Baixe Reversible electric control of exchange bias in a multiferroic field-effect device e outras Notas de estudo em PDF para Engenharia Elétrica, somente na Docsity! ARTICLES PUBLISHED ONLINE: 25 JULY 2010 | DOI: 10.1038/NMAT2803 Reversible electric control of exchange bias in a multiferroic field-effect device S. M. Wu1,2*, Shane A. Cybart1,2, P. Yu1,2, M. D. Rossell1, J. X. Zhang1, R. Ramesh1,2 and R. C. Dynes1,2,3 Electric-field control of magnetization has many potential applications in magnetic memory storage, sensors and spintronics. One approach to obtain this control is through multiferroic materials. Instead of using direct coupling between ferroelectric and ferromagnetic order parameters in a single-phase multiferroic material, which only shows a weak magnetoelectric effect, a unique method using indirect coupling through an intermediate antiferromagnetic order parameter can be used. In this article, we demonstrate electrical control of exchange bias using a field-effect device employing multiferroic (ferroelectric/antiferromagnetic) BiFeO3 as the dielectric and ferromagnetic La0.7Sr0.3MnO3 as the conducting channel; we can reversibly switch between two distinct exchange-bias states by switching the ferroelectric polarization of BiFeO3. This is an important step towards controlling magnetization with electric fields, which may enable a new class of electrically controllable spintronic devices and provide a new basis for producing electrically controllable spin-polarized currents. New physical phenomena at artificially constructedheterointerfaces are at present an exciting area ofcondensed-matter science1. Oxides of 3d transition-metals, such as the cuprates, manganites and more recently the multifer- roics, present a fascinating playground to explore the interactions of charge2, spin3, orbital4 and lattice degrees of freedom at such heterointerfaces, which eventually lead to new states of matter. One model system is the interface between LaAlO3 and SrTiO3, nominally two band insulators, that exhibit unusual electronic reconstruction and transport phenomena when adjoined as a heterointerface2,5,6. The emergence of charge-transfer-driven orbital ordering and ferromagnetism in a (Y,Ca)Ba2Cu3O7 layer at the interface with the doped manganite La0.67Ca0.33MnO3 has been demonstrated, thus bringing to bear the role of the orbital degree of freedom4,7. Such an epitaxial heterointerface is illustrated schematically in Fig. 1, which describes the various degrees of freedom. Electric-field control of such an interfacial ferromag- netic state would be a significant step towards magnetoelectric devices8–12. The possible electronic reconstructions at the interface will undoubtedly be influenced by the ground-state electronic structure of the transition-metal species at the interface. For example, the d5 electronic state of Fe3+ is stable to perturbations and therefore is unlikely to undergo reconstruction13. With this as the background, we are exploring one such d5 model system, manifested in ferroelectric (FE) and antiferromagnetic (AFM) BiFeO3 (BFO), which is epitaxially juxtaposed at the interface to a multivalent transition-metal ion such as Mn3+/Mn4+ in ferromagnetic La0.7Sr0.3MnO3 (LSMO). We have observed an unexpected ferromagnetic order that is induced in the Fe sublattice at the interface as a consequence of a complex interplay between the orbital degree of freedom and its coupling to the spin degree of freedom14. This ferromagnetic state in the Fe sublattice gives rise to a significant exchange-bias interaction with the ferromagnetic LSMO (a shift in the magnetic hysteresis curve along the applied-field axis in a coupled ferromagnetic (FM)/AFM system, when magnetically field cooled through a blocking temperature (TB; refs 15,16)). The discovery of a correlation between the electronic orbital structure 1Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA, 2Department of Physics, University of California, Berkeley, California 94720, USA, 3Department of Physics, University of California, San Diego, La Jolla, California 92093, USA. *e-mail: stephenw@berkeley.edu. at the interface and exchange bias suggests the possibility of using an electric field to control the magnetization of the ferromagnet, which is precisely the focus of this paper. Recently, there has been significant effort to electrically control