Paramagnetic spin-fluctuation effects

UIr2 is paramagnetic down to 4.2 K. The susceptibility is nearly temperature-independent and has a slight maximum at 60 K (x3ook = L48 X 10 8 m3/mol). The T2 dependence of the resistivity at low temperatures together with the relatively high y value [62 mJ/mol K2 (Brodsky et al. 1976)] are indicative of spin-fluctuation effects. 

UCoAl is a paramagnet with spin-fluctuation effects at low temperatures (Sechovsky et al. 1986a,b). 

The main equation for the d-electron GF in PAM coincides with the equation for the Hubbard model if the hopping matrix elements t, ) in the Hubbard model are replaced by the effective ones Athat are V2 and depend on frequency. By iteration of this equation with respect to Aij(u>) one can construct a perturbation theory near the atomic limit. A singular term in the expansions, describing the interaction of d-electrons with spin fluctuations, was found. This term leads to a resonance peak near the Fermi-level with a width of the order of the Kondo temperature. The dynamical spin susceptibility in the paramagnetic phase in the hydrodynamic limit was also calculated. 

We applied the generating functional approach to the periodic Anderson model. Calculation of the electron GFs gdd, 9ds, 9sd and gss reduces to calculation of only the d-electron GF. For this, an exact matrix equation was derived with the variational derivatives. Iterations with respect to the effective matrix element Aij(to) allow to construct a perturbation theory near the atomic limit. Along with the self-energy, the terminal part of the GF Q is very important. The first order correction for it describes the interaction of d-electrons with spin fluctuations. In the paramagnetic phase this term contains a logarithmic singularity near the Fermi-level and thus produces a Kondo-like resonance peak in the d-electron density of states. The spin susceptibility of d-electrons... 

Whereas the paramagnetic shift of the nuclear magnetic resonance frequency for a given applied field is related to the strength of the local hyperfine field at the nuclear site, induced by the electronic moments, the nuclear spin-lattice relaxation rate yields information about the low-frequency spectrum of thermally induced spin fluctuations. The influence of pair-correlation effects on the NMR relaxation in paramagnets was analysed experimentally and theoretically by... 

In paramagnetic solids, an important consideration arises from the fact that the correlation time Tq of the ionic spin fluctuations is shorter than the characteristic nuclear precession time t in the effective hyperfine field of (18.9). Thus, Tc < T, or Tc > ASIh. Under this condition, the nuclear moment senses only the average hyperfine field which is proportional to the average value of the spin. 

In conclusion, the molecular motion seems to be well described, and the decomposition of the electron spin dynamics from the dipole-dipole interaction is a good approximation. However, the calculated electron spin relaxation was too slow to account for the paramagnetic relaxation, either because the ZFS was too small in magnitude or fluctuating too fast. The reorientation of the water could have a large effect on the ZFS, but unfortunately this was not included in the treatment due to the problems with describing it from symmetry modes. Also, non-linear terms in the property surface might be of importance for a proper description of the ZFS fluctuations. 

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