APPLIED PHYSICS LETTERS
VOLUME 74, NUMBER 7
15 FEBRUARY 1999
Epitaxial ferroelectric/giant magnetoresistive heterostructures for magnetosensitive memory cell A. M. Grishin,a) S. I. Khartsev,b) and P. Johnsson Department of Condensed Matter Physics, Royal Institute of Technology, S-100 44 Stockholm, Sweden
~Received 13 August 1998; accepted for publication 15 December 1998! Epitaxial ferroelectric/giant magnetoresistive Pb~Zr0.52Ti0.48!O3 /La0.67Ca0.33MnO3 ~PZT/LCMO! heterostructures have been grown onto LaAlO3 ~001! single crystal by KrF pulsed laser deposition. Main processing parameters have been optimized to preserve giant magnetoresitivity in the LCMO film after deposition of the top ferroelectric layer. High degree of c-axis orientation and strong in-plane texture, coherent with the substrate, both in template LCMO and PZT layers, high dielectric permittivity of 720, remnant polarization of 17 mC/cm2 of PZT, and magnetoresistivity in LCMO of 28% at H50.5 T indicate excellent characteristics of coexisting magnetoresistive and ferroelectric properties. We compare the performance of magnetosensitive memory cells with La0.67Ca0.33MnO3 and La0.75Sr0.25MnO3 giant magnetoresistive electrodes. © 1999 American Institute of Physics. @S0003-6951~99!02307-4#
Perovskite ferroelectric thin films have tremendous potential for data storage, sensor, and microelectromechanical system technologies. Recent advances in pulsed laser deposition of epitaxial ferroelectric/conductive oxide heterostructures have enabled the most remarkable breakthrough achieved in ferroelectric memories.1,2 The use of cuprate superconductor ~La, Sr!CoO3, and SrRuO3 conductive oxide electrodes has brought the oxygen migration, responsible for fatigue, under control as well as unified the competing superconducting and ferroelectric ordering in a single monolithic film structure.3–6 Recently, doped rare-earth manganates exhibiting colossal magnetoresistivity have been proposed to be used as semiconductor channel material for ferroelectric field effect transistors ~FET!.7 The operation of the FET is based on the depletion and accumulation of hole carriers in compounds of mixed valency, such as manganates with low Mn31 content, by an electric stray field of reversed polarization in adjacent ferroelectric. Furthermore, colossal magnetoresistivity ~CMR! of manganate might endow such FET with high magnetosensitivity. Although the feasibility of an all-perovskite ferroelectric FET has been demonstrated,7 the magnetoresistive properties of manganate/ferroelectric film structures have to our knowledge not been explored. We present results on KrF pulsed laser deposition and characterization of epitaxial ferroelectric/giant magnetoresistive Pb~Zr0.52Ti0.48!O3 /La0.67Ca0.33MnO3 ~PZT/LCMO! heterostructures. There was no indication of interdiffusion at the interface, LCMO has high crystalline quality and smooth surface, promoting growth of high quality PZT film. A high degree of c-axis orientation and strong in-plane texture, coherent with the substrate, both in template LCMO and top PZT layers, high dielectric permittivity of 720, remnant polarization of 17 mC/cm2, and magnetoresistivity of 28% at a!
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b!
