Biomedical subjects

K Samwer

Publications and source records attributed to K Samwer.

11 recordsLinked to original sources

PubMed2006

A-site ordering versus electronic inhomogeneity in colossally magnetoresistive manganite films.

Epitaxial La(3/4)Ca(1/4)MnO3/MgO(100) (LCMO) thin film shows an unusual rhombohedral (R-3c) structure with a new perovskite superstructure at room temperature due to the CE-type ordering of La and Ca with modulation vector q=1/4[011]. A-site ordered film was found to be electronically homogeneous down to the 1 nm scale as revealed by scanning tunnelling microscopy/spectroscopy. In contrast, orthorhombic and A-site disordered LCMO demonstrate a mesoscopic phase separation far below the Curie temperature (TC). Unique La/Ca ordering compensates the cation mismatch stress within one supercell, a(S) approximately 1.55 nm, and enhances the electronic homogeneity. The phase separation does not seem to be a unique mechanism for the colossal magnetoresistance (CMR) as very large CMR approximately 500% was also observed in A-site ordered films.

Journal Article↗

PubMed2005

A universal criterion for plastic yielding of metallic glasses with a (T/Tg) 2/3 temperature dependence.

Room temperature (TR) elastic constants and compressive yield strengths of approximately 30 metallic glasses reveal an average shear limit gammaC=0.0267+/-0.0020, where tauY=gamma CG is the maximum resolved shear stress at yielding, and G the shear modulus. The gammaC values for individual glasses are correlated with t=TR/Tg , and gamma C for a single glass follows the same correlation (vs t=T/Tg). A cooperative shear model, inspired by Frenkel's analysis of the shear strength of solids, is proposed. Using a scaling analysis leads to a universal law tauCT/G=gammaC0-gammaC1(t)2/3 for the flow stress at finite T where gammaC0=(0.036+/-0.002) and gammaC1=(0.016+/-0.002).

Journal Article↗

PubMed2003

Structural phase transition at the percolation threshold in epitaxial (La0.7Ca0.3MnO3)1-x:(MgO)x nanocomposite films.

'Colossal magnetoresistance' in perovskite manganites such as La0.7Ca0.3MnO3 (LCMO), is caused by the interplay of ferro-paramagnetic, metal-insulator and structural phase transitions. Moreover, different electronic phases can coexist on a very fine scale resulting in percolative electron transport. Here we report on (LCMO)1-x:(MgO)x (0 < x < or = 0.8) epitaxial nano-composite films in which the structure and magnetotransport properties of the manganite nanoclusters can be tuned by the tensile stress originating from the MgO second phase. With increasing x, the lattice of LCMO was found to expand, yielding a bulk tensile strain. The largest colossal magnetoresistance of 10(5)% was observed at the percolation threshold in the conductivity at xc 0.3, which is coupled to a structural phase transition from orthorhombic (0 < x < or 0.1) to rhombohedral R3c structure (0.33 < or = x < or = 0.8). An increase of the Curie temperature for the Rc phase was observed. These results may provide a general method for controlling the magnetotransport properties of manganite-based composite films by appropriate choice of the second phase.

Calcium Compounds↗

PubMed2002

Intrinsic inhomogeneities in manganite thin films investigated with scanning tunneling spectroscopy.

Thin films of La0.7Sr0.3MnO3 on MgO show a metal insulator transition and colossal magnetoresistance. The shape of this transition can be explained by intrinsic spatial inhomogeneities, which give rise to a domain structure of conducting and insulating domains at the submicrometer scale. These domains then undergo a percolation transition. The tunneling conductance and tunneling gap measured by scanning tunneling spectroscopy were used to distinguish and visualize these domains.

Journal Article↗

PubMed2001

Model for intrinsic stress formation in amorphous thin films.

Metallic amorphous thin films evaporated on a substrate can be characterized by different growth regimes in dependence of the film thickness concerning surface morphology and intrinsic film stresses, independent of the details of the applied material systems. Here, a model is presented to link the surface topography and characteristic surface measures with the observed film stresses. This allows quantitative prediction of stresses in dependence of film preparation parameters for a tailored film production.

Journal Article↗

PubMed1995