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Pulsed laser deposited superlattices based on perovskite oxides
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Superlattices and Microstructures, Vol. 19, No. 3, 1996
Pulsed laser deposited superlattices based on perovskite oxides C. K, Q L, K.-C. K, M. C. R, Z. T, J. L. P, R. L. G, A. M. R, C. L, R. D, H. D. D, R. R, T. V Center for Superconductivity Research, University of Maryland, College Park, MD 20742, USA
K.-M. H, R. S Department of Physics, The Ohio State University, Columbus, OH 43210, USA (Received 21 August 1995) Recent advances made in the field of pulsed laser deposited superlattices made up of perovskite oxides are reviewed. The superconducting properties and the effect of adjacent layers in ultra-thin YBCO are studied in YBCO/(Pr Y )BCO superlattices. We describe the effects x 1~x of fundamental studies and possible appliof layering in (La Ba )MnO for the purposes 0.7 0.3 3 cations of these newly recognized colossal magnetoresistance materials. ( 1996 Academic Press Limited
1. Introduction The pulsed laser deposition (PLD) technique has been getting an increased attention since the successful growth of high temperature superconducting films [1], and the realization that the composition of the target is reproduced in the film under appropriate conditions of deposition. Since then PLD has been applied to the deposition of new classes of materials such as ferroelectrics, ferromagnets, electrooptic oxides, and so on [2,3]. In this paper, we will review the recent advances made at the University of Maryland, Center for Superconductivity Research in the pulsed laser deposited superlattices based on superconducting YBCO and ferromagnetic doped manganates. Experiments involving these layers, described in this paper, illustrate the versatility of the pulsed laser deposition technique for generating a variety of novel and interesting structures for both technological applications and basic studies. The sample fabrication procedure and the thickness calibration methods are described in Section 2. In Section 3, the study of the adjacent layers effect on the superconducting properties of ultra-thin YBCO films are summarized. We present strong experimental evidence of charge transfer at YBCO/(Pr Y )BCO interfaces. The new results of (La Ba )MnO /SrTiO superlattices are x 1~x 0.7 0.3 3 3 presented in Section 4. (La Ba )MnO (LBMO) is a member of the family of doped manganites 0.7 0.3 3 which show colossal magnetoresistance (CMR).
2. Sample fabrication A standard PLD system with a multiple-target holder was used for the deposition of single layer films and superlattices. For better control of the layer thicknesses, a computer controlled laser triggering and target exchange system was used to sequentially deposit different materials in situ. For the deposition of YBCO superlattices, the substrate was held at D750°C. The laser energy density was 0749–6036/96/030169]13 $18.00/0
( 1996 Academic Press Limited
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Fig. 1. X-ray h–2h scan around (001) and (002) peaks of a YBCO (2 unit-cells)/PrBCO (10 unit-cells) superlattice sample. The satellite peaks are marked with arrows.
typically D1.7 J cm~2. The oxygen pressure was kept at 100 mTorr during deposition and subsequently the samples were cooled to room temperature in D200 Torr oxygen. The CMR superlattices were deposited at a temperature of 625°C with laser energy density of 2 J cm~2 and oxygen pressure of 400 mTorr. The CMR superlattices were cooled down to room temperature in 300 Torr oxygen. No further annealing was performed on the samples. Special efforts were made to control the film thickness. Single layer films were separately deposited. A step was created by covering part of the sample and etching the rest. The step height was measured with a profilometer to give the deposition rate per one laser shot. The deposition rate calibrations were made immediately before and after the superlattice depositions to ensure the consistency of the calibration. The thickness of each layer in a sample was then determined by the number of laser pulses. The thickness of the superlattices was also cross-calibrated using a h–2h x-ray diffraction spectrum from the position of the satellite peaks. Figure 1 shows h–2h x-ray diffraction spectrum of a YBCO/PrBCO superlattice showing prominent satellite peaks adjacent to the main (001) peaks indicating atomically sharp interfaces and a periodic modulation of compositions.
3. YBCO/(PrxY1[x)BCOsuperlattices One of the characteristic features of cuprate superconductors is their layered structure. Since the ˚ , which is shorter than superconducting coherence length n along the c-axis in YBCO is about 5 A 0 the c-axis lattice parameter, the superconductivity is confined within the Cu-O planes. Many at2 tempts have been made to study the superconducting properties of a single-unit-cell sheet of YBCO, in order to understand the T in a bi-CuO layer and the effect of coupling between the layers. c 2 Experiments on superlattices with nominal thickness of 1.2 nm YBCO separated by thick PrBCO layers indicate that one unit-cell thick YBCO layer is in fact superconducting [4–8]. The experiments on the trilayer structure of YBCO sandwiched between (Pr Y )BCO buffer and cap layers conx 1~x firmed the superconducting transition in a one unit-cell thick YBCO layer [9,10]. However, the T c values depend strongly on the composition x in the (Pr Y )BCO adjacent layers. This led us to x 1~x study the effects of adjacent layers on the superconducting properties of ultra-thin YBCO layers.
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Fig. 2. Resistive transitions of 1, 2, and 4 unit-cell thick YBCO layers sandwiched between (Pr Y )BCO x 1~x adjacent layers with x\1 and 0.6.
Understanding the role of adjacent layers is important not only for the fundamental physics but also for the development of new higher T superconductors. c Various experiments have been conducted to determine the most important properties of the adjacent layers for achieving high T of YBCO. We conclude that charge transfer between YBCO c and the adjacent layers plays the primary role. However, charge transfer by itself is not enough to explain all the experimental results indicating the importance of other effects such as interlayercoupling and dimensional effects. 3.1 Superconducting transitions in ultra-thin YBCO layers While there are some reports on the superlattices of YBCO/(Pr Y )BCO, a peculiar long range x 1~x interaction across the (Pr Y )BCO layers hinders the study of intrinsic properties in YBCO. The x 1~x effect of adjacent layers on the superconducting properties of YBCO layers can be more easily seen in a single layer YBCO sandwiched between (Pr Y )BCO adjacent layers. The superconducting x 1~x properties of ultra-thin YBCO layers in trilayer structures are characterized by ac susceptibility, dc resistance, and critical current density. Figure 2 shows the resistive transitions of 1, 2, and 4 unit-cell thick YBCO layers with different x values in the adjacent (Pr Y )BCO layers. (Pr Y )BCO is a semiconductor for 0.5¹x¹1 and x 1~x x 1~x the resistivity increases rapidly with increasing x. For x¹0.5, (Pr Y )BCO is a superconductor. x 1~x As shown in Fig. 2, the resistive transition of ultra-thin YBCO is generally broader and T is lower c than in thick films. With decreasing x, T increases and the transition width narrows. The dependence c of T on the x values for 1 and 2 unit-cell thick YBCO sandwiched between (Pr Y )BCO layers is c x 1~x shown in Fig. 3. The variation in T for a given x and YBCO thickness is about 5 K from sample to c sample. The figure indicates that the 1 unit-cell thick YBCO layer is superconducting for all the x values, while T increases monotonically with decreasing x in the adjacent layers. c
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Fig. 4. The superconducting transitions of 1, 4, and 24 unit-cell thick YBCO layers after subtracting the (Pr Y )BCO contribution. 0.6 0.4
Since the thicknesses of the YBCO layers are so small, the temperature dependence of the resistance shown in Fig. 2 gets a substantial contribution from the adjacent layers. If we subtract the resistance of the adjacent layers, inferred from the resistance of a 30 nm thick (Pr Y )BCO layer x 1~x made under identical conditions, the temperature dependence of the resisitivity (q ) of the ultra-thin Y YBCO films shows metallic behavior in all cases (Fig. 4). For the calculation of q (T ) we assumed Y a parallel resistor model in which YBCO and two (Pr Y )BCO layers were connected in parallel x 1~x without any interface layers. q (T ) at 100 K does not change (within the experimental error; ^10%) Y with varying thickness of the YBCO layer suggesting no change of the charge carrier density and the scattering time with reduced thickness. The thickness independence of q was also seen by Cieplak Y
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Fig. 5. A, ac measurement of 1 unit-cell YBCO sandwiched between (Pr Y )BCO adjacent layers. B, 0.6 0.4 between PrBCO layers. The onset of transition is marked with an arrow.
