0018-8646/98/$5.00 (C) 1998 IBM Magnetotransport in doped manganate perovskites by J. Z. Sun, L. Krusin-Elbaum, A. Gupta, Gang Xiao, P. R. Duncombe, and S. S. P. Parkin Recent progress in oxide perovskite thin-film technology has led to the discovery of a large negative magnetoresistance at room temperature in doped manganate perovskite thin films. These films may have potentials for magnetic sensing applications. In this paper we review the basic phenomena and physics of magnetotransport in this class of materials. We also discuss our recent demonstration of a large low-field magnetoresistance effect, and the associated challenges that lie ahead. Introduction Large magnetoresistance (MR) was observed in the 1970s and 1980s in bulk ceramic and single-crystal forms of doped manganate perovskites. Kusters et al. [1] observed a negative magnetoresistance in the bulk doped perovskite manganate Nd0.5Pb0.5MnO3, for which a magnetic-field-induced percentage change in resistance DeltaR/R(H = 0) > 50% was found near its ferromagnetic transition at a temperature of 184 K. Here R(H = 0) is the resistance in zero field H = 0, and DeltaR = R(H = 0) - R(H not= 0). A negative magnetoresistance is defined as R(H = 0) > R(H not= 0). Earlier, Searle and Wang [2] had reported spin-dependent metal-insulator transitions in single crystals of (LaPb)MnO3. The recent interest in magnetoresistance in doped perovskite manganates was initiated by the discovery of a large room-temperature magnetoresistance in epitaxial thin films [3]. Thin films with large magnetoresistance at room temperature open up new possibilities for applications in diverse areas of technology such as magnetic random access memories and read heads for hard disk drives. Thin-film deposition technology for complex oxide materials has seen rapid progress, thanks to the effort in the commercialization of the high-temperature superconductors--a class of doped cuprate perovskites with materials characteristics similar to those of the doped manganate perovskites. Successful synthesis of the doped manganate perovskite thin films dates back to at least 1990, when Kasai and Ohno [4] first reported their work on the superconducting proximity effect observed in sandwiched epitaxial YBa2Cu3O7/La0.7Ca0.3MnO3/YBa2Cu3O7 thin films. Also in 1990, Cho et al. [5] reported successful deposition of La0.7Sr0.3MnO3 in epitaxial form using rf diode magnetron sputtering. In 1993, Von Helmolt et al. [3] reported a large negative magnetoresistance effect, of the order DeltaR/R(H = 0) > 60% at room temperature in a 7-tesla field, in laser-deposited thin-film La0.67Ba0.33MnOx. This was soon followed by the report of McCormack et al. [6] and Jin et al. [7] of their findings that thin-film La0.67Ca0.33MnO3 exhibits DeltaR/R(H = 6 T) = 127000% at 77 K. This large magnetoresistance effect has since been referred to as "colossal magnetoresistance" (CMR). In these early studies, a postdeposition anneal was critical for obtaining large magnetoresistance [3,6,7]. A typical annealing condition involves a high-temperature (around 900 degrees C) short-time (about 30 min) treatment in an oxygen atmosphere. Soon afterward it was demonstrated that chemical substitution on the trivalent site could have an effect equivalent to a controlled anneal. With 7% replacement of lanthanum by yttrium, Jin et al. [8] showed that large magnetoresistance with DeltaR/R(H = 6 T) approximately 10**3 can also be obtained in samples of bulk ceramic form at a temperature close to 140 K. Sun et al. [9] demonstrated that, using the same 7% yttrium substitution, values of DeltaR/R(H = 6 T) as high as approximately 200 can be obtained around 100 K with as-grown thin films that were laser-deposited at 600 degrees C without postdeposition anneal. For CMR effects in both bulk and thin-film materials, the basic phenomena seem to be the same. The resistivity peaks near the Curie temperature, around which the largest magnetoresistance is obtained. Above the Curie temperature, the resistivity shows a thermally activated behavior [10]. Below the Curie temperature, the resistivity decreases with decreasing temperature as the magnetic moment grows. A correlation has been found between the resistivity and magnetization [9,11,12]. A larger pseudoperovskite cell size seems to correlate with a lower Curie temperature, and a lower Curie temperature leads to a higher resistivity peak, which is accompanied by a larger magnetoresistance effect. All reports of large magnetoresistances involve the preparation of the materials in such a way as to suppress its resistance-peak temperature to below 150 K. The saturation magnetic field necessary to achieve a full CMR effect in the generic materials is large, usually of the order of several tesla, as illustrated in Figure 1. This limitation has recently been overcome for temperatures below 100 K, when large magnetoresistances in low fields were demonstrated in trilayer manganate/SrTiO3/manganate devices [13]. In this paper, we review the current state of affairs in this very dynamic area of research. We summarize some basic materials data established to date, as well as some of the building blocks of our understanding. We discuss in greater detail the issues we have highlighted above, especially with regard to the possible mechanism that produces the CMR effect and its implications for low-field magnetoresistance in the manganates. Materials properties Manganate perovskite oxides were first systematically studied in the 1950s [14,15]. The parent compounds La3+Mn3+O3 and Ca2+Mn4+O3 both have a perovskite structure. La3+Mn3+O3 is a layered antiferromagnet, while Ca2+Mn4+O3 is an antiferromagnet with opposite spin orientations for nearest-neighbor Mn4+ spins [18]. La3+Mn3+O3 is an antiferromagnetic insulator; Figure 2(a) is a schematic