Spin–singlet competition responsible for magnetization plateau associated with magnetocaloric effect in a tetrameric chain

The quantum phase transitions (QPTs) and magnetization plateau properties, together with magnetocaloric effect (MCE) of the tetrameric chain are investigated by means of Green’s function theory. We reveal the uniform, dimeric and gapped (gapless) tetrameric phases, which are explicitly confirmed by the field dependence of magnetization. The spin–singlet competition is proposed to make clear the magnetization plateaus and QPTs. Simultaneously, the QPTs and quantum critical points are identified by the dips of the isoentropes, or equivalently the sharp maxima of entropy at ultra-low temperature, as well as the local minima of specific heat and the valley–peak structure of magnetic cooling rate with its sign changed. In addition, the temperature (T) and magnetic field (h) dependence of magnetic entropy change (ΔS) exhibits prominent inverse magnetocaloric effect (IMCE), implying adiabatic magnetization can generate cooling, which follows a power law dependence of h: ΔS∼hn. The local exponent n≈2 is independent of h and T at low fields, which was observed in the antiferromagnetic materials experimentally. It is also found that the stronger the dimerization is, the larger the IMCE is, which would provide a clue to design antiferromagnetic refrigerant materials for use in magnetic cooling devices with low field controlling.