Robust isothermal electric control of exchange bias at room temperature, Notas de estudo de Engenharia Elétrica
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Robust isothermal electric control of exchange bias at room temperature, Notas de estudo de Engenharia Elétrica

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Robust isothermal electric control of exchange bias at room temperature

ARTICLES PUBLISHED ONLINE: 20 JUNE 2010 | DOI: 10.1038/NMAT2785

Robust isothermal electric control of exchange bias at room temperature Xi He1, Yi Wang1, NingWu1, Anthony N. Caruso2, Elio Vescovo3, Kirill D. Belashchenko1, Peter A. Dowben1 and Christian Binek1* Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with high- speed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr2O3 has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr2O3 single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Néel temperature.

Spintronics strives to exploit the spin degree of freedom ofelectrons for an advanced generation of electronic devices1,2.In particular, voltage-controlled spin electronics is of vital importance to continue progress in information technology. The main objective of such an advanced technology is to reduce power consumption while enhancing processing speed, integration density and functionality in comparison with present- day complementary metal–oxide–semiconductor electronics3–6. Almost all existing and prototypical solid-state spintronic devices rely on tailored interface magnetism, enabling spin-selective transmission or scattering of electrons. Controlling magnetism at thin-film interfaces, preferably by purely electrical means, is a key challenge to better spintronics7–10.

The absence of direct coupling between magnetization and electric field makes the electric control of collective magnetism in general, and surface and interface magnetism in particular, a scientific challenge. The significance of controlled interface magnetism started with the exchange-bias effect. Exchange bias is a coupling phenomenon at magnetic interfaces that manifests itself most prominently in the shift of the ferromagnetic hysteresis loop along the magnetic-field axis and is quantified by the magnitude µ0HEB of the shift11. The exchange-bias pinning of ferromagnetic thin films is employed in giant magnetoresistance and tunnelling magnetoresistance structures ofmagnetic-field sensors andmodern magnetic read heads12.

Electric control of exchange bias has been proposed for various spintronic applications that go beyond giantmagnetoresistance and tunnelling magnetoresistance technology5. One approach to such voltage control requires a reversible, laterally uniform, isothermal electric tuning of the exchange-bias field at room temperature, which remains a significant challenge.

Early attempts in electrically controlled exchange bias tried to exploit the linear magnetoelectric susceptibility of the antiferromagnetic material Cr2O3 as an active exchange-bias

1Department of Physics & Astronomy and the Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588-0111, USA, 2Department of Physics, 257 Flarsheim Hall, University of Missouri, 5110 Rockhill Road, Kansas City, Kansas 64110, USA, 3Brookhaven National Laboratory, National Synchrotron Light Source, Upton, New York 11973, USA. *e-mail: cbinek2@unl.edu.

pinning system13. In a magnetoelectric material an applied electric field induces a net magnetic moment, which can be used to electrically manipulate the magnetic states of an adjacent exchange-coupled ferromagnetic film14. The small value of the maximum parallel magnetoelectric susceptibility αme

‖(T = 263K) ≈ 4.13 psm−1 of Cr2O3 (ref. 15) led many researchers to the conclusion that multiferroic materials are better suited for this purpose. Such materials have two or more ferroic order parameters, such as ferroelectric polarization and (anti)ferromagnetic order16.

The potential for an increased magnetoelectric response, for the multiferroic materials, was dictated by the maximum possible value of αme ij . It is determined by the geometric mean of the ferroic susceptibilities, both of which can individually be very high inmultiferroics17–20. Coupling between these order parameters has been demonstrated21. However, it is typically weak, and the theoretical upper limit of αme ij is rarely reached16.