H50.5 T indicate excellent characteristics of coexisting magnetoresistive and ferroelectric properties. A brief description of the processing technique is as follows: a 248 nm KrF excimer laser ~Lambda Physik-300! was used to ablate stoichiometric ceramic targets of the following compositions: La0.67Ca0.33MnO3 and Pb~Zr0.52Ti0.48!O3. Both layers were made at a laser radiation energy density of 3–4 J/cm2, pulse repetition rate of 10 Hz, and distance between target and substrate of 65 mm. The background pressure did not exceed 1027 Torr. Deposition of LCMO onto the single crystal LaAlO3 substrate was carried out in an oxygen pressure of about 300 mTorr at a substrate temperature of 750 °C and was followed by annealing at the same temperature in 600 Torr oxygen for 10 min. The second PZT layer was grown at 560 °C in 250 mTorr oxygen pressure, and finally the whole heterostructure was annealed in an oxygen pressure of 600 Torr at 500 °C for 30 min, and then cooled down to room temperature. Typical thickness of LCMO and PZT layers measured by atomic force microscopy ~AFM! was 500 and 400 nm correspondingly. X-ray diffraction measurements have been performed to evaluate the structural quality and confirm the phase purity of each of the layers. Figures 1–3 show x-ray diffracradiation of the tion patterns in Cu Ka Pb~Zr0.52Ti0.48!O3~400 nm!/La0.67Ca0.33MnO3~500 nm! heterostructure. The (00l) reflections in u –2u-scan in Fig. 1 and the rocking curves ~v scan! in Fig. 2 clearly exhibit a high degree of c-axis orientation both in PZT and LCMO layers. Narrow rocking curves about the PZT-002 and LCMO-002 Bragg peaks yielded a full-width at half-maximum ~FWHM! of 0.63° and 0.49° correspondingly despite the twinning of LaAlO3 substrate clearly seen from LaAlO3-002 rocking curve. Figure 3 shows w scan of PZT-113 ( u sam564.350°, LCMO-103 ( u sam557.474°, 2 u det 2 u det577.830°), 577.460°), and LaAlO3-103 ( u sam558.530°, 2 u det 580.035°) ‘‘off-normal’’ plane reflections. The fourfold symmetry of the w scan and the 45° shift of the PZT-113 peaks with respect to the LCMO- and LaAlO3-103 peaks bring us to the following cube-on-cube epitaxial relationship
0003-6951/99/74(7)/1015/3/$15.00 1015 © 1999 American Institute of Physics Downloaded 06 Dec 2006 to 159.226.131.10. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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Appl. Phys. Lett., Vol. 74, No. 7, 15 February 1999
FIG. 1. u –2u scan x-ray diffraction ~XRD! pattern of the Pb~Zr0.52Ti0.48!O3~400 nm!/La0.67Ca0.33MnO3~500 nm! heterostructure in Cu K a radiation.
of the PZT/LCMO bilayer on LaAlO3 substrate: ~001!PZTi~001!LCMOi~001!LaAlO3, @100#,@010#PZTi@100#,@010#LCMOi@100#,@010#LaAlO3. Here all the crystallographic planes and directions are presented in the pseudocubic ~perovskite! reference system, except for PZT for which the tetragonal system is used. Ferroelectric hysteresis ( P – E) and capacitance–voltage (C – V) characteristics have been measured by pulsed testing Radiant Technologies setup RT66A using a five-2-ms-wide triangular pulse train. Gold contact pads (B50.6 mm) for ferroelectric characterization have been thermally evaporated through a contact mask on the top of PZT layer at room temperature. The resistivity of the PZT layer was in the range of 231011 V cm and thus keeps the leakage current in order of 1026 A/cm2 at applied electric fields up to 100 kV/ cm. Figure 4 shows a typical polarization P – E loop which has been found to be almost symmetrical with remnant polarization of 17 mC/cm2 and coercivity E c 529 kV/cm. The inset shows the C – V characteristics. The incremental dielectric permittivity e 8 was found to be 720 at zero bias and reach its maximum value of 2000 at a field around 40 kV/cm.
Grishin, Khartsev, and Johnsson
FIG. 3. XRD w scan of off-normal planes measured at oblique geometry: detector positions 2 u det577.830°, 77.460°, 80.035°; sample positions u sam 564.350°, 57.474°, 58.530° gave maxima of the PZT-113, LCMO-103, and LaAlO3-103 reflections correspondingly.
