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Register a new userRebuttal of The multicaloric effect in multiferroic materials
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The paper refutes the model and claims published in the Solid State Communications [1] as well as elsewhere. The theoretical approach proposed in The multicaloric effect in multiferroic materials by Melvin M.~Vopson has a number of inaccuracies and mistakes. The Author of [1] does not pay necessary attention to the range of applicability of the derived equations and confuses the dependent and independent variables. The resulting equations for electrically and magnetically induced multicaloric effects are incorrect and cannot be used for measurement interpretation.
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We have investigated multiple caloric effects in multiferroic Y2CoMnO6. Polycrystalline sample prepared by solid state method has shown a ferromagnetic Curie temperature 75 K with second order phase transition; a maximum magneto entropy change -$Delta$S_{Mmax}) of ~ 7.3 J/kg K with reasonable relative cooling power 220 J/kg is found without thermal and magnetic hysteresis loss. Electric field driven entropy change (-$Delta$S_{Emax}) of ~ 0.26 J/m^3 K obtained using the Maxwells relation and estimated magnetically induced total temperature change of 5.45 K around Curie temperature that confirms the multicaloric effect in the sample.
The relation between unusual Mexican-hat band dispersion, ferromagnetism and ferroelasticity is investigated using a combination of analytical, first-principles and phenomenological methods. The class of material with Mexican-hat band edge is studied using the $alpha$-SnO monolayer as a prototype. Such band edge causes a van Hove singularity diverging with $frac{1}{sqrt{E}}$, and in p-type material leads to spatial and/or time-reversal spontaneous symmetry breaking. We show that an unexpected multiferroic phase is obtained in a range of hole density for which the material presents ferromagnetism and ferroelasticity simultaneously.
We derive a sum rule to demonstrate that the static magnetoelectric (ME) effect is governed by optical transitions that are simultaneously excited via the electric and magnetic components of light. By a systematic analysis of magnetic point groups, we show that the ME sum rule is applicable to a broad variety of non-centrosymmetric magnets including ME multiferroic compounds. Due to the dynamical ME effect, the optical excitations in these materials can exhibit directional dichroism, i.e. the absorption coefficient can be different for counter-propagating light beams. According to the ME sum rule, the magnitude of the linear ME effect of a material is mainly determined by the directional dichroism of its low-energy optical excitations. Application of the sum rule to the multiferroic Ba$_2$CoGe$_2$O$_7$, Sr$_2$CoSi$_2$O$_7$ and Ca$_2$CoSi$_2$O$_7$ shows that in these compounds the static ME effect is mostly governed by the directional dichroism of the spin-wave excitations in the GHz-THz spectral range. On this basis, we argue that the studies of directional dichroism and the application of ME sum rule can promote the synthesis of new materials with large static ME effect.
The optical magnetoelectric effect, which is an inherent attribute of the spin excitations in multiferroics, drastically changes their optical properties compared to conventional materials where light-matter interaction is expressed only by the dielectric permittivity and magnetic permeability. Our polarized absorption experiments performed on multiferroic Ca2CoSi2O7 and Ba2CoGe2O7 in the THz spectral range demonstrate that such magnetoeletric spin excitations show quadrochroism, i.e. they have different colours for all the four combinations of the two propagation directions (forward or backward) and the two orthogonal polarizations of a light beam. We found that quadrochroism can give rise to peculiar optical properties, such as one-way transparency and zero-reflection of these excitations, which can open a new horizon in photonics. One-way transparency is also related to the static magnetoelectric phenomena, hence, these optical studies can provide guidelines for the systematic synthesis of new materials with large dc magnetoelectric effect.
Multiferroics are materials where two or more ferroic orders coexist owing to the interplay between spin, charge, lattice and orbital degrees of freedom. The explosive expansion of multiferroics literature in recent years demon-strates the fast growing interest in this field. In these studies, the first-principles calculation has played a pioneer role in the experiment explanation, mechanism discovery and prediction of novel multiferroics or magnetoelectric materials. In this review, we discuss, by no means comprehensively, the extensive applications and successful achievements of first-principles approach in the study of multiferroicity, magnetoelectric effect and tunnel junc-tions. In particular, we introduce some our recently developed methods, e.g., the orbital selective external potential (OSEP) method, which prove to be powerful tools in the finding of mechanisms responsible for the intriguing phe-nomena occurred in multiferroics or magnetoelectric materials. We also summarize first-principles studies on three types of electric control of magnetism, which is the common goal of both spintronics and multiferroics. Our review offers in depth understanding on the origin of ferroelectricity in transition metal oxides, and the coexistence of fer-roelectricity and ordered magnetism, and might be helpful to explore novel multiferroic or magnetoelectric materi-als in the future.
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