Artificial two-phase multiferroics have been studied extensively. Such piezoelectric/ferromagnetic heterosystems allow for electric control of anisotropy22,23. However, strain-induced non-hysteretic magnetoelastic effects are often not stable (persistent) in the absence of an applied field (that is, volatile). Removing the electric field from a linear piezoelectric element releases the strain in the ferromagnetic component and hence restores the anisotropy of the piezoelectrically unstrained film. When using a ferroelectric material, to induce piezoelectric strain control, one may take advantage of the ferroelectric hysteresis to impose some residual strain that will persist after removing the electric field. In contrast to this electric control of magnetic anisotropy in two-phase multiferroics, we report on a non-volatile electric control of unidirectional magnetic anisotropy.

The most promising multiferroic single-phase materials used for electrically controlled exchange bias are YMnO3 and BiFeO3 (refs 24,25). Complete suppression of the exchange bias has been

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ARTICLES NATURE MATERIALS DOI: 10.1038/NMAT2785

achieved at 2 K in an YMnO3/NiFe (permalloy) heterostructure. This effect, however, is irreversible. Moreover, the limitation of low temperatures makes YMnO3 unsuitable for applications. The situation is better with BiFeO3. In BiFeO3/CoFe heterostructures, local magnetization reversal on a lateral length scale of up to 2 µm has been demonstrated25,26. However, global magnetization reversal, which could be revealed in macroscopic magnetic hysteresis, has not been achieved. Global, but not isothermal magnetoelectric switching has been achieved in the pioneering Cr2O3/CoPt heterostructure13. However, each sign reversal of the exchange-bias field required a new magnetoelectric annealing procedure, in which the pinning layer is cooled from T > TN to T < TN in the presence of both electric and magnetic fields. Isothermal electric control of exchange bias has been attempted by various groups, but with only marginal success27,28. The result was that reversible and global electrically controlled exchange bias carried out isothermally at room temperature remained elusive.

Here we reveal an unconventional ferromagnetism at the (0001) surface of the magnetoelectric antiferromagnet Cr2O3 and demonstrate its suitability for electrically controlled exchange bias and magnetization. New insights were achieved by combining first-principles calculations, general symmetry arguments, spin- resolved photoemission spectroscopy and magnetometry (see Supplementary Information) for the Cr2O3 (0001) surface and its interface in an exchange-bias heterostructure. We used a molecular beam epitaxy (MBE)-grown chromia thin film (see the Methods section) for the spin- and energy-resolved ultraviolet photoemission spectroscopy (UPS), whereas the isothermal electric control of exchange bias was done on a heterostructure involving an oriented chromia single crystal with (0001) surface. The choice of a high-quality single crystal for the exchange-bias system completely rules out sample heating induced by leakage currents because of the virtually perfect insulating properties of single- crystalline chromia. The UPS measurements have been carried out in zero electric field after magnetoelectric initialization of the antiferromagnetic domain state. The non-zero conductivity of thin films is a well-known experimental advantage used for the photoemission investigation of samples that otherwise are virtually perfectly insulating in the bulk. The finite conductivity of the thin film prevents charge accumulation, which could lead to misrepresented photoelectron energies.

On the basis of the understanding of the surface ferromagnetism of Cr2O3 (0001), a new concept of Cr2O3 (0001)-based exchange bias is implemented. As a result, a reversible, isothermal and global electric control of exchange bias is demonstrated at room temperature by reproducible electrically induced discrete shifts of the global magnetic hysteresis loop along the magnetic-field axis (see Supplementary Movie).

Magnetically uncompensated surfaces of antiferromagnetically ordered single crystals have been a subject of intense investigations, in particular in the framework of exchange bias11. The surface magnetization of an uncompensated antiferromagnetic surface with roughness usually averages out, so that only a small non- equilibrium statistical fluctuation remains for exchange coupling with the adjacent ferromagnet29.

