We show that low field magnetoelectric (ME) properties of helimagnets ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{1.5}{\mathrm{Zn}}_2({\mathrm{Fe}}_{1-x}{\mathrm{Al}}_x{)}_{12}{\mathrm{O}}_{22}$ can be efficiently tailored by the Al-substitution level. As $x$ increases, the critical magnetic field for switching electric polarization is systematically reduced from $\sim 1\mathrm{T}$ down to $\sim 1\mathrm{mT}$, and the ME susceptibility is greatly enhanced to reach a giant value of $2.0\times {10}^4\mathrm{ps}/\mathrm{m}$ at an optimum $x=0.08$. We find that control of the nontrivial orbital moment in the octahedral Fe sites through the Al substitution is crucial for fine-tuning the magnetic anisotropy and obtaining the conspicuously improved ME characteristics.

• Received 5 August 2009
• Corrected 21 January 2010

DOI:https://doi.org/10.1103/PhysRevLett.104.037204