of its cell structure. A Jahn-Teller distortion lifts the double degeneracy of the Eg orbitals and defines the direction along which the Mn3+ moments order antiferromagnetically. A schematic of the layered antiferromagnetic Mn3+ moment arrangement is shown in Figure 2(b). From neutron diffraction studies, Wollan et al. [18] concluded that the Mn3+ moment must in this case lie in the plane of the Mn-O sheet, although the relative orientation of the moment with respect to the cell edge could not be determined from powder diffraction data. Recent work on thin-film La0.7Sr0.3MnO3 grown using laser ablation [19] suggests that a magnetic easy axis may exist for the magnetic moments to align along the (110) direction of the pseudocubic perovskite cell, as shown in Figure 2(b). CaMnO3 is also an antiferromagnetic insulator, with a Mn4+ moment aligned antiferromagnetically with a cubic symmetry [18]. It has a Neel temperature of 120 K. Between these two end compounds exists a continuous solid solution whose structural, magnetic, and transport properties depend sensitively on doping level x. Figure 3(a) shows a summary of the lattice constant of La1-xCaxMnO3 as a function of x. The Jahn-Teller distortion-related orthorhombicity disappears for x above 0.2, around which a metallic conductivity sets in, together with a large increase of the ferromagnetic moment. The magnetic phase diagram of the La1-xCaxMnO3 system is shown in Figure 3(b) [20]. This phase diagram is also qualitatively true for other manganate perovskites such as La1-xBaxMnO3 and La1-xSrxMnO3 [21-23]. The Curie temperature peaks around x approximately 0.3. The maximum Curie temperature of 380 K was observed in La0.7Sr0.3MnO3 [22,24]. Table 1 contains a summary of some representative compounds with x approximately equal to 0.3 [24]. Around x approximately 0.5, the system exhibits a complex first-order ferromagnetic-antiferromagnetic transition, accompanied by a metal-insulator transition [25-29]. Long-range ordering of the dopant charge is believed to exist at this doping level and is responsible for the first-order phase transition. Pr1-xBaxMnO3, Nd1-xSrxMnO3, and La1-xPbxMnO3 have also been studied at selected compositions [10], particularly around x approximately 0.3 and x approximately 0.5. The LaxMnO3-delta (0.67 <= x <= 1) self-doped system also exhibits ferromagnetic ordering, with a maximum Curie temperature around 300 K [30,31]. The magnetic and transport properties of doped manganate perovskites show a sensitive pressure dependence. Upon the application of hydrostatic pressure, the Curie temperature rises, the resistivity decreases, and the effective amount of magnetoresistance decreases. Results for polycrystalline Pr0.7Ca0.3MnO3 and for La0.7Ca0.3MnO3 were reported by Hwang et al. [32]. Pressure-dependence studies of other systems have also been reported. Arnold et al. [33] reported hydrostatic-pressure-dependent magnetic and transport properties of polycrystalline La0.60Y0.07Ca0.33MnO3, and obtained an average dTc/dP of around 2.5 K/kbar for Tc around 160 K. Khazeni et al. [34] studied the hydrostatic pressure dependence of magnetotransport in single-crystal Nd0.5Sr0.36Pb0.14MnO3-delta, and found a dTc/dP of 1.9 K/kbar for a Tc around 200 K. All three studies reveal a positive dTc/dP, around 2 K/kbar. The structural properties of the doped manganate perovskites show a strong correlation to their magnetic state. When the value of doping level x is increased, a metallic and ferromagnetic state develops, accompanied by a diminishing orthorhombic distortion. This can be seen in Figure 3(a), where the difference in a1 and a3 virtually disappears for x >= 0.2. Significant changes in lattice constants also occur when samples go through their ferromagnetic transition. Radaelli et al. [35] observed a large magnetovolume effect in polycrystalline samples of La1-xCaxMnO3 with x = 0.25 and 0.50. For x = 0.25, a lattice contraction immediately below the Curie temperature was observed, with a volume discontinuity of DeltaV/V approximately equal to 0.13%. For samples with x = 0.50, a much larger discontinuity was observed in lattice constants at the antiferromagnetic-to-ferromagnetic transition of 160 K, but with very little net volume change. For single-crystal La1-xSrxMnO3 at x = 0.170, Asamitsu et al. [36] reported a first-order structural phase transition between the orthorhombic form (low temperature, ferromagnetic) and the rhombohedral form (high temperature, paramagnetic) that can be triggered either by varying temperature near the Curie point or by the application of a magnetic field of the order of 1 tesla. A more dramatic first-order striction-coupled metal-insulator phase transition was observed in the single-crystal compound of (Nd1-ySmy)1/2Sr1/2MnO3 at y = 0.938, for which a resistivity change of more than three orders of magnitude, from 20 ohm-cm to 4 x 10**-4 ohm-cm, was observed at the Curie temperature of 110 K [25]. Near the Curie temperature, the phase transition could be driven by the application of a relatively small magnetic field. In a low field of 2.5 kOe, a DeltaR/R(H) of 100 was observed within a narrow temperature range of a few degrees above the Curie temperature. New compounds with similar magnetic and transport behaviors are rapidly being discovered. Moritomo et al. [37] recently reported successful synthesis of the layered perovskite system (La1-xSrx)n+1MnnO3n+1. A resistance change of close to two orders of magnitude in 7 tesla of field was demonstrated for the n = 2 compound at x = 0.4 near its Curie temperature of around 130 K. Additionally, a large magnetoresistance was observed in the pyrochlore compound Tl2MnO7-delta, which has a Curie temperature around 150 K [38,39]. Hall measurements [38] indicate that the majority carriers are electrons in