大学物理实验报告英文版.docx

1、大学物理实验报告英文版大学物理实验报告Ferroelectric Control of Spin PolarizationABSTRACTA current drawback of spintronics is the large power that is usually required for magnetic writing, in contrast with nanoelectronics, which relies on “zero-current,” gate-controlled operations. Efforts have been made to control the

2、 spin-relaxation rate, the Curie temperature, or the magnetic anisotropy with a gate voltage, but these effects are usually small and volatile. We used ferroelectric tunnel junctions with ferromagnetic electrodes to demonstrate local, large, and nonvolatile control of carrier spin polarization by el

3、ectrically switching ferroelectric polarization. Our results represent a giant type of interfacial magnetoelectric coupling and suggest a low-power approach for spin-based information control.Controlling the spin degree of freedom by purely electrical means is currently an important challenge in spi

4、ntronics (1,2). Approaches based on spin-transfer torque (3) have proven very successful in controlling the direction of magnetization in a ferromagnetic layer, but they require the injection of high current densities. An ideal solution would rely on the application of an electric field across an in

5、sulator, as in existing nanoelectronics. Early experiments have demonstrated the volatile modulation of spin-based properties with a gate voltage applied through a dielectric. Notable examples include the gate control of the spin-orbit interaction in III-V quantum wells (4), the Curie temperatureTC(

6、5), or the magnetic anisotropy (6) in magnetic semiconductors with carrier-mediated exchange interactions; for example, (Ga,Mn)As or (In,Mn)As. Electric fieldinduced modifications of magnetic anisotropy at room temperature have also been reported recently in ultrathin Fe-based layers (7,8).A nonvola

7、tile extension of this approach involves replacing the gate dielectric by a ferroelectric and taking advantage of the hysteretic response of its order parameter (polarization) with an electric field. When combined with (Ga,Mn)As channels, for instance, a remanent control ofTCover a few kelvin was ac

8、hieved through polarization-driven charge depletion/accumulation (9,10), and the magnetic anisotropy was modified by the coupling of piezoelectricity and magnetostriction (11,12). Indications of an electrical control of magnetization have also been provided in magnetoelectric heterostructures at roo

9、m temperature (1317).Recently, several theoretical studies have predicted that large variations of magnetic properties may occur at interfaces between ferroelectrics and high-TCferromagnets such as Fe (1820), Co2MnSi (21), or Fe3O4(22). Changing the direction of the ferroelectric polarization has be

10、en predicted to influence not only the interfacial anisotropy and magnetization, but also the spin polarization. Spin polarization ., the normalized difference in the density of states (DOS) of majority and minority spin carriers at the Fermi level (EF) is typically the key parameter controlling the

11、 response of spintronics systems, epitomized by magnetic tunnel junctions in which the tunnel magnetoresistance (TMR) is related to the electrode spin polarization by the Jullire formula (23). These predictions suggest that the nonvolatile character of ferroelectrics at the heart of ferroelectric ra

12、ndom access memory technology (24) may be exploited in spintronics devices such as magnetic random access memories or spin field-effect transistors (2). However, the nonvolatile electrical control of spin polarization has not yet been demonstrated.We address this issue experimentally by probing the

13、spin polarization of electrons tunneling from an Fe electrode through ultrathin ferroelectric BaTiO3(BTO) tunnel barriers (Fig. 1A). The BTO polarization can be electrically switched to point toward or away from the Fe electrode. We used a half-metallic (25) bottom electrode as a spin detector in th

14、ese artificial multiferroic tunnel junctions (26,27). Magnetotransport experiments provide evidence for a large and reversible dependence of theTMRon ferroelectric polarization direction.Fig. 1(A) Sketch of the nanojunction defined by electrically controlled nanoindentation. A thin resist is spin-co

15、ated on the BTO(1 nm)/LSMO(30 nm) bilayer. The nanoindentation is performed with a conductive-tip atomic force microscope, and the resulting nano-hole is filled by sputter-depositing Au/CoO/Co/Fe. (B) (Top) PFM phase image of a BTO(1 nm)/LSMO(30 nm) bilayer after poling the BTO along 1-by-4m stripes

16、 with either a negative or positive (tip-LSMO) voltage. (Bottom) CTAFM image of an unpoled area of a BTO(1 nm)/LSMO(30 nm) bilayer. , ohms. (C) X-ray absorption spectra collected at room temperature close to the Fe L3,2(top), Ba M5,4(middle), and Ti L3,2(bottom) edges on an AlOx nm)/Al nm)/Fe(2 nm)/

17、BTO(1 nm)/LSMO(30 nm)(D) HRTEM and (E) HAADF images of the Fe/BTO interface in a Ta(5 nm)/Fe(18 nm)/BTO(50 nm)/LSMO(30 nm)The white arrowheads in (D) indicate the lattice fringes of 011 planes in the iron layer. 110 and 001 indicate pseudotetragonal crystallographic axes of the BTO perovskite.The tu