The surfaces of single-domain antiferromagnetic magneto- electrics, such as the (0001) surface of the antiferromagnetically long-range ordered Cr2O3, are remarkable exceptions. The free energy of this system, with a boundary, depends on the polar vector n (external normal) as a macroscopic parameter. The existence of the magnetization at the boundary can be deduced from the reduction of the bulk magnetic point group by the presence of an invariant vector n. As both n and E are polar vectors, the boundary reduces the symmetry in a similar way to the electric field, E, in the bulk. An equilibrium magnetization must therefore exist at the surface of a magnetoelectric antiferromagnet, or at

its interface with another material. This argument automatically includes equilibrium surface roughness; a more detailed analysis will be published elsewhere.

Both the bulk single crystal and the thin-film sample are confirmed to be (0001) oriented by X-ray diffraction. The surface topography of the bulk and the thin-film sample are mapped using atomic force microscopy (AFM). Figure 1 is organized in such a way that structural data of the bulk sample are shown in the upper panels of a and b. The corresponding data of the thin-film sample are shown in the lower panels of Fig. 1a,b. The (0001) orientation of the bulk surface is independently corroborated by the hexagonal reflection pattern obtained in low-energy electron diffraction. The prominent (0006) and (00012) X-ray peaks of the bulk sample are virtually identically reproduced in the thin film (compare peak positions in the upper and lower panels of Fig. 1a). The surface topography of the samples reveals a plateau with a root-mean-squared (r.m.s.) roughness of 0.88 nm for the surface of the bulk crystal (upper panel of Fig. 1b) and an even lower r.m.s. roughness of 0.19 nm (lower panel of Fig. 1b) for the thin-film sample measured along selected lines.

Figure 1c illustrates a configuration of the Cr2O3 (0001) surface. It is seen that the particular antiferromagnetic domain has an uncompensated surfacemagneticmonolayerwith alignedmoments on all surface Cr3+ ions, even if the surface is not atomically flat. Two features conspire to produce this property. First, the corundum lattice of Cr2O3 can be imagined as a layered arrangement of buckled Cr3+ ions sandwiched between the triangular layers of O2− ions30. The electrostatically stable charge-neutral surface of this crystal is terminated by a semi-layer of Cr; this termination can be viewed as the cleavage of the crystal in the middle of the buckled Cr3+ layer31. Second, Cr ions, which are structurally similar with respect to the underlying O layer, have parallel spins. As a result, a single-domain antiferromagnetic state has all surface Cr spins pointing in the same direction. Note that although we have shown the surface Cr ions in bulk-like positions in Fig. 1, this assumption is immaterial for the existence of the surfacemagnetization, as follows from the general symmetry argument.

In single-crystalline Cr2O3, the antiferromagnetic order allows two degenerate 180◦ antiferromagnetic domains14 (see Fig. 1 and Supplementary Fig. S1). These two domains have surface magnetizations of opposite sign. If the degeneracy of the two domain types is not lifted, the system develops a random multi- domain state with zero net surface magnetization when it is cooled below TN. However, magnetoelectric annealing allows for preferential selection of one of these 180◦ domains by exploiting the free-energy gain 1F = αEH (ref. 14). As a result, even a rough Cr2O3 (0001) surface becomes spin-polarized when an antiferromagnetic single-domain state is established. Evidence of this roughness-insensitive surface magnetism is revealed by magnetometry (Supplementary Fig. S2 and Discussion) as well as spin-resolved UPS. Interpretation of the latter is supported by calculations of the site-resolved density of states (DOS) revealing a spin-polarized surface band above the valence-band maximum, in agreement with experimental findings. The UPS carried out on our MBE-grown Cr2O3 (0001) sample is sensitive to occupied surface electronic states.

Figure 2a shows the spin-polarized photoelectron intensity versus binding energy measured at 100K. First, the MBE-grown Cr2O3 (0001) thin film has been cooled from T > TN in a small magnetic field of 30mT alone, into a multidomain antiferromagnetic state. Spin-up and spin-down photoelectron intensities /↑,↓ (red circles and blue squares) are virtually identical, indicating negligible net surfacemagnetization and polarization.

