Strain-Induced Magnetic Properties of Pr 0.67 Sr 0.33 MnO 3 Thin Films
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approximately cubic with lattice constant a=3.79 A and 3.90A respectively. Bulk PSMO is orthorhombic with a/142 = 3.879A, b/I-f 3.866A, and c/2 = 3.856A [9]. We reference our strain analysis to a pseudo-cubic cell of 3.866A. A fully strained PSMO / LAO film would then have a lattice compression -2.0% in the film plane, with an elongated out-of-plane c-axis determined by the film elastic constants, while the PSMO/STO films would have a 0.8% tensile strain in-plane, with a compressed c-axis. The PSMO films were grown epitaxially by pulsed laser deposition (PLD) [10]. The energy density and the repetition rate of the laser were 2 J / cm2 and 5 Hz.. The films were grown at 750-800'C in 0.75 mbar of 02. After deposition, the films were cooled to room temperature in 1 atmosphere of 02. Films of 10 nm thickness were used to minimize defect-induced strain relaxation. The c-axis lattice constants for 10nm film were determined by x-ray diffraction to be 3.92 A (e= = -1.4%) for LAO substrates, and 3.81/A (E= = 1.4%) for STO substrates. For thicker films, the c-axis relaxes back toward the bulk value. The in-plane lattice constant of PSMO / STO was determined from the 103 reflection to be 3.90A (e. = -0.8%), which indicates that the thin film grown on STO is fully strained. For fully strained films, ee /e= = -2v/(1 - v), where v is the poisson's ratio. For a volume-preserving distortion, eI/ex = -2, or v = 1/2. In both fully strained La0 67Ca 0.33MnO 3 and La0.8Ca 0 2MnO 3 films, the e- /e, ratio is approximately -1.25 (v = 0.38) [7,11]. For strained La 0 .67Sr0 .33MnO 3 (LSMO) film, ez/ez has been reported to be 1.42 (v = 0.42)[12]. For the fully strained PSMO / STO film in our study, the ez/e. ratio is 1.75 (v = 0.47), slightly larger than either LCMO or LSMO films. If the PSMO / LAO films were fully strained, the in-plane strain would be ef = 2.0%, and the out-of plane efZUlcould be estimated to be 3.5% from the ez/e. ratio. The much smaller measured c-axis strain suggests that the film grown on LAO has relaxed even at a 10nm thickness, which is consistent with the observation of LCMO / LAO [11]. The in-plane lattice constant of PSMO / LAO film was difficult to determine from the T03 peak due to the peak broadening. We have estimated the in-plane lattice constant to be 3.83A (e& = 0.9%) based on our measured c-axis lattice constant and a thicknessdependence study of lattice constants of Lao.gCao. 2MnO 3 films grown on LAO substrate [11 ]. Magnetization measurements of 10nm thick PSMO films were made in a SQUID magnetometer. The diamagnetic signal from the substrate was minimized by mechanically thinning the substrate to approximately 0.3mm. For accurate background correction, the susceptibility of blank substrates was measured at different temperatures. Although the LAO substrate showed a slight temperature dependence at low temperature, for T>20K the specific susceptibility had a temperature-independent value of -1.95x10- emulgm-Oe. The susceptibility of the STO substrate showed a temperature-independent value of -1x10" em
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