We have used high-resolution Extended X-ray Absorption Fine-Structure and diffraction techniques to measure the local structure of strained La 0.5 Sr 0.5 CoO 3 films under compression and tension. The lattice mismatch strain in these compounds affects both the bond lengths and the bond angles, though the larger effect on the bandwidth is due to the bond length changes. The popular double exchange model for ferromagnetism in these compounds provides a correct qualitative description of the changes in Curie temperature T C , but quantitatively underestimates the changes. A microscopic model for ferromagnetism that provides a much stronger dependence on the structural distortions is needed.Epitaxial strain in thin films is often used to modify a material's physical properties and improve device performance. For example, biaxial strain can introduce bondlength and bond-angle distortions in semiconductor alloys, [1,2,3,4]which greatly affect their performance in real applications. Room temperature ferroelectricity has been induced by lattice strain in SrTiO 3 thin films, a material that is not ferroelectric in the bulk [5,6]. Enhanced magnetoresistance has been achieved in La 0.8 Ba 0.2 MnO 3 thin films at room temperature [7], which makes it a potential candidate for magnetic devices and sensors. The modification of physical properties using strain is also an important tool for understanding the physics of correlated electron materials. One longstanding question in the field is the origin of ferromagnetism in several poorly conducting transition-metal oxides. The most popular model has been Zener's double exchange mechanism (DE) [8]. In this paper we report results comparing the Curie Temperature with detailed structural measurements in strained films of La 0.5 Sr 0.5 CoO 3 (LSCO). While the predictions appear qualitatively correct, they do not quantitatively predict the correct dependence on lattice parameter and, therefore, bandwidth. Thus, either another mechanism or a modification to DE is needed.The perovskite, transition-metal oxide that has been most studied as a function of strain is the colossal magnetoresistive manganites [9]. The strain has been induced in several ways in manganites including films. The analysis of these experiments has examined how strain has mediated the ferromagnetic coupling via modification of the bandwidth W [8,10]. In the tight-binding model, the bandwidth W depends on the overlap integrals between the Mn 3d and O 2p orbitals such that W ∝ d −3.5 cos ω [11,12], where d is Mn-O bond length, ω = (180 • − φ)/2 is the tilt angle, and φ is Mn-O-Mn bond buckling. A decrease in the lanthanide ion radius by chemical substitution leads to a reduction of T C that has been attributed to an increase of the Mn-O-Mn bond angle with little change in the Mn-O bond length [13,14]. The opposite case is that compressive hydrostatic pressure increases T C due to a reduction in Mn-O bond length with little decrease in Mn-O-Mn bond angle [15]. However, for film studies no full consensus has been reached...

This work presents a study of the electronic phase separation resulting from oxygen non-stoichiometry in SrCoOx. We report here that for oxygen content 2.88 ≤ x ≤ 3, SrCoOx exhibits a magnetic phase separation while maintaining a single crystallographic phase. Two magnetic components are formed which match those found in SrCoO2.88 and SrCoO3 with TC = 220 K and 280 K, respectively. In addition, a value of TC = 160 K is assigned to the previously identified SrCoO2.75 phase. A magnetic phase diagram with four line phases is proposed for SrCoOx (2.5 ≤ x ≤ 3).

The ferromagnetic properties of epitaxial La0.5Sr0.5CoO3 thin films have been studied. The magnetic transition is affected by both strain and finite thickness. We have used a series of films of different thickness and on different substrates in order to quantitatively determine the change in Curie temperature contributed by each effect. The phase diagram of TC versus in-plane strain suggests that the ferromagnetic transition temperature 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.Thin films of perovskite oxide materials have attracted great attention lately due to their potential technological applications based on a variety of appealing physical properties, such as colossal magnetoresistivity, ferroelectricity, and high-Tc superconductivity. The properties of films differ from the corresponding bulk typically due to a combination of three factors. Firstly, defect levels are often higher in films. Oxygen deficiency is the most common defect and will typically suppress the transition temperature due to the decrease of doped hole density or the destruction of metal-oxygen hopping pathways.[1] Secondly, finite size effects may be important. For example, the Curie temperature (T C ) for a ferromagnetic thin film will be reduced when the spin-spin correlation length exceeds the film thickness. The thickness-dependent Curie temperature has been most carefully studied in simple metallic films of Fe, Co, Ni and Gd. [2, 3] A similar scaling effect has also been found in ferroelectric materials. [4] Thirdly, strain incorporated into films due to effects such as a lattice mismatch with the substrate may also alter the phase transition through changes in fundamental interactions that depend upon atomic spacing. Strain in thin films is often thought of as analogous to that induced in high-pressure experiments on bulk materials. However, a much larger strain can be achieved in films than that in bulk and the strain in films is usually biaxial rather than hydrostatic or uniaxial as in most bulk pressure experiments. The induced strain can modify the lattice structure, the critical temperature for phase transition, and sometimes the nature of the phases present themselves. [5,6,7] La 0.5 Sr 0.5 CoO 3 (LSCO) is a highly doped ferromagnetic oxide material with perovskite structure and has desirable properties of high electrical conductivity and large magnetoresistance. [8,9] Thin film LSCO is a candidate for applications such as electrodes for fuel cells, ferroelectric memory and spin valve devices. The first concern noted above, the oxygen content in LSCO films, can be controlled by carefully optimizing growth conditions so that stoichiometry of oxygen can be maintained. [10] However, the finite size and strain effects are intrinsic to the film, and it is not trivial to separate the influence of these two effects on a particular film. Recently, Fuchs et al [11] and Andre...

No abstract

We carried out detailed high-pressure Raman scattering studies on lead-free perovskite piezoceramics KNbO3 (KNO) with a smaller pressure interval so that the successive phase transition pressures are precisely determined (PO→T: 8.2 GPa; PT→C: 10.9 GPa). The pressure-quenched phase does not return to the ambient orthorhombic phase completely. In this work, the pressure-induced phase transition sequence is discussed in detail. Also, based on the pressure-dependent Raman spectra of KNO, we have discovered clear evidence of a pressure-induced Fano resonance. It reveals the role played by the Raman-active continuum and discrete energy states in the condensed matter system under high pressure and enables the high-pressure Fano resonances engineering in arbitrary geometries [q∈(−∞, 0)]. These findings highlight the opportunity for designing and tuning the wave transmission and optical switch properties of lead-free piezoceramics KNO via high pressure.