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Deterministic switching of ferromagnetism at room temperature using an electric field

Abstract

The technological appeal of multiferroics is the ability to control magnetism with electric field1,2,3. For devices to be useful, such control must be achieved at room temperature. The only single-phase multiferroic material exhibiting unambiguous magnetoelectric coupling at room temperature is BiFeO3 (refs 4 and 5). Its weak ferromagnetism arises from the canting of the antiferromagnetically aligned spins by the Dzyaloshinskii–Moriya (DM) interaction6,7,8,9. Prior theory considered the symmetry of the thermodynamic ground state and concluded that direct 180-degree switching of the DM vector by the ferroelectric polarization was forbidden10,11. Instead, we examined the kinetics of the switching process, something not considered previously in theoretical work10,11,12. Here we show a deterministic reversal of the DM vector and canted moment using an electric field at room temperature. First-principles calculations reveal that the switching kinetics favours a two-step switching process. In each step the DM vector and polarization are coupled and 180-degree deterministic switching of magnetization hence becomes possible, in agreement with experimental observation. We exploit this switching to demonstrate energy-efficient control of a spin-valve device at room temperature. The energy per unit area required is approximately an order of magnitude less than that needed for spin-transfer torque switching13,14. Given that the DM interaction is fundamental to single-phase multiferroics and magnetoelectrics3,9, our results suggest ways to engineer magnetoelectric switching and tailor technologically pertinent functionality for nanometre-scale, low-energy-consumption, non-volatile magnetoelectronics.

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Figure 1: Two-step polarization switching.
Figure 2: Magnetoelectric switching path.
Figure 3: Magnetization reversal by electric field.
Figure 4: Magnetoelectric devices.

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Acknowledgements

This work was supported by the National Science Foundation (Nanosystems Engineering Research Center for Translational Applications of Nanoscale Multiferroic Systems) under grant number EEC-1160504 and the DOD-ARO MURI supported by the Army Research Office through agreement number W911NF-08-2-0032 and partially by the FENA-FAME and NSF/MRSEC (DMR-1120296), through the Cornell Center for Materials Research, programs. J.I. acknowledges financial support from MINECO-Spain (grant numbers MAT2010-18113 and CSD2007-00041). J.L.B., L.Y. and B.D.H. acknowledge support from the DOE ESPM (grant number DE-SC0005037). 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 number DE-AC02-05CH11231. We thank C. Fennie for discussions, C. Zollner for the figure edits, and H. Nair, G. Stiehl and N. Dawley for their comments on the manuscript.

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Authors and Affiliations

Authors

Contributions

J.T.H. designed and, with M.T., D.C.R., D.G.S. and R.R., directed the study, analysed results, and wrote the manuscript. J.L.B. and L.Y. performed the time-dependent PFM measurements. J.L.B. and B.D.H. analysed the results. Q.H. and J.L. performed the XMCD measurements and provided analysis. Y.G. and M.T. grew the BiFeO3/SrRuO3 bilayers. J.D.C. aided in device fabrication. C.W. aided in the measurement and analysis of the spin-valve devices. J.I. performed the ab initio calculations. S.S. evaluated the transport results. J.T.H., M.T., C.W., S.S., D.C.R., D.G.S., J.I., B.D.H. and R.R. all made contributions to writing the manuscript.

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Correspondence to J. T. Heron.

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Extended data figures and tables

Extended Data Figure 1 PFM rotational dependence after partial switching.

Schematics of the PFM tip and the possible polarization components measured for 0° (a) and 90° (b) (that is, the [010] and [100] directions, respectively), as well as the corresponding vertical and lateral PFM images upon partial switching. c, Schematic of polarization directions and vector PFM images taken before (left) and after partial switching (right). All scale bars are 400 nm.

Extended Data Figure 2 Compositions of the first and overall switching directions under positive and negative PFM tip bias.

a, Compositions under positive tip bias, which switches the initial or as-grown state. b, Under negative PFM tip bias, which switches the final state of a back into an initial configuration. OOP, out of plane; IP, in plane.

Extended Data Figure 3 Ab initio calculations of the BiFeO3 system under the constraint of a (110) DyScO3 substrate.

No consideration of the multidomain structure or the influence from an unswitched BiFeO3 matrix is given. a, The lowest-energy switching path calculated for a direct (one-step) 180° switch. A large energy separates the direct reversal of the polarization (black curve, top panel), described here as the Bi3+ shift (middle panel). The Bi3+ shift reverses following a trajectory directly through zero shift (that is, zero polarization). The O6-octahedral rotation (lower panel) remains unperturbed by the direct switch. Black, blue and red curves indicate orthogonal components (x1, x2, and x3) of Bi3+ displacement and O6-octahedral rotation in the reference cell. b, The lowest-energy switching path for polarization reversal calculated from all possible switching paths is a three-step sequence of sequential ferroelastic 71° switches. In this case the shift of the O6-octahedral rotation parallels the changes made by the polarization (Bi3+ displacement), leading to the reversal of the polarization, octahedral rotation and thus, the weak ferromagnetic moment of BiFeO3.