Tl2MnO7-delta, and the carrier density is only 0.001-0.005 conduction electrons per formula unit. This is in contrast to the doped perovskite manganates, for which the conduction carriers are holes and the doping density is of the order of 0.1-0.5 per formula unit. This raises the interesting question of how ferromagnetism develops in the pyrochlores, and whether the double-exchange interaction is the only mechanism operative for stabilizing the ferromagnetic and metallic states [38]. CMR in thin films In 1993, Von Helmolt et al. reported a large, room-temperature magnetoresistance effect in La0.7Ba0.3MnO3 epitaxial thin films made using pulsed laser deposition (PLD) in an off-axis geometry. This geometry reduces the number of particles formed on the film surface--deposition of particles a few thousand angstroms to a micron in size is a problem associated with laser deposition. On-axis deposition with a sintered-powder ceramic target typically results in a particle density of the order of approximately 10**5-10**6 cm**-2. Off-axis deposition is known to reduce the powder density by at least an order of magnitude [40]. Ceramic targets with stoichiometric composition were used. The substrates were SrTiO3 single crystal, (100) or (110) cut. Optimal deposition was obtained at a substrate temperature of 600 degrees C, in an oxygen background pressure of 300 mTorr. Curiously, they reported that higher substrate temperature resulted in polycrystalline films, which was not a commonly seen phenomenon. As-deposited films show paramagnetic response at room temperature. A subsequent anneal at 900 degrees C in air for 12 hours resulted in a marked increase in Curie temperature. The sample becomes ferromagnetic at room temperature, although its magnetic moment at 2 tesla is still only 89% of that observed in bulk material. A magnetoresistance of DeltaR/R(H = 7 T) approximately 150% was seen, as summarized in Figure 4. Jin et al. [7] and McCormack et al. [6] reported the synthesis of epitaxial La0.67Ca0.33MnO3 films that exhibited a field-driven change of resistivity of three orders of magnitude in a field of 6 tesla. Their films were about 1000 A thick, deposited using pulsed laser ablation on (100) LaAlO3 substrates. Their optimal deposition temperature was 650-700 degrees C in an oxygen atmosphere of 100-300 mTorr. The highest magnetoresistance, shown in Figure 1, was obtained by heat-treating the films after deposition at 900 degrees C for 30 min in 3 atm. of oxygen [6]. A large CMR effect in Nd0.7Sr0.3MnO3-delta thin films was reported by Xiong et al. [41]. Their films were prepared using laser ablation in 300 mTorr of N2O. The substrate temperature was in the range of 600-800 degrees C during deposition. Trajanovic et al. reported successful growth of La0.67Sr0.33MnO3 thin films on buffered silicon substrates using laser ablation [42]. Many more thin-film materials systems have since been investigated. Commonly used deposition techniques for perovskite oxide deposition include laser ablation, reactive sputtering, reactive ion beam sputtering, co-evaporation, and CVD. Table 2 gives a summary of some recent publications on efforts of synthesizing the doped manganate perovskites using these techniques and the basic properties of the resulting films. Laser ablation is most widely used in a laboratory environment for the deposition of oxides. It is by far the most straightforward deposition method for complex oxides because of the ease of obtaining stoichiometric transfer of materials from target to substrate. Deposition can be made in various gas atmospheres within a wide pressure range, typically from 0 to 1 Torr. The major drawback for laser ablation is its scalability. Deposition of films over a 2-in. diameter would require major engineering effort. Because of the short target-to-substrate distance (typically around 2 in.), it is currently very difficult to imagine laser ablation being scaled up for deposition of wafers of a size much larger than 4 in. in diameter. Another problem associated with laser ablation is the generation of particulates. These particles, typically 0.1-1 micro-m in size, could seriously affect the yield of devices at a high device density. Reactive sputtering excels in the deposition of large-area films. Deposition of oxide films, especially complex oxides involving more than one cation element, could become challenging because of the problem of nonstoichiometric transfer. Both the sputtering process at the target and the resputtering process at the substrate can affect the stoichiometry of resulting films. For oxides containing alkaline metal cations, large amounts of oxygen negative ions can be generated at the target. The resputtering effect of these negative ions at the substrate is to alter the resulting film composition from that of the target. There is evidence, however, that this problem may not be as serious for manganates as it is in the case of high-temperature superconducting cuprates--to the extent that one may not have to use off-axis sputtering to get good enough stoichiometric transfer [5,45,46]. For large-area deposition, reactive molecular beam epitaxy using thermal evaporation has been gaining momentum in recent years [47,49-54]. MOCVD has also been successfully applied to the growth of manganates [23]. Bae and Wang have reported successful synthesis of La0.67Ca0.33MnO3 epitaxial thin films using a sol-gel approach [55]. Most commonly used substrates are perovskite single crystals of SrTiO3 and LaAlO3. This is because a good lattice match to the manganates is required. There is, however, some recent progress in the successful growth of high-quality epitaxial thin films on buffered silicon substrates [42,45,48], which might be more interesting for technological reasons. One striking feature of the