Al doping effect on magnetic phase transitions of magnetoelectric hexaferrite Ba 0.7Sr 1.3Zn 2(Fe 1-xAl x) 12O 22

We investigated the effect of Al doping in magnetic properties of the Y-type hexaferrite Ba${}_{0.7}$Sr${}_{1.3}$Zn${}_{2}$(Fe${}_{1\ensuremath{-}x}$Al${}_{x}$)${}_{12}$O${}_{22}$ ($0\ensuremath{\le}x\ensuremath{\le}0.12$), which exhibit field-induced magnetoelectric polarization. We find that Al doping increases the pitch of a spin helix and enhances $c$-axis magnetization, stabilizing longitudinal conical phases. These conical phases eventually collapse at $x\ensuremath{\ge}$ 0.10. These results suggest that competitions between easy-axis and easy-plane anisotropy fields play a key role in generating stable magnetoelectric polarization in Y-type hexaferrites.