Abstract
The magnetocaloric response of the mixed spin-1/2 and spin-S () Ising model on a decorated square lattice is thoroughly examined in presence of the transverse magnetic field within the generalized decoration-iteration transformation, which provides an exact mapping relation with an effective spin-1/2 Ising model on a square lattice in a zero magnetic field. Temperature dependencies of the entropy and isothermal entropy change exhibit an outstanding singular behavior in a close neighborhood of temperature-driven continuous phase transitions, which can be additionally tuned by the applied transverse magnetic field. While temperature variations of the entropy display in proximity of the critical temperature a striking energy-type singularity , two analogous weak singularities can be encountered in the temperature dependence of the isothermal entropy change. The basic magnetocaloric measurement of the isothermal entropy change may accordingly afford the smoking gun evidence of continuous phase transitions. It is shown that the investigated model predominantly displays the conventional magnetocaloric effect with exception of a small range of moderate temperatures, which contrarily promotes the inverse magnetocaloric effect. It turns out that the temperature range inherent to the inverse magnetocaloric effect is gradually suppressed upon increasing of the spin magnitude S.
Highlights
Before proceeding to a discussion of the most interesting results for the magnetocaloric properties of the mixed spin-1/2 and spin-S Ising model on a decorated square lattice in a transverse magnetic field defined through the Hamiltonian (1), it is worthwhile to remark that the finite-temperature phase diagrams, the longitudinal and transverse magnetizations, the specific heat and susceptibility were extensively examined in our two preceding articles [25,26], to which readers interested in a more comprehensive understanding of temperature behavior of these magnetic and thermodynamic quantities are referred to
Let us begin with the finite-temperature phase diagrams of the mixed spin-1/2 and spin-S (S > 1/2) Ising model on a decorated square lattice, which are depicted in Figure 2 for four selected values of the spin magnitude S in the form of plots the critical temperature kB Tc /| J | versus the transverse magnetic field Ω/| J |
In the present article we have investigated in detail the basic magnetocaloric properties of the mixed spin-1/2 and spin-S (S > 1/2) Ising model on a decorated square lattice in a transverse magnetic field, which was exactly solved through a rigorous mapping correspondence with the zero-field Ising square lattice established with the help of generalized decoration-iteration transformation
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The molecularbased magnetic materials [M2 Nb] cannot be regarded as true experimental representatives of a mixed spin-1/2 and spin-S (S > 1/2) Ising model on the decorated square lattice, because the spin-1/2 Nb4+ magnetic ions from nodal lattice sites of a square lattice are affected by longitudinal as well as transverse magnetic fields due to their almost isotropic gyromagnetic g-factor [34,35] In spite of this fact we hope that the results presented in this article could stimulate a targeted design of structural analogs of the molecularbased magnets [M2 Nb], which would contain the Ising-like magnetic ions with the highly anisotropic g-factor instead of the nearly isotropic spin-1/2 Nb4+ magnetic ions. It should be pointed out that the investigated mixed spin-1/2 and spin-S (S > 1/2) Ising model on a decorated square lattice represents a prototypical example of an exactly solved lattice-statistical spin model, which allows a detailed examination of MCE in a vicinity of temperature-driven continuous phase transition tunable by the transverse magnetic field
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