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Magnetic properties of a spin-1/2 Ising-Heisenberg Cairo pentagonal model

Hamid Arian Zad, N.S. Ananikian

Published Aug 9, 2019• 0 citations
OpenAlex

Abstract

In the present paper, the specific heat and magnetic property studies of a spin-1/2 Ising-Heisenberg Cairo pentagonal structure are reported. The model has been comprehensively investigated by F. C. Rodrigues {\it et al.} in Ref. \cite{Rodrigues} without external magnetic field. Here, we consider the mentioned Cairo pentagonal model in the presence of an external tunable magnetic field and examine the low-temperature magnetization process, as well as, the specific heat of the model by using the transfer matrix approach. We find that the model shows magnetization intermediate plateaus at zero and $\frac{1}{2}$ of the saturation magnetization accompanied with the growing single Schottky peak in the specific heat curve versus temperature. Each magnetization jump is in accordance with the merging of each two peaks of the specific heat curve when the magnetic field increases. This is reminiscent of the ground state phase transition from one plateau to one another. We also deduce that the magnetic field and the isotropic coupling constant considered for the dimers have substantial effects on the temperature dependence of the specific heat.

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In the present paper, the specific heat and magnetic property studies of a spin-1/2 Ising-Heisenberg Cairo pentagonal structure are reported. The model has been comprehensively investigated by F. C. Rodrigues {\it et al.} in Ref. \cite{Rodrigues} without external magnetic field. Here, we consider the mentioned Cairo pentagonal model in the presence of an external tunable magnetic field and examine the low-temperature magnetization process, as well as, the specific heat of the model by using the transfer matrix approach. We find that the model shows magnetization intermediate plateaus at zero and 12\frac{1}{2}21​ of the saturation magnetization accompanied with the growing single Schottky peak in the specific heat curve versus temperature. Each magnetization jump is in accordance with the merging of each two peaks of the specific heat curve when the magnetic field increases. This is reminiscent of the ground state phase transition from one plateau to one another. We also deduce that the magnetic field and the isotropic coupling constant considered for the dimers have substantial effects on the temperature dependence of the specific heat.
The exactly solvable spin-1/2 Ising-Heisenberg distorted diamond chain in the presence of the external magnetic field is investigated for the case of antiferromagnetic Ising and ferromagnetic XXZ Heisenberg interactions. The influence of quantum fluctuations and the distortion on the ground state, magnetic and thermal properties of the model are studied in detail.In particular, it is established that the zero-temperature magnetization curve may involve intermediate plateaus just at zero and 1/3 of the saturation magnetization. It is demonstrated that the temperature dependence of the specific heat reveals up to four distinct peaks at zero magnetic field and up to five distinct peaks at a weak magnetic field. The physical origin of all observed additional peaks of the specific heat has been clarified on the grounds of dominating thermal excitations. We have shown that the quantum fluctuations give rise to an effective geometrical frustration in this chain.
Through the direct decoration transformation approach, we obtain a general solution for the pentagonal Ising model, showing its equivalence to the isotropic free-fermion eight-vertex model. We study the ground-state phase diagram, in which one ferromagnetic (FM) state, one ferrimagnetic (FIM) state, and one frustrated state are found. Using the exact solution of the pentagonal Ising model, we discuss the finite-temperature phase diagrams and find a phase transition between the FIM state and the disordered state as well as a phase transition between the disordered state and the FM state. We also discuss some additional remarkable properties of the model, such as the magnetization, entropy, and specific heat, at finite temperature and at its low-temperature asymptotic limit. Because of the influence of the second-order phase transition between the frustrated and ferromagnetic phases, we obtain surprisingly low values of the entropy and the specific heat until the critical temperature is reached.
We study the S=1/2 Heisenberg antiferromagnet on the Cairo pentagon lattice by the numerical-diagonalization method. We tune the ratio of two antiferromagnetic interactions coming from two kinds of inequivalent sites in this lattice, examining the magnetization process of the antiferromagnet; particular attention is given to one-third of the height of the saturation. We find that quantum phase transition occurs at a specific ratio and that a magnetization plateau appears in the vicinity of the transition. The plateau is accompanied by a magnetization jump on one side among the edges due to the spin-flop phenomenon. Which side the jump appears depends on the ratio.
The quantum spin-12\frac{1}{2}21​ orthogonal-dimer chain with the Heisenberg intradimer and Ising interdimer interactions in a magnetic field is considered by a rigorous approach. The model conserves the zzz component of total spin on vertical Heisenberg bonds, and this property is used to calculate exactly the partition function using the transfer-matrix method. We have found the ground-state phase diagram of the given model in a magnetic field as well as the macroscopic degeneracy along field-induced transitions accompanied with the magnetization jumps. The model exhibits two intermediate fractional plateaux at one quarter and one half of the saturation magnetization. We have examined the effect of the exchange anisotropy in the XXZXXZXXZ Heisenberg intradimer interaction on the ground state. It is shown that the one-quarter and one-half plateaux may disappear from the magnetization curve for the ferromagnetic Heisenberg intradimer interaction. We have also studied rigorously the effect of frustrated interactions on the thermodynamic and magnetic properties of the model and show how the macroscopic degeneracy of the ground state is reflected in the low-temperature behavior of the magnetization, entropy, and specific heat. A possibility of observing enhanced magnetocaloric effect during the adiabatic demagnetization is discussed in detail.
Low-temperature magnetization curves and thermodynamics of a frustrated spin-12\frac{1}{2}21​ Heisenberg triangular bilayer with the antiferromagnetic intradimer interaction and either ferromagnetic or antiferromagnetic interdimer interaction are investigated in a highly frustrated parameter region, where localized many-magnon eigenstates provide the most dominant contribution to magnetothermodynamics. Low-energy states of the highly frustrated spin-12\frac{1}{2}21​ Heisenberg triangular bilayer can be accordingly found from a mapping correspondence with an effective triangular-lattice spin-12\frac{1}{2}21​ Ising model in a field. A description based on the effective Ising model implies that the frustrated Heisenberg triangular bilayer with the ferromagnetic interdimer coupling displays in a zero-temperature magnetization curve discontinuous magnetization jump, which is reduced upon increasing of temperature until a continuous field-driven phase transition from the Ising universality class is reached at a certain critical temperature. The frustrated Heisenberg triangular bilayer with the antiferromagnetic interdimer coupling contrarily exhibits multistep magnetization curve with intermediate plateaus at 13\frac{1}{3}31​ and 23\frac{2}{3}32​ of the saturation magnetization, whereas discontinuous magnetization jumps observable at zero temperature change to continuous field-driven phase transitions from the universality class of three-state Potts model at sufficiently low temperatures. Exact results and Monte Carlo simulations of the effective Ising model are confronted with full exact diagonalization data for the Heisenberg triangular bilayer in order to corroborate these findings.
The spin-1 Ising-Heisenberg diamond chain with the second-neighbor interaction between nodal spins is rigorously solved using the transfer-matrix method. In particular, exact results for the ground state, magnetization process and specific heat are presented and discussed. It is shown that further-neighbor interaction between nodal spins gives rise to three novel ground states with a translationally broken symmetry, but at the same time, does not increases the total number of intermediate plateaus in a zero-temperature magnetization curve compared with the simplified model without this interaction term. The zero-field specific heat displays interesting thermal dependencies with a single- or double-peak structure.