scholarly journals Weak Singularities of the Isothermal Entropy Change as the Smoking Gun Evidence of Phase Transitions of Mixed-Spin Ising Model on a Decorated Square Lattice in Transverse Field

Entropy ◽  
2021 ◽  
Vol 23 (11) ◽  
pp. 1533
Author(s):  
Jozef Strečka ◽  
Katarína Karl’ová
Keyword(s):  
Magnetic Field ◽  
Phase Transitions ◽  
Ising Model ◽  
Magnetocaloric Effect ◽  
Entropy Change ◽  
Transverse Magnetic ◽  
Continuous Phase ◽  
Square Lattice ◽  
Mixed Spin ◽  
Weak Singularities

The magnetocaloric response of the mixed spin-1/2 and spin-S (S>1/2) 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 Tc a striking energy-type singularity (T−Tc)log|T−Tc|, 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.

Triple mixed-spin Ising model

2020 ◽  
Vol 34 (13) ◽  
pp. 2050129
Author(s):  
Erhan Albayrak
Keyword(s):  
Phase Transitions ◽  
Ising Model ◽  
Magnetic Fields ◽  
Exchange Interaction ◽  
Spin System ◽  
Bethe Lattice ◽  
Nearest Neighbors ◽  
Recursion Relations ◽  
Mixed Spin ◽  
Exchange Interaction Parameter

The A, B and C atoms with spin-1/2, spin-3/2 and spin-5/2 are joined together sequentially on the Bethe lattice in the form of ABCABC[Formula: see text] to simulate a molecule as a triple mixed-spin system. The spins are assumed to be interacting with only their nearest-neighbors via bilinear exchange interaction parameter in addition to crystal and external magnetic fields. The order-parameters are obtained in terms of exact recursion relations, then from the study of their thermal variations, the phase diagrams are calculated on the possible planes of our system. It is found that the model gives only second-order phase transitions in addition to the compensation temperatures.

Green Function Calculations of Properties for the Magnetocaloric Layered Structures Based Upon FeMnAsP

SPIN ◽  
2016 ◽  
Vol 06 (03) ◽  
pp. 1650010
Author(s):  
Osvaldo F. Schilling
Keyword(s):  
Magnetic Field ◽  
Phase Transitions ◽  
Green Function ◽  
Landau Theory ◽  
Random Phase ◽  
Layered Structures ◽  
Continuous Phase ◽  
Theoretical Treatment ◽  
Entropy Variation ◽  
Commercial Applications

The alternating Fe–Mn layered structures of the compounds FeMnAsxP[Formula: see text] display properties which have been demonstrated experimentally as very promising as far as commercial applications of the magnetocaloric effect are concerned. However, the theoretical literature on this and other families of magnetocaloric compounds still adopts simple molecular-field models in the description of important statistical mechanical properties like the entropy variation that accompanies applied isothermal magnetic field cycling, as well as the temperature variation following adiabatic magnetic field cycles. In the present paper, a random phase approximation Green function theoretical treatment is applied to such structures. The advantages of such approach are well known since the details of the crystal structure are easily incorporated in the model, as well as a precise description of correlations between neighbor spins can be obtained. We focus on a simple one-exchange parameter Heisenberg model, and the observed first-order phase transitions are reproduced by the introduction of a biquadratic term in the Hamiltonian whose origin is related both to the magnetoelastic coupling with the phonon spectrum in these compounds as well as with the values of spins in the Fe and Mn ions. The calculations are compared with experimental magnetocaloric data for the FeMnAsxP[Formula: see text] compounds. In particular, the magnetic field dependence for the entropy variation at the transition temperature predicted from the Landau theory of continuous phase transitions is reproduced even in the case of discontinuous transitions.

Magnetic Properties and Magnetocaloric Effect of Gd3Ni in Crystalline and Amorphous States

2012 ◽  
Vol 190 ◽  
pp. 355-358 ◽  
Author(s):  
D.A. Shishkin ◽  
N.V. Baranov ◽  
A.V. Proshkin ◽  
S.V. Andreev ◽  
A.S. Volegov
Keyword(s):  
Magnetic Field ◽  
Magnetocaloric Effect ◽  
Entropy Change ◽  
Rapid Quenching ◽  
Antiferromagnetic Order ◽  
Cooling Power ◽  
Ferromagnetic Behavior ◽  
Field Change ◽  
Ni Alloy ◽  
Magnetic Field Change

The liquid quenched Gd3Ni alloy is observed to exhibit a ferromagnetic behavior below TC = 117 K unlike crystalline compound having an antiferromagnetic order at T < TN = 99 K. Rapid quenching from the melt results in a considerable enhancement of the magnetocaloric effect in Gd3Ni at low magnetic fields. The maximal value of the isothermal magnetic entropy change at a magnetic field change of 20 kOe for the amorphous Gd3Ni surpasses by more than 8 times the SM value for the polycrystalline counterpart. The relative cooling power for the amorphous Gd3Ni alloy is estimated as 265 J kg-1 and 676 J kg-1 at a magnetic field change of 20 kOe and 50 kOe, respectively.

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