Incommensurate magnetic structure, Fe/Cu chemical disorder and magnetic interactions in the high-temperature multiferroic YBaCuFeO5

TL;DR

This study models the incommensurate magnetic structure of YBaCuFeO5, linking magnetic order, chemical disorder, and electric polarization, supported by neutron diffraction, ab-initio calculations, and analysis of Fe/Cu distributions.

Contribution

It provides the first detailed model of the incommensurate magnetic structure in YBaCuFeO5, integrating experimental data with ab-initio calculations and exploring the role of Fe/Cu disorder.

Findings

Incommensurate magnetic order appears below TN2 with a spiral structure.

Polarization scales with the magnetic modulation vector q.

Fe/Cu disorder influences magnetic interactions and order.

Abstract

Motivated by the recent observations of incommensurate magnetic order and electric polarization in YBaCuFeO5 up to temperatures TN2 as high as 230K [1,2] we report here for the first time a model for the incommensurate magnetic structure of this material that we complement with ab-initio calculations of the magnetic exchange parameters. Using neutron powder diffraction we show that the appearance of polarization below TN2 is accompanied by the replacement of the high temperature collinear magnetic order by a circular inclined spiral with propagation vector ki=(1/2, 1/2, 1/2+-q). Moreover, we find that the polarization approximately scales with the modulus of the magnetic modulation vector q down to the lowest temperature investigated (T=3K). Further, we observe occupational Fe/Cu disorder in the FeO5-CuO5 bipyramids, although a preferential occupation of such units by Fe-Cu pairs is supported by the observed magnetic order and by density functional calculations. We calculate exchange coupling constants for different Fe/Cu distributions and show that, for those containing Fe-Cu dimers, the resulting magnetic order is compatible with the experimentally observed collinear magnetic structure (kc=(1/2, 1/2, 1/2), TN2 > T > TN1 = 440K). Based on these results we discuss possible origins for the incommensurate modulation and its coupling with ferroelectricity.