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Epitaxial La0.67Sr0.33MnO3 magnetic tunnel junctions Article in Journal of Applied Physics · May 1997 DOI: 10.1063/1.364585 · Source: IEEE Xplore
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Epitaxial La0.67Sr0.33MnO3 magnetic tunnel junctions X. W. Li, Yu Lu, G. Q. Gong, and Gang Xiaoa) Department of Physics, Brown University, Providence, Rhode Island 02912
A. Gupta, P. Lecoeur, and J. Z. Sun IBM Research Division, T. J. Watson Research Center, Yorktown Heights, New York 10698
Y. Y. Wang and V. P. Dravid Materials Science and Engineering Department, Northwestern University, Evanston, Illinois 60208
We report the observation of a large magnetoresistance ~83%! at low magnetic fields of tens of Oe at 4.2 K in the epitaxial trilayer junction structure, La0.67Sr0.33MnO3 /SrTiO3 /La0.67Sr0.33MnO3. The spin-polarization parameter of the manganite has been determined from the magnetoresistance value. The switching fields of the two magnetic layers were designed by using the magnetic shape anisotropy. By limiting the sweeping field in a low field range ~;100 Oe!, we have achieved bistable resistive states at zero field, which is of potential interest for magnetoelectronic applications. © 1997 American Institute of Physics. @S0021-8979~97!73108-6#
The manganese perovskites in the form of La12x Dx MnO3 ~D5Ca, Sr, Ba, etc.! are strongly magnetoresistive materials. They exhibit ‘‘colossal’’ magnetoresistance ~CMR! effect when subjected to a high magnetic field of the order of Tesla.1–4 At present the electronic structures of the manganites have not been fully resolved. It has been predicted that the inherent double-exchange mechanism could induce a strong spin polarization, causing the material to become a ferromagnetic half-metal ~HM!.5 A perfect HM consists exclusively of majority charge carriers, and the other spin channel is insulating. One effective method to probe the electronic structure is electron tunneling. Magnetic tunnel junctions ~MTJs! allow the spin-polarization parameter to be measured.6–8 Furthermore MTJs could exhibit very large magnetoresistance at a magnetic field scale corresponding to the coercivities (H c ) of the magnetic electrodes.9–12 In manganites, although their H c values are only tens of Oesterds ~Oe!, the CMR effect appears at much higher fields which is required to suppress the magnetic thermal disorder.1,2 Because of this high field scale, there is much skepticism regarding the potential applications of manganites in magnetoelectronics. One approach to reduce the field scale12 is to incorporate the manganites into MTJ by taking advantage of their large spin polarization and low H c . In this paper, we describe our attempt to fabricate epitaxial MTJ using manganites in the form of @top electrode La0.67Sr0.33MnO3#/@barrier SrTiO3#/@bottom electrode La0.67Sr0.33MnO3# or LSMO/STO/LSMO. We have observed a large magnetoresistance ratio of 83% at low fields of a few tens of Oe at 4.2 K, which correspond to the coercivities of the magnetic layers. From the MR ratio we have obtained the spin-polarization parameter for LSMO. We have observed bistable resistive states at zero field, an effect useful for magnetoelectronic applications. Our results show that the field scale to achieve a large MR can indeed be substantially reduced in manganites. We have grown the manganite tunnel junctions using a multitarget pulsed laser deposition system.13 The LSMO/ a!
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J. Appl. Phys. 81 (8), 15 April 1997
STO/LSMO trilayers were grown in situ on ~100!-oriented STO substrates with no subsequent thermal treatment. The thicknesses of both LSMO electrodes are about 500 Å, and the STO barrier thickness is in the range of 30–60 Å. The as-prepared LSMO film has a sharp ferromagnetic transition temperature (T c ) of 347 K and a spontaneous magnetization of 622 emu/cm3 at 5 K. The cation stoichiometry of the films determined from Rutherford backscattering spectroscopy was within 5% of the nominal target compositions. After the blank trilayer films ~131 cm2! were deposited, we used a standard optical lithographic process to pattern the device structures down to micron scale. In Fig. 1, we show a schematic representation of an MTJ and a high-resolution crosssectional transmission electron microscopy ~TEM! lattice image of the interface region with the STO barrier ~30 Å! in one of the junctions. The TEM image reveals the heteroepitaxial growth of the layers and the sharp interfaces between the insulating STO and the metallic LSMO layers. The bottom electrodes are always large in area, whereas the top electrodes are patterned into rectangular shapes with micronscale dimensions. We have measured the I – V curves and the
FIG. 1. ~Left!: High resolution TEM micrograph of a cross-sectional lattice image obtained from ~a! La0.67Sr0.33MnO3/SrTiO3/La0.67Sr0.33MnO3 trilayer junction. The SrTiO3 barrier thickness is 3 nm. ~Right!: Schematic diagram of a magnetic tunnel junction. The thicknesses of the LSMO electrodes are about 50 nm.
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© 1997 American Institute of Physics
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FIG. 3. Temperature dependence of the maximum magnetoresistance ratio (DR/R p ) max and R p .
