M. A. Torija

Magnetocaloric effect and refrigerant capacity in charge-ordered manganites

The influence of first- and second-order magnetic phase transitions on the magnetocaloric effect (MCE) and refrigerant capacity (RC) of charge-ordered Pr0.5Sr0.5MnO3 has been investigated. The system undergoes a paramagnetic to ferromagnetic transition at TC∼255 K followed by a ferromagnetic charge-disordered to antiferromagnetic charge-ordered transition at TCO∼165 K. While the first-order magnetic transition (FOMT) at TCO induces a larger MCE (6.8 J/kg K) limited to a narrower temperature range resulting in a smaller RC (168 J/kg), the second-order magnetic transition at TC induces a smaller MCE (3.2 J/kg K) but spreads over a broader temperature range resulting in a larger RC (215 J/kg). In addition, large magnetic and thermal hysteretic losses associated with the FOMT below TCO are detrimental to an efficient magnetic RC, whereas these effects are negligible below TC because of the second-order nature of this transition. These results are of practical importance in assessing the usefulness of charge-o...

Atomic-Resolution Imaging of Spin-State Superlattices in Nanopockets within Cobaltite Thin Films

Certain cobalt oxides are known to exhibit ordered Co spin states, as determined from macroscopic techniques. Here we report real-space atomic-resolution imaging of Co spin-state ordering in nanopockets of La(0.5)Sr(0.5)CoO(3-δ) thin films. Unlike the bulk material, where no Co spin-state ordering is found, thin films present a strain-induced domain structure due to oxygen vacancy ordering, inside of which some nanometer sized domains show high-spin Co ions in the planes containing O vacancies and low-spin Co ions in the stoichiometric planes. First-principles calculations provide support for this interpretation.

Lattice mismatch accommodation via oxygen vacancy ordering in epitaxial La0.5Sr0.5CoO3-δ thin films

The properties of complex oxide films depend sensitively on epitaxial strain. This strain affects bond lengths and angles, and defect types and densities, thus impacting physical properties. In this work we perform detailed characterization of depth-dependent strain in epitaxial La0.5Sr0.5CoO3-δ (LSCO) films on SrTiO3(001), SrTiO3(110), and LaAlO3(001) substrates, combining high resolution x-ray diffraction and scanning transmission electron microscopy, in addition to geometric phase analysis. We elucidate a fundamental link between strain state and O vacancy ordering in LSCO films, where lattice mismatch and crystallographic orientation can be used to manipulate the modulation vector of the long-range vacancy order, thus providing a new approach to tailor the properties of such films.

Epitaxial La0.5Sr0.5CoO3 thin films: Structure, magnetism, and transport

La1−xSrxCoO3 has received considerable attention in bulk form. This is due to interest in the fundamental magnetic properties (spin-state transitions and magnetic phase separation) as well as potential applications in ferroelectric memory and solid-oxide fuel cells. The structure and properties in thin film form are not well understood, and the influence of dimensional confinement on effects such as magnetic phase separation is unknown. Here, we report a comprehensive investigation of structure, magnetism, and transport in strained epitaxial La0.5Sr0.5CoO3 (001) films deposited on SrTiO3 (001) substrates by reactive dc magnetron sputtering. The crystalline quality, phase purity, strain state, oxygen stoichiometry, morphology, and magnetic and electronic properties of the epilayers are all probed and are found to be particularly sensitive to the total sputtering gas pressure and the ratio of reactive to inert gas (PO2/PAr). The various structure-property relationships are discussed in detail, particularly ...

Chemically Driven Nanoscopic Magnetic Phase Separation at the SrTiO 3 (001)/La 1− x Sr x CoO 3 Interface

The degradation in magnetic properties in very thin film complex oxides is studied using SrTiO(3)(001)/La(1-x)Sr(x)CoO(3), providing unequivocal evidence for nanoscopic interfacial magnetic phase separation. Electron microscopy and spectroscopy reveal that this occurs due to inhomogeneity in local hole doping, driven by subtle, depthwise variations in the Sr and O stoichiometry. Simple thermodynamic and structural arguments for the origin of these variations are provided.

Magnetocaloric effect and critical behavior in Pr0.5Sr0.5MnO3: an analysis of the validity of the Maxwell relation and the nature of the phase transitions.