et al. [11]. Because of 2D nature of YBCO, it is not surprising to observe the invariance of charge carrier density in ultra-thin YBCO samples. However, the invariance of the charge carrier density measured in the normal state and the drop of T with reduced YBCO thickness seem like contradicc tory results. This will be discussed further later. All the samples were characterized by ac susceptibility method. Figures 5 (a) and (b) show the ac susceptibility results of 1 unit-cell thick YBCO with (Pr Y )BCO and PrBCO, respectively. The 0.6 0.4 prominent signal in both samples indicates that the superconductivity in a one unit-cell YBCO film is not coming from a single percolation path and that a significant portion of the sample is superconducting. T (R\0 X) measured by dc resistance matches the onset temperature of the ac suscepc tibility signal. As discussed before, T of a YBCO with PrBCO adjacent layers is lower than that with c (Pr Y )BCO adjacent layers. However, the width of the ac transition remains the same (D4 K) 0.6 0.4 suggesting that the level of uniformity in T is the same in both cases. Hence, the interdiffusion of c Y/Pr atoms and the non-uniformity of T cannot be the origin of T dependence on x. c c The critical current density was measured as a function of temperature and magnetic field. Figure 6 shows a universal temperature dependence of J (T )/J (0) versus T/T for 1, 2, 3, and 4 c c c unit-cells, and bulk YBCO (100 unit-cells) samples. J values of 2]106, 6]106, 1.2]107, and c 1.5]107 A cm~2 at 4.2 K and zero field are obtained for 1, 2, 3, and 4 unit-cell thick YBCO layers, respectively. The values of J are very close to that of the best thick film value (about 5]107 A cm~2) c [12]. The universal scaling in the temperature dependence of J suggests that the pinning for unit-cell c samples is the same as for thick YBCO films. The magnetic field dependence of J for field applied c
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Fig. 6. Normalized J versus T/T curves for 1, 2, 3, 4 unit-cells, and thick (120 mm) YBCO samples. c c 108
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parallel and perpendicular to the ab plane is shown for a 1 unit-cell sample and a 200 nm thick film in Fig. 7. Surprisingly, for the field perpendicular to the ab plane, the suppression of J is weak and a value of c J \4]105 A cm~2 was obtained even at 7 T for 1 unit-cell thick YBCO film. That indicates the critical c current density in our 1 unit-cell YBCO layer is not dominated by weak links as proposed by Pennycook et al. [13] for superlattices with the same thickness of YBCO layers. Although YBCO/(Pr Y )BCO x 1~x interfaces and the tensile stress on lattices, which differs from the thick film case, are expected to generate strong pinning centers, this tendency is not observed in the temperature and field dependence of J . c The ac susceptibility and critical current measurements on YBCO/(Pr Y )BCO shows that x 1~x the properties of YBCO layers in these structures are not controlled by the extrinsic effects such as weak links and interdiffusion of Y/Pr atoms. Thus, this is an ideal system to study the intrinsic properties of ultra-thin YBCO layers and to understand the effect of adjacent layers on T . c 3.2 Raman study of YBCO/PrBCO superlattices Raman measurements were performed on YBCO/PrBCO superlattices with individual layer thicknesses in the range of 1 to 11 unit-cells. The T of the samples was varied between 30 K and 82 K c
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A plot of various Raman frequencies at 5 K for different superlattices. The solid lines are guides to the
depending on the thickness of the layers. Figure 8 shows the frequencies of O(2)[O(3) out-of-phase vibration in the superlattices for PrBCO (D303 cm~1) and YBCO (337 cm~1) layers, O(2)]O(3) in-phase vibrations (437 cm~1), and axial motion of O(4) apical oxygen in YBCO (500 cm~1) and PrBCO (D524 cm~1). As evident in the figure, all these modes, except the O(4) vibrations associated with the PrBCO layers, remain unchanged as the layer thicknesses are varied. The apical oxygen vibration in the PrBCO layer softens as the thickness of the PrBCO layer (d ) is less that two Pr unit-cells. There is wide evidence that the O(4) atom plays a special role in the oxide superconductors. The softening of the apical oxygen in the PrBCO layer is consistent with the charge redistribution across the interfaces, since the carrier density of PrBCO is over two orders of magnitude smaller than that in the YBCO layers. This difference in carrier density should result in significant charge transfer across the interfaces. Given the low carrier density in PrBCO, the relative change in density would be large in the PrBCO layers and much weaker in the YBCO layers. These features would explain the observed softening of the O(4) mode in PrBCO layers thinner than three unit-cells and the minimal change in O(4) associated with YBCO. The softening is observed only in PrBCO layers thinner than unit-cells, indicating that the redistribution of charges occurs only over unit-cell dimensions. 3.3 Infra-red transmission measurements in YBCO/(PrxY1[x)BCO superlattices The charge carrier density of YBCO/PrBCO structures has been measured in the normal state by several groups [14,15]. The results have been inconclusive due to the carrier contribution from the
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Fig. 9. The frequency dependence of IR transmission in various temperatures for Sample A (a 2 layers of 6 unit-cell YBCO with (Pr Y )BCO) and Sample B (a 2 layers of 6 unit-cell YBCO with PrBCO). 0.6 0.4
adjacent layers. The problem with transport measurements has been the subtraction of the adjacent layer contributions. This requires knowledge of the role of adjacent layers which we do not know yet. Transport measurements are usually performed when a sample is still in the normal state. However, it is not clear whether the superconducting properties and the normal state transport are directly related. This can also be seen from the contradicting results of the same YBCO resistivities yet different T versus YBCO thickness in section 3.1. The infra-red transmission measurements can be c performed at low temperatures, where the sample is superconducting. Hence, the contribution of adjacent layers is negligible and the superconducting carrier density can be measured directly. Figure 9 shows the frequency dependence of IR transmission for various temperatures for Sample A (2 layers of 6 unit-cell YBCO layers among (Pr Y )BCO adjacent layers) and Sample 0.6 0.4 B (2 layers of 6 unit-cell YBCO layers among PrBCO adjacent layers). The solid line is a fit based on the two fluid model. From the data fit the carrier density in the normal state, n, the superconducting fraction, f , and scattering time, 1/s, are obtained. Sample A has n\(4.0^0.3)]1021 cm~3, s f \(0.35^0.04), and 1/s\(110^20) cm~1 at 5 K. Hence, the superconducting carrier density at 5 K s for the sample A is (1.4^0.3)]1021 cm~3. The same fit is performed for Sample B, and the parameters are n\(2.7^0.3)]1021 cm~3, f \(0.22^0.02) at 5 K. The superconducting carrier density s at 5 K is (0.6^0.1)]1021 cm~3. The only difference between sample A and B is the value x in the (Pr Y )BCO adjacent layers, which makes a big difference in the superconducting carrier density. x 1~x The IR transmission measurements show a decrease in both the superconducting fraction and the total number of carriers with increasing x value in (Pr Y )BCO layers. Being a direct measurement x 1~x
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Fig. 10. I-V curves of a 1 unit-cell sample at various temperatures on a log-log scale. The dotted line is VDI3 curve.
