Magnetism and thermodynamic properties of a spin-1/2 ferrimagnetic diamond XY chain in magnetic fields at finite temperatures

摘要

The elementary excitation spectra of a one-dimensional ferrimagnetic diamond chain in the spin-1/2 XY model at low temperatures have been calculated by using an invariant eigen-operator (IEO) method, the energies of elementary excitations in different specific cases are discussed, and the analytic solutions of three critical magnetic field intensities (H_(C1), H_(C2), and H_(peak)) are given. The magnetization versus external magnetic field curve displays a 1/3 magnetization plateau at low temperatures, in which H_(C1) is the critical magnetic field intensity from the disappearance of the 1/3 magnetization plateau to spin-flop states, H_(C2) is the critical magnetic field intensity from spin-flop states to the saturation magnetization, and H_(peak) is the critical magnetic field intensity when the temperature magnetization shows a peak in the external magnetic field. The temperature dependences of the magnetic susceptibility and the specific heat show a double peak structure. The entropy and the magnetic susceptibility versus external magnetic field curves also exhibit a double peak structure, and the positions of the two peaks correspond to H_(C1) and H_(C2), respectively. This derives from the competition among different types of energies: the temperature-dependent thermal disorder energy, the potential energy of the spin magnetic moment, the ferromagnetic exchange interaction energy, and the anti-ferromagnetic exchange interaction energy. However at low temperatures, the specific heat as a function of external magnetic field curve exhibits minima at the above two critical points (H_(C1) and H_(C2)). The origins of the above phenomena are discussed in detail.