of Pr and ESR of Gd in single crystals of PrNi5 are observed to exhibit a dramatic increase of the resonance linewidth with temperature in the range 1.2K ⩽ T ⩽ 10K. The observations are interpreted, within the framework of a crystal field spin Hamiltonian model calculation, in terms of low frequency fluctuations of the Pr ions induced by Pr-Pr exchange coupling. A fitting procedure of the theory to the experimental data enabled us to extract the various Pr-Pr and Pr-Gd exchange parameters. The former yield independent information concerning the lowest temperature that can be achieved in nuclear cooling experiments.

On the origin of the exchange interaction between rare-earth ions in Van Vleck pnictides☆

Abstract The exchange interaction parameters between the Gd ions and the host rare earth ions in the Van Vleck nitrides PrN and TmN are positive and opposite in sign to those previously observed in the heavy pnictides.

ESR of non S state ions in metallic Van Vleck pnictides

The ESR study is reported of Er in PrBi, TmBi and LaBi; Dy in TmBi and PrSb; and Yb in PrSb. The Er ions in these compounds exhibit Gamma 8(1) ground state crystalline field splitting the ground state of Dy and Yb is believed to be a Gamma 6. The resonance properties of the Er in PrBi and TmBi are interpreted in terms of Bleaneys spin Hamiltonian generalized to include the exchange Hamiltonian of Yang et al. (1976). Assuming that the Lea-Leask-Wolf parameter, x, as extracted for the nonmagnetic host LaBi:Er, is identical with those in PrBi:Er and TmBi:Er, the experimental results are consistent with a negligible anisotropic term in the Pr-Er and Tm-Er exchange interactions in these two compounds respectively. This is consistent with the conclusions of Rettori et al. (1975) for PrSb:Er but does not agree with experimental results and theoretical predictions in dilute alloys which indicate significant anisotropic exchange.

The crystalline electric field in some non-magnetic dialuminides compounds

Abstract Magnetization measurements performed in powdered LuAl 2 : RE ( RE = Dy, Er) and single crystals of YAl 2 :Er and LaAl 2 :Nd are reported. The results yield information concerning the levels scheme of the paramagnetic impurities in these compounds. The results in YAl 2 :Er are discussed in view of recent measurements.

Electron spin resonance study of Gd in the nuclear cooling agent: PrNi5 single crystals

The ESR of Gd in single crystals of PrNi5 is observed to exhibit significant angular dependence of the resonance position and linewidth at low temperatures. This is interpreted in terms of the axial spin Hamiltonian which takes the anisotropic susceptibility and the Gd-Pr exchange into consideration. A lineshape analysis enables the authors to derive the axial crystal field parameter D=165+or-30 G and an isotropic Gd-Pr exchange of 1.0+or-0.2 meV. The Gd ESR linewidth exhibits a dramatic increase with increasing temperature; the thermal broadening is angularly dependent. This dramatic increase of the linewidth versus temperature is similar to that observed by others for the Pr NMR in PrNi5 single crystals. Both the NMR and the ESR thermal broadenings are attributed to low-frequency fluctuations of the Pr ions induced by the Pr-Pr exchange coupling. A model, originally developed to explain the ionic relaxation rate in cubic Van-Vleck compounds, is adapted to the present case of hexagonal Van-Vleck compounds.

Relaxation phenomena and host exchange parameters in Tm van Vleck compounds

The ESR linewidth of Gd in TmP (Sugawara et al. 1975) TmSb and TmBi (measured in the present work and by Davidov and Baberschke 1975) exhibits an appreciable temperature dependence. This behaviour is attributed to the fluctuation spectra of the host Tm ions. A previous theory (Davidov et al. 1977) for impurity relaxation in weakly coupled van Vleck paramagnets based on the Bloch-Redfield kinetic equation is extended and applied to the interpretation of the ESR linewidth in the Tm pnictides. In particular the second moment calculation of the host fluctuation spectra has been extended to include both pair correlation and autocorrelation contributions. Explicit expressions are given for Tm and Pr cubic van Vleck compounds. The Tm-Tm host exchange has been estimated by fitting theory to the experimental results. The host exchange parameter in TmSb is very small confirming previous studies on this compound.

On the anisotropy in the exchange field experienced by Er ions in the metallic cubic compounds PrSb and TmSb

The electron spin resonance spectra of Er in PrSb and TmSb single crystals exhibit two lines with appreciable angular dependent resonance fields. The data is analysed in terms of a Lambda 8(1) crystalline field ground state together with a generalized exchange interaction between the Er ions and the host Pr (or Tm) rare earth ions. Using the crystalline field parameters as determined for the analogue compounds YSb:Er and LaSb:Er, the exchange parameters for PrSb:Er were found. Unlike previous predictions, no evidence for a significant orbital anisotropy in the exchange was observed.

Relaxation narrowing of Gd in cubic Van-Vleck compound

Abstract We present the first observation of fine structure relaxation narrowing of Gd in single crystals of cubic Van-Vleck compound: PrSb. A lineshape analysis provides evidence that the narrowing effect is mainly associated with Gd relaxation due to its coupling with low frequency Pr fluctuations induced by the Pr–Pr exchange coupling.

Paramagnetic fluctuations in cubic Van-Vleck pnictides

Abstract The lineshape of the paramagnetic fluctuation spectra in cubic Van-Vleck compounds is discussed in conjunction with the relaxation of an impurity or that of the host nucleus in these systems.

ESR of Impurity as a Probe for the Host Exchange Interaction in Tm Van-Vleck Compounds

The relaxation rate of an impurity in Tm Van-Vleck compounds with cubic structure is given for two limiting cases. Analysis of the ESR linewidth of Gd in TmP, TmSb and TmBi in terms of this theory yields information concerning the nature of the host fluctuation spectra and the host exchange parameters.