of the superconducting carrier density, this is a strong evidence of charge transfer in YBCO/ (Pr Y )BCO structures. x 1~x Our results suggest that charge transfer at YBCO/(Pr Y )BCO interfaces is the primary x 1~x cause of the adjacent layer effect. However, it is hard to explain the broadening of the transition width and the low zero resistance T only through charge transfer. The 2D nature of ultra-thin samples c makes them very susceptible to the thermal fluctuations and subsequently the creation of free vortices. We have also studied the existence of the Kosterlitz–Thouless transition in ultra-thin YBCO films. 3.4 Kosterlitz‒Thouless transition study Trilayer samples with a 1 unit-cell thick YBCO layer sandwiched between (Pr Y )BCO adjacent 0.6 0.4 layers were used in this study. Figure 10 shows I-V characteristics of a 1 unit-cell sample at various temperatures in a log-log scale. At a high temperature when the sample is in the normal state, the I-V curves show a linear behavior with an exponent a\1 (VDIa(T)); i.e. Ohmic. Upon lowering the temperature, a non-linear region appears on the I-V curves. The curve marked by an arrow has a region with an exponent 3; however, it changes back to being a\1 at low current. There is no definite discontinuity in the behavior between curves which are linear (a\1) and those which are non-linear
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(a[1) as expected from a K-T transition. The residual magnetic field inside the cryostat was measured using a Hall sensor, and was less than 0.1 mG limited by the sensitivity of the device. Hence, the Ohmic behavior at low currents is an intrinsic sample characteristic. Even though there is no K-T transition in 1 unit-cell YBCO samples, the thermally generated vortices may still be the origin of the linear tail in the I-V characteristics. When the physical size of a film (either the width or the length of a sample) is bigger than the magnetic screening length, one expects that free vortices exist in the film even below the nominal K-T transition temperature. This means that there would be a tail with a\1 in the I-V curves which has an exponent greater than 3, as we have seen. Other possible origin such as noise rounding is under investigation. Using YBCO/(Pr Y )BCO multilayer and superlattice structures we have investigated the x 1~x origin of the adjacent layer effects and the possibility of higher transition temperature. We conclude that charge transfer is the primary origin of the dependence of the superconducting properties on the adjacent layers. However, charge transfer by itself is not enough to explain all the experimental results. Other effects such as interlayer-coupling, thermal fluctuation, dimensional effects play a secondary role. From the application point of view, our findings can be applied toward fabrication of devices which require high quality ultra-thin superconducting layers such as a superconductor field effect device (Su-FET).
4. (La0.7Ba0.3)MnO3/SrTiO3 superlattices The giant magnetoresistance (GMR) behavior in metallic multilayers and granular materials has been studied actively due to both its scientific interest and its potential technological importance for magnetic sensors, especially for hard disk drive read heads. Recently, colossal magnetoresistance (CMR) effects were reported for doped perovskite manganites, such as La-Ba-Mn-O, La-Ca-Mn-O, La-Sr-Mn-O, Nd-Pb-Mn-O, and Nd-Sr-Mn-O [16–20]. The largest magnetoresistance (MR) ratio reported so far was over 1]106% at 60 K and 8 T in Nd Sr MnO thin films [16]. A heat treat0.7 0.3 3 ment at 900°C is normally required to obtain a large MR ratio, and the resistivity and MR ratio are very sensitive to the sample preparation conditions. It is still not clear why and how these processing conditions affect doped manganite films. We utilized a superlattice structure to investigate the possibility of device applications and to study the intrinsic properties of CMR manganites. Our work was motivated by the latest developments in the ultra-thin metallic ferromagnet layers and superlattices. As shown in the YBCO/(Pr Y )BCO superlattices, the superlattice structure can be used as x 1~x a model system to study a variety of physical phenomena. There have been several reports concerning growth of superlattice structures made up of YBa Cu O and doped manganite layers due to the structural similarity of the perovskite com2 3 7 pounds [21,22], but the structures were studied in the context of superconductivity in the YBCO layers. We report growth, resistivity, and magnetoresistance of superlattice structures with La Ba MnO (LBMO) and insulating SrTiO (STO) layers. The individual layer thickness of 0.7 0.3 3 ˚ to 55 A ˚ while3 the thickness of the STO layer was fixed at 160 A ˚. LBMO (d ) ranged from 2300 A LBMO ˚ thick LBMO layers. As the Our results demonstrate that a resistivity peak (T ) exists down to 55 A p thickness decreases, the resistivity increases and T shifts to lower temperatures. A large MR is p ˚ , a large MR is observed well below observed for all samples near T . In the case of d ¹110 A p LBMO T and localization behavior is seen at low temperatures. Possible origins will be discussed. p Ion channeling in Rutherford Back Scattering (RBS) measured on a single layer of LBMO deposited at the same conditions as the superlattices reveals that the film is a high quality crystal with a minimum yield of 4.5%. A FWHM of 0.12° was obtained from the rocking curve of the (002) LBMO peak.
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˚ )/LBMO (160 A ˚ ) superlattice sample. Fig. 11. X-ray h–2h scan around (002) and (002) peaks of a STO (55 A The satellite peaks are marked with arrows.
A set of LBMO/STO superlattices was made for this study. The total thickness of the doped ˚ to 55 A ˚ . The manganite layers was fixed and the individual layer thickness was varied from 1100 A ˚ thick STO. The insulating STO separation between doped manganite layers was fixed with 160 A layers were used to decouple the LBMO layers in order to study the properties of ultra-thin LBMO ˚ )/STO(160 A ˚ ) superlattice is shown layers. The x-ray diffraction of the (002) peaks of a LBMO(55 A in Fig. 11. The satellite peaks adjacent to the main (002) peaks are marked by arrows. (001) peaks of both LBMO and STO layers without any other phase peaks indicate an epitaxial c-axis growth. The observation satellite peaks in both the LBMO and STO layers unambiguously proves the periodic structure and chemical modulation of the superlattice. All superlattices were characterized by x-ray diffraction and show prominent satellite structure. Transport measurements were performed on each sample using a 4-probe method. Electrical contacts were made in such a way that a current was flowing in all layers. Figure 12 shows the resistive transition in zero field and MR at 5 T for three LBMO/STO superlattices with the individual ˚ , 110 A ˚ , and 55 A ˚ . The resistivity is calculated using the total LBMO thickness (d ) of 1100 A LBMO ˚ LBMO is the same as those thickness of LBMO layers. The resistive transition, T , and q of 1100 A p p of thicker films. The resistive peak is observed in all samples with T at 325 K, 280 K, and 275 K for p ˚ , 110 A ˚ , and 55 A ˚ , respectively. The minimum resistivity d \1100 A of samples below T increases LBMO p ˚ with reducing d and an increase of resistivity at low temperature is observed for d \110 A LBMO LBMO thick samples. Applying a magnetic field reduces the resistivity significantly in all samples. As seen in Fig. 12, the MR ratio at a field of 5 T has a peak at a slightly lower temperature than the resistivity ˚ peak. There also exists a significant magnetoresistance below the resistivity peak in the thinner (110 A ˚ ) LBMO layers. However, the peak MR is almost constant in all thicknesses. and 55 A The deviation of the resistive transition from thick LBMO films starts in the thickness range ˚ . The change of T and q is very gradual as observed in the case of 110 A ˚ and of a few hundred A p p ˚ LBMO samples. It is very surprising 55 A that the basic characteristics of the thick film are preserved ˚ thick layers. This is another indication of the high quality of the samples. From h–2h down to 55 A x-ray diffraction we observed an increase in the c-axis lattice parameter of the LBMO layers ˚ in 1100 A ˚ LBMO; c\3.922 A ˚ in 55 A ˚ LBMO]. Possible origins of this gradual change [c\3.911 A are a change of the Mn-O-Mn bond as a result of strain, oxygen deficiency, interface scattering, and interdiffussion.
