Thickness dependent magnetotransport in ultra-thin manganite films

To understand the near-interface magnetism in manganites, uniform, ultrathin films of La0.67Sr0.33MnO3 were grown epitaxially on single crystal (001) LaAlO3 and (110) NdGaO3 substrates. The temperature and magnetic field dependent film resistance is used to probe the film’s structural and magnetic properties. A surface and/or interface related dead-layer is inferred from the thickness dependent resistance and magnetoresistance. The total thickness of the dead layer is estimated to be ∼ 30 Å for films on NdGaO3 and ∼ 50 Å for films on LaAlO3. PACS numbers: 75.70.-i, 73.50.Yg, 68.55.Jk, 75.50 Typeset using REVTEX 1 One issue in manganite trilayer junction is the premature disappearance of magnetoresistance (MR) upon temperature increase. The MR in La0.67Sr0.33MnO3 based trilayer junctions disappears above 150K, well below the electrode’s Curie temperature of 360 K [1–7]. Recently, spin-resolved photoemission measured the spin-polarization of surface electronic density-of-state of epitaxial thin films of La0.67Sr0.33MnO3 [8,9]. It revealed, as a function of increasing temperature, a more rapid decrease of the surface spin polarization than the film’s overall magnetization. Both transport and photoemission experiments suggest the possible existence of a surface dead-layer with depressed magnetic order at elevated temperatures. Questions remain as to the depth of this dead-layer, and its physical origin. In this paper, we experimentally establish some estimates about the dead-layer thickness. The approach is to study the thickness dependent electrical transport, including resistance and magnetoresistance, in ultra-thin epitaxial films of La0.67Sr0.33MnO3 (LSMO). To estimate the effect of lattice strain, films on two types of substrates are compared — those grown on (001) LaAlO3(LAO) and on (110) NdGaO3 (NGO). The film thickness range covered by this study is between 15 Å and 240 Å. Films were grown epitaxially using laser ablation from a stoichiometric La0.67Sr0.33MnO3 target. A Nd-YAG laser was used for ablation, operating in frequency-tripled mode at 355 nm, with a pulse energy of 140mJ/pulse at 10Hz repetition. The deposition rate was about 0.6 Å/ sec. A series of films were made in the thickness range between 15 Å and 240 Å. Two substrates, one LAO and one NGO, each 1 cm×2mm×0.5mm in size, were loaded sideby-side for simultaneous deposition at each thickness. The substrate holder’s temperature during growth was 750C, and the growth ambient consisted of 300mτ of oxygen at a flow rate of 50 sccm. The films were subsequently coated with four in-line contact pads, 1 mm diameter and 2.2 mm apart from each other, for transport measurement. These contact pads, usually silver or gold, about 1000 Å thick, were sputter deposited through a stencil mask. The four-point resistance thus measured was assumed to be the resistance-square R2. The film thickness was deduced from deposition time normalized to calibration runs.