Furthermore, multiple measurements were undertaken for the single-antiferromagnetic-domain states, each with a fresh sample preparation. Subsequent sample preparations involve alternating

580 NATUREMATERIALS | VOL 9 | JULY 2010 | www.nature.com/naturematerials

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Cr2O3 (0006)

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Figure 1 | Structural characterization. a, θ–2θ X-ray diffraction pattern of chromia bulk single crystal (upper panel) and thin film (lower panel) showing the chromia (0006) and (00012) peaks, respectively. The film is deposited on a sapphire (0001) substrate, giving rise to (0006), (00012), Kα and Kβ(∗) peaks and a weak structure-factor-forbidden (0009) peak. The inset shows a room-temperature low-energy electron diffraction pattern of the hexagonal chromia (0001) surface measured at an electron energy of 140 eV. b, Real-space topography of the chromia (0001) surface of bulk single crystal (upper panel) and thin film (lower panel) measured by AFM. The respective main frames show cross-sectional analysis along indicated lines. A r.m.s. roughness of 0.88 nm is calculated in the region between scanning position 0.15 and 0.81 µm for the bulk single crystal. The r.m.s. roughness of 0.19 nm of the thin film is measured between 0.04 and 0.50 µm. c, The spin structure of a Cr2O3 single crystal with a stepped (0001) surface is shown for one of its two antiferromagnetic single-domain states. Up (red) and down (dark blue) spins of the Cr3+ ions (green spheres) point along the c axis.

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Figure 2 | Spin-polarized UPS measurements and layer-resolved DOS. a, The intensity of photoelectrons (occupied states) versus binding energy from a Cr2O3 (0001) surface measured at T = 100 K after cooling in µ0H= 30 mT and E=0 from T>TN. Spin-up and spin-down intensities are shown by red circles and blues squares, respectively. Inset: The result of a first-principles calculation of the layer-resolved DOS. Colour code follows the experiment. The green line indicates a surplus surface state with spin-up polarization. b, Spin-up (red circles) and spin-down (blue squares) intensities after magnetoelectric annealing in E= 3.85× 10−4 kV mm−1

and µ0H= 30 mT. The lines are best fits of multiple-peak Gaussian functions. The diamonds show Cr 3d spin-up (red and green) and spin-down (blue) contributions extracted from the fits. The Gaussian fit shown by the green diamonds reflects specific surface states. Colour code matches the theoretical DOS data. The triangles show the contrast, P, in the spin-dependent intensities versus binding energy. The green triangles highlight the contribution from the surface state. Maximum absolute errors in P are indicated by bars.

magnetoelectric field cooling from above the Néel temperature to the target temperature ofT =100K. Themagnetoelectric annealing in alternating fields gives rise to alternating surface magnetization. This leads to a reproducible reversal of the spin-polarization mea- sured at T = 100K. Such data are then summed to provide an average overall net polarization, independent of instrumental asym- metry, as is the standard practice in spin-polarized photoemission and spin-polarized inverse photoemission.

The signal is clearly spin-split after magnetoelectric annealing in E = 3.85 × 10−4 kVmm−1 and µ0H = 30mT (compare red circles and blue squares in Fig. 2b), demonstrating high net spin- polarization at the surface. The spin contrast P= (/↑− /↓)/(/↑+ /↓), exhibited by triangles in Fig. 2b, is seen to increase significantly close to the valence-band maximum, EF, (green triangles). We identify this feature with the contribution from the spin-polarized surface

states of Cr 3d character. To corroborate this interpretation, we decompose the spin-dependent photoemission spectra /↑,↓ into contributions from Cr 3d bulk and surface states. The contribution above the valence-bandmaximum (green) is interpreted as an extra spin-polarized surface state.

