Magnetic Measurements on Stressed and Stress Relieved La 0.66 Ca 0.33 MnO 3 Thin Films
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ABSTRACT In general, epitaxially grown thin films are biaxially stressed if the lattice mismatch of substrate and film is below 2%. We have used epitaxial strain as an extrinsic source of tensile stress exerted to Lao.66 Cao.33MnO 3 thin films deposited on SrTiO 3 single crystal substrates. The average stress in the film is a function of film thickness and post deposition annealing. Thickness variation and annealing procedures have been used to explore the stress dependence of the characterisics of magnetization curves of the stressed films. It could be shown that the inflection point of the magnetization curve which determines the transition from Blochwall movements to rotations of the magnetization vector and the Rayleigh constant of the virgin magnetization curve are correlated with the stress of the films in analogy to results obtained for plastically deformed single crystals of conventional ferromagnetic metals. INTRODUCTION The discovery of high temperature superconductivity in hole doped copper oxide compounds has generated renewed interest in doped Mott insulators with strong electron correlation. Consequently, the electronic properties of doped rare earth manganites in perovskite structure of the type RE1-.AMnO 3 [ RE = La, Pr, Nd, A = Ca, Sr ] have been revisited [1,2,3]. The magnetic field driven shift of a first-order metal-insulator transition in a temperature window around the ferromagnetic ordering of the Mn spins causes a colossal negative magnetoresistance (CMR),which can exceed I0W [4]. From a fundamental point of view the underlying physical processes leading to the metal-insulator transition and the magnetic ordering are still not fully understood. The exploration of methods in order to tailor properties such as tuning the temperature window for the magnetic ordering and the enhancement of the magnetic field sensitivity for low magnetic fields remains a challenge. Currently, doped LaMnO 3 is being reexamined as a possible next generation magnetoresistance sensor material [5]. The CMR materials have basically a perovskite-type crystal structure which can be rhombohedrically or orthorhombically distorted. They show a rich variety of magnetic ordering which is influenced by a cooperative tilting of the MnO 6 octahedra due to the ion-size mismatch of the A-site cations and the Jahn-Teller distortion as well. The LaCaMnO 3 system, e.g. , shows ferromagnetic ordering in the Mn-O layers of the a-b planes and a doping dependent antiferromagnetic (x < 0.2 and x > 0.5) or ferromagnetic (0.2 < x < 0.5) ordering along the c-axis. The first-order metal-insulator phase transition associated with the magnetic ordering can be shifted in a wide temperature range by doping. The valence state conversion of the Mn ions from Mn' to Mn"+ by a partial substitution of the trivalent RE' by the divalent A2' leads to a mixed valence system as already proposed by Jonker and van Santen [6]. The striking correlation between ferromagnetism and metallic conductivity is due to the coupled dynamics of charge and spin essentially
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