Effect of strain on the electrical conduction in epitaxial films ofLa0.7Ca0.3MnO

Abstract

We have used scanning tunneling microscopy and potentiometry to study the microstructure and nanoscopic current transport in thin films of La 0.7 Ca 0.3 MnO 3 grown on SrTiO 3 and NdGaO 3 substrates. Thin ͑50 nm͒ films, which are strained, show step-terrace growth, whereas relatively thicker film ͑200 nm͒, which is strain relaxed, shows well-connected grains. Charge transport in these films is inhomogeneous on the scale of nanometers. There are large variations in the local potential at grain boundaries as well as step edges. The value of the average field distribution at grain boundaries and step edges is found to be dependent on the strain in the film. Within the terraces local variations in potentials correlate with the nature of the strain in the film and this gives rise to inhomogeneous current flow in them.Electronic transport in rare-earth manganites which show colossal magnetoresistance ͑CMR͒ has been a topic of much current interest. 1,2 In these materials the resistivity () shows a peak at a temperature T p which is close to the ferromagnetic transition temperature T c . For a given chemical composition and oxygen stoichiometry the values of T p , T c , and depend on the physical state of the sample. This is particularly true for thin films. For instance, films of the same CMR material grown epitaxially on different substrates can have very different , T p , and T c and, consequently, magnetoresistance ͑MR͒. It has been proposed that the presence of biaxial strain ⑀ bi stabilizes the Jahn-Teller distortion around the Mn 3ϩ ions and reduces T p and increases . 3 The effect of ⑀ bi is expected to be important in thin films of manganites. Recent experiments have established this. 4 However, strain is not the only factor. In thin films of CMR materials it is likely that the strain and microstructure together determine the nature of the charge transport. While techniques like x-ray diffraction ͑XRD͒, which measure lattice constants averaged over a large length scale and can characterize the strain as ''uniform,'' however, in mesoscopic or submicron length scales the strain can be inhomogeneous. The film will also contain internal boundaries, discontinuities, and steps. It is therfore expected that the electrical conduction in these materials will be inhomogeneous on submicron length scales. As a result an average parameter like ⑀ bi is not enough to describe the current transport in these films. In this paper we address specifically these issues. By using a combination of macroscopic and nanoscopic techniques, we can identify that the inhomogeneities seen in the current path depend on the strain in the film.͑001͒-oriented La 0.7 Ca 0.3 MnO 3 ͑LCMO͒ films were grown using pulsed laser deposition ͑PLD͒ on SrTiO 3 ͑STO͒ and NdGaO 3 ͑NGO͒ substrates. 5 Three types of films were studied: namely, 50-nm-thick film grown on STO ͓LCMO/ STO͑50͔͒, 200-nm-thick film grown on STO ͓LCMO/ STO͑200͔͒ and 50-nm-thick film grown on NGO ͓LCMO/ NGO͑50͔͒. In Table I, we give some of the relevant details of the samples used. We ha...

Thickness dependent transport properties of compressively strained La0.88Sr0.12MnO3 ultrathin films

“…Therefore, the clarification of the detailed role of biaxial strain is essential and has been recently addressed in several experimental studies. [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] Strain has also been found to significantly affect the conduction noise properties. 36 Among the manganites, the divalent-doped La 1−x Sr x MnO 3 ͑0.1Ͻ x Ͻ 0.5͒, because of its large one electron bandwidth, is of great interest.…”

Exotic magnetic anisotropy map in epitaxial La0.7Ca0.3MnO3films

“…Therefore, conventional phase segregation as that observed in many LCMO systems (see for instance Ref. 33 on phase segregation of LCMO on STO and NGO) cannot be at the root of the observed behavior in LCMO/BTO. Certainly, the observed magnetic behavior results from an inhomogeneous structural scenario within the epitaxial film, and since inhomogeneous strains can result in different magnetic interactions; our magnetically granular system could be considered as a kind of phase-segregated system only in a very broad sense.…”

“…This is in contrast to what is observed in LCMO/STO, a system for which a canonical PS scenario is ascribed. 6,33 …”

Anisotropic behaviour in the magnetic field dependence of the low temperature electrical resistance of calcium-doped lanthanum manganate thin films grown by RF magnetron sputtering

“…In the case of the film grown on LaAlO 3 , only (h00) pseudo-cubic reflections were observed in the X-ray diffraction pattern. [23] and epitaxial thin films [8,24,25], whereas the low temperature resistivity of the film grown on MgO is two orders of magnitude larger. This suggests that the contribution of possible small-angle grain boundaries [26] or microstructural defects [24] to the overall electrical resistance of the film grown on LaAlO 3 remains limited.…”