manganate thin films is the sensitivity of their properties to postdeposition heat treatment, as can be seen clearly from the data presented in Table 2. This is probably related to the strong mutual dependence of the magnetic state and the local lattice configuration, especially that of the Mn-O-Mn bond length and bond angle, as has previously been discussed. Direct comparison of data is difficult between thin-film and bulk samples, either single-crystal or polycrystalline, because of the possible existence of a large amount of residual uniaxial stress in thin films. For manganate films that are epitaxial, their precise cell dimensions and structures cannot be determined completely from simple theta-2theta X-ray diffraction. This may contribute partially to the lack of direct correlation between their Tp and lattice parameters, as shown in Table 2. There might be some interaction between the grain boundaries and the magnetic domain boundaries that could lead to the pinning of magnetic domain boundaries at the grain boundary, resulting in increased spin-dependent scattering of carriers at the grain boundary [56]. On the whole, however, the role of the grain boundary is much less significant for the transport properties of manganate thin film than it is for high-temperature superconducting cuprates. This is also evident from the fact that similar magnetotransport properties can be obtained from epitaxial thin films and polycrystalline bulk samples with only minor tuning of the processing conditions. Magnetic properties The saturation magnetic moment of La1-xCaxMnO3 peaks around x = 0.3 [18]. This moment corresponds to about 3.6 Bohr magnetons (muB) per Mn site, or a low-temperature saturation magnetization of 4piM approximately equal to 7400 Oe. As-grown films appear to be fairly soft magnetically, with a coercivity of the order of 10-500 Oe, depending on the type of material, its growth condition, and the operating temperature. The magnetization of most laser-deposited films appears to lie in the film plane, as one would expect from the large shape anisotropy. It is not yet clear whether there are preferential directions for the magnetic moment to align in-plane. Optical Kerr contrast microscopy [19] on as-grown La0.67Sr0.33MnO3 thin films shows a possible in-plane easy axis along the pseudocubic cell axis of the manganates, at least over an area tens of microns in size. No significant difference is seen, however, in magnetic hysteresis loops of samples several millimeters in size when the field is aligned in the (100) or (110) direction of the pseudocubic axis. Uniaxial strain or stress in films can also complicate matters to some degree. Observation of perpendicular magnetic anisotropy in sputter-deposited La1-xSrxMnO3 polycrystalline films with x = 0.21 has been reported by Cho et al. [5]. A significant amount of linewidth broadening in the ferromagnetic resonance spectrum was observed [57] for as-grown thin films of La0.67Ba0.33MnO3, suggesting the presence of magnetic inhomogeneity. High-temperature annealing of the films reduces their FMR linewidth and the dc resistivity. An extrapolated zero-temperature spin-wave stiffness constant of D(0) approximately equal to 115 meV-A**2 was obtained for La0.67Ca0.33MnO3 thin films at 10 GHz. This agrees with the estimate obtained from recent neutron diffraction studies [58]. Possible origins of CMR Goodenough et al. [59,60] proposed a qualitative theory for the magnetic interaction in the manganates based on a special type of covalent Mn-O-Mn bond, which he denotes as the semicovalent bond. For this type of bond, the distance between the ions is very important. For magnetic moments of ions sharing the oxygen ion, a small lattice constant would lead to antiparallel or negative exchange coupling, while a longer bond length would lead to parallel or positive exchange coupling. For the doped manganate perovskite family of compounds, there is a weak magnetic exchange coupling between the Mn3+ ions, a negative interaction between the Mn4+ ions, and a strong positive interaction between the Mn3+ and Mn4+ ions [14]. The type and strength of magnetic exchange coupling between adjacent manganese ions in the Mn-O-Mn bond depends sensitively on the valency of the Mn, or the doping level x of the compound. One peculiar feature of the doped manganate perovskites is the close association of ferromagnetism with metallic conduction. Zener [61] proposed the mechanism of double exchange that could explain this correlation. Doping of the trivalent rare-earth site by divalent ions causes a corresponding number of Mn3+ ions to become Mn4+. The displacement of these holes (sometimes referred to as Zener carriers [62]) increases the conductivity. The strong positive exchange coupling between the Mn3+ and Mn4+ ions in (Mn3+)-(O)-(Mn4+) provides a mechanism for ferromagnetic ordering. A resonance hybrid between the two states Psi1: (Mn3+)-(O2-)-(Mn4+) and Psi2: (Mn4+)-(O2-)-(Mn3+) is energetically favored. For such a simplified model, the transfer integral for one electron becomes [63,64] tij = bij cos (thetaij/2), where thetaij is the angle between the two ionic spins and bij is the coupling constant. When considered in the environment of an extended lattice, this interplay between dopant level and the magnetic ground state leads to the theoretical proposal of either a canted or a spiral ground state for the Mn spin, with thetaij determined by dopant concentration x [65]. Double exchange provides a mechanism for the explanation of the simultaneous onset of metallicity and ferromagnetism. On the other hand, Millis et al. [66] showed that a Hamiltonian containing only double exchange is insufficient to account for the large magnetoresistance observed in these CMR compounds. The calculated resistance is too small. It