A theoretical insight into an isentropic strategy for enhancing magnetoelectric coupling of organic multiferroics

Comparative study between charge and spin thermoelectric figure of merits in an antiferromagnetic ring

Considering the unique and diverse characteristic features of antiferromagnetic (AFM) systems, here in our work, we explore spin-dependent thermoelectric behavior in an AFM ring geometry. A reasonably large (≫1) spin figure of merit, referred to asZST, is obtained under suitable input conditions. Two important prerequisites are (i) breaking the symmetry among up and down spin sub-Hamiltonians and (ii) generating different asymmetric transmission line shapes across a Fermi energy for two opposite spin electrons. Describing the physical system within a tight-binding framework, where spin-dependent scattering occurs due to the interaction of itinerant electrons with local magnetic moments via the usual spin-moment exchange interaction, we compute all the thermoelectric quantities based on Landauer integrals following the Green's function technique. The behavior of charge figure of merit, denoted asZCT, is also discussed along withZST. ThoughZCTreaches above unity, it is much smaller compared toZST. This is the key finding of our investigation. To make the present communication a self-contained one, we compare the results with another arrangement of magnetic moments in a ring-like geometry and in a chain-like one. Our analysis gives a suitable hint that in the presence of spin-dependent scattering, much more favorable energy conversion can be substantiated.

Perfect spin Seebeck effect, spin-valve, spin-filter and spin-rectification based on the heterojunction of sawtooth graphene and graphyne nanoribbons

In order to develop low-power-consumption electronic devices, we studied a heterojunction composed of a 5,4-sawtooth graphene nanoribbon and 6,6,12-graphyne nanoribbon by using first-principles calculations combined with the non-equilibrium Green's function approach. Some spin caloritronic and spintronic devices can be realized based on the heterojunction, exhibiting excellent and multifunctional properties: a net spin current and perfect spin Seebeck effect are generated, a thermal induced and bias induced colossal magnetoresistance is obtained, and a spin filter with 100% spin polarization and a spin rectification with a high rectification ratio are acquired. Our results indicate that the proposed heterojunction is a promising candidate for multi-effect and low-power-consumption electronic devices.

A theoretical strategy for pressure-driven ferroelectric transition associated with critical behavior and magnetoelectric coupling in organic multiferroics

In organic multiferroics, the charge or spin coupled to the lattice induces lattice symmetry breaking, which is responsible for the ferroelectric (FE) transition. We propose a quantum spin model to describe the ferroelectricity of organic multiferroics, in which the pressure-driven spin-lattice coupling is controlled both by a jump function and a pressure power function. The T-p phase diagram shows different scaling relationships at low and high pressure regions, respectively, which is in accordance with the experimental observation. It is found that the pressure can not only enhance the FE polarization, but also enhance the transition temperature T_c as well as the electrocaloric effect (ECE). The electrocaloric critical scaling laws are proposed to verify the order and universality of the FE transition based on the Banerjee and Franco's criteria. In addition, we propose a temperature mediated mechanism within an isentropic process based on the ECE combined with the pyromagnetic effect, together with multiple physically (magnetic field and pressure jointly) controlled means, to enhance the magnetoelectric coupling around room-temperature, which will provide thermodynamic and quantum controlled means for realizing multi-state logic memory.