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Atomically engineered ferroic layers yield a room-temperature magnetoelectric multiferroic

Nature volume 537, pages 523–527 (2016)Cite this article

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Abstract

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 magnetism1,2. Such materials are exceedingly rare, however, owing to competing requirements for displacive ferroelectricity and magnetism3. Despite the recent identification of several new multiferroic materials and magnetoelectric coupling mechanisms4,5,6,7,8,9,10,11,12,13,14,15, 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 applications2. 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 LuFeO3—the geometric ferroelectric with the greatest known planar rumpling16—we introduce individual monolayers of FeO during growth to construct formula-unit-thick syntactic layers of ferrimagnetic LuFe2O4 (refs 17, 18) within the LuFeO3 matrix, that is, (LuFeO3)m/(LuFe2O4)1 superlattices. The severe rumpling imposed by the neighbouring LuFeO3 drives the ferrimagnetic LuFe2O4 into a simultaneously ferroelectric state, while also reducing the LuFe2O4 spin frustration. This increases the magnetic transition temperature substantially—from 240 kelvin for LuFe2O4 (ref. 18) to 281 kelvin for (LuFeO3)9/(LuFe2O4)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.

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Figure 1: HAADF-STEM images.
Figure 2: Magnetic and ferroelectric characterization of (LuFeO3)m/(LuFe2O4)n superlattices.
Figure 3: First-principles calculations of the spin configuration of LuFe2O4.
Figure 4: Magnetoelectric coupling in the (LuFeO3)9/(LuFe2O4)1 superlattice.

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Acknowledgements

We acknowledge discussions with G. Stiehl, R. Haislmaier, A. SenGupta, V. Gopalan, W. Wang, W. Wu and E. Barnard and technical support with the electron microscopy from E. J. Kirkland, M. Thomas and J. Grazul. Research primarily supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award No. DE-SC0002334, which supported the work of J.A.Mu. (2010–2014), C.M.B., M.E.H., J.A.Mo., H.D., A.F.R., R.He., Q.M., H.P., R.M., C.J.F., P.S., D.A.M. and D.G.S. Substrate preparation was performed in part at the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant ECCS-15420819). The electron microscopy studies made use of the electron microscopy facility of the Cornell Center for Materials Research, a National Science Foundation (NSF) Materials Research Science and Engineering Centers programme (DMR 1120296) and NSF IMR-0417392. X-ray dichroism was performed at the Advanced Light Source at Lawrence Berkeley National Laboratory. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No. DE-AC02-05CH11231. J.A.Mu. acknowledges fellowship support from the Army Research Office in the form of a National Defense Science and Engineering Graduate Fellowship and from the National Science Foundation in the form of a Graduate Research Fellowship. J.A.Mu. was funded (July 2015–) by C-SPINS, one of six centres of STARnet, a Semiconductor Research Corporation programme, sponsored by MARCO and DARPA. J.T.H. acknowledges support from the Semiconductor Research Corporation (SRC) under grant 2014-IN-2534. J.D.C. acknowledges support from SRC-FAME, one of six centres of STARnet, a Semiconductor Research Corporation programme sponsored by MARCO and DARPA. S.M.D. acknowledges the support of a National Research Council NIST postdoctoral research associateship. Z.L. acknowledges support from the NSF under Grant No. EEC-1160504 NSF Nanosystems Engineering Research Center for Translational Applications of Nanoscale Multiferroic Systems (TANMS). A.F. is supported by the Swiss National Science Foundation. R.Ho. and L.F.K. acknowledge support by the David and Lucile Packard Foundation. E.P. acknowledges support from the National Science Foundation in the form of a Graduate Research Fellowship (DGE-1144153). Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

Author information

Author notes

  1. Julia A. Mundy, Charles M. Brooks and Megan E. Holtz: These authors contributed equally to this work.

Authors and Affiliations

  1. School of Applied and Engineering Physics, Cornell University, Ithaca, 14853, New York, USA

    Julia A. Mundy, Megan E. Holtz, Hena Das, Alejandro F. Rébola, Robert Hovden, Elliot Padgett, Qingyun Mao, Lena F. Kourkoutis, Craig J. Fennie & David A. Muller

  2. Department of Materials Science and Engineering, Cornell University, Ithaca, 14853, New York, USA

    Charles M. Brooks, John T. Heron, Rainer Held, Hanjong Paik & Darrell G. Schlom

  3. Department of Physics and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, 61801, Illinois, USA

    Jarrett A. Moyer & Peter Schiffer

  4. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, 48103, Michigan, USA

    John T. Heron

  5. Department of Materials Science and Engineering, University of California, Berkeley, 94720, California, USA

    James D. Clarkson, Zhiqi Liu & Ramamoorthy Ramesh

  6. NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, 20899, Maryland, USA

    Steven M. Disseler, Julie A. Borchers & William D. Ratcliff

  7. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, 94720, California, USA

    Alan Farhan, Elke Arenholz & Andreas Scholl

  8. Department of Physics, Pennsylvania State University, University Park, 16802, Pennsylvania, USA

    Rajiv Misra

  9. Kavli Institute at Cornell for Nanoscale Science, Ithaca, 14853, New York, USA

    Lena F. Kourkoutis, David A. Muller & Darrell G. Schlom

  10. Department of Physics, University of California, Berkeley, 94720, California, USA

    Ramamoorthy Ramesh

  11. Materials Sciences Division, Lawrence Berkeley National Laboratory, California, 94720, USA

    Ramamoorthy Ramesh

Authors
  1. Julia A. Mundy
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  2. Charles M. Brooks
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Contributions