has an incorrect temperature and field dependence when compared with experiment. They concluded that a strong electron-phonon interaction, in this case mediated by the Jahn-Teller coupling of the Mn3+ ions, must be included. The 3d4 Jahn-Teller distortion of Mn3+ is known to be strong. With a Jahn-Teller energy of the order of [66] 1 eV, it is the underlying driving force for the tetragonal-to-orthorhombic phase transition observed for x <= 0.2, as was shown in Figure 3(a). It is also responsible for the cubic-tetragonal transition observed at T* approximately equal to 800 K in LaMnO3 [59]. Roder et al. [67] presented a calculation that incorporated the coupling to longitudinal optical phonons in the double exchange model, and showed that such coupling causes a suppression of the mean-field magnetic transition temperature, with the amount of suppression dependent on the Jahn-Teller coupling strength. Millis et al. [68] suggested that for x > 0.2 and above Curie temperature, slowly fluctuating local Jahn-Teller distortions localize the conduction-band electrons into polarons. The polaron effect is turned off as temperature is decreased through Tc, permitting the formation of a metallic state. The competition between electron itineracy and self-trapping is controlled by the ratio of the Jahn-Teller self-trapping energy EJ-T and an electron itineracy energy which is parameterized by an effective hopping matrix element teff. Double exchange causes teff to be affected by the degree of magnetic order, and spin disorder leads to its reduction. When EJ-T/teff exceeds a certain critical value, the phonon effect dominates and polarons form, localizing the electrons. This "dynamic Jahn-Teller" polaron model is gaining support from some recent structural [35] and neutron diffraction studies. A large oxygen isotope effect on the ferromagnetic transition temperature in La0.8Ca0.2MnO3 was reported by Zhao et al. [69]. Evidence for polaron-dominated conduction was reported by Jaime et al. [70,71]. Their recent transport measurements on La2/3Ca1/3MnO3 at temperatures above Tc revealed a large and field-independent difference between the activation energies for resistivity and for thermopower, which is a characteristic of Holstein polarons. An explicit band calculation of the magnitude of Mn d-O p hybridization using local spin density approximation was recently reported by Pickett and Singh [16]. Their conclusions are as follows: 1. The electronic structure near the Fermi surface is found to be very nearly half metallic for La1-xCaxMnO3 with x = 1/3. 2. A spin-dependent hybridization is found. For the minority channel, the O p bands and Mn d bands are nonoverlapping and hybridize much more weakly than is the case for the majority O p and Mn d bands that do overlap and mix very strongly at the Fermi level. 3. The Ca/La local environment disorder leads to variations in the Mn d site energy that create a tendency toward incoherence (i.e., localization) in the minority states near EF. These effects for the relatively broad majority bands should be minor. The likely result is that EF lies below a mobility edge, giving nonconducting minority states. 4. The half metallicity is a local effect that persists near flipped Mn spins (e.g., abrupt "domain walls"). The lack of a Stoner continuum and the possibility of minority spin polarons in a half-metallic ferromagnet provide possibilities for transport anomalies and for a large negative magnetoresistance. The strong field dependence of the magnetic properties suggests the presence of magnetic inhomogeneity. Experimentally, one finds that in an applied field of several tesla, the ferromagnetic transition broadens significantly, the low-temperature saturation magnetic moment increases, and a large reduction of resistance appears. An example is shown in Figure 5 for a set of data taken on a La0.75Ca0.25MnO3 polycrystalline sample [20]. Zhang and Yang [72,73] offered a theoretical model for the description of fluctuating magnetic clusters and their possible relation to magnetic excitations in the system. Sun et al. [74] analyzed this situation within a classical mean-field approximation [75]. The effective magnetic moment mu at high field saturates at a value of around 20 muB, suggesting a rather small ferromagnetic cluster containing only four or five Mn ions. There is also evidence showing a possible correlation between the density of such magnetic clusters and the dopant concentration on the La site [74]. These magnetic clusters may be related to the Jahn-Teller distortion-based magnetic polaron described by Millis et al. [68]. In light of such a magnetically inhomogeneous system, the CMR effect may be related to the spin-dependent transport between these ferromagnetic (and thus metallic) clusters, with the amount of intercluster conductivity determined by the relative alignment of the magnetic moment of the clusters. As first pointed out by Byers and Rubinstein [76], this is similar to granular ferromagnetic tunneling systems such as Ni-SiO2 [75], although here the mechanism for spin-dependent conductivity between clusters is still under investigation. Here, the large saturation field for CMR effect is simply a reflection of the small size of such magnetic clusters. If one could create a macroscopic interface for the relative rotation of magnetic moments from one side to another, one might expect large magnetoresistive effects for transport across such an interface. The field necessary to cause relative rotation of the macroscopic magnetic moment should be close to that of the coercivity of the manganate material, which is of the order of 10**1-10**2 Oe. Thus, low-field CMR may be achievable. The role of magnetic inhomogeneities has been fairly widely discussed in the literature (from different perspectives). For