FIG. 2. Magnetoresistance vs magnetic field at different temperatures for a tunnel junction with a rectangular 2.5312.5 mm2 top electrode. The moment orientations of both electrodes are shown at various fields.
tunneling resistance (R) as a function of field using conventional electronics. The arrangement of the current and voltage terminals is shown in Fig. 1. Figure 2 shows the magnetoresistance ~MR! at various temperatures of an MTJ with a rectangular top electrode of area 2.5312.5 mm2. The resistance R is defined as dV/dI at zero bias. The magnetic field was applied along the easy axis of the rectangle ~see the inset of Fig. 2!. The orientations of the magnetization vectors of the top and bottom electrodes are marked at some specific field strengths. The change of MR is large at low fields of the order of tens of Oe, in sharp contrast to the Tesla-scale field required to observe the CMR effect. The maximum change in resistance, (DR/R p ) max is 83% at T54.2 K between the parallel and antiparallel moment configurations. The parameter R p is the tunneling resistance for the parallel alignment. We have found that the I – V curves of the junction are very nonlinear, but are of the shape characteristic of electron tunneling.14 The lead resistance over the area of the junction is about 8 V, and the junction resistance exceeds 4 kV. Therefore it is expected that the current density is uniform within the junctions. The first switching field at about 56 Oe is rather sharp. The magnetotunneling effect results from the asymmetry in the density of states ~DOS! of the majority and minority bands in magnetic metals. According to the model of spinpolarized tunneling,6,7 the MR ratio is given by ~ DR/R p ! max5 ~ R ↑↓ 2R ↑↑ ! /R ↑↑ 52 P 2 / ~ 12 P 2 ! ,
~1!
where R ↑↑ and R ↑↓ are the resistances for the parallel and 5510
J. Appl. Phys., Vol. 81, No. 8, 15 April 1997
antiparallel M configurations, respectively ~note R ↑↑ [R p !, P is the spin-polarization parameter for LSMO. If we take (DR/R p ) max'83%, relation ~1! leads to P'0.54 for LSMO. This value is much larger than that of a typical magnetic transition metal. Pickett and Singh5 have calculated the band structures of La12x Cax MnO2 . Their results reveal that the manganites at x50.33 resembles a half-metal, i.e., the minority DOS at the Fermi surface is very low. The calculated DOS leads to a P'0.37, which is smaller than our experimental P'0.54 for LSMO. Figure 3 also shows the adverse effect of temperature on magnetotunneling. We have obtained the maximum MR, (DR/R p ) max from each MR curve at a specific T. We present in Fig. 3 the T dependence of (DR/R p ) max . Below 100 K, (DR/R p ) max decreases slowly with increasing T. But the drop in (DR/R p ) max accelerates above 100 K and the MR vanishes at about 200 K, which is much lower than the T c of 347 K. In order to observe magnetotunneling, the bottom and the top electrodes should have different switching fields, H c1 and H c2 , so that the parallel and antiparallel moment configurations can be obtained over certain field ranges. We achieve this by taking advantage of the magnetic shape anisotropy. The bottom electrode has a large area and its coercivity is close to the bulk value. In Fig. 4 we show the T dependence of H c of a pure LSMO thin film. At T54.2 K H c is about 49 Oe, and it decreases slowly at high T. The two insets in Fig. 4 show the magnetic hysteresis curves at 5 and 290 K from which we obtained the values of H c . The lower switching field, H c1 , obtained from the MR curves in Fig. 2 is identified as that due to the bottom electrode. We also present H c1 as a function of T in Fig. 4. As can be seen the H c1 is about the same magnitude as H c obtained from the bulk thin film, and both H c1 and H c exhibit a similar T dependence. The second switching field, H c2 , is larger than H c1 . At T54.2 K, H c2 is about 160 Oe. We attribute H c2 to the top electrode. Using its dimensions ~the film thickness d550 nm and width w52.5 mm! and magnetization ~M s 5622 emu/cm3!, we estimated its demagnetization field, Li et al.
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FIG. 5. Magnetoresistance curves obtained by limiting the magnitude of the sweeping field to 110 Oe. The magnetic moment of the top electrode is fixed along the positive ~negative! field direction for the left ~right! figure.
FIG. 4. Temperature dependence of the coercivity (H c ) of a bulk LSMO film and the switching field (H c1 ) of the magnetic tunnel junction shown in Fig. 2.
H K 54 p M s d/w5156 Oe, which is close to the measured value. Another way to observe magnetotunneling is to keep the moment of the magnetically hard electrode undisturbed, while applying a sweeping field with magnitude less than H c2 , but large than H c1 . In this case only the soft electrode can switch its moment direction, which leads to the MTJ oscillating between the high and low resistance state. The experimental realization is shown in Fig. 5, where the MR curves were obtained as we swept the field between 2110 and 110 Oe ~Note H c1 ,110 Oe,H c2 !. In the left figure, the moment of the top electrode is fixed along the positive field direction. In the right figure, it is along the negative field direction. Clearly, as the moment of the bottom electrode switches back and forth, two distinctive resistive states appear at zero field. This property is particularly useful for some magnetoelectronic applications. Another interesting observation in Fig. 5 is that, in addition to the large and abrupt resistive changes, there exist a few smaller resistance jumps in the hysteresis loops. These minor jumps are not artifacts, but rather due to the domain wall motions in the junction. In other words, the junction electrodes are not magnetic single domain, but consist of a few domains. In summary, the La0.67Sr0.33MnO3 epitaxial tunnel junction can exhibit a large magnetoresistance of up to 83% at low temperatures and at magnetic fields of only tens of Oe.
J. Appl. Phys., Vol. 81, No. 8, 15 April 1997
The mechanism is the spin-polarized tunneling between the ferromagnetic metals. The switching fields of the electrodes can be designed by taking advantage of the magnetic shape anisotropy. We have demonstrated bistable resistive states at zero field which is of potential interest for magnetoelectronic applications. Acknowledgments: The authors wish to thank W. J. Gallagher, J. C. Slonczewski, P. L. Trouilloud, and T. R. McGuire for stimulating discussions and help. This work was supported by National Science Foundation Grant Nos. DMR-9414160 and DMR-9258306, IBM, and US Department of Energy Grant No. DE-FG02-92ER45475. 1
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