the magnetization of a ferromagnetic thin film8,9. One technique has been to use a magnetoelectric multiferroic material with directly coupled FE and FM order parameters, such that there is direct control of magnetization by the electric field17. One problem with this type of direct coupling is that most single- phase FM–FE multiferroics show only a weak magnetoelectric effect10,11. In addition, there are only a small number of materials that can exhibit this form of direct coupling, because of a large number of material constraints, some of which are mutually exclusive18,19. There has been some progress in direct single-phase magnetoelectric coupling in multiferroic materials. For example, electrical control of magnetization in domain walls of GdFeO3 has been shown20. Although this is a significant achievement, the relative change in magnetization may not be sufficient for practical applications that require stronger magnetoelectric coupling or full reversal ofmagnetization. In contrast to the few single-phase FE/FM multiferroics, there are several known FE/AFM multiferroics. A different approach to achieve stronger magnetoelectric coupling, using FE/AFM multiferroics, employs a bilayer consisting of a thin FMmaterial that is coupled to themultiferroic. In this case, theAFM order parameter in themultiferroic acts as a medium that indirectly couples the FM ordering of an FM thin film and the FE ordering of the multiferroic. The mechanism for coupling between the AFM and the FM is exchange bias. Two ideal candidate materials for this type of indirect coupling are multiferroic (AFM/FE) BFO and ferromagnetic LSMO, which were previously mentioned to show significant exchange bias owing to interfacial orbital reconstruction. BFO is a suitable multiferroic for this application because it has strong coupling between its AFM and FE order parameters21. Controlling this exchange bias may allow for the manipulation of magnetization, by biasing at amagnetic field and electrically shifting the magnetic hysteresis curve beyond the coercive field in either direction. This behaviour is schematically shown in Fig. 2a. 756 NATURE MATERIALS | VOL 9 | SEPTEMBER 2010 | www.nature.com/naturematerials © 2010 Macmillan Publishers Limited. All rights reserved. NATURE MATERIALS DOI: 10.1038/NMAT2803 ARTICLES a Oxide A Oxide B b SrTiO3 LaAlO3 La0.67Ca0.33MnO3 (Y,Ca)Ba2Cu3O7 La0.7Sr0.3MnO3 BiFeO3 Charge transfer Charge transfer/ orbital reconstruction Orbital reconstruction High-mobility two-dimensional electron gas Interface ferromagnetism/ suppressed interfacial superconductivity Interface ferromagnetism/ exchange bias Orbital Lattice Spin Charge Emergent interfacial phenomenon Figure 1 | The interplay between heterointerfacial degrees of freedom. a, A schematic showing the interplay between the different degrees of freedom at play (charge, orbital, spin and lattice) at heteroepitaxially grown interfaces between different oxide materials. b, Some canonical examples of interfacial reconstruction and the new emergent interfacial phenomenon that occur in these systems. Electric control of magnetism has been attempted by using a field-effect device with the magnetoelectric material Cr2O3 as an AFM dielectric and an FM Co/Pt multilayer channel22. It was shown that through cooling with both electric and magnetic fields it was possible to affect the polarity of the exchange bias with respect to the direction of the magnetic cooling field. Later, it was demonstrated with multiferroic thin-film YMnO3 and permalloy bilayers23 that it was possible to set an exchange bias with field cooling and remove it with ferroelectric switching. After removal it was not possible to return to the field-cooled state. Using a different approach we have built a multiferroic field-effect device with a heteroepitaxially grown, ferromagneticmanganite (LSMO) channel layer. In our systemwe have been able to electrically switch between two exchange-bias states reversibly, withoutmagnetic-field cooling. To build our electric-field-effect device, a thin-film heterostruc- ture of LSMO and BFO was heteroepitaxially grown by pulsed laser deposition on a SrTiO3 (STO) (100) substrate. The LSMO was chosen to be 3–5 nm thick so that exchange-bias effects at the interface will dominate when probed with electrical-transport measurements. In addition, because of the FM-layer-thickness