example, Ju et al. [77] investigated the correlation between magnetoresistance and magnetization in polycrystalline La0.67Ba0.33MnO3 samples, and pointed out the possible importance of magnetic domain walls in determining the magnetoresistance. Hwang et al. [78] compared magnetoresistance of bulk single-crystal and polycrystalline La2/3Sr1/3MnO3 samples, and suggested that spin-dependent tunneling might be responsible for intergranular transport in the polycrystalline samples; such a tunneling mechanism could be responsible for the additional low-field cusp in the magnetoresistance observed in the polycrystalline samples. Low-field magnetoresistance For the manganates to be useful in magnetic field sensing and memory applications, the saturation field of their CMR effect must be significantly reduced from its bulk value of several tesla. Cheong et al. [79], using a pair of ferromagnetic flux focusers attached to a ceramic piece of La0.67Ca0.33MnO3 to locally concentrate magnetic field seen by the manganates, obtained a factor of 5900 enhancement of magnetoresistance response in fields below 10 Oe. An attempt at using a superconducting YBaCuO thin film to focus flux was made by Dong et al. [80]. Alternatively, Bozovic et al. [51,81] used 2D MBE growth to fabricate lateral superlattices of manganates with different composition, so as to emphasize interface-related magnetic scattering of carriers. They observed an in-plane anisotropic resistance and reported a low-field magnetoresistance slope of (1/R)(dR/dH) = 36 per tesla [51]. The likelihood of strongly spin-dependent transport at an interface with misaligned magnetic moment prompts the search for isolation of a single interface by experimental means. Magnetic domain boundaries are considered natural candidates, and the observation of pinning of magnetic domain walls at grain boundaries has led to the investigation of magnetotransport studies in polycrystalline materials, as well as in thin films grown on polycrystalline substrates. Gupta et al. [56] investigated the possible consequences of magnetic domain boundary pinning by polycrystalline grain boundaries and observed an enhanced magnetoresistance in the low-temperature region compared to that observed for epitaxial thin films grown on single-crystal substrates. Another approach is to fabricate a perpendicular transport structure with a trilayer thin film composed of underlying and overlying manganate layers, separated by a thin layer of foreign material so as to disrupt the magnetic exchange coupling but maintain some sort of electrical contact; and study the spin-dependent electrical transport across this structure using a current-perpendicular geometry. Sun et al. have successfully fabricated such a structure to demonstrate a large low-field CMR effect [13]. A resistance change of a factor of 2 was obtained with a switching field of less than 200 Oe at 4.2 K. The structure was lithographically fabricated with La0.67X0.33MnO3/SrTiO3/La0.67X0.33MnO3 trilayers (with X = Ca, Sr). Figure 6 shows a schematic of the fabrication process. An example of the change in magnetoresistance as a function of sweeping field is shown in Figure 7. The mechanism of the spin-dependent transport process across SrTiO3 is yet to be fully understood. Nevertheless, the successful demonstration of such a device structure serves as an existence proof that a large magnetoresistive response can be achieved in low fields using the manganates. Large low-field magnetoresistance has also been observed recently in layered manganite single-crystal La1.4Sr1.6Mn2O7 by Kimura et al. [82]. Their transport measurement with current perpendicular to the Mn-O planes revealed a magnetoresistance as high as 240% in fields below 1 kOe and at temperatures below 100 K. They suggested interplane spin-dependent tunneling as a possible mechanism for such low-field magnetoresistance. Thus far, large low-field magnetoresistance in the manganates has been observed only at reduced temperatures, usually below 100 K. One possibility is that at high temperatures, a large leakage current across a defect-populated barrier layer acts as a shunt to the magnetoresistance [83]. Summary Interest in the doped perovskite manganates has been reactivated in the recent past. This followed from the observation of a large negative magnetoresistance in this class of materials, in thin-film form and at room temperature, which has made this class of materials potentially useful for magnetic-field-sensing applications such as hard-disk read heads. The magnitude of the magnetoresistance of the doped manganates can be orders of magnitude larger than that of metal alloy superlattices such as Cu/Co multilayers. The only other class of materials exhibiting such a large magnetoresistive effect is the class of magnetic semiconductors [84,85] such as Eu1-xGdxSe. However, so far the effect there is limited to temperatures below 50 K. The large magnetoresistance of the doped manganates, however, persists to temperatures above ambient. For the generic doped manganate material to reach its full magnetoresistance, a saturation field of the order of a few tesla is usually needed. However, for certain device structures such as those involving use of a current-perpendicular trilayer transport junction, large magnetoresistances can be obtained at a low field. The mechanism that governs the large spin-dependent transport in the doped manganates is not yet fully understood. It is likely that double exchange plays a central role in determining the local magnetic and electronic structure. However, there is ample evidence for the presence of magnetic inhomogeneities, perhaps coupled to local structural distortions in forms of magnetic polarons mediated through a Jahn-Teller distortion-based electron-phonon coupling. The