We have performed magnetotransport measurements on La 2/3 Sr 1/3 MnO3 / SrTiO3 / La 2/3 Sr 1/3 MnO3 magnetic tunnel junctions. A magnetoresistance ratio of more than 1800 % is obtained at 4K, from which we infer an electrode spin polarization of at least 95 %. This result strongly underscores the half-metallic nature of mixed-valence manganites and demonstrates its capability as a spin analyzer. The magnetoresistance extends up to temperatures of more than 270K. We argue that these improvements over most previous works may result from optimizing the patterning process for oxide heterostructures.PACS numbers: 73.40Rw, 71.20.Eh Magnetic tunnel junctions (MTJ) have been studied actively from the mid 90's [1] due to both the underlying physics and their potential applications as magnetic memories (MRAMs) or sensors. These structures consist of two ferromagnetic metallic electrodes (FM) sandwiching a thin insulating barrier (I). When a bias voltage V DC is applied, electrons near the FM/I interface tunnel through the barrier and, since they are spin-polarized, the resistance depends on the relative orientation of the electrodes' magnetization. The tunneling magnetoresistance (TMR) ratio is defined aswhere R AP and R P are the resistances of the junction in the antiparallel and parallel configurations, respectively. In Julliere's model [2], the TMR ratio is related to the spin polarizations P 1 and P 2 of the two ferromagnetic electrodes as:Within this simple model, large TMR ratios result from electrodes, or from electrode-barrier interfaces [3], with large effective spin polarization values. Junctions which integrate amorphous barriers such as Al 2 O 3 and transition ferromagnets, for which the spin polarization does not exceed around 50 % [1, 4], do not show TMR ratios larger than 60 %. Preliminary work has been reported[5] on obtaining large interfacial spin polarizations owing to band structure effects, but the simplest route to achieving large TMR ratios relies on the use of so-called "half-metals" with a nearly total intrinsic spin polarization.A regarding their half-metallicity. Indeed, whereas spinpolarized photoemission spectroscopy experiments [10] have confirmed the half-metallic character of LSMO, the maximum spin polarization as inferred from tunneling experiments does not exceed 86% in LCMO [11] and 83% in LSMO [12].In this letter, we report a TMR ratio of more than 1800 % at T=4.2K and V DC =1mV in La 2/3 Sr 1/3 MnO 3 / SrTiO 3 / La 2/3 Sr 1/3 MnO 3 fully epitaxial MTJs, from which we deduce a spin polarization of at least 95 % for LSMO. This result confirms for the first time the transport half-metallic nature [13] of this material, which can therefore be used as a spin analyzer in tunneling experiments [3]. We argue that this large TMR value arises both from preserving the quality of the LSMO / STO (STO : SrTiO 3 ) interfaces during our upgraded patterning process, and from designing junctions of small size. The TMR extends to temperatures of about 280K, an improvement compared to previous resul...

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.

The phase diagram of high-temperature superconductors is still to be understood 1 . In the low-carrier-doping regime, a loss of spectral weight in the electronic excitation spectrum-the so-called pseudogap-is observed above the superconducting temperature T c , and below a characteristic temperature T * (ref. 2). First observed in the spin channel by NMR measurements, the pseudogap has also been observed in the charge channel by scanning probe microscopy and photoemission experiments, for instance 2 . An important issue to address is whether this phenomenon is related to superconductivity or to a competing 'hidden' order. In the superconductivity case, it has been suggested that superconducting pairing fluctuations may be responsible, but this view remains to be tested experimentally. Here, we have designed a Josephson-like experiment to probe directly the fluctuating pairs in the normal state. We show that fluctuations survive only in a restricted range of temperature above T c , well below T * , and therefore cannot explain the opening of the pseudogap at higher temperature.Angle-resolved photoemission spectroscopy 3,4 and scanning tunnelling spectroscopy 5 showed a characteristic energy of the pseudogap that merges with the superconducting gap when the temperature is lowered below T c . This reveals a smooth crossover rather than a sharp transition line between the pseudogap regime and the superconducting state, and has led to the superconducting precursor scenario. As opposed to the conventional Bardeen-Cooper-Schrieffer (BCS) transition, where pairing and condensation occur simultaneously at T c , in underdoped cuprates fluctuating pairs may form at T * , with no long-range coherence, and condense in the superconducting state at T c (refs 6,7). Difficulties in confirming (or invalidating) this scenario arise from the fact that most of the experimental techniques used to investigate the pseudogap are sensitive only to the one-particle excitations, and therefore cannot provide a test of pairing above T c . Owing to its ability to probe the properties of the superconducting wavefunction, the Josephson effect is a natural way to address the fluctuation issue.In a second-order phase transition, the susceptibility is given by the linear response of the order parameter to a suitable external field. In the case of the superconducting phase transition, the role of the external field can be played by the rigid pair field of a second superconductor below its own T c (refs 8,9). In a Josephson junction in which one side of the junction is the fluctuating superconductor of interest above its T c , whereas the other side is a superconductor below its T c , the coupling between the pairing fluctuations and the well-established pair field gives rise to an excess current I ex proportional to the imaginary part of the frequency-and wavenumber-dependent pair susceptibility χ(ω, q). For a conventional superconductor above its T c (ref. 9)where Γ 0 = (16k B /h)(T − T c ) is the relaxation rate of the fluctuations, ξ(T ) is the ...