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The exchange of Sr and Ba through the interdiffusion increases T and decreases the resistivity p in LBMO layers, hence it is very unlikely that the interdiffusion causes the changes in superlattices. The oxygen deficient polycrystalline LBMO [23] has similar properties such as the increase of the resistivity, reduction of T , and substantial MR at low temperatures, however it is hard to believe p the samples made at the same deposition condition has different oxygen content. If there is a nonuniformity of oxygen among the layers in the superlattices because of the diffusion process, it will broaden the resistive peak without changing the peak temperature. Since the mean free path es˚ , the interface timated using the resistivity of LBMO at low temperatures is in the order of a few A ˚ ) cannot be dominating scattering determined by the physical thickness of the sample (about 100 A the transport properties. The observation of an increase in the c-axis lattice parameter of the LBMO layers and the importance of the Mn-O-Mn bond for the properties of this material lead us to suggest that the induced strain due to the multiple layering of the two materials is a more probable cause for the observed behavior. More study is needed. The CMR materials have potential applications for a magnetic sensors using the large magnetoresistance. MR of 10% at 100 Oe is required for the hard disk read head application; the DC resistance drop in these materials is presently less than 1% at 100 Oe. However MR of 10% at a few thousand Oe is observed in CMR materials which can be useful for the magnetic sensor applications in automobiles. We demonstrated the growth of LBMO/STO superlattices and showed that the resistivity peak ˚ thick LBMO layers. With reduced LBMO layer thickness, the resistivity gradexists even for 55 A
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ually increases while T gradually decreases, respectively. A large MR is observed for all samples near p ˚, a T , however MR does not depend strongly on the layer thickness. In the case of d ¹110 A p LBMO large MR is observed well below T and localization behavior is seen at low temperatures. p Acknowledgements—The CMR superlattice research is supported by Office of Naval Research under contract number.
References [1] A. Inam, M. S. Hegde, X. D. Wu, T. Venkatesan, P. England, P. E. Miceli, E. W. Chase, C. C. Chang, J. M. Tarascon and J. B. Wachtman, Appl. Phys. Lett. 53, 908 (1988). [2] K. Hubler and D. B. Chrisey, eds Pulsed Laser Deposition of Thin Films, John Wiley & Sons, New York: (1994). [3] O. Auciello and J. Engemann, eds Multicomponent and Multilayed Thin Films for Advanced Microelectronics: Techniques, Fundamentals and Devices, Kluwer Academic Publication: (1993). [4] J.-M. Triscone, O. Fischer, O. Brunner, L. Antognazza, A. D. Kent, and M. G. Karkut, Phys. Rev. Lett. 64, 804 (1990). [5] Qi Li, X. X. Xi, X. D. Qu, A. Inam, S. Vadlamannati, W. L. McLean, T. Venkatesan, R. Ramesh, D. M. Hwang, J. A. Martinez and L. Nazar, Phys. Rev. Lett. 64, 3086 (1990). [6] Qi Li et al., Phys. Rev. Lett. 69, 2713 (1992). [7] D. H. Lowndes, D. P. Norton and J. D. Budai, Phys. Rev. Lett. 65, 1160 (1990). [8] D. P. Norton, D. H. Lowndes, S. J. Pennycook and J. D. Budai, Phys. Rev. Lett. 67, 1359 (1991). [9] C. Kwon, Qi Li, X. X. Xi, S. Bhattacharya, C. Doughty, T. Venkatesan, H. Zhang, J. W. Lynn, J. L. Peng, Z. Y. Li, N. D. Spencer and K. Feldman, Appl. Phys. Lett. 62, 1289 (1993). [10] T. Terashima, Y. Bando, K. Iijima, K. Yamamoto, K. Hirata, K. Hayashi, K. Kamigaki and H. Terauchi, Phys. Rev. Lett. 67, 1362 (1991). [11] M. Z. Cieplak, S. Guha, S. Vadlamannati, T. Giebultowicz and P. Lindenfeld, Phys. Rev. B 50, 12 876 (1994). [12] B. Roas, L. Schultz and G. Saemann-Ischenko, Phys. Rev. Lett. 64, 479 (1990). [13] S. J. Pennycook, M. F. Chisholm, D. E. Jesson, D. P. Norton, D. H. Lowndes, R. Feenstra, H. R. Kerchner and J. O. Thomson, Phys. Rev. Lett. 67, 765 (1991). [14] M. Affronte, J.-M. Triscone, O. Brunner, L. Antognazza, L. Mieville, M. Decroux, Phys. Rev. B 43, 11 484 (1991). [15] Y. Matsuda, S. Komiyama, T. Terashima, K. Shimura and Y. Bando, Phys. Rev. Lett. 69, 3228 (1992). [16] G. C. Xiong, Qi Li, H. L. Ju, R. L. Greene and T. Venkatesan, Appl. Phys. Lett. 66, 1689 (1995). [17] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz and K. Samwer, Phys. Rev. Lett. 71, 2331 (1993). [18] K.-I. Chahara, T. Ohno, M. Kasai and Y. Kozono, Appl. Phys. Lett. 63, 90 (1993). [19] S. Jin, T. H. Teifel, M. McCormack, R. A. Fastnacht, R. Ramesh and L. H. Chen, Science 264, 413 (1994). [20] H. L. Ju, C. Kwon, Q. Li, R. L. Greene and T. Venkatesan, Appl. Phys. Lett. 65, 2109 (1994). [21] G. Jakob, V. V. Moshchalkov and Y. Bruynseraede, Appl. Phys. Lett. 66, 2564 (1995). [22] Y. Kozono, M. Kasai, Y. Kanke, T. Ohno, M. Hanazono and Y. Sugita, Physica C 185‒189, 1919 (1991). [23] H. L. Ju, J. Goaplakrishnan, J. L. Peng, Qi Li, G. C. Xiong, T. Venkatesan and R. L. Greene, Phys. Rev. B 51, 6143 (1995).
