The role of grain boundaries in the magnetoresistance ͑MR͒ properties of the manganites has been investigated by comparing the properties of epitaxial and polycrystalline films of La 0.67 D 0.33 MnO 3Ϫ␦ (DϭCa, Sr, or vacancies͒. While the MR in the epitaxial films is strongly peaked near the ferromagnetic transition temperature and is very small at low temperatures, the polycrystalline films show large MR over a wide temperature range down to 5 K. The results are explained in terms of switching of magnetic domains in the grains and disorder-induced canting of Mn spins in the grain-boundary region. ͓S0163-1829͑96͒51746-7͔

We report on the fabrication of a new class of trilayer epitaxial thin film devices based on the doped perovskite manganates La–Ca–Mn–O and La–Sr–Mn–O. We show that large resistance changes, up to a factor of 2, can be induced by a moderate applied magnetic field below 200 Oe in these trilayers supporting current-perpendicular-to-plane transport. These results show that low-field spin-dependent transport in manganates can be accomplished, the magnitude of which is suitable for magnetoresistive field sensors.

We report significant enhancements of magnetoresistance in granular (La0.67Ca0.33MnO3)x/(SrTiO3)1−x. The system exhibits a conduction threshold at x=xc∼60%, around which magnetoresistance versus x has a maximum. The composition xc at which maximum enhancement in magnetoresistance is observed is the same at high (around 5 T) and at low (a few hundred Oersted) fields. The enhancement is consistent with the disorder-driven amplification of spin-dependent transport at the structural boundaries of the mixture.

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Insulated-gate field-effect transistors (IGFETs) (Fig. 1) comprising pentacene as the semiconducting layer have been fabricated and studied previously.[1±5] The characteristics and performance of pentacene IGFETs make them competitive for applications requiring large area coverage, mechanical flexibility, and low overall cost. Such applications include thin-film transistor (TFT) switching devices for active matrix liquid-crystal displays (AMLCDs), where hydrogenated amorphous silicon (a-Si:H) TFTs are currently used, active matrix organic light-emitting diode displays (AMOLEDs), and low end data storage (e.g., smart cards and identification tags). Pentacene TFTs have produced the highest field effect mobility values reported for organic IGFETs. A mobility of 0.62 cm 2 V ±1 s ±1 and a current on/off ratio of 10 8 have been reported for devices comprising vapor-deposited purified pentacene films grown at substrate temperatures of approximately 120 C. [3,5] These values of mobility and on/off ratio are similar to the ones reported for a-Si:H TFTs. However, an operating voltage swing from ±100 to +100 V is required in order to achieve such high performance from pentacene devices. This is too high compared to a-Si:H and for any practical applications in general. [6] This shortcoming is also demonstrated by comparing the subthreshold slopes, s, of a-Si:H devices (about 0.5 V/decade) and those of pentacene-based IGFETs (5±12 V/decade). The best subthreshold slope value for pentacene IGFETs reported to date is s^1.6 V/decade. [5] This was achieved by the use of a self-assembled monolayer (SAM) of octadecyltrichlorosilane deposited between the SiO 2 insulator and pentacene. Still, the required voltage span had to be larger than 100 V for high mobility and high current on/off ratio to be achieved in addition to low s. Recently, a mobility of more than 1 cm 2 V ±1 s ±1 was reported for a pentacene-based device that also required high operating voltages (about ±80 V).[7]A dependence of the calculated mobility of pentacene IGFETs on gate voltage has been observed earlier. [2,8] In the present paper, we have used our understanding of the gate voltage dependence of mobility in pentacene devices [8] to demonstrate high-performance pentacene IGFETs exhibiting mobility similar to (a-Si:H) TFT, good current modulation, and excellent subthreshold slopes at operating voltages of about 5 V. For this purpose we used a relatively high dielectric constant e metal oxide film as the gate insulator, specifically barium strontium titanate (BST), deposited by means of chemical solution processing. The typical IGFET configuration is depicted in Figure 1. Pentacene films were deposited using vapor deposition. As-received pentacene (97+ % FLUKA Chemical Co.) was used. It has been shown previously that both mobility and current modulation (I on /I off ) increase when pentacene is purified before deposition.[5] Thus we expect that our results could be improved substantially by using purified pentacene. Further details on the vacuum chamber configu...