Colossal Temperature Coefficient of Resistivity in Epitaxial Colossal Magnetoresistive La 0.67 Ca 0.33 MnO 3 Films
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Colossal Temperature Coefficient of Resistivity in Epitaxial Colossal Magnetoresistive La0.67Ca0.33MnO3 Films S.I. Khartsev, A.M. Grishin KTH, Department of Microelectronics and Information Technology, S-164 40 Kista, SWEDEN. ABSTRACT Epitaxial La0.67Ca0.33MnO3 (LCMO) films have been grown by pulsed laser deposition technique on three single crystal substrates: LaAlO3, SrTiO3, and NdGaO3 having lattice mismatch with LCMO film of + 1.26 %, - 0.9 %, and + 0.18 %. We found the transport colossal magnetoresistive (CMR) properties were improved with the reduction of a film/substrate mismatch. Processing conditions for LCMO film on NdGaO3 substrate were optimized as well as post-annealing process was employed to achieve a superior temperature coefficient of resistivity TCR = 35 %K-1 and the magnetoresistance MR = 66 % at 7 kOe. To our knowledge, these are the record CMR parameters achieved so far.
INTRODUCTION Thin manganite perovskite films demonstrate colossal magnetoresistivity thus considered as candidates for new generation of magnetic reading heads for ultradense magnetic storage. Also, recently we have demonstrated feasibility to use CMR films for uncooled and moderately cooled infrared (IR) bolometers. [1] Both effects are caused by a very sharp insulator-to-metal phase transition at critical temperature Tc. The temperature coefficient of resistivity (TCR=1/ρ×dρ/dT) is the main parameter showing how narrow the phase transition is, thus indicating the crystalline performance and phase homogeneity of a CMR film. For bolometric applications TCR is a figure of an IR-detector merit. As for CMR phenomenon, the apparent magnetoresistance MR = 1 ρ(H)/ρ(0) is also determined by TCR: applied magnetic field shifts the peak of ρ-T dependence as much as ∆Tc(H) to higher temperatures, therefore MR = ∆Tc×TCR appears to be proportional to TCR. In highly crystalline CMR films even a small shift of the sharp ρ-T curve results in essential magnetoresistivity. In present research we chose widely explored La0.67Ca0.33MnO3 (LCMO) composition to achieve a superior TCR by the optimization of pulsed laser deposition (PLD) process and filmto-substrate crystallographic affinity. EXPERIMENTAL LCMO films were fabricated by KrF excimer PLD process. Three different (001) oriented single crystals were used as substrates: LaAlO3 (LAO), SrTiO3 (STO), and NdGaO3 (NGO) having film-tosubstrate mismatch afilm/asubstrate - 1= + 1.26%, - 0.9% and + 0.18%, respectively. Independently of the deposition parameters, the best film performance has been achieved for NdGaO3 single crystal with a minimum mismatch to LCMO film. For example, at substrate temperature of 800oC, oxygen pressure of 250 mTorr, and laser fluence of 3 J/cm2 we have obtained the following transition temperature Tc and TCR: 272 K and 9.44 %/K @ 253 K for NGO, 252 K and 6.9 %/K @ 235 K for LAO, and 237 K and 7.1 %/K @ 207 K for STO. We chose NdGaO3 substrate as the basis for technological quest. F7.12.1
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