1. Electrical Transport Properties of La 0.33 Ca 0.67 MnO 3 R Schmidt, S Cox, J C Loudon, P A Midgley, N D Mathur University of Cambridge, Department of Materials Science & Metallurgy, Pembroke Street, Cambridge CB2 3QZ, United Kingdom Introduction The La 1-x Ca x MnO 3 (LCMO) system has attracted considerable interest due to its colossal magnetoresistance (CMR), charge and orbital ordering (CO/OO), phase competition or coexistence, and in general the rich variety of crystallographic, electronic and magnetic phases. The classical CO state (for x>0.5) is believed to arise from Mn 3+ /Mn 4+ cation stacking and thus exhibits a charge modulation wave vector q coupled to the doping level by q ≈ (1-x) a*, with a* being the reciprocal space vector. In LCMO with x=0.52, q would be ≈ 0.48, and was claimed to contain 2 components, the x=0.5 Mn 3+ /Mn 4+ /Mn 3+ /Mn 4+ stacking, and additionally the stacking fault modulation of an extra Mn 4+ cation every 50st unit cell, which corresponds to q/a* ≈ 1/50 or a stacking fault distance of approximately 7.8 nm. In a recent TEM study we have obtained quite different findings [1]. By reducing the electron spot size of the TEM below 7.8 nm, the images obtained by working in diffraction mode showed a uniform periodicity of q/a* ≈ 0.48, which rules out full CO based on Mn 3+ /Mn 4+ stacking. Instead, this modulation could be an indication for the formation of charge-density waves (CDWs), which has been proposed in Pr 0.63 Ca 0.37 O 3 [2]. In the TEM diffraction patterns below, the full lines indicate reflections from the parent lattice, the dotted lines the charge modulation reflections, which are slightly off the q/a* = 0.5 values in both cases. These findings have inspired us to study the electrical transport properties of polycrystalline La 0.33 Ca 0.67 MnO 3 bulk material to search for evidence of CDW type conduction. In the future it is intended to investigate a possible anisotropy of electrical transport in epitaxial films of La 0.33 Ca 0.67 MnO 3 along the charge modulated (CM) a and non- modulated c lattice direction. We have grown 40 nm thick films (CM below 270K) on NdGaO 3 substrates by pulsed laser deposition. [1] Loudon J C, Cox S, Williams A J, Attfield J P, Littlewood P B, Midgley P A, Mathur N D, cond ‑ mat/0308581, 2004 [2] Wahl A, Mercone S, Pautrat A, Pollet M, Simon C, Sedmidubski D, cond-mat/0306161, 2003 Convergent beam electron diffraction pattern: FWHM = 3.6 nm Aperture diameter = 500 nm q/a* = 0.473 ± 0.005 q/a* = 0.468 ± 0.003 Pulsed Laser Deposition of La 0.33 Ca 0.67 MnO 3 1. Vacuum annealing of the substrate at 700°C for 1 h 2. Deposition at 15 Pa O 2 partial pressure - 27 minutes deposition time - 215.6 Joule laser energy - 1 Hz laser pulse frequency 3. Post deposition annealing at 60 kPa O 2 partial pressure at 700°C for 1 hour Current in Ampere Voltage in Volt 140 K 150 K 160 K 170 K 180 K 190 K 220 K 230 K 200 K 210 K Current vs Voltage Characteristics of Bulk La 0.33 Ca 0.67 MnO 3 Conclusions From the voltage versus current characteristics it can be seen clearly that electrical transport is non-linear at low temperatures. This is in agreement with the expected I-V characteristics of sliding CDW conduction. Still, it has to be proven that the non-linearity does not just reflect a simple self-heating effect of the sample as commonly seen in spinel NTCR thermistor manganates. The epitaxial La 0.33 Ca 0.67 MnO 3 films on NdGaO 3 showed good crystallinity and uniform strain and are suitable for transport property investigations. Structural Analysis of Epitaxial La 0.33 Ca 0.67 MnO 3 Atomic Force Microscopy X-Ray Diffractometry 46.8 47.247.448.0 48.4 48.8 L F 48.048.448.8 2Theta (°) S = Substrate F = Fringe L = Layer F F L F The film thickness was calculated from the fringe distance to be 40 ± 2 nm. The rocking curves for the substrate and layer showed a FWHM of 0.0039° and 0.0057° in omega respectively, indicating good crystallinity of both. The layer peak showed a positive shift of 0.753° in 2theta in respect to the expected angle, which can be explained by the strain induced by the lattice mismatch of substrate and film. Reciprocal Space Mapping La 0.33 Ca 0.67 O 3 on NdGaO 3 [040] reflection[165] reflection La 0.33 Ca 0.67 MnO 3 orthorhombic distorted perovskite structure space group Pnma: a ≈ c ≈ b/ ≈ a C (cubic) b a c a = 5.3864 Å b = 7.5687 Å c = 5.3812 Å NdGaO 3 orthorhombic distorted perovskite structure space group Pnma: a ≈ c ≈ b/ ≈ a C a = 5.4979 Å b = 7.7078 Å c = 5.4272 Å lattice mismatch : Layer Substrate [1] Radaelli P G, Cox, D E, Capogna L, Cheong S W, Marezio M, Phys Rev B, 59 (22), 1999 [2] Ubizskii S B, Vasylechko L O, Savytskii D I, Matkovskii A O, Syvorotka I M, Supercond Sci Techn, 7 (10), 1994 [1] [2] a : 2.0 % b : 0.8 %