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Using the strong coupling diagram technique, magnetic and spectral propertiesof the two-dimensional repulsive Hubbard model are investigated in the rangesof repulsions $t\leq U\leq 10t$, temperatures $0.1t\lesssim T\lesssim 4t$ andelectron concentrations $0.6\lesssim\bar{n}\leq 1$ with $t$ the hoppingconstant. The approach takes into account interactions of electrons with spinand charge fluctuations of all ranges and fulfils the Mermin-Wagner theorem.Temperature and concentration dependencies of the uniform magneticsusceptibility, the variation of the double occupancy with the repulsion andthe temperature dependence of the square of the site spin are in satisfactoryagreement with Monte Carlo results. Three types of the temperature variation ofthe electron energy spectrum can be distinguished at half-filling. For$U\lesssim 3t$, at low temperatures, there are two nonintersecting bands, whichapproach each other on the boundary of the magnetic Brillouin zone. Withincreasing $T$ these bands merge into one band crossing the Fermi level. For$4t\lesssim U \lesssim 6t$, the low-temperature picture described above issupplemented with a low-intensity spin-polaron band located near the Fermilevel. As its counterpart in the strong-correlation case, the band is formed bybound states of electrons and spin excitations. However, in contrast to theformer case, the band exists even at half-filling and occupies the entireBrillouin zone. As for lower $U$, with increasing temperature, all bandscoalesce into a single band. For $U\gtrsim 7t$ and low temperatures thespectrum has a pronounced four-band structure, which with increasing $T$transforms into two Hubbard subbands.

Magnetic properties and temperature variation of spectra in the Hubbard model