magnetic interaction of these local magnetic clusters resembles that of a granular magnetic system, and the spin-dependent electronic transport across these local magnetic clusters is probably the origin of the large magnetoresistive effect. Large low-field magnetoresistance has been demonstrated only at reduced temperature. The noise characteristic of the doped manganates is still under investigation; preliminary studies show the presence of large 1/f noise, perhaps due to magnetic domain wall motions [86-88]. The high-frequency response of their magnetoresistance would have to be determined, and associated fabrication processes compatible with the microelectronics technology would have to be developed before the applicability of this class of materials to magnetic sensing applications could be assessed. Acknowledgments We wish to thank J. C. Slonczewski, D. P. DiVincenzo, J. J. Connolly, S. L. Brown, R. A. Altman, L. S. Yu-Jahnes, C. Jahnes, Yu Lu, G. Q. Gong, R. B. 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He received a B.S. degree in physics from Fudan University, Shanghai, China, in 1984, and M.S. and Ph.D. degrees in applied physics from Stanford University in 1986 and 1989, respectively. For the following two years, he worked at Superconductor Technologies Inc. in Santa Barbara, California, as a member of their technical staff. He subsequently joined IBM at the Thomas J. Watson Research Center, where he has worked on superconducting and magnetic thin films and devices. Dr. Sun is a member of the American Physical Society and the Materials Research Society. Lia Krusin-Elbaum IBM Research Division, Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598 (ELBAUM at YKTVMV, elbaum@watson.ibm.com). Dr. Krusin-Elbaum received a Ph.D. degree in solid state physics from New York University in 1979. She subsequently joined the IBM Thomas J. Watson Research Center, where she is currently a Research Staff Member in the Physical Sciences Department. She has pursued her interests in a variety of subjects, such as spin-glasses, electronic transport in thin metal films, and semiconductor and superconducting devices. Since 1988, her efforts have been focused primarily on the exploration of the macroscopic magnetic behavior of high-Tc superconductors. She has contributed to establishing basic superconducting parameters and to the understanding of the pinning of magnetic vortices and vortex dynamics in high-Tc cuprate single crystals, films, and wires/tapes--important for future applications. Dr. Krusin-Elbaum is associated with an orders-of-magnitude enhancement of the current-carrying capability of the cuprate superconductors via doping with extended columnar defects by irradiation with high-energy heavy ions and GeV protons. Her current interests include the magnetic and transport properties of manganates and of small magnetic structures. Dr. Krusin-Elbaum has received four IBM Invention Achievement Awards, and she holds seven U.S. patents. She is a Fellow of the American Physical Society. Arunava Gupta IBM Research Division, Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598 (AGUPTA at YKTVMV, agupta@watson.ibm.com). Dr. Gupta is a Research Staff Member in the Physical Sciences Department at the IBM Thomas J. Watson Research Center. He received an M.S. degree in chemistry from the Indian Institute of Technology in 1976 and a Ph.D. degree in chemical physics from Stanford University in 1980. He subsequently joined the Allied Signal Corporation in Morristown, New Jersey, where he worked on catalysis and laser processing of materials. In 1985 Dr. Gupta joined the Thomas J. Watson Research Center, where he has worked on various photothermal and photochemical laser techniques for processing and patterning of materials. His present interests are in areas related to the growth and properties of oxide thin films. He has pioneered the development and use of the pulsed-laser deposition technique for epitaxial and layer-by-layer growth of oxide films, for which he received an IBM Outstanding Technical Achievement Award in 1992. Dr. Gupta is a member of the Materials Research Society and the American Physical Society. Gang Xiao Physics Department, Brown University, Providence, Rhode Island 02912 (xiao@physics.brown.univ). Dr. Xiao is an Associate Professor of Physics at Brown University in the areas of magnetism and high-vacuum thin-film deposition and characterization. He received M.Sc. and Ph.D. degrees in physics from The Johns Hopkins University, serving as a Postdoctoral Fellow there. He was a Sloan Research Fellow and a recipient of an NSF Young Investigator Award. In 1995 he received the Outstanding Young Scientist Award from the Overseas Chinese Physicists Association. Professor Xiao has published 102 journal articles in the area of condensed-matter physics and materials science. He has presented 30 invited talks at institutions and at national and international conferences. He holds a patent on magnetic recording media. Professor Xiao is a member of the American Physical Society. Peter R. Duncombe IBM Research Division, Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598 (PRD at YKTVMV, prd@watson.ibm.com). Mr. Duncombe is a Senior Associate Engineer in the Exploratory Silicon Science and Technology Department at the IBM Thomas J. Watson Research Center, working on thin-film materials and processes for DRAM, MRAM, NVFRAM, and other applications of ferroelectric and high-permittivity ceramics. Since joining IBM in 1985, he has worked on sintering, superconductivity, GMR, and ceramic packaging. Mr. Duncombe received a Research Division Award for his contributions to the glass ceramic composite via in 1992 and a First Plateau Invention Achievement Award in 1995. He received a B.A. in chemistry from SUNY New Paltz in 1980 and