good refinement for x = 0.06 when the fraction of Al ions at the octahedral (tetrahedral) sites are 0.9920 (0.0080), directly supporting that Al ions mostly replaces the oc- tahedral Fe sites for x = 0.06. These results support that Al ions in Ba 0 . 5 Sr 1 . 5 Zn 2 (Fe 1$_{−}$ $_{x}$Al$_{x}$ ) 12 O 22 highly prefer to occupy the octahedral Fe sites. 

Moreover, magnetic circular dichroism measurements at the Fe L$_{2}$$_{,}$$_{3}$ -edge in the undoped sample revealed a con- siderable orbital magnetic moment of 0.30 ± 0.1 µ$_{B}$ /f.u. along the ab -plane [20]. Note that ionic Fe 3+ has a half- filled d 5 configuration and its total angular momentum L = 0. Thus the orbital moment is expected to vanish. If the Fe 3+ ion, however, is not at the center of an octa- hedron, i.e., off-centered, then the hybridization with O 2 p becomes anisotropic and contributes a non-vanishing orbital moment, as observed in GaFeO 3 [21]. Accord- ing to our structural refinements as well as those in Ref. [17], the Fe 3+ ions in the green octahedra (Fig. 1(a)) are indeed off-centered, being shifted along the c -axis, which results in the nontrivial orbital moment and the easy spin axis in the ab -plane. Based on the octahedral site prefer- ence of the Al 3+ ions, it is likely that the main role of the Al-substitution is to reduce the magnetic anisotropy en- ergy, through either dilution of the off-centered Fe 3+ sites or reduction of their off-centering deformation. Indeed, subsequent measurements for a x = 0.08 sample showed decrease of the in-plane orbital magnetic moment by ∼ 20 % [20]. As a consequence, the magnetic moment can tilt away from the ab -plane more easily with Al-substitution, resulting in reduced B$_{c}$$_{,}$$_{L}$ at low x , and the heliconical spin state seems to emerge for x = 0.06 - 0.08 with re- duced in-plane anisotropy. Therefore, decrease of the in- plane orbital moment with the Al-substitution seems to be an essential process for lowering magnetic anisotropy and thus improving the low field ME response [22]. 

The mechanism uncovered here is also applicable to Ba 2 Mg 2 Fe 12 O 22 [15, 16], in which Mg 2+ ions occupy both tetrahedral and octahedral Fe sites in an arbi- trary fraction. Here, the partial occupation of Mg 2+ in the octahedral Fe-sites seems to also reduce the mag- netic anisotropy [23], via the decrease of the orbital mo- ment. In comparison, Al-substitution in BSZFO seems to be more effective for systematic control of magnetic anisotropy and B$_{c}$$_{,}$$_{L}$ as it can directly control the orbital moment in the system. 

Finally, it is noteworthy that the helical spin order- ing temperatures of Al-substituted BSZFO remain rela- tively high (Fig. 4(b)); for instance, T$_{N}$ = 337 K at x = 0.00 and 263 K at x = 0.08. Although the FE phase boundaries could not be determined above 220 K due to electrical leakage, we expect that the low B -induced FE switching will be observable up to T$_{N}$ upon further optimization of the resistivity. In our recent work on a heat-treated BSZFO sample, B$_{c}$$_{,}$$_{L}$ could be determined up to T$_{N}$ = 315 K due to dramatic increase of resis- tivity [24]. Therefore, there is high hope that the Al- 

substituted BSZFO also exhibits low B -induced ferro- electricity even near room temperature. 

In summary, we have shown that Al-substitution into hexaferrites provides an unprecedented opportunity to decrease effectively the critical magnetic field for inducing ferroelectricity as well as to obtain gigantic low field mag- netoelectricity through the control of orbital moment. Upon optimization of spin ordering temperature and elec- trical resistivity, the helimagnetic insulators may provide a pathway to the realization of novel magnetoelectric de- vices at room temperature under low magnetic fields. 

This work is supported by NRL (M10600000238), GPP (K20702020014-07E0200-01410), and Basic Sci- ence Research (2009-0083512) programs. JHP is sup- ported by National Creative Initiative Center and WCU program (R31-2008-000-10059-0) and JHC by KOSEF (20090059529) and BAERI programs. SHC is supported by Seoul R&BD (10543) and Seoul Science Fellowship. 

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