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)

Abstract

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