Superlattices and Microstructures, Vol. 19, No. 3, 1996
Pulsed laser deposited superlattices based on perovskite oxides C. K, Q L, K.-C. K, M. C. R, Z. T, J. L. P, R. L. G, A. M. R, C. L, R. D, H. D. D, R. R, T. V Center for Superconductivity Research, University of Maryland, College Park, MD 20742, USA
K.-M. H, R. S Department of Physics, The Ohio State University, Columbus, OH 43210, USA (Received 21 August 1995) Recent advances made in the field of pulsed laser deposited superlattices made up of perovskite oxides are reviewed. The superconducting properties and the effect of adjacent layers in ultra-thin YBCO are studied in YBCO/(Pr Y )BCO superlattices. We describe the effects x 1~x of fundamental studies and possible appliof layering in (La Ba )MnO for the purposes 0.7 0.3 3 cations of these newly recognized colossal magnetoresistance materials. ( 1996 Academic Press Limited
1. Introduction The pulsed laser deposition (PLD) technique has been getting an increased attention since the successful growth of high temperature superconducting films [1], and the realization that the composition of the target is reproduced in the film under appropriate conditions of deposition. Since then PLD has been applied to the deposition of new classes of materials such as ferroelectrics, ferromagnets, electrooptic oxides, and so on [2,3]. In this paper, we will review the recent advances made at the University of Maryland, Center for Superconductivity Research in the pulsed laser deposited superlattices based on superconducting YBCO and ferromagnetic doped manganates. Experiments involving these layers, described in this paper, illustrate the versatility of the pulsed laser deposition technique for generating a variety of novel and interesting structures for both technological applications and basic studies. The sample fabrication procedure and the thickness calibration methods are described in Section 2. In Section 3, the study of the adjacent layers effect on the superconducting properties of ultra-thin YBCO films are summarized. We present strong experimental evidence of charge transfer at YBCO/(Pr Y )BCO interfaces. The new results of (La Ba )MnO /SrTiO superlattices are x 1~x 0.7 0.3 3 3 presented in Section 4. (La Ba )MnO (LBMO) is a member of the family of doped manganites 0.7 0.3 3 which show colossal magnetoresistance (CMR).
2. Sample fabrication A standard PLD system with a multiple-target holder was used for the deposition of single layer films and superlattices. For better control of the layer thicknesses, a computer controlled laser triggering and target exchange system was used to sequentially deposit different materials in situ. For the deposition of YBCO superlattices, the substrate was held at D750°C. The laser energy density was 0749–6036/96/030169]13 $18.00/0
( 1996 Academic Press Limited
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Fig. 1. X-ray h–2h scan around (001) and (002) peaks of a YBCO (2 unit-cells)/PrBCO (10 unit-cells) superlattice sample. The satellite peaks are marked with arrows.
typically D1.7 J cm~2. The oxygen pressure was kept at 100 mTorr during deposition and subsequently the samples were cooled to room temperature in D200 Torr oxygen. The CMR superlattices were deposited at a temperature of 625°C with laser energy density of 2 J cm~2 and oxygen pressure of 400 mTorr. The CMR superlattices were cooled down to room temperature in 300 Torr oxygen. No further annealing was performed on the samples. Special efforts were made to control the film thickness. Single layer films were separately deposited. A step was created by covering part of the sample and etching the rest. The step height was measured with a profilometer to give the deposition rate per one laser shot. The deposition rate calibrations were made immediately before and after the superlattice depositions to ensure the consistency of the calibration. The thickness of each layer in a sample was then determined by the number of laser pulses. The thickness of the superlattices was also cross-calibrated using a h–2h x-ray diffraction spectrum from the position of the satellite peaks. Figure 1 shows h–2h x-ray diffraction spectrum of a YBCO/PrBCO superlattice showing prominent satellite peaks adjacent to the main (001) peaks indicating atomically sharp interfaces and a periodic modulation of compositions.
3. YBCO/(PrxY1[x)BCOsuperlattices One of the characteristic features of cuprate superconductors is their layered structure. Since the ˚ , which is shorter than superconducting coherence length n along the c-axis in YBCO is about 5 A 0 the c-axis lattice parameter, the superconductivity is confined within the Cu-O planes. Many at2 tempts have been made to study the superconducting properties of a single-unit-cell sheet of YBCO, in order to understand the T in a bi-CuO layer and the effect of coupling between the layers. c 2 Experiments on superlattices with nominal thickness of 1.2 nm YBCO separated by thick PrBCO layers indicate that one unit-cell thick YBCO layer is in fact superconducting [4–8]. The experiments on the trilayer structure of YBCO sandwiched between (Pr Y )BCO buffer and cap layers conx 1~x firmed the superconducting transition in a one unit-cell thick YBCO layer [9,10]. However, the T c values depend strongly on the composition x in the (Pr Y )BCO adjacent layers. This led us to x 1~x study the effects of adjacent layers on the superconducting properties of ultra-thin YBCO layers.
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Fig. 2. Resistive transitions of 1, 2, and 4 unit-cell thick YBCO layers sandwiched between (Pr Y )BCO x 1~x adjacent layers with x\1 and 0.6.
Understanding the role of adjacent layers is important not only for the fundamental physics but also for the development of new higher T superconductors. c Various experiments have been conducted to determine the most important properties of the adjacent layers for achieving high T of YBCO. We conclude that charge transfer between YBCO c and the adjacent layers plays the primary role. However, charge transfer by itself is not enough to explain all the experimental results indicating the importance of other effects such as interlayercoupling and dimensional effects. 3.1 Superconducting transitions in ultra-thin YBCO layers While there are some reports on the superlattices of YBCO/(Pr Y )BCO, a peculiar long range x 1~x interaction across the (Pr Y )BCO layers hinders the study of intrinsic properties in YBCO. The x 1~x effect of adjacent layers on the superconducting properties of YBCO layers can be more easily seen in a single layer YBCO sandwiched between (Pr Y )BCO adjacent layers. The superconducting x 1~x properties of ultra-thin YBCO layers in trilayer structures are characterized by ac susceptibility, dc resistance, and critical current density. Figure 2 shows the resistive transitions of 1, 2, and 4 unit-cell thick YBCO layers with different x values in the adjacent (Pr Y )BCO layers. (Pr Y )BCO is a semiconductor for 0.5¹x¹1 and x 1~x x 1~x the resistivity increases rapidly with increasing x. For x¹0.5, (Pr Y )BCO is a superconductor. x 1~x As shown in Fig. 2, the resistive transition of ultra-thin YBCO is generally broader and T is lower c than in thick films. With decreasing x, T increases and the transition width narrows. The dependence c of T on the x values for 1 and 2 unit-cell thick YBCO sandwiched between (Pr Y )BCO layers is c x 1~x shown in Fig. 3. The variation in T for a given x and YBCO thickness is about 5 K from sample to c sample. The figure indicates that the 1 unit-cell thick YBCO layer is superconducting for all the x values, while T increases monotonically with decreasing x in the adjacent layers. c
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Fig. 4. The superconducting transitions of 1, 4, and 24 unit-cell thick YBCO layers after subtracting the (Pr Y )BCO contribution. 0.6 0.4
Since the thicknesses of the YBCO layers are so small, the temperature dependence of the resistance shown in Fig. 2 gets a substantial contribution from the adjacent layers. If we subtract the resistance of the adjacent layers, inferred from the resistance of a 30 nm thick (Pr Y )BCO layer x 1~x made under identical conditions, the temperature dependence of the resisitivity (q ) of the ultra-thin Y YBCO films shows metallic behavior in all cases (Fig. 4). For the calculation of q (T ) we assumed Y a parallel resistor model in which YBCO and two (Pr Y )BCO layers were connected in parallel x 1~x without any interface layers. q (T ) at 100 K does not change (within the experimental error; ^10%) Y with varying thickness of the YBCO layer suggesting no change of the charge carrier density and the scattering time with reduced thickness. The thickness independence of q was also seen by Cieplak Y
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Fig. 5. A, ac measurement of 1 unit-cell YBCO sandwiched between (Pr Y )BCO adjacent layers. B, 0.6 0.4 between PrBCO layers. The onset of transition is marked with an arrow.
