Control of magnetoelectric coupling in the Co 2 Y -type hexaferrites

We comprehensively investigated the magnetic, ferroelectric, and ME properties of ${\\mathrm{Ba}}_{2\\ensuremath{-}x}{\\mathrm{Sr}}_{x}{\\mathrm{Co}}_{2}{({\\mathrm{Fe}}_{1\\ensuremath{-}y}{\\mathrm{Al}}_{y})}_{12}{\\mathrm{O}}_{22}$ single crystals in broad doping ranges of Sr $(1.0\\ensuremath{\\le}x\\ensuremath{\\le}1.8)$ and Al $(0.00\\ensuremath{\\le}y\\ensuremath{\\le}0.08)$. Most of the investigated compounds exhibit an intriguing coexistence of two apparently competing magnetic phases: a transverse conical (TC) and alternating longitudinal conical (ALC) spin structure. The magnetic properties show that the ${\\mathrm{Ba}}_{0.2}{\\mathrm{Sr}}_{1.8}{\\mathrm{Co}}_{2}{({\\mathrm{Fe}}_{0.96}{\\mathrm{Al}}_{0.04})}_{12}{\\mathrm{O}}_{22}$ crystal has the highest ordering temperature and largest volume fraction of the ALC phase at zero H; further, after the application of an in-plane H, it exhibits a maximized volume fraction of the metastable TC phase, resulting in the highest ME susceptibility and electric polarization at all temperatures below 300 K. Our findings demonstrate that securing the thermal stability of the ALC phase is a crucial prerequisite to achieve optimized ME coupling in ${\\mathrm{Co}}_{2}\\mathrm{Y}$-type hexaferrites, pointing to a general strategy applicable to other hexaferrites as well.