The Maxwell relation, the Clausius-Clapeyron equation, and a non-iterative method to obtain the critical exponents have been used to characterize the magnetocaloric effect (MCE) and the nature of the phase transitions in Pr0.5Sr0.5MnO3, which undergoes a second-order paramagnetic to ferromagnetic (PM-FM) transition at TC ~ 247 K, and a first-order ferromagnetic to antiferromagnetic (FM-AFM) transition at TN ~ 165 K. We find that around the second-order PM-FM transition, the MCE (as represented by the magnetic entropy change, ΔSM) can be precisely determined from magnetization measurements using the Maxwell relation. However, around the first-order FM-AFM transition, values of ΔSM calculated with the Maxwell relation deviate significantly from those calculated by the Clausius-Clapeyron equation at the magnetic field and temperature ranges where a conversion between the AFM and FM phases occurs. A detailed analysis of the critical exponents of the second-order PM-FM transition allows us to correlate the short-range type magnetic interactions with the MCE. Using the Arrott-Noakes equation of state with the appropriate values of the critical exponents, the field- and temperature-dependent magnetization [Formula: see text] curves, and hence the [Formula: see text] curves, have been simulated and compared with experimental data. A good agreement between the experimental and simulated data has been found in the vicinity of the Curie temperature TC, but a noticeable discrepancy is present for [Formula: see text]. This discrepancy arises mainly from the coexistence of AFM and FM phases and the presence of ferromagnetic clusters in the AFM matrix.

Large amplitude oscillatory shear of block copolymer spheres on a body-centered cubic lattice: are micelles like metals?

Small-angle X-ray diffraction experiments have uncovered a remarkable mechanism of grain alignment during plastic deformation of ordered sphere-forming diblock copolymer micelles when subjected to large amplitude dynamic shearing. A nearly monodisperse poly(styrene-b-ethylene-alt-propylene) (SEP) diblock copolymer with block molecular weights of 42,000 and 60,000 was mixed with squalane (C(30)H(62)), an EP selective solvent, at a concentration of 10 wt%. After high temperature annealing, the sample formed an ordered polydomain morphology containing glassy S cores at room temperature. SAXS powder patterns confirm body-centered cubic (BCC) symmetry and reveal the development of a complex array of two-dimensionally resolved Bragg reflections following the application, and cessation, of oscillatory shearing. These diffraction results are interpreted on the basis of the classic mechanism of crystalline slip, which accounts for plastic deformation of ductile materials such as metals. Four distinct slip systems are shown to be active in this work, suggesting a robust basis for deforming and mixing of soft ordered solids.

Applications of aberration corrected scanning transmission electron microscopy and electron energy loss spectroscopy to thin oxide films and interfaces

Abstract Aberration correction in the scanning transmission electron microscope allows spatial resolutions of the order of one Ångström to be routinely achieved. When combined with electron energy loss spectroscopy, it is possible to simultaneously map the structure, the chemistry and even the electronic properties of materials in one single experiment. Here we will apply these techniques to the characterization of thin films and interfaces based on complex oxides with the perovskite structure. The relatively large lattice parameter of these materials combined with the fact that most of them have absorption edges within the reach of the spectrometer optics makes these materials ideal for these experiments. We will show how it is possible to map the chemistry of interfaces atomic plane by atomic plane, including light elements such as O. Applications to cobaltite and titanate thin films will be described.

Aberration Corrected STEM-EELS: Applications to Magnetic Materials

The macroscopic properties of magnetic materials, thin films and nanosystems are intimately related to their atomic structure and the presence of defects, interfaces, etc. Electron energy loss spectroscopy (EELS) in the aberration corrected scanning transmission electron microscope (STEM) allows simultaneous exploration of structure, chemistry and electronic properties in real space with atomic resolution. Furthermore, derived techniques such as electron magnetic chiral dichroism allow studies of magnetization in real space [1]. These techniques combine to provide a unique tool to understand systems such as the La0.7Sr0.3MnO3/SrTiO3 (LSMO/STO) ferromagnetic/insulating superlattices where unexpected behaviors such as a magnetic moment on Ti atoms have been measured. EELS images show that charge transfer takes place due to the presence of an extra interfacial La0.7Sr03-O plane [2]. In this talk we will review applications of STEM-EELS to such systems and show how interface charge transfer explains the observed ferromagnetism. Other examples to be described include the sensitivity of the EELS fine structure to the spin state of atoms [3]. For example, Co ions in A1-xBxCoO3 perovskites (where A and B are divalent/trivalent elements) exhibit a competition between the crystal field splitting and Hund’s-rule exchange energy in the 3d states, which determines the spin state of the individual Co ions. These materials provide an ideal test bed to explore how environmental effects, such as O vacancies or impurities, affect the resulting Co spin state. The ordering of O vacancies along with the presence of epitaxial strain in these compounds can stabilize a superlattice in the Co spin state which can be measured by EELS [4]. This example shows how column-by-column spectroscopy offers the possibility of atomic resolution mapping of spin states.