The multiferroic RMn2O5 family, where R is rare-earth ion or Y, exhibits rich physics of multiferroicity which has not yet well understood, noting that multiferroicity is receiving attentions for promising application potentials. DyMn2O5 is a represe
ntative member of this family. The ferroelectric polarization in DyMn2O5 is claimed to have two anti-parallel components: one (PDM) from the symmetric exchange striction between the Dy3+-Mn4+ interactions and the other (PMM) from the symmetric exchange striction between the Mn3+-Mn4+ interactions. We investigate the evolutions of the two components upon a partial substitution of Mn3+ by nonmagnetic Al3+ in order to tailor the Mn-Mn interactions and then to modulate component PMM in DyMn2-x/2Alx/2O5. It is revealed that the ferroelectric polarization can be successfully reversed by the Al-substitution via substantially suppressing the Mn3+-Mn4+ interactions and thus the PMM. The Dy3+-Mn4+ interactions and the polarization component PDM can sustain against the substitution until a level as high as x=0.2. In addition, the independent Dy spin ordering is shifted remarkably down to an extremely low temperature due to the Al3+ substitution. The present work not only confirms the existence of the two anti-parallel polarization components but also unveils the possibility of tailoring them independently.
We report the evolution of thermal transport properties of iron-based superconductor K$_x$Fe$_{2-y}$Se$_2$ with sulfur substitution at Se sites. Sulfur doping suppresses the superconducting $T_c$ as well as the Seebeck coefficient. The Seebeck coeffi
cient of all crystals in the low temperature range can be described very well by diffusive thermoelectric response model. The zero-temperature extrapolated value of Seebeck coefficient divided by temperature $S/T$ gradually decreases from $-0.48 mu V/K^2$ to a very small value $sim$ 0.03 $mu$V/K$^2$ where $T_c$ is completely suppressed. The normal state electron Sommerfeld term ($gamma_n$) of specific heat also decreases with the increase of sulfur content. The dcrease of $S/T$ and $gamma_n$ reflects a suppression of the density of states at the Fermi energy, or a change in the Fermi surface that would induce the suppression of correlation strength.
Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of sev
eral order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review article, we outline some of the progressive milestones in this stimulating field, specially for those single phase multiferroics where magnetism and ferroelectricity coexist. Firstly, we will highlight the physical concepts of multiferroicity and the current challenges to integrate the magnetism and ferroelectricity into a single-phase system. Subsequently, we will summarize various strategies used to combine the two types of orders. Special attentions to three novel mechanisms for multiferroicity generation: (1) the ferroelectricity induced by the spin orders such as spiral and E-phase antiferromagnetic spin orders, which break the spatial inversion symmetry, (2) the ferroelectricity originating from the charge ordered states, and (3) the ferrotoroidic system, will be paid. Then, we will address the elementary excitations such as electromagnons, and application potentials of multiferroics. Finally, open questions and opportunities will be prospected.