et al. [11]. Because of 2D nature of YBCO, it is not surprising to observe the invariance of charge carrier density in ultra-thin YBCO samples. However, the invariance of the charge carrier density measured in the normal state and the drop of T with reduced YBCO thickness seem like contradicc tory results. This will be discussed further later. All the samples were characterized by ac susceptibility method. Figures 5 (a) and (b) show the ac susceptibility results of 1 unit-cell thick YBCO with (Pr Y )BCO and PrBCO, respectively. The 0.6 0.4 prominent signal in both samples indicates that the superconductivity in a one unit-cell YBCO film is not coming from a single percolation path and that a significant portion of the sample is superconducting. T (R\0 X) measured by dc resistance matches the onset temperature of the ac suscepc tibility signal. As discussed before, T of a YBCO with PrBCO adjacent layers is lower than that with c (Pr Y )BCO adjacent layers. However, the width of the ac transition remains the same (D4 K) 0.6 0.4 suggesting that the level of uniformity in T is the same in both cases. Hence, the interdiffusion of c Y/Pr atoms and the non-uniformity of T cannot be the origin of T dependence on x. c c The critical current density was measured as a function of temperature and magnetic field. Figure 6 shows a universal temperature dependence of J (T )/J (0) versus T/T for 1, 2, 3, and 4 c c c unit-cells, and bulk YBCO (100 unit-cells) samples. J values of 2]106, 6]106, 1.2]107, and c 1.5]107 A cm~2 at 4.2 K and zero field are obtained for 1, 2, 3, and 4 unit-cell thick YBCO layers, respectively. The values of J are very close to that of the best thick film value (about 5]107 A cm~2) c [12]. The universal scaling in the temperature dependence of J suggests that the pinning for unit-cell c samples is the same as for thick YBCO films. The magnetic field dependence of J for field applied c
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Fig. 6. Normalized J versus T/T curves for 1, 2, 3, 4 unit-cells, and thick (120 mm) YBCO samples. c c 108
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parallel and perpendicular to the ab plane is shown for a 1 unit-cell sample and a 200 nm thick film in Fig. 7. Surprisingly, for the field perpendicular to the ab plane, the suppression of J is weak and a value of c J \4]105 A cm~2 was obtained even at 7 T for 1 unit-cell thick YBCO film. That indicates the critical c current density in our 1 unit-cell YBCO layer is not dominated by weak links as proposed by Pennycook et al. [13] for superlattices with the same thickness of YBCO layers. Although YBCO/(Pr Y )BCO x 1~x interfaces and the tensile stress on lattices, which differs from the thick film case, are expected to generate strong pinning centers, this tendency is not observed in the temperature and field dependence of J . c The ac susceptibility and critical current measurements on YBCO/(Pr Y )BCO shows that x 1~x the properties of YBCO layers in these structures are not controlled by the extrinsic effects such as weak links and interdiffusion of Y/Pr atoms. Thus, this is an ideal system to study the intrinsic properties of ultra-thin YBCO layers and to understand the effect of adjacent layers on T . c 3.2 Raman study of YBCO/PrBCO superlattices Raman measurements were performed on YBCO/PrBCO superlattices with individual layer thicknesses in the range of 1 to 11 unit-cells. The T of the samples was varied between 30 K and 82 K c
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depending on the thickness of the layers. Figure 8 shows the frequencies of O(2)[O(3) out-of-phase vibration in the superlattices for PrBCO (D303 cm~1) and YBCO (337 cm~1) layers, O(2)]O(3) in-phase vibrations (437 cm~1), and axial motion of O(4) apical oxygen in YBCO (500 cm~1) and PrBCO (D524 cm~1). As evident in the figure, all these modes, except the O(4) vibrations associated with the PrBCO layers, remain unchanged as the layer thicknesses are varied. The apical oxygen vibration in the PrBCO layer softens as the thickness of the PrBCO layer (d ) is less that two Pr unit-cells. There is wide evidence that the O(4) atom plays a special role in the oxide superconductors. The softening of the apical oxygen in the PrBCO layer is consistent with the charge redistribution across the interfaces, since the carrier density of PrBCO is over two orders of magnitude smaller than that in the YBCO layers. This difference in carrier density should result in significant charge transfer across the interfaces. Given the low carrier density in PrBCO, the relative change in density would be large in the PrBCO layers and much weaker in the YBCO layers. These features would explain the observed softening of the O(4) mode in PrBCO layers thinner than three unit-cells and the minimal change in O(4) associated with YBCO. The softening is observed only in PrBCO layers thinner than unit-cells, indicating that the redistribution of charges occurs only over unit-cell dimensions. 3.3 Infra-red transmission measurements in YBCO/(PrxY1[x)BCO superlattices The charge carrier density of YBCO/PrBCO structures has been measured in the normal state by several groups [14,15]. The results have been inconclusive due to the carrier contribution from the
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Fig. 9. The frequency dependence of IR transmission in various temperatures for Sample A (a 2 layers of 6 unit-cell YBCO with (Pr Y )BCO) and Sample B (a 2 layers of 6 unit-cell YBCO with PrBCO). 0.6 0.4
adjacent layers. The problem with transport measurements has been the subtraction of the adjacent layer contributions. This requires knowledge of the role of adjacent layers which we do not know yet. Transport measurements are usually performed when a sample is still in the normal state. However, it is not clear whether the superconducting properties and the normal state transport are directly related. This can also be seen from the contradicting results of the same YBCO resistivities yet different T versus YBCO thickness in section 3.1. The infra-red transmission measurements can be c performed at low temperatures, where the sample is superconducting. Hence, the contribution of adjacent layers is negligible and the superconducting carrier density can be measured directly. Figure 9 shows the frequency dependence of IR transmission for various temperatures for Sample A (2 layers of 6 unit-cell YBCO layers among (Pr Y )BCO adjacent layers) and Sample 0.6 0.4 B (2 layers of 6 unit-cell YBCO layers among PrBCO adjacent layers). The solid line is a fit based on the two fluid model. From the data fit the carrier density in the normal state, n, the superconducting fraction, f , and scattering time, 1/s, are obtained. Sample A has n\(4.0^0.3)]1021 cm~3, s f \(0.35^0.04), and 1/s\(110^20) cm~1 at 5 K. Hence, the superconducting carrier density at 5 K s for the sample A is (1.4^0.3)]1021 cm~3. The same fit is performed for Sample B, and the parameters are n\(2.7^0.3)]1021 cm~3, f \(0.22^0.02) at 5 K. The superconducting carrier density s at 5 K is (0.6^0.1)]1021 cm~3. The only difference between sample A and B is the value x in the (Pr Y )BCO adjacent layers, which makes a big difference in the superconducting carrier density. x 1~x The IR transmission measurements show a decrease in both the superconducting fraction and the total number of carriers with increasing x value in (Pr Y )BCO layers. Being a direct measurement x 1~x
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Fig. 10. I-V curves of a 1 unit-cell sample at various temperatures on a log-log scale. The dotted line is VDI3 curve.
