4 research outputs found
Persistent Type-II Multiferroicity in Nanostructured MnWO<sub>4</sub> Ceramics
Persistent
Type-II Multiferroicity in Nanostructured
MnWO<sub>4</sub> Ceramic
Incorporation of JahnāTeller Cu<sup>2+</sup> Ions into Magnetoelectric Multiferroic MnWO<sub>4</sub>: Structural, Magnetic, and Dielectric Permittivity Properties of Mn<sub>1ā<i>x</i></sub>Cu<sub><i>x</i></sub>WO<sub>4</sub> (<i>x</i> ā¤ 0.25)
Polycrystalline
samples of Mn<sub>1ā<i>x</i></sub>Cu<sub><i>x</i></sub>WO<sub>4</sub> (<i>x</i> ā¤ 0.5)
have been prepared by a solid-state synthesis as well
as from a citrate synthesis at moderate temperature (850 Ā°C).
The goal is to study changes in the structural, magnetic, and dielectric
properties of magnetoelectric type-II multiferroic MnWO<sub>4</sub> caused by replacing JahnāTeller-inactive Mn<sup>2+</sup> (d<sup>5</sup>, <i>S</i> = 5/2) ions with JahnāTeller-active
Cu<sup>2+</sup> (d<sup>9</sup>, <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 ā¤ <i>x</i> ā¤ 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 MnWO<sub>4</sub> structure and suggest random distributions of JahnāTeller-distorted
CuO<sub>6</sub> octahedra in the solid solution. Magnetic susceptibility
reveals that only 5% of Cu substitution suppresses the nonpolar collinear
AF1 antiferromagnetic structure observed in pure MnWO<sub>4</sub>.
Type-II multiferroicity survives a weak Cu substitution rate (<i>x</i> < 0.15). Multiferroic transition temperature and NeĢel
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 MnWO<sub>4</sub> strengthens the overall antiferromagnetic interaction and
reduces the magnetic frustration
Pair Distribution Function and Density Functional Theory Analyses of Hydrogen Trapping by Ī³āMnO<sub>2</sub>
In the presence of āAg<sub>2</sub>Oā as a promoter,
Ī³-MnO<sub>2</sub> traps dihydrogen in its (2 Ć 1) and (1
Ć 1) tunnels. The course of this reaction was examined by analyzing
the X-ray diffraction patterns of the H<sub><i>x</i></sub>MnO<sub>2</sub>/āAg<sub>2</sub>Oā system (0 ā¤ <i>x</i> < 1) on the basis of pair distribution function and
density functional theory (DFT) analyses. Hydrogen trapping occurs
preferentially in the (2 Ć 1) tunnels of Ī³-MnO<sub>2</sub>, which is then followed by that in the (1 Ć 1) tunnels. Our
DFT analysis shows that this process is thermodynamically favorable
Increasing the Phase-Transition Temperatures in Spin-Frustrated Multiferroic MnWO<sub>4</sub> by Mo Doping
Ceramic samples of MnW<sub>1ā<i>x</i></sub>Mo<sub><i>x</i></sub>O<sub>4</sub> (<i>x</i> ā¤
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 Mn<sup>2+</sup> ions in the spin spiral multiferroic MnWO<sub>4</sub>. Samples were characterized by X-ray diffraction, optical
spectroscopy, magnetization, and dielectric permittivity measurements.
It was observed that the NeĢel temperature <i>T</i><sub>N</sub>, the spin spiral ordering temperature <i>T</i><sub>M2</sub>, and the ferroelectric phase-transition temperature <i>T</i><sub>FE2</sub> of MnWO<sub>4</sub> increased upon the nonmagnetic
substitution of Mo<sup>6+</sup> for W<sup>6+</sup>. Like pure MnWO<sub>4</sub>, the ferroelectric critical temperature <i>T</i><sub>FE2</sub>(<i>x</i>) coincides with the magnetic ordering
temperature <i>T</i><sub>M2</sub>(<i>x</i>). A
density functional analysis of the spin-exchange interactions for
a hypothetical MnMoO<sub>4</sub> that is isostructural with MnWO<sub>4</sub> suggests that Mo substitution increases the strength of the
spin-exchange couplings among Mn<sup>2+</sup> in the vicinity of a
Mo<sup>6+</sup> ion. Our study shows that the Mo-doped MnW<sub>1ā<i>x</i></sub>Mo<sub><i>x</i></sub>O<sub>4</sub> (<i>x</i> ā¤ 0.3) compounds are spin-frustrated materials
that have higher magnetic and ferroelectric phase-transition temperatures
than does pure MnWO<sub>4</sub>