arXiv:1001.1226v2 [cond-mat.mtrl-sci] 21 Jan 2010 Realization of giant magnetoelectricity in helimagnets We show that low field magnetoelectric (ME) properties of helimagnets Ba 0 . 5 Sr 1 . 5 Zn 2 (Fe 1$_{−}$ x Al x ) 12 O 22 can be efficiently tailored by Al-substitution level. As x in- creases  the critical magnetic field for switching electric polarization is systematically reduced from ∼ 1 T down to ∼ 1 mT  and the ME susceptibility is greatly enhanced to reach a giant value of 2.0 × 10 4 ps/m at an optimum x = 0.08. We find that control of nontrivial orbital moment in the octahedral Fe sites through the Al-substitution is crucial for fine tuning of magnetic anisotropy and obtaining the conspicuously improved ME characteristics. PACS numbers: 75.80.+q  77.80.Fm  71.45.Gm  75.30.Gw Sae Hwan Chun  1 Yi Sheng Chai  1 Yoon Seok Oh  1 Deepshikha Jaiswal-Nagar  1 So Young Haam  1 Ingyu Kim  1 Bumsung Lee  1 Dong Hak Nam  1 Kyung-Tae Ko  2 Jae-Hoon Park  2 Jae-Ho Chung  3 and Kee Hoon Kim 1 $^{1}$FPRD  Department of Physics and Astronomy  Seoul National University  Seoul 151-747  South Korea $^{2}$Department of Physics & Division of Advanced Materials Science  POSTECH  Pohang 790-784  South Korea $^{3}$Department of Physics  Korea University  Seoul 136-713  South Korea In recent years  extensive researches on multiferroics have been performed with motivations to understand the nontrivial cross-coupling mechanism between magnetism and ferroelectricity as well as to search for new materials applicable in next-generation devices [1–8]. Numerous studies have focused particularly on the class of so-called magnetic ferroelectrics [1–3  9  10]  in which ferroelec- tricity is induced by magnetic order through either in- verse Dzyaloshinskii-Moriya effects [6–8] or the exchange striction mechanism [10]. Although dramatic variation of electric polarization P with magnetic field B   often re- alized in the magnetic ferroelectrics [3]  might be useful for application  the phenomena occur mostly at low tem- peratures [1  2  9  10] and related ME susceptibility is yet too small [5]. Hence  it is a longstanding challenge in the research of multiferroics to improve both the operating temperature [11  12] and the ME sensitivity [13–15]. The hexaferrite Ba 0 . 5 Sr 1 . 5 Zn 2 Fe 12 O 22 (BSZFO) with helical spin order is currently a unique candidate that can show the B -induced ferroelectricity above room tempera- ture up to ∼ 340 K [12]. However  at 300 K  its ferroelec- tric (FE) phase is expected to emerge at B ∼ 1 T  which is too high for memory applications. Moreover  its ME coupling is rather weak; the MES α$_{ME}$ ≡ µ$_{0}$ dP dB at 30 K shows a maximum value of ∼ 1.3 × 10 3 ps/m at B ∼ 400 mT [12]  which is one or two orders of magnitude smaller than the highest α$_{ME}$ ∼ 10 4 - 10 5 ps/m realized in hetero- geneous films [13] or strain-coupled composites [5  14]. When Zn is replaced by Mg to form a Mg 2 Y-type hex- aferrite  Ba 2 Mg 2 Fe 12 O 22   the critical magnetic field for inducing P becomes extremely low  ∼ 30 mT [15  16]. On the other hand  a maximum operation temperature of the Mg 2 Y-type hexaferrite is expected to be lower than 195 K. Moreover  a microscopic understanding for the low- ered critical magnetic field remains unclear. Therefore  systematic studies are further required to understand the origin of the intricate ME effect and improve multiferroic properties in the hexaferrite system. In this letter  we demonstrate that Al-substituted Single crystals of Ba 0 . 5 Sr 1 . 5 Zn 2 (Fe 1$_{−}$ $_{x}$Al$_{x}$ ) 12 O 22 were grown from Na 2 O-Fe 2 O 3 flux in air [17]. Crystals were cut into a rectangular form for electric polarization P measurements along the ab -plane while B was applied along the direction normal to the P vector in the ab - plane. Before the ME current measurements  each spec- imen was electrically poled in its paraelectric state  for BSZFO  i.e. Ba 0 . 5 Sr 1 . 5 Zn 2 (Fe 1$_{−}$ $_{x}$Al$_{x}$ ) 12 O 22 greatly im- proves the multiferroic characteristics  resulting in the highest α$_{ME}$ of single-phase multiferroics near-zero mag- netic field. We find that predominant substitution of Al ions into octahedral Fe sites with nontrivial orbital mo- ment is crucial for fine tuning of the magnetic anisotropy and thus for tailoring the ME coupling. FIG. 1. (a) Crystal structure of the Zn 2 Y type-hexaferrite Ba 0 . 5 Sr 1 . 5 Zn 2 (Fe 1$_{−}$ x Al x ) 12 O 22 that has alternating stacks of magnetic S and L blocks along the c -axis. Schematic illus- tration of rotating magnetic moments in the L ( ⃗ µ$_{L}$ ) and S blocks ( ⃗ µ$_{S}$ ) in the (b) helical ( x = 0.00) and (c) heliconical ( x = 0.08) phases under in-plane B = 0 T and 10 mT. ⃗ k 0 is the spin modulation wave vector parallel to [001]. (c) (b) [100] [010] [001] (a) S$_{4}$ L$_{3}$ S$_{3}$ S$_{2}$ S$_{1}$ L$_{2}$ L$_{1}$ S$_{2}$ S$_{1}$ L$_{2}$ L$_{1}$ tetrahedral site octahedral off-centered  octahedral x  = 0.00 x  = 0.08 B  = 0 B  = 10 mT P  = 0 P  ≠ 0 B  = 0 B  = 10 mT k$_{0}$ S$_{2}$ S$_{1}$ L$_{2}$ L$_{1}$ µ$_{S}$ µ$_{L}$ P  = 0 P  = 0 k$_{0}$ k$_{0}$ k$_{0}$ Ba$_{0.5}$Sr$_{1.5}$Zn$_{2}$(Fe$_{1-}$$_{x}$$_{ }$Al$_{x}$)$_{12}$O$_{22}$