Helimagnets by disorder: Its role on the high-temperature magnetic spiral in the YBaCuFeO5 perovskite

Abstract

Most of the spiral magnetoelectric multiferroics investigated in recent years are geometrically or exchange-frustrated magnets, where the presence of triangular or other frustrated spin networks produce low magnetic transition temperatures. This critically limits their potential uses. The exceptional stability of the spiral magnetic order (at ${T}_{S}$) in the layered structure of the ${\mathrm{YBaCuFeO}}_{5}$ double perovskite involves a nonconventional mechanism: spiral order by disorder. The model has been theoretically developed by Scaramucci et al. [Phys. Rev. Res. 2, 013273 (2020)] after the discovery of a huge impact of cation disorder on ${T}_{S}$ [M. Morin et al., Nat. Commun. 7, 13758 (2016)]. In this work the influence of disorder (and only disorder) on the magnetic phase diagram is studied on a quantitative basis extending the range of previous studies. We thoroughly investigate the impact of frustration due to $B$-site disorder (${n}_{d}$) on the magnetic spirals in the reference composition ${\mathrm{YBaCuFeO}}_{5}$. The interplay between disorder, stability, and the detailed features of the incommensurate spiral magnetic orders were systematic, quantitative, and methodically investigated in samples of identical composition, spanning a wide range of ${n}_{d}$ values. Three different regimes are distinguished in the ${\mathrm{YBaCuFeO}}_{5}$ phase diagram versus disorder. A triple point is found in ${\mathrm{YBaCuFeO}}_{5}$ driven by Fe/Cu disorder that sets limits to ${T}_{S}$ and the cycloidal component of the helicoidal order. These layered materials appear as a very efficient realization of the avenue ``spiral order by disorder'' to supply functional helimagnets at normal working temperatures.