The magnetoresistance of the LCMO underlayer has been measured in an electromagnet with magnetic fields up to 0.5 T by standard four-probe direct current ~dc! technique with subtraction of the thermoelectric power contribution. The four silver contact pads (B50.8 mm) were deposited by dc magnetron sputtering in the corners of the heterostructure where the upper PZT layer has been scraped off. The magnetoresistance ratio is defined as: D r / r [( r 0 2 r 0.5 T)/ r 0 . Figure 5 shows almost identical temperature dependencies of resistivity r (T) and magnetoresistance Dr/r in the test single layer LCMO and the epitaxial PZT/LCMO heterostructure. r vs T dependence has a bell shape, which is typical for a metal–semiconductor ~ferromagnetic–paramagnetic! phase transition, with maximum at T c 5262 °C. The magnetoresistance Dr/r vs T reaches its maximum value of 28% at H 55 kOe at the temperature where the resistivity curve r (T) has the maximum slope. The evident similarity of magnetotransport properties of the test LCMO single layer and the epitaxial PZT/LCMO heterostructure indicates that the giant magnetoresistivity effect in LCMO has been preserved
FIG. 4. Typical ferroelectric P – E and C – V hysteresis loops in Au/PZT/La0.67Ca0.33MnO3 400-nm-thick, 0.6-mm-diam capacitive cell measured by 2 ms pulse technique ~d symbols!. The resistivity was in the range of 231011 V cm, P r 517 m C/cm2, E c 529 kV/cm, incremental dielectric permittivity e 8 5720 at zero bias and reaches its maximum value of 2000 FIG. 2. XRD rocking curves of PZT-002, LCMO-002, and LaAlO3-002 at a field around 40 kV/cm. Ferroelectric properties of reflections for PZT/LCMO thin film heterostructure grown onto single crysAu/PZT/La0.75Sr0.25MnO3 capacitor are shown for comparison ~n symbols!. tal LaAlO3 substrate. Downloaded 06 Dec 2006 to 159.226.131.10. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
Appl. Phys. Lett., Vol. 74, No. 7, 15 February 1999
FIG. 5. Temperature dependencies of resistivity r (T) and magnetoresistance D r / r [( r 0 2 r 5 kOe)/ r 0 of 500-nm-thick La0.67Ca0.33MnO3 films measured at a sensing current of 100 mA. d symbols correspond to epitaxial PZT/La0.67Ca0.33MnO3 heterostructure while s symbols show the data for La0.67Ca0.33MnO3 test single layer grown onto the LaAlO3 at the same processing conditions. r and Dr/r vs T curves for a PZT/La0.75Sr0.25MnO3 heterostructure are shown for comparison ~n symbols!.
after deposition of the coating ferroelectric layer. We choose the La0.67Ca0.33MnO3 compound because it has an unbeatable combination of CMR properties among all known manganates: the highest magnetoresistivity at the highest T c . 8 However, La0.67Ca0.33MnO3 is probably not the best candidate for a magnetosensitive FET. It is reasonable to introduce the following figure of merit F5
Dr 1 Dr 1 • } • r t r r •C
to characterize the performance of a magnetosensitive memory cell. t is the access time. High resistivity of manganates at operation temperature leads to a larger t 5RC time constant and lower signal to noise ratio, which would be of concern since it could reduce the operating speed of the
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memory and the detectivity of the magnetosensitive field effect transistor. For comparison we present the ferroelectric properties ~Fig. 4! and temperature dependencies of resistivity and magnetoresistance ~Fig. 5! for a PZT/La0.75Sr0.25MnO3 epitaxial heterostructure fabricated at processing conditions similar to PZT/La0.67Ca0.33MnO3. It is clearly seen that the 15-fold gain in conductivity ~near the room temperature! of La0.75Sr0.25MnO3 compensates the three times loss in the peak of magnetoresitivity. Thus the choice of optimum magnetoresistor remains a matter that demands thorough consideration. In conclusion, the use of giant magnetoresistive La0.67Ca0.33MnO3 as an epitaxial electrode on LaAlO3 single crystal substrate and the growth of an epitaxial Pb~Zr0.52Ti0.48!O3 film on this electrode has been successfully demonstrated. The close lattice matching of similar perovskite structures yielded epitaxial quality and superior ferroelectric and magnetoresistive performance of fabricated heterostructures. This work is supported by the Go¨ran Gustafssons Foundation and the Swedish Natural Science Research Council ~NFR!. 1
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