Persistent Type-II Multiferroicity in Nanostructured MnWO₄ Ceramics

Persistent Type-II Multiferroicity in Nanostructured MnWO₄ Ceramic

Incorporation of Jahn–Teller Cu²⁺ Ions into Magnetoelectric Multiferroic MnWO₄: Structural, Magnetic, and Dielectric Permittivity Properties of Mn_1–<i>x</i>Cu_<i>x</i>WO₄ (<i>x</i> ≤ 0.25)

Polycrystalline samples of Mn_1–<i>x</i>Cu_<i>x</i>WO₄ (<i>x</i> ≤ 0.5) have been prepared by a solid-state synthesis as well as from a citrate synthesis at moderate temperature (850 °C). The goal is to study changes in the structural, magnetic, and dielectric properties of magnetoelectric type-II multiferroic MnWO₄ caused by replacing Jahn–Teller-inactive Mn²⁺ (d⁵, <i>S</i> = 5/2) ions with Jahn–Teller-active Cu²⁺ (d⁹, <i>S</i> = 1/2) ions. Combination of techniques including scanning electron microscopy, powder X-ray and neutron diffraction, and Raman spectroscopy demonstrates that the polycrystalline samples with low copper content 0 ≤ <i>x</i> ≤ 0.25 are solid solution that forms in the monoclinic <i>P</i>2/c space group. Rietveld analyses indicate that Cu atoms substitutes for Mn atoms at the Mn crystallographic site of the MnWO₄ structure and suggest random distributions of Jahn–Teller-distorted CuO₆ octahedra in the solid solution. Magnetic susceptibility reveals that only 5% of Cu substitution suppresses the nonpolar collinear AF1 antiferromagnetic structure observed in pure MnWO₄. Type-II multiferroicity survives a weak Cu substitution rate (<i>x</i> < 0.15). Multiferroic transition temperature and Néel temperature increase as the amount of Cu increases. New trends in some of the magnetic properties and in dielectric behaviors are observed for <i>x</i> = 0.20 and 0.25. Careful analysis of the magnetic susceptibility reveals that the incorporation of Cu into MnWO₄ strengthens the overall antiferromagnetic interaction and reduces the magnetic frustration

Pair Distribution Function and Density Functional Theory Analyses of Hydrogen Trapping by γ‑MnO₂

In the presence of “Ag₂O” as a promoter, γ-MnO₂ traps dihydrogen in its (2 × 1) and (1 × 1) tunnels. The course of this reaction was examined by analyzing the X-ray diffraction patterns of the H_<i>x</i>MnO₂/“Ag₂O” system (0 ≤ <i>x</i> < 1) on the basis of pair distribution function and density functional theory (DFT) analyses. Hydrogen trapping occurs preferentially in the (2 × 1) tunnels of γ-MnO₂, which is then followed by that in the (1 × 1) tunnels. Our DFT analysis shows that this process is thermodynamically favorable

Increasing the Phase-Transition Temperatures in Spin-Frustrated Multiferroic MnWO₄ by Mo Doping

Ceramic samples of MnW_1–<i>x</i>Mo_<i>x</i>O₄ (<i>x</i> ≤ 0.3) solid solution were prepared by a solid-state route with the goal of increasing the magnitude of the spin-exchange couplings among the Mn²⁺ ions in the spin spiral multiferroic MnWO₄. Samples were characterized by X-ray diffraction, optical spectroscopy, magnetization, and dielectric permittivity measurements. It was observed that the Néel temperature <i>T</i>_N, the spin spiral ordering temperature <i>T</i>_M2, and the ferroelectric phase-transition temperature <i>T</i>_FE2 of MnWO₄ increased upon the nonmagnetic substitution of Mo⁶⁺ for W⁶⁺. Like pure MnWO₄, the ferroelectric critical temperature <i>T</i>_FE2(<i>x</i>) coincides with the magnetic ordering temperature <i>T</i>_M2(<i>x</i>). A density functional analysis of the spin-exchange interactions for a hypothetical MnMoO₄ that is isostructural with MnWO₄ suggests that Mo substitution increases the strength of the spin-exchange couplings among Mn²⁺ in the vicinity of a Mo⁶⁺ ion. Our study shows that the Mo-doped MnW_1–<i>x</i>Mo_<i>x</i>O₄ (<i>x</i> ≤ 0.3) compounds are spin-frustrated materials that have higher magnetic and ferroelectric phase-transition temperatures than does pure MnWO₄