Teresa Castán

Thermodynamics of ferrotoroidic materials: Toroidocaloric effect

The three primary ferroics, namely ferromagnets, ferroelectrics and ferroelastics exhibit corresponding large (or even giant) magnetocaloric,electrocaloric and elastocaloric effects when a phase transition is induced by the application of an appropriate external field. Recently the suite of primary ferroics has been extended to include ferrotoroidic materials in which there is an ordering of toroidic moments in the form of magnetic vortex-like structures, examples being LiCo(PO_4)_3 and Ba_2CoGe_2O_7. In the present work we formulate the thermodynamics of ferrotoroidic materials. Within a Landau free energy framework we calculate the toroidocaloric effect by quantifying isothermal entropy change (or adiabatic temperature change) in the presence of an applied toroidic field when usual magnetization and polarization may also be present simultaneously. We also obtain a nonlinear Clausius-Clapeyron relation for phase coexistence.

Tweed in martensites: A potential new spin glass

Weve been studying the tweed precursors above the martensitic transition in shape-memory alloys. These characteristic cross-hatched modulations occur for hundreds of degrees above the first-order shape-changing transition. Our two-dimensional model for this transition, in the limit of infinite elastic anisotropy, can be mapped onto a spin-glass Hamiltonian in a random field. We suggest that the tweed precursors are a direct analogy of the spin-glass phase. The tweed is intermediate between the high-temperature cubic phase and the low-temperature martensitic phase in the same way as the spin-glass phase can be intermediate between ferromagnet and antiferromagnet.

Unified mean-field study of ferro- and antiferromagnetic behavior of the Ising model with external field

A simultaneous mean-field treatment, of both phenomena, ferro- and antiferromagnetism, in the framework of the Bragg–Williams theory, shows the importance of the interplay between the two corresponding order parameters in determining the features of the phase diagram. The problems of phase separation and order–disorder transition in binary alloys are also included in the solution. The different projections of the phase diagram are discussed.

Magnetocaloric and barocaloric responses in magnetovolumic systems

By means of a mean-field model extended to include magnetovolumic effects we study the effect of external fields on the thermal response characterized either by the isothermal entropy change and/or the adiabatic temperature change. The model includes two different situations induced by the magnetovolumic coupling. (i) A first order para- ferromagnetic phase transition that entails a volume change. (ii) An inversion of the effective exchange interaction that promotes the occurrence of an antiferromagnetic phase at low temperatures. In both cases, we study the magneto- and baro-caloric effects as well as the corresponding cross caloric responses. By comparing the present theoretical results with available experimental data for several materials we conclude that the present thermodynamical model reproduces the general trends associated with the considered caloric and cross caloric responses.

Thermodynamics of Multiferroic Materials

This chapter deals with the Thermodynamics of multiferroic materials. We first discuss the symmetry of the four primary ferroic order parameters related to ferromagnets, ferroelectrics, ferroelastics and ferrotoroidics and set up basic field equations. We then develop a general thermodynamic description based on generalized displacements and the corresponding conjugated fields. We use these ideas to calculate multicaloric effects in these materials, in particular focussing on the toroidocaloric effect. Next we develop Landau free energy models for multiferroic materials and illustrate various ideas through three representative examples: (i) magnetoelectric transitions, (ii) magnetic shape memory transitions where elasticity plays a crucial role, and (iii) ferrotoroidic transitions. We also briefly discuss the role of disorder and its consequences for the existence of ferroic tweed as well as ferroic glass. Finally, we provide an outlook in terms of the open problems in this exciting field of research.

Statistico‐thermodynamic properties of a binary alloy

We use the antiferromagnetic Ising model to obtain the equilibrium properties of a bcc binary alloy AxB1−x which undergoes an order–disorder transition. We use Monte Carlo simulations to obtain the μ–T phase diagram, for different values of x, and discuss how to interpret the results in relation to experiments.

Intermittent dynamics in externally driven ferroelastics and strain glasses

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Ordering Processes in FCC and BCC Binary Alloys: A Comparative Study

We study the problem of ordering kinetics in a binary alloy rapidly quenched through an order-disorder transition by Monte Carlo simulations. We consider two particular cases: (i) an AB bcc alloy relaxing from the disordered to an ordered B2 phase (e.g. FeAl) and (ii) an A 3 B fcc alloy relaxing from a disordered to an ordered L1 2 phase (e.g. Cu 3 Au). The alloys, that are supposed to contain a little concentration of vacancies, are modeled by means of a spin-1 Ising model. A modified Kawasaki dynamics in which only exchanges between atoms and vacancies are permitted, has been used in the simulations. The main differences in the behaviour of both systems during the coarsening regime are analyzed.