This interpretation is in accordance with our first-principles calculation of the layer-resolved DOS of the Cr2O3 (0001) surface shown in the inset of Fig. 2a. The DOS of a representative central layer with spin-up sublattice (majority/minority in red/blue) magnetization is shown by the lower two curves of the inset. The DOS of the surface layer is shown by the two upper curves in the inset of Fig. 2a. Note that in addition to the bulk states, a surplus spin-up density of states (green) appears above the valence- band maximum. This is consistent with our experimental findings in photoemission (Fig. 2b) and magnetometry (Supplementary Fig. S2 and Discussion).

Experimental and theoretical evidence together point very strongly to the existence of a roughness-insensitive ferromagnetic state at the Cr2O3 (0001) surface when the underlying Cr2O3 single crystal is in an antiferromagnetic single-domain state. Our findings indicate that the surface has a magnetization and is spin-polarized, despite the roughness that is evidently present according to our AFM investigations. Although the roughness may have some effect on the magnitude of the surface magnetization, its mere presence is unusual, and further supported by the experiments on electrically switched exchange bias.

The ferromagnetic surface moment can be isothermally switched by electrical means, giving rise to reversible switching of large exchange-bias fields in our perpendicular exchange-bias heterostructure Cr2O3(0001)/Pd 0.5 nm/(Co 0.6 nm Pd 1.0 nm)3. The Cr2O3 substrate in the exchange-bias heterostructure is a (0001) bulk single crystal. The temperature dependence of the exchange bias and its relation to the temperature dependence of the chromia surface magnetization are discussed in the Supplementary Discussion.

Figure 3 demonstrates large isothermal electric switching of the exchange-bias field. It is achieved by leaving the realm of the linear magnetoelectric effect, which gives rise to only a minuscule electric control effect27,28. In contrast to this small linear effect, significant electrically controlled switching requires that a critical threshold given by the product |EH |c, where E andH are isothermally applied axial electric and magnetic fields, is overcome. Initially the het- erostructure has been magnetoelectrically annealed in EH > 0 with E=0.1 kVmm−1 andµ0H =77.8mT down to T =303K. The hys- teresis loops are measured isothermally at T =303K and E=0. The red squares in Fig. 3a,b show the same virgin loop with positive ex- change bias ofµ0HEB=+6mT.Next, without changing the temper- ature, a field product EH <0 of individual fields E=−2.6 kVmm−1 and µ0H =+154mT is applied for less than a second. During the time when an electric field is applied, the electric current is moni- tored to stay below 0.01 µA, resulting in virtually zero sample heat- ing. After applying the electric- and magnetic-field product a mag- netic hysteresis loop is measured in E = 0. Green triangles (Fig. 3a) show the resulting loop with a pronounced negative exchange bias of µ0HEB ≈−13mT. The same field product is achieved with E=+2.6 kVmm−1 andµ0H =−154mT, having the same effect on the exchange bias as shown in Fig. 3b by blue circles. The isothermal switching of the exchange-bias field implies a field-induced switch- ing of the antiferromagnetic single-domain state of Cr2O3 into the opposite antiferromagnetic registration. This switching is accom- panied by a reversal of the interface magnetization. Figure 3c shows a sequence of switched exchange-bias fields obtained by switching the electric field back and forth between E=+2.6 kVmm−1 and E=−2.0 kVmm−1 at constant set field µ0H =−154mT, all at a constant temperature T=303K. The reproducible switching shows no signs of ageing. The asymmetry between positive and negative

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Figure 3 | Isothermal electric switching of the exchange-bias field. a, Exchange-biased hysteresis loops of Cr2O3 (0001)/Pd 0.5 nm/ (Co 0.6 nm Pd 1.0 nm)3 at T = 303 K after initial magnetoelectric annealing in E=0.1 kV mm−1 and µ0H= 77.8 mT. Hysteresis loops are measured by polar Kerr magnetometry in E = 0, respectively. The red squares show the virgin curve with a positive exchange-bias field of µ0HEB=+6 mT. Isothermal-field exposure in E=−2.6 kV mm−1 and µ0H=+154 mT gives rise to a loop with a negative exchange-bias field of µ0HEB≈−13 mT (green triangles). b, The red squares show the same virgin reference loop. The blue circles show the hysteresis loop after isothermal-field exposure in E =+2.6 kV mm−1 and µ0H=−154 mT, giving rise to the same negative exchange bias of µ0HEB=−13 mT. c, µ0HEB versus number of repeated isothermal switching through exposure to E=+2.6 kV mm−1 (blue circles) and E=−2 kV mm−1 (red squares) at constant µ0H=−154 mT, respectively.

