Antiferromagnetism prediction

Isostructural compounds A2Crp4 have not been prepared yet. The same arguments as those above lead to the prediction of ferromagnetism within the MeF4 -sheets of octahedra, exactly as observed in fluorides ACrFa, but this time without the possibility of compensation by antiferromagnetic coupling between the layers. 

The last results we shall discuss concern antiferromagnetic UO2 (Tn = 30.5 K) which is a semiconductor, and for which precise data exist as well as for Th02 The spin wave low temperature predictions fit the experiment data at low temperature with reasonable agreement and an excess found in the temperature range 30 K - 120 K is attributed to the Jahn-Teller effect. The entropy change at the transition is within 9% of the Rln 3 value expected for the F5 triplet ground state. 

Our model is thus of a metal, with a small number of carriers in the conduction band and 3d- or 4f-moments antiferromagnetically coupled to each other by direct exchange. The carriers may be either inserted by doping or by overlap from the lower Hubbard band (cf. Chapter 4, Section 3). The most striking prediction of the model, however, is that the degenerate electron gas should have a much enhanced Pauli magnetism. Suppose that EF is the Fermi... 

We emphasize that, in the insulating state, the gap is given by U- j(B1+ B2), and that it depends on the existence of moments and not on whether or not they are ordered. The gap is not related to the crystal structure. Indeed, if the crystal structure is such as to predict a gap, no antiferromagnetic lattice can form, unless the gap resulting from the Hubbard U is greater than that derived from the crystal structure. 

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