Study of electrical and magnetic properties of LCSMO thin films

Abstract

Perovskite-type manganese oxides R1-xAxMnO3 (where R and A are trivalent rare-earth and divalent alkaline-earth ions, respectively) exhibit a colossal magnetoresistance (CMR) effect. Such an effect has attracted considerable interest from both the fundamental and practical application points of view. Theoretical and experimental studies have shown that the spin, charge, and lattice degrees of freedom in R1-xAxMnO3 are strongly correlated, which means that one of the degrees of freedom can be controlled by other degrees of freedom via the coupling among them. Therefore, the CMR property of these perovskites can be varied to a great extent by lattice strain and cation doping. The La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 compounds are typical manganites that show metal-insulator and ferromagnetic-paramagnetic phase transitions at the transition temperatures Tc of 382 and 260 K, respectively. They have a perovskite structure with lattice constants (pseudocubic with a~b~c~3.88 A) that are similar to those of (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 single crystal (a~b~c~4.02 A). By depositing La0.7(Ca1-xSrx)0.3MnO3 (LCSMO) thin films with different compositions on piezoelectric 70Pb(Mg1/3Nb2/3)O3-30PbTiO3 (PMN-PT) substrate, Tc can be tuned. In this thesis, we have successfully grown LCSMO thin films with different thicknesses and compositions (x=0, 0.25, 0.5, 0.75 and 1) in LCSMO/PMN-PT and LCSMO/PMN-PT/Terfenol-D heterostructures. Influence of strain induced by converse piezoelectric effect on the electrical resistance of the manganite thin films has been studied. It has been found that after the PMN-PT substrate has been polarized, the lattice strains in the PMN-PT substrate and the LCSMO can be continuously modulated by applying a dc or ac electric field across the PMN-PT substrate. As a result, the resistance of the LCSMO film is modulated due to the strong coupling between the charge and lattice degrees of freedom in the film. These results indicate that the strain induced by the converse piezoelectric effect in the PMN-PT substrates have been effectively transmitted to the film. The induced lattice strain in the films leads to a change in their electrical properties. Moreover, we have found that the ferroelectric field effect in the LSMO/PMN-PT and LCMO/PMN-PT is negligibly small compared to the lattice strain effect. The transition temperatures of these thin films vary in a wide range from 70 K to as high as about 350 K and Tc of each sample can be tuned by up to 10 K upon application of a dc electric field of 10 kV/cm across the PMN-PT substrate. Besides, we have also studied magnetic field induced strain effect on the electrical property of LCMO thin film by constructing LCMO/PMN-PT/Terfenol-D heterostructure. The strain is generated by application of magnetic field to the magnetostrictive Terfenol-D and then transferred to the thin film layer across the substrate. Resistance of the LCMO films was measured before and after bonding Terfenol-D to the PMN-PT substrate. It has been found that the resistance of the LCMO thin film decreases and the Curie temperature increases after bonding the Terfenol-D to the PMN-PT substrate. Considering the thermal expansion mismatch between Terfenol-D and PMN-PT, this change is probably caused by contraction in the structure introduced during subsequent cooling after curing of the silver epoxy. The evolution of magnetic domain patterns with temperature in LSMO thin film on (110) NdGaO3 (NGO) single crystal substrate has been characterized by a magnetic force microscope (MFM). Highly ordered stripe ferromagnetic (FM) domain patterns are observed and proved to originate from anisotropic stress in the film plane. The FM domain contrast is found to continuously decrease upon warming until the transition temperature has been reached, at which the patterns almost completely vanish. The evolution of magnetic domain patterns coincides with resistivity change with temperature. This correspondence confirms the role of carrier scattering by thermal spin fluctuation in determining the transport property in CMR manganites.