Itinerant magnetism

We have used the multisublattice generalization of the coherent potential approximation (CPA) in conjunction with the Linear-MufRn-Tin-Orbital (LMTO) method in the atomic sphere approximation (ASA). The LMTO-ASA is based on the work of Andersen and co-workers and the combined technique allows us to treat all phases on equal footing. To treat itinerant magnetism we have employed for the local spin density approximation (LSDA) the Vosko-Wilk-Nusair parameterization". 

From an experimental point of view, it appears that the resonant f level is the best starting hypothesis for most U, Np and Pu compounds. Only in some cases of strong hybridization (particularly for Laves phase and AuCua-type structure intermetallics) it will broaden into true bands and we shall try to give criteria for itinerant magnetism. 

At the beginning of this section we said that a localized picture is very often the best starting point for U, Np and Pu compounds. Is there any clear-cut criterion to direct us in the way of itinerant magnetism Good experimental evidence seems to be - a reduced ordered moment (in particular when it is incompatible with the effective moment in the crystal field scheme ) and - a large electronic specific heat (> 10 mJ/mol K ) and - a reduced magnetic entropy at the transition ( RLn2)... 

We present here some examples of itinerant magnetism. 

These experimental results received theoretical support from band calculation results, Brooks found that for UN and PuN the Stoner criterion was just satisfied and that these nitrides were itinerant magnetic systems. 

NpOs2 this intermetallic Laves phase is ferromagnetic below T = 7.5 K with p d = 0.4 Pb while p ff = 3.3 Pb - The magnetie entropy is 0.2 RLn2 while y = 205 mJ/mol j 238) y dditional evidence for itinerant magnetism comes from the very large superimposed susceptibility in the ordered state... 

Compared with the situation for the 3d series, the 4d orbitals are relatively more diffuse and hence can overlap more efficiently. One result of this diffuseness is the absence of permanent magnets amongst the 4d solids the 4d wavefunctions cannot support local moments. This can be rationalized in terms of the Stoner theory of itinerant magnetism which also has a cluster or molecular analog L Briefly, a magnetic instability may occur if... 

For a deeper discussion of the subject, I refer to a book by Yosida [127] and, for itinerant magnetism and general solid state physics with any relation to magnetism, a book by Kiibler [128]. 

It would not be appropriate here to discuss the many points at issue in the intense international debate which has been going on for some time in this area. The comments of the discussion panel at the international conference on magnetism in Kyoto, ICM 82, provide a convenient summary (Kanamori 1983) and a book dedicated to itinerant magnetism is now available (CapeUmann 1987). We shall therefore give only a brief outline of the expectations of those theories which are currently used by the majority of experimentalists in attempting to explain the observed ferromagnetic behaviour of the transition metal intermetallic compounds. 

AnCo2 compounds exist for U, Np, Pu and Am. For AmCo2, however, no magnetic data are available. In contrast to AnFe2 compounds the Co isotypes display much weaker itinerant magnetic properties. 

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