Abstract
Preparing materials which simultaneously exhibit spontaneous magnetic and electrical polarisations is challenging as the electronic features which are typically used to stabilise each of these two polarisations in materials are contradictory. Here we show that by performing low-temperature cation-exchange reactions on a hybrid improper ferroelectric material, Li2SrTa2O7, which adopts a polar structure due to a cooperative tilting of its constituent TaO6 octahedra rather than an electronically driven atom displacement, a paramagnetic polar phase, MnSrTa2O7, can be prepared. On cooling below 43 K the Mn2+ centres in MnSrTa2O7 adopt a canted antiferromagnetic state, with a small spontaneous magnetic moment. On further cooling to 38 K there is a further transition in which the size of the ferromagnetic moment increases coincident with a decrease in magnitude of the polar distortion, consistent with a coupling between the two polarisations.
Highlights
Preparing materials which simultaneously exhibit spontaneous magnetic and electrical polarisations is challenging as the electronic features which are typically used to stabilise each of these two polarisations in materials are contradictory
The “hybrid improper” (HIP) mechanism offers an alternative method for inducing the noncentrosymmetric polar crystal structures necessary for ferroelectric behaviour[16,17,18,19,20]
When combined with the high B-site charges needed in these frameworks, this size requirement rules out the inclusion of the vast majority of paramagnetic transition-metal cations, and to date, the only candidate magnetoelectric multiferroic materials based on the HIP mechanism are Ca3Mn2O716,24,30 and [Ca0.69Sr0.46Tb1.85Fe2O7]0.85[Ca3Ti2O7]0.1531, the latter of which uses the introduction of some large diamagnetic Ti4+ ions onto the perovskite B site, via alloying the magnetic iron phase with diamagnetic Ca3Ti2O7, to help stabilise the required distorted perovskite framework
Summary
Preparing materials which simultaneously exhibit spontaneous magnetic and electrical polarisations is challenging as the electronic features which are typically used to stabilise each of these two polarisations in materials are contradictory. One of the first materials predicted to exhibit HIP ferroelectric behaviour, Ca3Mn2O716, adopts a canted antiferromagnetic state (weak ferromagnet) below 115 K29, in which magnetoelectric coupling has been observed[24,30], in line with theory predictions[16].
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