dependence in exchange-bias systems, minimizing the thickness of the LSMO layer also maximizes the magnitude of the exchange bias15. As a precautionary measure, the BFO layer was chosen to be 600 nm thick to prevent pinhole shorts in the dielectric, which can cause gate leakage24. Structural characterization using X-ray diffraction revealed single-phase BFO and LSMO layers, and high-resolution transmission electronmicroscopy experiments confirmed a high-quality interface, as illustrated in Supplementary Fig. S1. Detailed electron energy-loss spectroscopy scans (Supple- mentary Fig. S1) revealed very little interdiffusion at the inter- face and hence the transition-metal site. The ferroelectric domain structure of the BFO layer exhibited a typical stripe-like structure consisting of a predominantly 71◦ domain-wall pattern, shown in Supplementary Fig. S2. After switching the FE polarization with an electric field, this domain pattern primarily switches by 180◦ from the original polarization state, retaining the stripe-like pattern (Supplementary Fig. S2). Magnetization hysteresis curves for the as-grown films were measured with superconducting quantum interference device (SQUID) magnetometry and exhibit exchange bias (Fig. 2b). After being cooled in a magnetic field of ±1 T, applied parallel to the interface, there is a corresponding shift in the hysteresis loop. Cooling the sample in positive magnetic field results in a negative shift of the hysteresis loop and vice versa. These results were very reproducible; over 100 heterostructure samples with LSMO thicknesses in the range of 2–10 nm were grown and exhibited exchange bias like the data presented in Fig. 2b. The magnitude of the exchange coupling systematically decreased with LSMO layer thickness, consistent with the conventional picture of exchange bias as originating from the interface15,16. To test that this shift is AFM–FM exchange bias, we measured a sample with a thin 2 nm layer of STO inserted between the AFM and the FM. The magnetic hysteresis curve of this sample (Fig. 2b (inset)) shows no observable shift of the loop when field cooled. Furthermore, from these experiments the blocking temperature of this systemwas determined to be ∼100–120K. This is important because it sets an upper temperature limit for where exchange bias can be observed, and thus where devices can be operated. To investigate electrical transport in these heterostructures three samples with 3-nm-thick LSMO and 600-nm-thick BFO were patterned with 45 gated Hall-bar structures schematically shown in Fig. 3. This LSMO thickness was chosen because it exhibited the largest exchange bias while still remaining conducting. The details of the device-fabrication process are described in the Methods section. After fabrication, sheet resistance as a function of temperature (RS–T ) was measured to ensure that the material was not damaged during device processing and to verify that ohmic contacts were made to the buried LSMO layer. Figure 4a illustrates two typical RS–T plots for the LSMO layer, one for each FE polarization of the BFO. The inset shows a larger temperature range for data taken for the as-grownBFOFEpolarization state. The result is consistent with measurements for similarly prepared bare 3 nm LSMO films. It is interesting to note the similarity of the shape of the RS–T plots for the two gate polarities. These data show both a large vertical translation of resistance and a small multiplicative change in RS(T ) between the two FE polarization states. We interpret the vertical translation as a change in residual resistivity Rs0. From Matthiessen’s rule, this suggests a change in the density NATURE MATERIALS | VOL 9 | SEPTEMBER 2010 | www.nature.com/naturematerials 757 © 2010 Macmillan Publishers Limited. All rights reserved. ARTICLES NATURE MATERIALS DOI: 10.1038/NMAT2803 0 20 60 0 ¬60 0.1 3.0 2.5 2.0 3.0 2.5 2.0 0 0 ¬0.2 ¬0.1 V G ( V ) H EB / H C ¬MR +MR H EB / H C 40 Pulse number 60 Rs (k Ω / ) Rs (k Ω / ) R S ( kΩ / ) R S ( kΩ / ) H ¬ H ¬H + H +2.91 2.90 2.89 2.88 1.99 1.98 1.97 ¬4 ¬2 0 Magnetic field (kOe) Magnetic field (kOe) 2 4 ¬3 ¬2 ¬1 0 1 2 3 a b c d e Figure 5 | Electric-field control of exchange bias. a, The gate-voltage-pulse sequence used for the measurements. b,c, Measurements of normalized exchange bias and peak resistance for the gate-pulse sequence shown in a. Each point