Control of intrinsic disorder in magnetoelectric materials

Perovskite titanates ATiO3 are among the most promising materials in the rapidly-developing field of oxide electronics.  Their dielectric and transport properties are easily modulated by epitaxial strain, field-effect or chemical doping, hence facilitating their integration with conventional charge-based electronics.  However, the past decade has also seen an intensive search for a multiferroic perovskite with strong magnetoelectric coupling, which would enjoy numerous spintronic applications.  The spot-light soon fell on EuTiO3 , whose large magnetic moment (S = 7/2 per unit cell) and quantum paraelectricity suggest that it is a magnetic analog of SrTiO3, the “workhorse” oxide for electronic devices.

Although EuTiO3 has been known as a G-type antiferromagnet (AF) below T_Neel ∼ 5.5 K since the 1960s, its magnetoelectric properties were only revealed in 2001 by a 7% drop in the dielectric constant at T_Neel.  Early efforts to model EuTiO3 shared one critical feature: an assumption of cubic crystal symmetry throughout the phase diagram.  The recent discovery of a cubic-tetragonal transition at 283 K in powdered samples obliges us to revise our views, since tetragonal symmetry pushes EuTiO3 closer to a ferromagnetic/ferroelectric (FM/FE) phase boundary and permits the breakage of spatial inversion symmetry.  Theoretical and X-ray diffraction analysis suggests that tetragonal EuTiO3 may be disordered, thus opening the door to local symmetry-breaking and electric dipole formation via the Dzyaloshinskii-Moriya interaction.

More generally, nanoscale heterogeneity limits applications for many complex oxides, since functional electronic materials must display phase purity over lengthscales greater or equal to their intended device dimensions.  It is therefore an urgent priority to identify methods for controlling or suppressing such disorder.  In a recent work of ours we employed the magnetic anisotropy of EuTiO3 as a structural probe and showed that monocrystalline cubic EuTiO3 undergoes a transition to an intrinsically nano-disordered tetragonal phase with an easy-plane AF ground state.  However, the disorder may be reduced by
cooling under an electric field.  This mechanism of homogeneity control may prove crucial for phase management in future devices made from elastically hard oxides.

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