exchange-bias values is a consequence of a difference in the interface magnetization SCr2O3 for negative and positive exchange bias.

A nonlinear magnetoelectric switching of the antiferromagnetic single-domain state of Cr2O3 was reported as far back as 1966 by Martin and Anderson32. Their work illustrated that the isothermal switching between the two different antiferromagnetic domains of Cr2O3 is possible if sufficiently strong field products E ·H are applied along the c axis32. This switching is a thermally activated process. At constant temperature there is a critical value |EH |c, above which the system settles into the single-domain state with the lowest free energy, even if this requires a switching of the entire antiferromagnetic spin structure. This hysteretic switching of Cr2O3 is directly reflected in the hysteresis of the electric-field dependence of the exchange-bias field.

Figure 4 shows the threshold character of electric switching at T = 303K. All data are taken after magnetoelectric annealing in E= 0.1 kVmm−1 and µ0H = 77.8mT. The hysteretic electric- field dependence, µ0HEB versus E , is determined from individual magnetic hysteresis loopsmeasured in E=0. Each data point results from a loop measured after isothermal exposure of the sample to one of various E-fields and fixed magnetic field µ0H =−115mT (circles), µ0H = −154mT (triangles) and µ0H = −229mT (squares), respectively. Note that the same values of the field products can be achieved for various corresponding positive magnetic fields µ0H =+115,+154 and +229mT. The resulting

electrically controlled switching is shown in Supplementary Fig. S4. Two main characteristics are observed in the µ0HEB versus E data. First, for a given positive magnetic field there is a critical negative and positive electric field, Ec, where switching of the exchange-bias field takes place. The rectangular hysteresis µ0HEB versus E is in perfect agreement with the isothermal switching of the antiferromagnetic domain state of Cr2O3 reported in ref. 32. This includes details such as the asymmetry between the negative and the positive switching field.

The insets of Fig. 4 show that the critical switching fields of the exchange bias obey the relation |EH |c= const corresponding to the switching of the Cr2O3 antiferromagnetic single domain32. Solid squares are data points of Ec for magnetic fieldsµ0H =−115,−154 and−229mT. The lines are fits of the functional formH=const/Ec. This shows that the switching effect originates from the coherent flip of the antiferromagnetic registration of the Cr2O3 pinning system. The inversion of the antiferromagnetic spin structure is accompanied by the reversal of the Cr2O3 (0001) interface magnetization, which in turn causes switching of the exchange-bias field. As this switching is induced at a threshold of the product |EH |c, the H -field can be made arbitrarily small when E is scaled up accordingly. There is plenty of room for E-field increase by shrinking the thickness of the pinning layer down to the nanoscale. It is feasible to use nanostructured arrays of permanent magnetic nanopillars to apply magnetic stray

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Figure 4 | Hysteretic electric-field dependence of the exchange-bias field. µ0HEB versus E, measured at T= 303 K from individual Kerr loops. Data are taken after initial magnetoelectric annealing of Cr2O3 (0001)/ Pd 0.5 nm/(Co 0.6 nm Pd 1.0 nm)3 in axial fields E=0.1 kV mm−1 and µ0H= 77.8 mT. Kerr loops are measured in E=0 after isothermal E-field and simultaneous H-field exposure of the sample. For a given µ0HEB versus E curve the magnetic field is constant. The three µ0HEB versus E data sets correspond to µ0H=−115 mT (circles), µ0H=−154 mT (triangles) and µ0H=−229 mT (squares), respectively. The solid squares in the insets show the data points of electric switching fields and corresponding magnetic fields µ0H=−115,−154 and−229 mT. The lines are single-parameter fits of the functional form H= const/Ec.

