Optimized magnetoelectric coupling at room temperature via the control of spin anisotropy and phase competition in the Co2Y-type hexaferrite single crystals

Even though the Co₂Y-type hexaferrites have been known to have great potential for realizing the large magnetoelectric (ME) coupling at room temperature, there are no consensus how to to optimize their magnitude. Not only to optimize the ME coupling but to seek for the physical origin of having larger ME coupling near room temperature, we have grown Co₂Y-type hexaferrites single crystals Ba_2-xSr_xCo₂(Fe_1-yAl_y)₁₂O₂₂ with systematic variation of Ba/Sr and Fe/Al ratio. We have observed that, among the grown crystals, the nominal composition with x = 1.8 and y = 0.04 has the largest ME coupling coefficient, dP/dH, and magnetic controllable electric polarization, ΔP_max, in the various temperature range. Moreover, electric phase diagram and magnetic structures are established by bulk magnetization, dielectric constant and neutron scattering studies. We suggest that optimization of spin anisotropy and phase competition between transverse cone and alternating longitudinal cone, which are driven by Sr and Al substitution, result in the largest ME coupling in the x = 1.8 and y = 0.04 composition. The role of Sr, Co and Al ions in the spin frustration and magnetic anisotropy will be also discussed.