of the superconducting carrier density, this is a strong evidence of charge transfer in YBCO/ (Pr Y )BCO structures. x 1~x Our results suggest that charge transfer at YBCO/(Pr Y )BCO interfaces is the primary x 1~x cause of the adjacent layer effect. However, it is hard to explain the broadening of the transition width and the low zero resistance T only through charge transfer. The 2D nature of ultra-thin samples c makes them very susceptible to the thermal fluctuations and subsequently the creation of free vortices. We have also studied the existence of the Kosterlitz–Thouless transition in ultra-thin YBCO films. 3.4 Kosterlitz‒Thouless transition study Trilayer samples with a 1 unit-cell thick YBCO layer sandwiched between (Pr Y )BCO adjacent 0.6 0.4 layers were used in this study. Figure 10 shows I-V characteristics of a 1 unit-cell sample at various temperatures in a log-log scale. At a high temperature when the sample is in the normal state, the I-V curves show a linear behavior with an exponent a\1 (VDIa(T)); i.e. Ohmic. Upon lowering the temperature, a non-linear region appears on the I-V curves. The curve marked by an arrow has a region with an exponent 3; however, it changes back to being a\1 at low current. There is no definite discontinuity in the behavior between curves which are linear (a\1) and those which are non-linear
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(a[1) as expected from a K-T transition. The residual magnetic field inside the cryostat was measured using a Hall sensor, and was less than 0.1 mG limited by the sensitivity of the device. Hence, the Ohmic behavior at low currents is an intrinsic sample characteristic. Even though there is no K-T transition in 1 unit-cell YBCO samples, the thermally generated vortices may still be the origin of the linear tail in the I-V characteristics. When the physical size of a film (either the width or the length of a sample) is bigger than the magnetic screening length, one expects that free vortices exist in the film even below the nominal K-T transition temperature. This means that there would be a tail with a\1 in the I-V curves which has an exponent greater than 3, as we have seen. Other possible origin such as noise rounding is under investigation. Using YBCO/(Pr Y )BCO multilayer and superlattice structures we have investigated the x 1~x origin of the adjacent layer effects and the possibility of higher transition temperature. We conclude that charge transfer is the primary origin of the dependence of the superconducting properties on the adjacent layers. However, charge transfer by itself is not enough to explain all the experimental results. Other effects such as interlayer-coupling, thermal fluctuation, dimensional effects play a secondary role. From the application point of view, our findings can be applied toward fabrication of devices which require high quality ultra-thin superconducting layers such as a superconductor field effect device (Su-FET).
4. (La0.7Ba0.3)MnO3/SrTiO3 superlattices The giant magnetoresistance (GMR) behavior in metallic multilayers and granular materials has been studied actively due to both its scientific interest and its potential technological importance for magnetic sensors, especially for hard disk drive read heads. Recently, colossal magnetoresistance (CMR) effects were reported for doped perovskite manganites, such as La-Ba-Mn-O, La-Ca-Mn-O, La-Sr-Mn-O, Nd-Pb-Mn-O, and Nd-Sr-Mn-O [16–20]. The largest magnetoresistance (MR) ratio reported so far was over 1]106% at 60 K and 8 T in Nd Sr MnO thin films [16]. A heat treat0.7 0.3 3 ment at 900°C is normally required to obtain a large MR ratio, and the resistivity and MR ratio are very sensitive to the sample preparation conditions. It is still not clear why and how these processing conditions affect doped manganite films. We utilized a superlattice structure to investigate the possibility of device applications and to study the intrinsic properties of CMR manganites. Our work was motivated by the latest developments in the ultra-thin metallic ferromagnet layers and superlattices. As shown in the YBCO/(Pr Y )BCO superlattices, the superlattice structure can be used as x 1~x a model system to study a variety of physical phenomena. There have been several reports concerning growth of superlattice structures made up of YBa Cu O and doped manganite layers due to the structural similarity of the perovskite com2 3 7 pounds [21,22], but the structures were studied in the context of superconductivity in the YBCO layers. We report growth, resistivity, and magnetoresistance of superlattice structures with La Ba MnO (LBMO) and insulating SrTiO (STO) layers. The individual layer thickness of 0.7 0.3 3 ˚ to 55 A ˚ while3 the thickness of the STO layer was fixed at 160 A ˚. LBMO (d ) ranged from 2300 A LBMO ˚ thick LBMO layers. As the Our results demonstrate that a resistivity peak (T ) exists down to 55 A p thickness decreases, the resistivity increases and T shifts to lower temperatures. A large MR is p ˚ , a large MR is observed well below observed for all samples near T . In the case of d ¹110 A p LBMO T and localization behavior is seen at low temperatures. Possible origins will be discussed. p Ion channeling in Rutherford Back Scattering (RBS) measured on a single layer of LBMO deposited at the same conditions as the superlattices reveals that the film is a high quality crystal with a minimum yield of 4.5%. A FWHM of 0.12° was obtained from the rocking curve of the (002) LBMO peak.
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˚ )/LBMO (160 A ˚ ) superlattice sample. Fig. 11. X-ray h–2h scan around (002) and (002) peaks of a STO (55 A The satellite peaks are marked with arrows.
A set of LBMO/STO superlattices was made for this study. The total thickness of the doped ˚ to 55 A ˚ . The manganite layers was fixed and the individual layer thickness was varied from 1100 A ˚ thick STO. The insulating STO separation between doped manganite layers was fixed with 160 A layers were used to decouple the LBMO layers in order to study the properties of ultra-thin LBMO ˚ )/STO(160 A ˚ ) superlattice is shown layers. The x-ray diffraction of the (002) peaks of a LBMO(55 A in Fig. 11. The satellite peaks adjacent to the main (002) peaks are marked by arrows. (001) peaks of both LBMO and STO layers without any other phase peaks indicate an epitaxial c-axis growth. The observation satellite peaks in both the LBMO and STO layers unambiguously proves the periodic structure and chemical modulation of the superlattice. All superlattices were characterized by x-ray diffraction and show prominent satellite structure. Transport measurements were performed on each sample using a 4-probe method. Electrical contacts were made in such a way that a current was flowing in all layers. Figure 12 shows the resistive transition in zero field and MR at 5 T for three LBMO/STO superlattices with the individual ˚ , 110 A ˚ , and 55 A ˚ . The resistivity is calculated using the total LBMO thickness (d ) of 1100 A LBMO ˚ LBMO is the same as those thickness of LBMO layers. The resistive transition, T , and q of 1100 A p p of thicker films. The resistive peak is observed in all samples with T at 325 K, 280 K, and 275 K for p ˚ , 110 A ˚ , and 55 A ˚ , respectively. The minimum resistivity d \1100 A of samples below T increases LBMO p ˚ with reducing d and an increase of resistivity at low temperature is observed for d \110 A LBMO LBMO thick samples. Applying a magnetic field reduces the resistivity significantly in all samples. As seen in Fig. 12, the MR ratio at a field of 5 T has a peak at a slightly lower temperature than the resistivity ˚ peak. There also exists a significant magnetoresistance below the resistivity peak in the thinner (110 A ˚ ) LBMO layers. However, the peak MR is almost constant in all thicknesses. and 55 A The deviation of the resistive transition from thick LBMO films starts in the thickness range ˚ . The change of T and q is very gradual as observed in the case of 110 A ˚ and of a few hundred A p p ˚ LBMO samples. It is very surprising 55 A that the basic characteristics of the thick film are preserved ˚ thick layers. This is another indication of the high quality of the samples. From h–2h down to 55 A x-ray diffraction we observed an increase in the c-axis lattice parameter of the LBMO layers ˚ in 1100 A ˚ LBMO; c\3.922 A ˚ in 55 A ˚ LBMO]. Possible origins of this gradual change [c\3.911 A are a change of the Mn-O-Mn bond as a result of strain, oxygen deficiency, interface scattering, and interdiffussion.