Extended Data Figure 4 Ab initio calculations of the Fe displacement in single- and two-step switching events.

Fe3+ displacements for the single-step and two-step switching events shown in, and plotted with the data from, Fig. 2a. In each case, the Fe3+ shift mimics the Bi3+ shift; however, the Fe3+ shifts are smaller than those for Bi3+. a.u., arbitrary units. f.u., formula units.

Extended Data Figure 5 Domain configuration during switching process.

A vector PFM image obtained partially through the switching process. Throughout the entire switching process, and in this image, (dark blue) domains only touch (orange) and (light blue) domains; the other polarization directions behave analogously. Scale bar is 500 nm.

Extended Data Figure 6 In-plane and out-of-plane PFM images of the boundary between switched and as-grown (initial) regions.

The arrows indicate the in-plane and out-of-plane (inset) components of the polarizations. The domain walls across the boundary appear to be continuous, suggesting that the unswitched matrix (as-grown region) has an influence on the final polarization and domain states. Scale bar is 500 nm.

Extended Data Figure 7 Magnetic hysteresis curves from BiFeO3/Co0.9Fe0.1 (2.5 nm) and BiFeO3/SrTiO3 (1 nm)/Co0.9Fe0.1 (2.5 nm) heterostructures.

a, Schematic of the BiFeO3/Co0.9Fe0.1 heterostructure with the directions of the net in-plane polarization (Pnet IP, the vector sum of the (001)p surface projections of the two polarization variants) and a 200 Oe magnetic field (Hgrowth) applied during the deposition of the Co0.9Fe0.1. The growth field was used to test the strength of the coupling to BiFeO3, given that the growth field should attempt to induce a uniaxial anisotropy in that direction. Magnetic hysteresis loops taken along different in-plane angles rotating from the direction of the growth field show that despite the growth field the easy axis is found to parallel the axis set by Pnet IP. b, Schematic of an experimental configuration similar to that in a; however, a 1-nm-thick layer of insulating and non-magnetic SrTiO3 has been deposited onto the BiFeO3 before the deposition of Co0.9Fe0.1. The magnetic loops from the BiFeO3/SrTiO3/Co0.9Fe0.1 heterostructure are plotted in blue and have a uniaxial anisotropy in the direction of the growth field (orthogonal to the axis set by Pnet IP) with reduced strength (lower saturation and switching fields) compared to those obtained from the BiFeO3/Co0.9Fe0.1 heterostructure in a (red curves).

Extended Data Figure 8 Null electric-field control of magnetism measurement on a BiFeO3/SrTiO3 (1 nm)/Co0.9Fe0.1 (2.5 nm) heterostructure.

a, Anisotropic magnetoresistance obtained from the BiFeO3/SrTiO3/Co0.9Fe0.1 heterostructure taken at 20 Oe after the corresponding electric field was applied. No change in the phase of the anisotropic magnetoresistance curves was observed, indicating no switching of the magnetization.

Extended Data Figure 9 Description of the magnetoresistance in relation to the domain structure of the unpinned (not in contact with BiFeO3) and pinned (in contact with BiFeO3) Co0.9Fe0.1 layers.

As the magnetic field is swept from positive to negative field (open purple circles) along the easy axis of the device the domain structure of the pinned layer evolves from single-domain to a stripe-like structure and back to single-domain. The numbers correlate to the schematics of the domain structures to the spin-valve resistance. At large, positive magnetic field the free and pinned layers are monodomain with magnetizations parallel (1, light blue box) and the device resistance is low. At low, positive magnetic field the pinned layer breaks up into two domain variants owing to the exchange coupling with BiFeO3 while the free layer remains largely monodomain (2, black box). Both net magnetizations are parallel but the device resistance increases due to domain formation in the pinned layer. The purple box (3) encloses the region of magnetic field where the unpinned layer breaks up into domains during switching and the device resistance increases rapidly. In box 4 (red) the net magnetizations of the two layers are antiparallel but not fully antiparallel as the pinned layer is broken into domains and the device resistance is high. At high, negative magnetic field the device is again in a low-resistance state and the two layers are monodomain with parallel magnetization. A similar evolution of the domain structure occurs as the magnetic field is increased from negative to positive values (open red circles).

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Heron, J., Bosse, J., He, Q. et al. Deterministic switching of ferromagnetism at room temperature using an electric field. Nature 516, 370–373 (2014). https://doi.org/10.1038/nature14004

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