24. S. Rotzler, H. Schramek, H. R. Brenner, Nature 349, 337 (1991); J. L. McManaman, J. C. Blosser, S. H. Appel, J. Neurosci. 1, 771 (1981). 25. D. J. Perkel, J. J. Petrozzino, R. A. Nicoll, J. A. Connor, Neuron 11, 817 (1993); J. A. Cummings, R. M. Mulkey, R. A. Nicoll, R. C. Malenka, ibid. 16, 825 (1996). 26. T. Manabe, P. Renner, R. A. Nicoll, Nature 355, 50 (1992). 27. D. V. Lissin, R. C. Carroll, R. A. Nicoll, R. C. Malen- kavon, M. Zastrow, J. Neurosci. 15, 1263 (1999); R. C. Carroll, D. V. Lissin, M. V. Zastrow, R. A. Nicoll, R. C. Malenka, Nature Neurosci. 2, 454 (1999). 28. J. L. Gooch, M. R. Suchyta, J. M. Balbierz, J. H. Petajan, T. P. Clemmer, Crit. Care Med. 19, 1125 (1991); V. Segredo et al., N. Engl. J. Med. 327, 524 (1992); T. L. Kurt, J. Toxicol. Clin. Toxicol. 34, 253 (1996). 29. This work was supported by a Human Frontier Sci- ence Program Long-Term Fellowship (M.A.) and the NIH and the Muscular Dystrophy Association ( J.W.L.). We thank the members of our laboratory for helpful discussions about this work. 14 July 1999; accepted 26 August 1999 R EPORTS Role of Metal-Oxide Interface in Determining the Spin Polarization of Magnetic Tunnel Junctions Jose Maria De Teresa, Agne `s Barthe ´le ´my, Albert Fert,* Jean Pierre Contour, Franc ¸ois Montaigne, Pierre Seneor The role of the metal-oxide interface in determining the spin polarization of electrons tunneling from or into ferromagnetic transition metals in magnetic tunnel junctions is reported. The spin polarization of cobalt in tunnel junctions with an alumina barrier is positive, but it is negative when the barrier is strontium titanate or cerium lanthanite. The results are ascribed to bonding effects at the transition metal– barrier interface. The influence of the electronic structure of metal-oxide interfaces on the spin polarization raises interesting fundamental problems and opens new ways to optimize the magnetoresistance of tunnel junctions. A tunnel junction consists of two metallic lay- ers (electrodes) separated by a thin insulating layer. When the electrodes are ferromagnetic, the tunneling of electrons across the insulating barrier is spin-polarized, and this polarization reflects that of the density of states (DOS) at the Fermi level (E F ) of the electrodes. This spin polarization is the origin of the tunneling mag- netoresistance (TMR), which is currently a hot topic of research in magnetism (1) and very promising for applications (2). Paradoxically, even though applications have already begun to be developed, there are still gaps in our under- standing of spin-polarized tunneling. For exam- ple, the physics governing the spin polarization of tunneling electrons is not clearly understood. Previously, the spin polarization P of electrons tunneling from a given ferromagnetic electrode was generally thought to reflect a characteristic intrinsic spin polarization of the DOS in the ferromagnet, P = N 1 (E F ) - N 2 (E F ) N 1  E F ) + N 2 (E F ) (1) However, recent findings show that the ampli- tude of the spin polarization, and even its sign, depends on the choice of barrier material (3, 4). Here, we describe a series of TMR experiments on Co/I/La 0.7 Sr 0.3 MnO 3 (LSMO) tunnel junc- tions, where the barrier I can be SrTiO 3 (STO), Ce 0.69 La 0.31 O 1.845 (CLO), or Al 2 O 3 (ALO). The effective polarization of Co was found to be positive (higher tunneling probability for majority spin electrons) when I is ALO, and negative (higher tunneling probability for mi- nority spin electrons) when I is STO or CLO. Moreover, the bias dependence of the TMR is completely different in these two cases. The strong influence of the electronic structure of the barrier and barrier-electrode interface in tunnel junctions raises interesting fundamental prob- lems and presents new ways to tailor the TMR. The first piece of information on the spin polarization of electrons tunneling from a ferromagnetic metal (F) comes from experi- ments on F/I/S junctions, in which the second electrode is a superconductor (S). The spin splitting of the quasi-particle DOS of S, in- duced by a magnetic field, can be used to analyze the spin polarization of the tunneling current. Extensive data have been obtained with F/ALO/Al junctions, and a positive po- larization has been found for all the ferro- magnetic metals and alloys that have been studied (5). This is surprising, especially for metals like Co or Ni in which a negative polarization is expected from the smaller DOS at E F for the majority spin direction (the majority spin d subband is below E F ). This problem has not been clearly solved, even though it is frequently argued that s-character electrons should tunnel more easily, so that the experimental positive polarization can re- flect only that of the s-character DOS (6, 7). Some theoretical justification has been pro- vided by ab initio calculations of the electron- ic structure at a Co-ALO interface. Nguyen- Mahn et al.(8) determined the DOS of the tunneling electrons on the first Al atoms at the interface and found that, because of an sp-d bonding mechanism between Al and Co, this DOS is positively polarized. This can be viewed as an interface filtering effect control- ling the starting point of the polarized eva- nescent wave in the barrier. In junctions with two ferromagnetic elec- trodes, F 1 /I/F 2 , spin-polarized tunneling gives rise to TMR because the resistance of the junction depends on whether the electrodes have parallel or antiparallel magnetizations. This change can be large, typically 15 to 40% at room temperature, so that TMR has great relevance for the technology of MRAM (magnetic random access memory) or read heads. The experimental results at low bias are generally interpreted according to Jul- lie `re’s expression, R R = R AP - R P R AP = 2P 1 P 2 1 + P 1 P 2 (2) where R AP and R P are the resistances in the antiparallel and parallel states, respectively, and P 1 and P 2 are the electron spin polariza- tions of the two electrodes. In junctions studied up to now, mostly with ferromagnetic transition-metal elec- trodes and ALO barriers, a normal TMR has been found; that is, the tunnel resistance is smaller when the magnetizations of F 1 and F 2 are parallel. This behavior is expected when the sign of the polarization coefficient P is the same for both electrodes and is consistent with the aforementioned uniformly positive spin polarization found for various transition metals in F/ALO/Al junctions. However, two recent results have indicated that, with types of barrier other than ALO, the spin polariza- tion of electrons tunneling from Co or NiFe (permalloy) can also be negative. Sharma et Unite ´ Mixte de Physique, CNRS–Thomson CSF, Labo- ratoire Central de Recherche, Domaine de Corbeville, 91404 Orsay, France, and Universite ´ Paris-Sud, 91405 Orsay, France. *To whom correspondence should be addressed. E- mail: fert@lcr.thomson-csf.com R ESEARCH A RTICLES www.sciencemag.org SCIENCE VOL 286 15 OCTOBER 1999 507