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0.0 350
Fig. 12. The resistive transitions in zero field and the temperature dependence of MR (R(0T)-R(5T))/R(0T) for ˚ , 110 A ˚ , and 55 A ˚. three STO/LBMO superlattices with the individual LBMO thickness of 1100 A
The exchange of Sr and Ba through the interdiffusion increases T and decreases the resistivity p in LBMO layers, hence it is very unlikely that the interdiffusion causes the changes in superlattices. The oxygen deficient polycrystalline LBMO [23] has similar properties such as the increase of the resistivity, reduction of T , and substantial MR at low temperatures, however it is hard to believe p the samples made at the same deposition condition has different oxygen content. If there is a nonuniformity of oxygen among the layers in the superlattices because of the diffusion process, it will broaden the resistive peak without changing the peak temperature. Since the mean free path es˚ , the interface timated using the resistivity of LBMO at low temperatures is in the order of a few A ˚ ) cannot be dominating scattering determined by the physical thickness of the sample (about 100 A the transport properties. The observation of an increase in the c-axis lattice parameter of the LBMO layers and the importance of the Mn-O-Mn bond for the properties of this material lead us to suggest that the induced strain due to the multiple layering of the two materials is a more probable cause for the observed behavior. More study is needed. The CMR materials have potential applications for a magnetic sensors using the large magnetoresistance. MR of 10% at 100 Oe is required for the hard disk read head application; the DC resistance drop in these materials is presently less than 1% at 100 Oe. However MR of 10% at a few thousand Oe is observed in CMR materials which can be useful for the magnetic sensor applications in automobiles. We demonstrated the growth of LBMO/STO superlattices and showed that the resistivity peak ˚ thick LBMO layers. With reduced LBMO layer thickness, the resistivity gradexists even for 55 A
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ually increases while T gradually decreases, respectively. A large MR is observed for all samples near p ˚, a T , however MR does not depend strongly on the layer thickness. In the case of d ¹110 A p LBMO large MR is observed well below T and localization behavior is seen at low temperatures. p Acknowledgements—The CMR superlattice research is supported by Office of Naval Research under contract number.
References [1] A. Inam, M. S. Hegde, X. D. Wu, T. Venkatesan, P. England, P. E. Miceli, E. W. Chase, C. C. Chang, J. M. Tarascon and J. B. Wachtman, Appl. Phys. Lett. 53, 908 (1988). [2] K. Hubler and D. B. Chrisey, eds Pulsed Laser Deposition of Thin Films, John Wiley & Sons, New York: (1994). [3] O. Auciello and J. Engemann, eds Multicomponent and Multilayed Thin Films for Advanced Microelectronics: Techniques, Fundamentals and Devices, Kluwer Academic Publication: (1993). [4] J.-M. Triscone, O. Fischer, O. Brunner, L. Antognazza, A. D. Kent, and M. G. Karkut, Phys. Rev. Lett. 64, 804 (1990). [5] Qi Li, X. X. Xi, X. D. Qu, A. Inam, S. Vadlamannati, W. L. McLean, T. Venkatesan, R. Ramesh, D. M. Hwang, J. A. Martinez and L. Nazar, Phys. Rev. Lett. 64, 3086 (1990). [6] Qi Li et al., Phys. Rev. Lett. 69, 2713 (1992). [7] D. H. Lowndes, D. P. Norton and J. D. Budai, Phys. Rev. Lett. 65, 1160 (1990). [8] D. P. Norton, D. H. Lowndes, S. J. Pennycook and J. D. Budai, Phys. Rev. Lett. 67, 1359 (1991). [9] C. Kwon, Qi Li, X. X. Xi, S. Bhattacharya, C. Doughty, T. Venkatesan, H. Zhang, J. W. Lynn, J. L. Peng, Z. Y. Li, N. D. Spencer and K. Feldman, Appl. Phys. Lett. 62, 1289 (1993). [10] T. Terashima, Y. Bando, K. Iijima, K. Yamamoto, K. Hirata, K. Hayashi, K. Kamigaki and H. Terauchi, Phys. Rev. Lett. 67, 1362 (1991). [11] M. Z. Cieplak, S. Guha, S. Vadlamannati, T. Giebultowicz and P. Lindenfeld, Phys. Rev. B 50, 12 876 (1994). [12] B. Roas, L. Schultz and G. Saemann-Ischenko, Phys. Rev. Lett. 64, 479 (1990). [13] S. J. Pennycook, M. F. Chisholm, D. E. Jesson, D. P. Norton, D. H. Lowndes, R. Feenstra, H. R. Kerchner and J. O. Thomson, Phys. Rev. Lett. 67, 765 (1991). [14] M. Affronte, J.-M. Triscone, O. Brunner, L. Antognazza, L. Mieville, M. Decroux, Phys. Rev. B 43, 11 484 (1991). [15] Y. Matsuda, S. Komiyama, T. Terashima, K. Shimura and Y. Bando, Phys. Rev. Lett. 69, 3228 (1992). [16] G. C. Xiong, Qi Li, H. L. Ju, R. L. Greene and T. Venkatesan, Appl. Phys. Lett. 66, 1689 (1995). [17] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz and K. Samwer, Phys. Rev. Lett. 71, 2331 (1993). [18] K.-I. Chahara, T. Ohno, M. Kasai and Y. Kozono, Appl. Phys. Lett. 63, 90 (1993). [19] S. Jin, T. H. Teifel, M. McCormack, R. A. Fastnacht, R. Ramesh and L. H. Chen, Science 264, 413 (1994). [20] H. L. Ju, C. Kwon, Q. Li, R. L. Greene and T. Venkatesan, Appl. Phys. Lett. 65, 2109 (1994). [21] G. Jakob, V. V. Moshchalkov and Y. Bruynseraede, Appl. Phys. Lett. 66, 2564 (1995). [22] Y. Kozono, M. Kasai, Y. Kanke, T. Ohno, M. Hanazono and Y. Sugita, Physica C 185‒189, 1919 (1991). [23] H. L. Ju, J. Goaplakrishnan, J. L. Peng, Qi Li, G. C. Xiong, T. Venkatesan and R. L. Greene, Phys. Rev. B 51, 6143 (1995).