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.
At a glance
- The renaissance of magnetoelectric multiferroics. Science 309, 391–392 (2005) &
- Multiferroic and magnetoelectric materials. Nature 442, 759–765 (2006) , &
- Why are there so few magnetic ferroelectrics? J. Phys. Chem. B 104, 6694–6709 (2000)
- Some Properties of ferromagnetoelectric nickel-iodine boracite, Ni3B7O13I. J. Appl. Phys. 37, 1404–1405 (1966) , , &
- Magnetic control of ferroelectric polarization. Nature 426, 55–58 (2003) et al.
- Epitaxial BiFeO3 multiferroic thin film heterostructures. Science 299, 1719–1722 (2003) et al.
- A strong ferroelectric ferromagnet created by means of spin-lattice coupling. Nature 466, 954–958 (2010) et al.
- Strain-induced artificial multiferroicity in Pb(Zr0.53Ti0.47)O3/Pb(Fe0.66W0.33)O3 layered nanostructure at ambient temperature. J. Mater. Sci. 44, 5113–5119 (2009) , , , &
- Near-room temperature relaxor multiferroic. Appl. Phys. Lett. 97, 202910 (2010) , , , &
- Room temperature ferroelectric and magnetic investigations and detailed phase analysis of Aurivillius phase Bi5Ti3Fe0.7CoO15 thin films. J. Appl. Phys. 112, 052010 (2012) et al.
- A polar corundum oxide displaying weak ferromagnetism at room temperature. J. Am. Chem. Soc. 134, 3737–3747 (2012) et al.
- Deterministic switching of ferromagnetism at room temperature using an electric field. Nature 516, 370–373 (2014) et al.
- Near room-temperature multiferroic materials with tunable ferromagnetic and electrical properties. Nat. Commun. 5, 4021 (2014) et al.
- Tilt engineering of spontaneous polarization and magnetization above 300 K in a bulk layered perovskite. Science 347, 420–424 (2015) et al.
- Designing switchable polarization and magnetization at room temperature in an oxide. Nature 525, 363–366 (2015) et al.
- Bulk magnetoelectricity in the hexagonal manganites and ferrites. Nat. Commun. 5, 2998 (2014) , , , &
- Ferroelectricity from iron valence ordering in the charge-frustrated system LuFe2O4. Nature 436, 1136–1138 (2005) et al.
- Three-dimensional magnetic correlations in multiferroic LuFe2O4. Phys. Rev. Lett. 100, 107601 (2008) et al.
- Oxide interfaces—an opportunity for electronics. Science 327, 1607–1611 (2010) &
- Dielectric properties of charge-ordered LuFe2O4 revisited: the apparent influence of contacts. Phys. Rev. Lett. 109, 016405 (2012) , , , &
- Intrinsic electrical properties of LuFe2O4. Phys. Rev. B 88, 085130 (2013) et al.
- XRD and HREM studies of epitaxially stabilized hexagonal orthoferrites RFeO3 (R = Eu–Lu). Chem. Mater. 16, 1751–1755 (2004) et al.
- Noncentrosymmetric structure of LuFeO3 in metastable state. Jpn. J. Appl. Phys. 49, 09ME06 (2010) , , , &
- Room-temperature multiferroic hexagonal LuFeO3 films. Phys. Rev. Lett. 110, 237601 (2013) et al.
- Magnetic structure and ordering of multiferroic hexagonal LuFeO3. Phys. Rev. Lett. 114, 217602 (2015) et al.
- Direct observation of interlocked domain walls in hexagonal RMnO3 (R =Tm, Lu). Phys. Rev. B 85, 020102 (2012) et al.
- Unit-cell scale mapping of ferroelectricity and tetragonality in epitaxial ultrathin ferroelectric films. Nat. Mater. 6, 64–69 (2007) et al.
- Atomic-resolution chemical imaging of oxygen local bonding environments by electron energy loss spectroscopy. Appl. Phys. Lett. 101, 042907 (2012) , , , &
- The adsorption-controlled growth of LuFe2O4 by molecular-beam epitaxy. Appl. Phys. Lett. 101, 132907 (2012) et al.
- Magnetic property of single crystal Lu2Fe3O7. J. Magn. Magn. Mater. 104–107, 827–828 (1992) , &
- Probing ferroelectricity in PbZr0.2Ti0.8O3 with polarized soft x rays. Phys. Rev. B 82, 140103(R) (2010) et al.
- X-ray linear dichroism dependence on ferroelectric polarization. J. Phys. Condens. Matter 24, 245902 (2012) et al.
- Intrinsic magnetic properties of hexagonal LuFeO3 and the effects of nonstoichiometry. APL Mater. 2, 012106 (2014) et al.
- Hard and soft x-rays XAS characterization of charge ordered LuFe2O4. J. Phys. Conf. Ser. 592, 012121 (2015) et al.
- A decade of piezoresponse force microscopy: progress, challenges, and opportunities. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 53, 2226–2252 (2006) , &
- Scanning Probe Microscopy: Electrical and Electromechanical Phenomena at the Nanoscale Vol. 1 (eds & ) 173–214 (Springer, 2007) , , & in
- Electromechanical imaging and spectroscopy of ferroelectric and piezoelectric materials: state of the art and prospects for the future. J. Am. Ceram. Soc. 92, 1629–1647 (2009) , , &
- Structures of LuFeCoO4 and LuFe2O4. Acta Crystallogr. C 46, 1917–1918 (1990) , , &
- Enhanced Curie temperatures and magnetoelastic domains in Dy/Lu superlattices and films. Phys. Rev. Lett. 70, 3502–3505 (1993) et al.
- Strain-dependent magnetic phase diagram of epitaxial La0.67Sr0.33MnO3 thin films. Appl. Phys. Lett. 76, 2421–2423 (2000) , , &
- Oxygen stoichiometry and magnetic properties of LuFe2O4+δ. J. Appl. Phys. 113, 063909 (2013) et al.
- Charge order in LuFe2O4: an unlikely route to ferroelectricity. Phys. Rev. Lett. 108, 187601 (2012) et al.
- Competing ferri- and antiferromagnetic phases in geometrically frustrated LuFe2O4. Phys. Rev. Lett. 108, 037206 (2012) et al.
- Two-dimensional magnetic order in hexagonal LuFe2O4. J. Phys. Colloq. 49, 1497–1498 (1988) , , , &
- First-principles calculations of the electronic structure and spectra of strongly correlated systems: the LDA + U method. J. Phys. Condens. Matter 9, 767–808 (1997) , &
- Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996) , &
- Ab initio molecular dynamics for liquid metals. Phys. Rev. B 47, 558–561 (1993) &
- Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 54, 11169–11186 (1996) &
- WIEN2k: An Augmented Plane Wave Plus Local Orbitals Program for Calculating Crystal Properties http://www.wien2k.at/reg_user/textbooks/usersguide.pdf (Tech. Univ. Wien, 2002) , , , &
- Cryogenic PEEM at the advanced light source. J. Electron Spectrosc. Relat. Phenom. 185, 340–346 (2012) et al.
- Element-specific magnetic microscopy with circularly polarized X-rays. Science 259, 658–661 (1993) et al.
Extended data figures and tables
Extended Data Figures
- Extended Data Figure 1: X-ray diffraction characterization of the (LuFeO3)m/(LuFe2O4)n superlattices. (358 KB)
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. (122 KB)
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. (355 KB)
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. (174 KB)
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. (482 KB)
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. (293 KB)
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. (168 KB)
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. (244 KB)
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. (700 KB)
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. (691 KB)
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.