18、nnel junctions that we used in this study are based on BTO(1 nm)/LSMO(30 nm) bilayers grown epitaxially onto (001)-oriented NdGaO3(NGO) single-crystal substrates (28). The large (180) and stable piezoresponse force microscopy (PFM) phase contrast (28) between negatively and positively poled areas (F

19、ig. 1B, top) indicates that the ultrathin BTO films are ferroelectric at room temperature (29). The persistence of ferroelectricity for such ultrathin films of BTO arises from the large lattice mismatch with the NGO substrate (%), which is expected to dramatically enhance ferroelectric properties in

20、 this highly strained BTO (30). The local topographical and transport properties of the BTO(1 nm)/LSMO(30 nm) bilayers were characterized by conductive-tip atomic force microscopy (CTAFM) (28). The surface is very smooth with terraces separated by one-unit-cellhigh steps, visible in both the topogra

21、phy (29) and resistance mappings (Fig. 1B, bottom). No anomalies in the CTAFM data were observed over lateral distances on the micrometer scale.We defined tunnel junctions from these bilayers by a lithographic technique based on CTAFM (28,31). Top electrical contacts of diameter 10 to 30 nm can be p

22、atterned by this nanofabrication process. The subsequent sputter deposition of a 5-nm-thick Fe layer, capped by a Au(100 nm)/CoO nm)/Co nm) stack to increase coercivity, defined a set of nanojunctions (Fig. 1A). The same Au/CoO/Co/Fe stack was deposited on another BTO(1 nm)/LSMO(30 nm) sample for ma

23、gnetic measurements. Additionally, a Ta(5 nm)/Fe(18 nm)/BTO(50 nm)/LSMO(30 nm) sample and a AlOx nm)/Al nm)/Fe(2 nm)/BTO(1 nm)/LSMO(30 nm) sample were realized for structural and spectroscopic characterizations.We used both a conventional high-resolution transmission electron microscope (HRTEM) and

24、the NION UltraSTEM 100 scanning transmission electron microscope (STEM) to investigate the Fe/BTO interface properties of the Ta/Fe/BTO/LSMO sample. The epitaxial growth of the BTO/LSMO bilayer on the NGO substrate was confirmed by HRTEM and high-resolution STEM images. The low-resolution, high-angl

25、e annular dark field (HAADF) image of the entire heterostructure shows the sharpness of the LSMO/BTO interface over the studied area (Fig. 1E, top).Figure 1Dreveals a smooth interface between the BTO and the Fe layers. Whereas the BTO film is epitaxially grown on top of LSMO, the Fe layer consists o

26、f textured nanocrystallites. From the in-plane (a) and out-of-plane (c) lattice parameters in the tetragonal BTO layer, we infer thatc/a= , in good agreement with the value of found with the use of x-ray diffraction (29). The interplanar distances for selected crystallites in the Fe layer ., (Fig. 1

27、D, white arrowheads) are consistent with the 011 planes of body-centered cubic (bcc) Fe.We investigated the BTO/Fe interface region more closely in the HAADF mode of the STEM (Fig. 1E, bottom). On the BTO side, the atomically resolved HAADF image allows the distinction of atomic columns where the pe

28、rovskite A-site atoms (Ba) appear as brighter spots. Lattice fringes with the characteristic 100 interplanar distances of bcc Fe ( ) can be distinguished on the opposite side. Subtle structural, chemical, and/or electronic modifications may be expected to occur at the interfacial boundary between th

29、e BTO perovskite-type structure and the Fe layer. These effects may lead to interdiffusion of Fe, Ba, and O atoms over less than 1 nm, or the local modification of the Fe DOS close toEF, consistent with ab initio calculations of the BTO/Fe interface (1820).To characterize the oxidation state of Fe,

30、we performed x-ray absorption spectroscopy (XAS) measurements on a AlOx nm)/Al nm)/Fe(2 nm)/BTO(1 nm)/LSMO(30 nm) sample (28). The probe depth was at least 7 nm, as indicated by the finite XAS intensity at the La M4,5edge (28), so that the entire Fe thickness contributed substantially to the signal.

31、 As shown inFig. 1C(top), the spectrum at the Fe L2,3edge corresponds to that of metallic Fe (32). The XAS spectrum obtained at the Ba M4,5edge (Fig. 1C, middle) is similar to that reported for Ba2+in (33). Despite the poor signal-to-noise ratio, the Ti L2,3edge spectrum (Fig. C, bottom) shows the t

32、ypical signature expected for a valence close to 4+ (34). From the XAS, HRTEM, and STEM analyses, we conclude that the Fe/BTO interface is smooth with no detectable oxidation of the Fe layer within a limit of less than 1 nm.After cooling in a magnetic field of 5 kOe aligned along the 110 easy axis of pseudocubic LSMO (which is parallel to the orthorhombic 100 axis of NGO), we characterized the transport properties of the junctions at low temperature K).Figure 2A(middle) shows a typical resistanceversusmagnetic fieldR(H) cycle recorded at a bias voltage of 2 mV (positive bias correspo