Multiferroic magnetic spirals induced by random magnetic exchanges

TL;DR

This paper proposes a new high-temperature mechanism for multiferroic magnetic spirals driven by random frustration in a Heisenberg model, supported by Monte Carlo and density functional theory calculations.

Contribution

It introduces a novel mechanism where random frustration induces magnetic spirals at high temperatures, differing from traditional competing exchange interactions.

Findings

Spiral phases can exist at high temperatures with random frustration.

Randomly introduced frustrating bonds prevent spin glass order.

The mechanism is generalizable to other materials.

Abstract

Multiferroism can originate from the breaking of inversion symmetry caused by magnetic-spiral order. The usual mechanism for stabilizing a magnetic spiral is competition between magnetic exchange interactions differing by their range and sign, such as nearest-neighbor and next-nearest- neighbor interactions. Since the latter are usually weak the onset temperatures for multiferroism via this mechanism are typically low. By considering a realistic model for YBaCuFeO$_5$ we propose an alternative mechanism for magnetic-spiral order, and hence for multiferroism, that occurs at much higher temperatures. We show using Monte-Carlo simulations and electronic structure calculations based on density functional theory that the Heisenberg model on a geometrically non-frustrated lattice with only nearest-neighbor interactions can have a spiral phase up to high temperature when frustrating bonds are introduced randomly along a single crystallographic direction as caused, e.g., by a particular type of chemical disorder. This long-range correlated pattern of frustration avoids ferroelectrically inactive spin glass order. Finally, we provide an intuitive explanation for this mechanism and discuss its generalization to other materials.