Magnetism in the large-U single-band Hubbard model

Using an exact expression of the t-J model in terms of fermionic holon and local spin operators, we investigate magnetism in the large-U limit of the Hubbard model as a function of doping (x) away from half-filling. The characteristic dopant concentration xc below which the local spins remain antiferromagnetically correlated, and above which they are ferromagnetically coupled, is estimated. In the antiferromagnetic regime x < xc, a large reduction of the holon bandwidth is found and explicitly related to the zero-point motion of the spin fluctuations. The basic picture of this regime is that of a Heisenberg antiferromagnet defected by a more or less static distribution of polaronic holons. In the ferromagnetic regime, x < xc, and for a three-dimensional lattice, the model exhibits coexistence of charge itinerancy and localized magnetic moment behavior. At low temperature T the magnetization follows the Bloch T3/2 law, while above the Curie temperature (Tc) the paramagnetic susceptibility follows the Curie-Weiss law. The Bohr magneton number is 1 − x and, hence, non integer. For T < Tc≪ Tc the specific heat varies as Cv/T = A + BT1/2, where the first term is contributed by itinerant holons and the second by spin reversals. These results are consistent with the observed ferromagnetism in the 3d transition metals, suggesting that the large-U Hubbard model shares many important physical properties with the intermediate-U model. However, we point out that there are significant differences between the large-U ferromagnetism and the Stoner-Slater band ferromagnetism, and that these differences can be measured by spin polarization experiments at low temperature.