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Register a new userTopology and manipulation of multiferroic hybrid domains in MnWO$_4$
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Authors
D. Meier
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An investigation of the spatially resolved distribution of domains in the multiferroic phase of MnWO$_4$ reveals that characteristic features of magnetic and ferroelectric domains are inseparably entangled. Consequently, the concept of multiferroic hybrid domains is introduced for compounds in which ferroelectricity is induced by magnetic order. The three-dimensional structure of the domains is resolved. Annealing cycles reveal a topological memory effect that goes beyond previously reported memory effects and allows one to reconstruct the entire multiferroic multidomain structure subsequent to quenching it.
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Different methods of texturing polycrystalline materials are developed over years to use/probe anisotropic material properties with relative ease, where complicated and expensive single crystal growth processes could be avoided. In this paper, particle morphology assisted texturing in multiferroic MnWO$_4$ has been discussed. Detailed powder x-ray diffraction vis-a-vis scanning electron microscopic studies on differently annealed and processed samples have been employed to probe the giant texturing effect in powdered MnWO$_4$. A quantitative measure of the texturing has been carried out by means of Rietveld analysis technique. Qualitative presentation of magnetic and dielectric data on textured pellet demonstrated the development of clear anisotropic physical properties in polycrystalline pellets. Finally, we established that the highly anisotropic plate like particles are formed due to easy cleavage of the significantly large crystalline grains.
Neutron spherical polarimetry, which is directly sensitive to the absolute magnetic configuration and domain population, has been used in this work to unambiguously prove the multiferroicity of (ND4)2[FeCl5(D2O)]. We demonstrate that the application of an electric field upon cooling results in the stabilization of a single-cycloidal magnetic domain below 6.9 K, while poling in the opposite electric field direction produces the full population of the domain with opposite magnetic chirality. We prove the complete switchability of the magnetic domains at low temperature by the applied electric field, which constitutes a direct proof of the strong magnetoelectric coupling. Additionally, we refine the magnetic structure of the ordered ground state, determining the underlying magnetic space group consistent with the direction of the ferroelectric polarization, and we provide evidence of a collinear amplitude-modulated state with magnetic moments along the a-axis in the temperature region between 6.9 and 7.2 K.
BaMnF$_4$ microsheets have been prepared by hydrothermal method. Strong room-temperature blue-violet photoluminescence has been observed (absolute luminescence quantum yield 67%), with two peaks located at 385 nm and 410 nm, respectively. More interestingly, photon self-absorption phenomenon has been observed, leading to unusual abrupt drop of luminescence intensity at wavelength of 400 nm. To understand the underlying mechanism of such emitting, the electronic structure of BaMnF$_4$ has been studied by first principles calculations. The observed two peaks are attributed to electrons transitions between the upper-Hubbard bands of Mns $t_{2g}$ orbitals and the lower-Hubbard bands of Mns $e_g$ orbitals. Those Mott gap mediated d-d orbital transitions may provide additional degrees of freedom to tune the photon generation and absorption in ferroelectrics.
The BaAl$_4$ prototype crystal structure is the most populous of all structure types, and is the building block for a diverse set of sub-structures including the famous ThCr$_2$Si$_2$ family that hosts high-temperature superconductivity and numerous magnetic and strongly correlated electron systems. The MA$_4$ family of materials (M=Sr, Ba, Eu; A=Al, Ga, In) themselves present an intriguing set of ground states including charge and spin orders, but have largely been considered as uninteresting metals. Using electronic structure calculations, symmetry analysis and topological quantum chemistry techniques, we predict the exemplary compound BaAl$_4$ to harbor a three-dimensional Dirac spectrum with non-trivial topology and possible nodal lines crossing the Brillouin zone, wherein one pair of semi-Dirac points with linear dispersion along the $k_z$ direction and quadratic dispersion along the $k_x/k_y$ direction resides on the rotational axis with $C_{4v}$ point group symmetry. Electrical transport measurements reveal the presence of an extremely large, unsaturating positive magnetoresistance in BaAl$_4$ despite an uncompensated band structure, and quantum oscillations and angle-resolved photoemission spectroscopy measurements confirm the predicted multiband semimetal structure with pockets of Dirac holes and a Van Hove singularity (VHS) remarkably consistent with the theoretical prediction. We thus present BaAl$_4$ as a new topological semimetal, casting its prototype status into a new role as building block for a vast array of new topological materials.
Materials with long-range order like ferromagnetism or ferroelectricity exhibit uniform, yet differently oriented three-dimensional regions called domains that are separated by two-dimensional topological defects termed domain wallscite{Tagantsev2010,AlexHubert1998}. A change of the ordered state across a domain wall can lead to local non-bulk properties such as enhanced conductance or the promotion of unusual phasescite{Seidel2009,Meier2012,Farokhipoor2014}. Although highly desirable, controlled transfer of these exciting properties between the bulk and the walls is usually not possible. Here we demonstrate this crossover from three- to two-dimensions for confining multiferroic Dy$_{0.7}$Tb$_{0.3}$FeO$_3$ domains into multiferroic domain walls at a specified location within a non-multiferroic environment. This process is fully reversible; an applied magnetic or electric field controls the transformation. Aside from the aspect of magnetoelectric functionality, such interconversion can be key to tailoring elusive domain architectures such as in antiferromagnets.
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