is determined from an MR sweep at 5.5 K after pulsing the gate with voltage VG. The exchange bias modulates with ferroelectric polarization; the data shown for b were obtained with a negative remanent magnetization in the LSMO channel whereas the data shown for c were obtained in positive remanent magnetization. Error analysis was done on b by taking multiple MR sweeps. Standard deviations of the individual peak locations were obtained and the error was calculated for each point using standard error propagation techniques. Owing to the large number of data and the long time it takes to run such an experiment, the corresponding measurement was not made on c. Both measurements were made without field cooling the system at any time. The maximum normalized values of exchange bias correspond to shifts of∼100 Oe in b and∼−200 Oe in c. d,e, Examples of individual MR curves from the upper and lower resistive states where the exchange-bias values were determined. with our RS(T ) results, in Fig. 4a, that show a large increase in temperature-independent scattering. Finally, we can imagine an atomic-scale mechanism that involves the position of the Fe ion in BFO relative to the Mn ion in the LSMO at the interface and the consequent influence on the Fe–Mn coupling. Clearly, owing to themultiplicity of physical phenomena that are likely to be involved, a considerable number of further experiments including different devices are required to fully understand the phenomenon at hand. Furthermore, similar materials systems need to be thoroughly investigated to find higher blocking temperatures to enable room-temperature operation. This reversible electric modulation of exchange bias is an important step towards the electrical control of magnetism. Because of the half-metallic properties of LSMO (ref. 32), devices like this may be used to create electrically controlled fully spin-polarized currents. This type of low-current, low-power, electrical switching has far-reaching implications for the field of spintronics. Methods Single-crystal STO(001) was chosen as the substrate owing to the small lattice mismatch with both LSMO and BFO. Before the growth, the substrate was wet-etched with buffered hydrofluoric acid, followed by a thermal annealing process. This forms a TiO2-terminated atomically flat top surface on the STO (ref. 33). LSMO and BFO were epitaxially grown on the prepared STO from stoichiometric targets, at a laser energy density of ∼1.5 or 1 J cm−2 with repetition rates of 1 or 3Hz, respectively. The substrate temperature was held at 700 ◦C in an oxygen ambience of 200mtorr and 100mtorr for LSMO and BFO respectively. After the growth, the samples were cooled to room temperature in an oxygen ambience of 760 torr at a rate of 5 ◦Cmin−1. After growth, a 2 nm Ti layer and a 150 nm Au layer were deposited using thermal evaporation to serve as the gate electrode. The device structure was then patterned using contact photolithography and Ar+ ion milling. A second lithography and Ar+ ion milling patterns the gate electrode. A final evaporation of 150-nm-thick Pd for contact pads was carried out using a lift-off process34. Received 15 December 2009; accepted 14 June 2010; published online 25 July 2010 References 1. Heber, J. 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The authors also thank J. S. Lee, D. A. Arena and C. C. Kao for X-ray magnetic circular dichroism measurements, L. W. Martin for discussions, Glenair Inc. for providing us with Nano Miniature connectors used in our experiment at 5 K, Y. P. Chen for circuit board layout and J. Clarke for use of his laboratory. Author contributions This work was a collaborative effort between the R.C.D. and R.R. groups. In the R.C.D. group, S.M.W. and S.A.C. designed the device and determined the processing steps. Device fabrication was carried out by S.M.W. The experimental measurement set-up was built and designed by S.A.C., and both S.M.W. and S.A.C. carried out the measurements. In the R.R. group, P.Y. grew all of the films and made SQUID magnetometer measurements. J.X.Z. carried out the PFM work and M.D.R. carried out the TEM study, both shown in the Supplementary Information. 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 S.M.W. NATURE MATERIALS | VOL 9 | SEPTEMBER 2010 | www.nature.com/naturematerials 761 © 2010 Macmillan Publishers Limited. All rights reserved.