The thin films were synthesized by C.M.B. and J.A.Mu. with assistance from R.He. and H.P. DFT calculations were performed by H.D., A.F.R. and C.J.F. The films were characterized by SQUID by J.A.Mo., R.M. and P.S.; by STEM by M.E.H., J.A.Mu., R.Ho., E.P., L.F.K. and D.A.M.; by variable-temperature STEM by Q.M., M.E.H. and D.A.M.; by neutron scattering by S.M.D., J.A.B. and W.D.R.; by transport by J.T.H.; by PFM by J.D.C., J.T.H. and R.R.; by X-ray spectroscopy by J.A.Mu., Z.L. and E.A.; by PEEM by A.F., Z.L., J.D.C., R.R. and A.S. J.A.Mu., C.J.F. and D.G.S. wrote the manuscript. The study was conceived and guided by D.G.S. All authors discussed results and commented on the manuscript.

Corresponding author

Correspondence to Darrell G. Schlom.

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Competing interests

The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks M. Fiebig, T. Kimura and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 X-ray diffraction characterization of the (LuFeO3)m/(LuFe2O4)n superlattices.

a, θ–2θ XRD scans for the (LuFeO3)m/(LuFe2O4)n films for which either n or m is equal to 1. The composition is labelled (m-n) on the right. The asterisk (*) indicates the 111 XRD peak from the (111) YSZ substrate. b, Rocking-curve XRD scan of the 005 film peak of the (LuFeO3)1/(LuFe2O4)1 film (blue) compared with the 111 peak of the YSZ substrate (black). FWHM, full-width at half-maximum.

Extended Data Figure 2 Relation between the lutetium displacements and polarization.

The magnitude of the lutetium displacement d can be measured by HAADF-STEM. Using first-principles calculations, this displacement can be directly related to the polarization of the structure. Lutetium is shown in turquoise, iron in yellow and oxygen in brown.

Extended Data Figure 3 Magnetic characterization of the (LuFeO3)m/(LuFe2O4)n superlattices.

a, M–T curves for a series of (LuFeO3)m/(LuFe2O4)1 superlattices cooled in a 1-kOe field. b, M–T curves for a series of (LuFeO3)1/(LuFe2O4)n superlattices cooled in a 1-kOe field. c, The “excess magnetization” is found by subtracting the bulk magnetization of the LuFe2O4 and LuFeO3 from the measured moment. It is plotted normalized to the number of iron atoms in the LuFe2O4 layers in the sample. The composition is plotted according to the fraction of iron atoms in the LuFeO3 layers in the (LuFeO3)m(LuFe2O4)n structure. d, Loops of the magnetization M as a function of the magnetic field H for the (LuFeO3)9/(LuFe2O4)1 superlattice. The M–H loop at 300 K has a distinctly different shape that is more reminiscent of the 250-K loop, demonstrating that ferromagnetic (or ferrimagnetic) fluctuations still exist at 300 K even if the entire film is not ferromagnetic (or ferrimagnetic). e, The saturation magnetization of the (LuFeO3)9/(LuFe2O4)1 superlattice at 70 KOe as a function of temperature. Although the remanent magnetization, as measured by the field-cooled curve, disappears around the Curie temperature of 281 K, ferromagnetic (or ferrimagnetic) fluctuations remain in this sample to temperatures above room temperature.

Extended Data Figure 4 Neutron diffraction of the (LuFeO3)6/(LuFe2O4)2 superlattice.

a, Magnetic reflections for the (LuFeO3)6/(LuFe2O4)2 superlattice were observed in neutron diffraction by scanning along the [10L] direction in reciprocal space at several temperatures between 5 K and 325 K. A single peak is observed showing considerable change in intensity between 5 K and room temperature. The offset from the 101 position is due to a slight misalignment of the sample. r.l.u. in a denotes reciprocal lattice units. b, Integrated intensity of the 101 magnetic reflection for the (LuFeO3)6/(LuFe2O4)2 superlattice as a function of temperature. The solid line is the mean-field fit. Error bars in a and b represent one standard deviation.

Extended Data Figure 5 HAADF-STEM images of the (LuFeO3)m/(LuFe2O4)1 superlattices.

a–d, Coloured overlays represent the local polarization for m = 1 (a), m = 3 (b), m = 7 (c) and m = 9 (d). Turquoise atoms have positive polarization and red atoms have negative polarization, as indicated by the colour bars. For each row of lutetium atoms, the mean lutetium displacement is plotted, with the bar representing the 20%–80% spread of the root-mean-square displacement. The colour of the bar indicates the direction of polarization.

