22 SEPTEMBER 2016 | VOL 537 | NATURE | 523 LETTER doi:10.1038/nature19343 Atomically engineered ferroic layers yield a room- temperature magnetoelectric multiferroic Julia A. Mundy 1 *, Charles M. Brooks 2 *, Megan E. Holtz 1 *, Jarrett A. Moyer 3 , Hena Das 1 , Alejandro F. Rébola 1 , John T. Heron 2,4 , James D. Clarkson 5 , Steven M. Disseler 6 , Zhiqi Liu 5 , Alan Farhan 7 , Rainer Held 2 , Robert Hovden 1 , Elliot Padgett 1 , Qingyun Mao 1 , Hanjong Paik 2 , Rajiv Misra 8 , Lena F. Kourkoutis 1,9 , Elke Arenholz 7 , Andreas Scholl 7 , Julie A. Borchers 6 , William D. Ratcliff 6 , Ramamoorthy Ramesh 5,10,11 , Craig J. Fennie 1 , Peter Schiffer 3 , David A. Muller 1,9 & Darrell G. Schlom 2,9 Materials that exhibit simultaneous order in their electric and magnetic ground states hold promise for use in next-generation memory devices in which electric fields control magnetism 1,2 . Such materials are exceedingly rare, however, owing to competing requirements for displacive ferroelectricity and magnetism 3 . Despite the recent identification of several new multiferroic materials and magnetoelectric coupling mechanisms 4–15 , known single- phase multiferroics remain limited by antiferromagnetic or weak ferromagnetic alignments, by a lack of coupling between the order parameters, or by having properties that emerge only well below room temperature, precluding device applications 2 . Here we present a methodology for constructing single-phase multiferroic materials in which ferroelectricity and strong magnetic ordering are coupled near room temperature. Starting with hexagonal LuFeO 3 —the geometric ferroelectric with the greatest known planar rumpling 16 — we introduce individual monolayers of FeO during growth to construct formula-unit-thick syntactic layers of ferrimagnetic LuFe 2 O 4 (refs 17, 18) within the LuFeO 3 matrix, that is, (LuFeO 3 ) m / (LuFe 2 O 4 ) 1 superlattices. The severe rumpling imposed by the neighbouring LuFeO 3 drives the ferrimagnetic LuFe 2 O 4 into a simultaneously ferroelectric state, while also reducing the LuFe 2 O 4 spin frustration. This increases the magnetic transition temperature substantially—from 240 kelvin for LuFe 2 O 4 (ref. 18) to 281 kelvin for (LuFeO 3 ) 9 /(LuFe 2 O 4 ) 1 . Moreover, the ferroelectric order couples to the ferrimagnetism, enabling direct electric-field control of magnetism at 200 kelvin. Our results demonstrate a design methodology for creating higher-temperature magnetoelectric multiferroics by exploiting a combination of geometric frustration, lattice distortions and epitaxial engineering. Advances in thin-film deposition have enabled materials to be rationally designed at the atomic-scale where the local chemistry, bonding and electronic environment can be tailored to stabilize emergent phenomena 19 . Here we exploit such techniques to directly perturb the structural environment of the frustrated hexagonal ferrimagnet LuFe 2 O 4 at the sub-ångström scale, tuning the magnetic order to construct a new magnetoelectric multiferroic. LuFe 2 O 4 was purported to be simultaneously ferrimagnetic and ferroelectric at 250 K—the highest temperature of any known material 17 . Although its ferrimagnetic ordering is widely affirmed 18 , recent studies find that LuFe 2 O 4 is not ferroelectric 20,21 . A robust high-temperature fer- roelectric with a closely related structure exists, however: hexagonal LuFeO 3 . Although metastable, hexagonal LuFeO 3 has been grown in thin-film form by epitaxial stabilization 22 . Isostructural to YMnO 3 , 1 School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA. 2 Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA. 3 Department of Physics and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA. 4 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48103, USA. 5 Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA. 6 NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA. 7 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. 8 Department of Physics, Pennsylvania State University, University Park, Pennsylvania 16802, USA. 9 Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA. 10 Department of Physics, University of California, Berkeley, California 94720, USA. 11 Materials Sciences Division, Lawrence Berkeley National Laboratory, California 94720, USA. *These authors contributed equally to this work. a LuFe 2 O 4 LuFeO 3 2 nm b Lu Fe O (LuFeO 3 ) m /(LuFe 2 O 4 ) 1 superlattices m = 1 m = 2 m = 3 m = 4 m = 5 m = 6 m = 7 m = 8 m = 9 m = 10 Figure 1 | HAADF-STEM images. a, End-members LuFe 2 O 4 (left) and LuFeO 3 (right). b, (LuFeO 3 ) m /(LuFe 2 O 4 ) 1 superlattice series for 1 ≤ m ≤ 10. Samples are imaged along the LuFeO 3 P6 3 cm [100] zone axis. LuFe 2 O 4 is imaged down the equivalent zone axis, which, owing to the primitive unit cell of LuFe 2 O 4 , is the [120] zone axis. Schematics of the LuFe 2 O 4 and LuFeO 3 crystal structures are shown in a with lutetium (Lu), iron (Fe) and oxygen (O) in turquoise, yellow and brown, respectively. © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.