The structure of the ground state of a narrow-band Hubbard magnet is i
nvestigated in the t-J model. The low-temperature thermodynamics of th
e ferromagnetic state is studied. The behaviour of the susceptibility
of SmBa2Cu3O6.3 such as a paramagnetic increase in 1/T law and a maxim
um at T approximate to 0.3 K, is explained in the model of a narrow ba
nd with metallic conductivity.