Extended Data Figure 6 Quantification of the ferroelectric displacements from HAADF-STEM images.

After identifying the position of the lutetium atom with sub-ångström precision, it is compared to the neighbouring atoms and the displacement is calculated. a, Schematics of the ‘down’, ‘up’ and non-polar polarization states. b, Average displacement of the lutetium atoms as a function of the number of LuFeO3 layers m in the (LuFeO3)m/(LuFe2O4)1 structure. The displacement of the end-member LuFeO3 is shown for reference; this displacement of 29 pm corresponds to approximately 4.3 μC cm−2. Error bars in a and b are s.e.m. c, A comparison of the distortion observed in the middle of the LuFeO3 block to those in the edge layers, for example, those adjacent to the LuFe2O4 bilayers. d, In situ TEM heating experiment of the (LuFeO3)m/(LuFe2O4)n superlattices. We infer the ferroelectric phase from where distortions in the lutetium rows are resolved. With increasing temperature, ferroelectricity disappears starting with lower m. Above T = 675 K, we see no ferroelectric distortions; however, the electrical noise in the images at these temperatures is quite large.

Extended Data Figure 7 X-ray linear dichroic spectroscopy of the Fe L2,3 edge.

a, b, The X-ray adsorption spectra for in-plane (blue) and out-of-plane (red) linearly polarized radiation are plotted in the top panels for the (LuFeO3)9/(LuFe2O4)1 (a) and (LuFeO3)1/(LuFe2O4)3 (b) superlattices at 300 K. The difference between the normalized spectra (black, bottom panels) is also plotted for each case. For the (LuFeO3)9/(LuFe2O4)1 sample, the peak dichroism is about 40% whereas the peak dichroism is only about 20% for the (LuFeO3)1/(LuFe2O4)3 superlattice.

Extended Data Figure 8 Exchange interactions in the COII structure of LuFe2O4.

a, Schematic of the COII LuFe2O4 structure with intra-layer, inter-layer and in-plane interactions labelled. The Fe–O–Fe bond angles in the undistorted structure are indicated by the black arrows. The red arrows demonstrate the change to the bond angles as the distortions turn on. Lutetium, Fe3+, Fe2+ and oxygen are shown in turquoise, yellow, green and brown, respectively. b, Calculated exchange interactions as a function of the lutetium distortion Q. Circles, squares and diamonds denote the DFT-estimated value of the exchange interactions between two Fe2+ spins, two Fe3+ spins and Fe2+–Fe3+ spins, respectively. We considered in-plane interactions, intra-bilayer interactions and the interaction between two FeO2 bilayers.

Extended Data Figure 9 Spin configurations of the COI and COII structures of LuFe2O4.

a, Left, calculated density of states (DOS) for LuFe2O4 with the COI magnetic ground state, along with the occupancy of the iron 3d channel. Upper and lower panels show the DOS for the Fe2+ and Fe3+ ions, respectively. Oxygen 2p states are plotted in each case. Right, the crystal field splitting from the trigonal bipyramid symmetry and occupancy of the iron 3d channel. b, Low-energy spin configurations of COI and COII states labelled with the corresponding magnetization. Although the ground states of COI and COII have magnetizations of 0.5μB/Fe and 1.17μB/Fe, respectively, each has additional low-energy configurations with M ranging from 0μB/Fe to 1.17μB/Fe. Lutetium, Fe3+, Fe2+ and oxygen are shown in turquoise, yellow (spins in red), green (spins in blue) and brown, respectively. c, Low-energy spin configurations of hole-doped COI and COII states labelled with the corresponding magnetization.

Extended Data Figure 10 Calculated stable structures for LuFe2O4 and the (LuFeO3)3/(LuFe2O4)1 superlattice.

Monoclinic structures of the LuFe2O4 system containing charge-ordered Fe2+/Fe3+. a, The antiferroelectric charge-ordered state (COI); b, the ferroelectric charge-ordered state (COII); and c, the non-polar charge-ordered state (COIII). Panels a and b are shown in Fig. 3a and b, respectively. d–f, Single-domain (d) and undoped-type (e) and doped-type structures of the (LuFeO3)3/(LuFe2O4)1 structure. Electrons transfer from the LuFe2O4 layers to the LuFeO3 layers in the doped-type configuration (orange arrows). The doped-type configuration also stabilizes charged ferroelectric domain walls. The density of states for the Fe3+ and Fe2+ ions are plotted in f in yellow and green, respectively. Lutetium, Fe3+, Fe2+ and oxygen are shown in turquoise or red (depending on the ferroelectric polarization), yellow, green and brown, respectively.

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Mundy, J., Brooks, C., Holtz, M. et al. Atomically engineered ferroic layers yield a room-temperature magnetoelectric multiferroic. Nature 537, 523–527 (2016). https://doi.org/10.1038/nature19343

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