Magnetism of perovskite oxides: The effect of strain and phase separation

The magnetic properties of perovskite oxides can be affected by various conditions such as doping concentration, finite size limitation, and mechanical strain, which are associated with a range of intriguing physical phenomena in highly correlated electron systems such as colossal magnetoresistance, high temperature superconductivity, and phase inhomogeneities. In this thesis, we studied several topics concerning the cobaltates and nickelates which are associated with magnetism in perovskite oxides. ^ La0.5Sr0.5CoO3 is a ferromagnetic material with Curie temperature TC of 250 K. In a form of thin films, we studied strain effect on its ferromagnetism. However, ferromagnetism in thin films is affected by both finite size effect and strain effect. We have used a series of films of different thicknesses and on different substrates to quantitatively determine the change in TC contributed by each effect. The phase diagram of TC versus in-plane strain suggests that TC is suppressed by tensile strain and enhanced by compressive strain. The general method of separating strain and finite thickness effects should be applicable to any ordering phase transition in thin films. The local structure of LSCO thin films was investigated by Extended X-ray Absorption Fine Structure technique. Our results suggest that the tensile strain elongates the Co-O bond length, while compressive strain shortens the bond length. The change of bond length is mainly responsible for the modulation of TC upon strain. This is contrary to assumptions generally used in literature on strained manganite films. Current double exchange model is not adequate to describe the ferromagnetic mechanism for cobaltate. ^ In a case of no La, we studied the magnetic properties of SrCoO 2.5+x. SrCoO2.5 is an antiferromaget with Néel temperature of 570 K. With a starting material of SrCoO 2.88, we have carried out oxidization and reduction experiments through an electrochemical method. The hole doping introduced into SrCoO2.5 by oxygen has shown to play a crucial role in determining the materials\u27 ferromagnetic characters such as Curie temperature, spin state, magnetic saturation moment, etc. A magnetic phase diagram of SrCoO2.5+x is thus proposed for the first time. The striking feature of the diagram is the existence of several line phases which contain antiferromagnetic and ferromagnetic phases. In high contrast with La1-ySryCoO 3, SrCoO2.5+x can be phase separated into a number of magnetic regions for which each one has a unique oxygen/hole concentration such as x = 0, 0.25, 0.375, and 0.5, respectively. Our results suggest that the mobile holes are playing a key role in ferromagnetism and phase separation in the system. ^ For perovskite nickelate, we have successfully grown high-quality epitaxial La1.67Sr0.33NiO4 films using pulsed laser deposition. For the first time, the x-ray diffraction superlattice peaks associated with charge stripe phase have been successfully observed in films. By studying the evolution of the stripe phase as the film thicknesses are decreased, we provide direct evidence for suppression of the stripe phase in thinner samples with thicknesses of less than 2600 Å. A scenario of short-range ordered stripes due to a lattice clamping effect is proposed to account for the suppression in light of a model of electronic stripe-glass.