Influence of non-uniform magnetic field on spatial orientation of Heisenberg dimers with spin S=1

The paper presents exact results for the Heisenberg dimer with spin S = 1 placed in a non-uniform magnetic field. It was assumed that the dimer has a degree of freedom of movement enabling the spatial arrangement of the dimer axis parallel or perpendicular to the field gradient. Based on the calculations of the Gibbs magnetic energy, the occurrence of magneto-spatial phase transitions of the 1st kind was found, in which the dimer changing its magnetization state simultaneously changes the spatial orientation relative to the field gradient. Ground state phase diagrams and temperature diagrams are presented, as well as calculations of the Gibbs energy, magnetization, entropy and dimer quadrupolar moment. These calculations form an internally consistent thermodynamic description and are presented in figures. The discussed model illustrates the possibility of a magnetic contribution to the self-assembly of dimers on the surface.