an M.S. in chemical engineering from SUNY Buffalo in 1983. He is a member of the American Chemical Society. Stuart S. P. Parkin IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120 (parkin@almaden.ibm.com). Educated in Britain, Dr. Parkin received his B.A. degree (1977) from the University of Cambridge, was elected a Research Fellow (1979) at Trinity College, Cambridge, and was awarded his Ph.D. degree (1980) at The Cavendish Laboratory, Cambridge. He joined IBM Research in San Jose as a World Trade Postdoctoral Fellow in 1982, becoming a permanent member of the staff the following year. Prior to coming to San Jose, he was awarded a Royal Society European Exchange Fellowship, which he spent as a Postdoctoral Fellow at the University of Paris, Orsay, in 1980 and 1981. At IBM, his interests have ranged from organic superconductors to ceramic high-temperature superconductors, and, in recent years, the study of magnetic thin-film structures and nanostructures. In 1991 Dr. Parkin discovered oscillations in the magnitude of the interlayer exchange coupling in transition-metal magnetic multilayered systems. In 1997 Dr. Parkin was awarded the Hewlett-Packard EuroPhysics Prize for Outstanding Achievement in Solid-State Physics for his "discovery and contribution to the understanding of Giant Magnetoresistance" and its "potential for technological applications." He has received other awards, including the Materials Research Society Outstanding Young Investigator Award (1991) and the Charles Vernon Boys Prize from the Institute of Physics, London (1991), as well as several awards from IBM, including three Outstanding Technical Achievement Awards and several awards for patents. He was elected a Member of the IBM Academy of Technology in 1997 and named a Master Inventor by the IBM Corporation in 1997. D. Parkin is a Fellow of the American Physical Society and a Consulting Professor at Stanford University. His work on magnetic multilayered structures is widely referenced. Recently it was reported in Science magazine that Dr. Parkin is the sixth most highly cited author in the physical sciences for the period 1990-1996. He was recently named a Centennial Lecturer by the American Physical Society. Dr. Parkin's present work involves the study of magnetic tunnel junctions and the development of an advanced nonvolatile magnetic random access memory based on magnetic tunnel junction storage cells. Table 1 Physical properties of (A0.73+B0.32+)MnO3 compounds, according to Coey et al. [24]. System a0^ Tc gamma**b thetaD# rho0 (nm) (K) (mJ-mol**-1 K**2) (K) (ohm-m) (Y0.7Sr0.3)MnO3 0.3858(4) 360(5) 8.1(3) 348(5) 5 x 10**-8 (La0.7Sr0.3)MnO3 0.3875 370 6.0 353 6 x 10**-7 (La0.7Ba0.3)MnO3 0.3885 330 6.1 333 1 x 10**-6 (La0.7Ca0.3)MnO3 0.3855 220 1 x 10**-4 (La0.7Ca0.3)MnO3b 0.3860 260 5.2 2 x 10**-4 (Nd0.7Ba0.3)MnO3b 0.3883 145 7 x 10**-1 (Nd0.7Sr0.3)MnO3 0.3872 115 8 x 10**1 (Nd0.7Ba0.3)MnO3 0.3885 110 8 x 10**3 ^Lattice parameter of the elementary perovskite cell. #Polycrystalline ceramic. Table 2 Some recent results on thin-film CMR materials. In the Material column, only cation ratios are listed. T denotes target composition, F denotes film composition, and Ts denotes deposition temperature. Results from as-grown films are denoted with a superscript a when necessary. Superscripts b and c correspond to different annealing conditions. Tp denotes the resistance-peak temperature. Room-temperature resistivity is denoted as rho0. Material Deposition Substrate Ts Pressure Annealing a0 Tp rho0 Maximum method (deg C) (mTorr) conditions (A) (K) (ohm-cm) DeltaR/R(H) T: La0.7Ba0.3 Off-axis SrTiO3 600 300 900 deg C N/A ~300 3 x 10**-4 150% PLD [3] (100) 12 hr at 7 T (110) air T: La0.67Ca0.33 PLD [7] LaAlO3 ~600-700 100 (O2) 700 deg C/30 minb 3.89a ~100a N/A 460%a 1400%b (100) 900 deg C/3 hrc ~200b 400%c 10**5%best 1 atm O2 ~280c at 6 T T: La0.67Ca0.33 PLD [6] LaAlO3 ~650-700 ~100- 900 deg C 3.89a ~100 N/A 1.25 x 10**5% (100) 300 (O2) 30 min at 6 T 3 atm O2 T: La0.7Ca0.3 PLD [24,43] 720 150 (O2) none 3.855 220 1 x 10**-2 T: La0.7Ba0.3 PLD [24,43] 720 150 (O2) none 3.885 330 1 x 10**-4 T: Y0.7Sr0.3 PLD [24,43] MgO 720 150 (O2) none 3.858 360 5 x 10**-6 T: La0.7Sr0.3 PLD [24,43] (100) 720 150 (O2) none 3.875 370 6 x 10**-5 T: Nd0.7Sr0.3 PLD [24,43] 720 150 (O2) none 3.872 115 8 x 10**3 T: Nd0.7Ba0.3 PLD [24,43] 720 150 (O2) none 3.885 110 8 x 10**5 T: Nd0.7Sr0.3 PLD [41] LaAlO3 ~600-800 300 (N2O) 900 deg C 3.93a (95a) 2 x 10**-2 3340%a at 5 T (100) 30 min 50 >=10**6% at 8 T 1 atm O2 T: La0.67Ca0.33 PLD [44] LaAlO3 700 100 (O2) 850 deg C N/A 110 N/A 1.1 x 10**6% (100) 1 hr, 3 atm O2 at 6 T T: La0.68Ca0.11 Ion beam MgO N/A 0.25 N/A 3.90 220 <1 x 10**-2 120% at 1 T F: La0.72Ca0.25 sputter [17], (100) Xe:O2 = 1:1.5 1 kV/80 mA F: La0.74Pb0.26 rf sputter [45], Si 500 40 800 deg C 3.86 325 3 x 10**2 30% at 2 T 2 in., 50 W (100) Ar:O2 = 4:1 2 hr, O2 T: La0.67Ca0.33 dc sputter [46], MgO 700 60 none a = b 86 12 460% at 0.82 T 380 V/100 mA (100) Ar:O2 = 1:1 = 7.76 c = 7.74 T: La0.67Ca0.33 dc sputter [46], MgO 700 60 950 deg C 226 <10**-3 130% at 0.82 T 380 V/100 mA (100) Ar:O2 = 1:1 6 hr, air F: La0.67Ca0.33 MOCVD [23] LaAlO3 ~500-700 3-4 950 deg C N/A ~260 2 x 10**-4 42% at 7 T (100) Torr (O2) ~1 hr (1 atm O2) F: La0.67Sr0.33 MOCVD [23] LaAlO3 ~500-700 3-4 950 deg C N/A 380 1.5 x 10**-4 27% at 7 T (100) Torr (O2) ~1 hr (1 atm O2) F: La0.58Ca0.33 MBE [47] SrTiO3 680 0.01 as grown 3.815 234 N/A 900% at 5.5 T (100) O-zone T: La0.75 PLD [31] SrTiO3 ~600-700 250 as growna N/A 255a 2 x 10**-4a 425%a at 4 T self-doped (100) (O2) and 850 deg C 30 min (O2) 315 N/A 130% at 4 T T: La0.67Sr0.33 PLD [42] Si (100) 670 400 as grown 3.88 380 1 x 10**-4 N/A Bi4Ti3O12 buffered T: La0.67Ca0.33 PLD [48] Si (100) ~600-750 200 as grown 3.83 ~120- ~2-4 x 10**-2 250% at 5 T (111) (O2) 210 YSZ buffered