fields below the coercive field of the CoPd film. For instance, the field 20 nm from the end of a Ni rod, 175 nm long and 20 nm diameter, has been estimated as ∼25mT (ref. 33). The insets of Fig. 4 show that a magnetic stray field of 20mT requires a maximum electric switching field of 16 kVmm−1 to reverse the exchange-bias field and ferromagnetic magnetization. Chromia is known for its excellent dielectric properties, and is used as a good insulator with high dielectric breakdown fields of 107 V cm−1 = 1,000 kVmm−1 at room temperature. Still it will remain a challenge to achieve these dielectric properties in thin films.

Electric control of magnetism, at room temperature, is the basis of advanced spintronics for post-metal–oxide–semiconductor technology. The intensive research efforts on multiferroic materials of recent years are to a large extent driven by the possibilities of electrically controlled magnetism at room temperature. We have shown by a combination of experiment and theory that Cr2O3, the archetypical magnetoelectric antiferromagnet, is revived as a candidate for reversible electric control of magnetism at room temperature. This control is made possible by the roughness-insensitive ferromagnetic spin state at the (0001) surface of Cr2O3. In the highly nonlinear magnetoelectric regime the antiferromagnetic order can be electrically switched along with the surface magnetization. This phenomenon takes place at the interface between Cr2O3 (0001) and a ferromagnetic Co/Pd multilayer film. In this perpendicular exchange-bias system, a reversible and global electric switching of the exchange-bias field was realized isothermally at room temperature. This observation opens up exciting prospects for spintronics applications.

Methods MBE is used for the sample growth. Ex situ structural characterization is done by X-ray diffraction techniques. The magnetic characterization is primarily based on

the polar magneto–optical Kerr effect and partially carried out with the help of a superconducting quantum interference device. The Cr2O3 (0001) surface of the c−Al2O3/Cr(110)[8 nm]/Cr2O3(0001)[103 nm] sample was cleaned by ion-beam sputtering and post-sputtering annealing procedures before the photoemission measurements. Spin-polarized angle-resolved photoemission spectra were acquired at the U5UA undulator spherical grating monochromator beamline at the National Synchrotron Light Source. The electronic structure calculations of the Cr2O3 (0001) surface were carried out using the projected augmented wave method as implemented in the Vienna ab initio simulation program34–36.

Received 30 December 2009; accepted 18 May 2010; published online 20 June 2010

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Acknowledgements This work is supported by NSF through Career DMR-0547887, by the Nebraska Research Initiative (NRI), by the NSF MRSEC Grant No. 0820521 and by the NRC/NRI supplement to MRSEC. K.D.B. is a Cottrell Scholar of Research Corporation. Technical

help from S-Q. Shi, V. R. Shah and L. P. Yue in the calculation of DOS, taking XRD and AFM data is acknowledged, respectively. We are thankful to Crystal GmbH for providing excellent Cr2O3 single crystals.

Author contributions X.H. and C.B. designed the study, in particular conceiving the electrically controlled exchange bias and electrically controlled magnetism. Y.W. and X.H. collected and analysed the magnetic data. N.W. led the photoemission experiments and data analysis. A.C. and E.V. supported the photoemission experiments. K.D.B. conceived the concept of roughness-insensitive surface magnetization and directed the electronic structure calculations. P.A.D. directed and conceived the photoemission experiments. C.B. directed the overall study. All authors contributed to the scientific process and the refinement of the manuscript. C.B. and X.H. wrote most of the paper.

Additional information The authors declare no competing financial interests. Supplementary information accompanies this paper on www.nature.com/naturematerials. Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions. Correspondence and requests formaterials should be addressed to C.B.

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