Spin fluctuations, effect

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. 

UPt3 belongs to the most exotic U-compounds because of the co-existence of superconductivity and pronounced spin-fluctuation effects at low temperatures. Due to very high y value ( = 400 mJ/mol K2) it can be classified as a heavy-fermion... 

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

Although such an anomalous magnetic low-temperature behaviour, as observed in URuAl and URuGa, is frequently associated with spin-fluctuation effects, other possible explanation cannot be omitted. Possible explanation in terms of band effects may be considered since no anomaly was found in the temperature dependence of the specific heat (no T3 In T term) and the y-values are 45 and 52 mJ/mol K2, respectively. By contrast the resistivity behaviour supports the first interpretation (Havela et al. 1985a, Sechovsky et al. 1986b). 

Magnetic Properties at Finite Temperatures Spin-Fluctuation Effects... 

We now turn to a discussion of the Neel temperature. Our estimated Stoner temperature for UCuj is 1000 K. Given the experimental value of 15K, this implies that Tn is totally controlled by spin fluctuation effects, as observed for a number of transition metal magnets. This prompted us to calculate using spin fluctuation theory (Lonzarich 1986, Moriya 1987). For antiferromagnets, this formalism has been derived by Nakayama and Moriya (1987). The only change we make is to replace the assumed form of the susceptibility of Nakayama and Moriya by the Lorentzian form observed in neutron scattering. 

At low temperatures (T < T f) the spin-fluctuation effects are dominating, giving rise to the following contributions to the physical properties (see Coqblin et al. 1978) ... 

When spin-fluctuation effects are taken into account, the state can no longer be regarded as magnetically uniform. Instead of the free energy mentioned in eq. (8) for an itinerant magnet, we must now study the free energy density per unit volume A/(r), which is given as (Shimizu 1981, Yamada 1991, 1993)... 

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