Biomedical subjects

S E Lofland

Publications and source records attributed to S E Lofland.

6 recordsLinked to original sources

PubMed2004

On the origin of high-temperature ferromagnetism in the low-temperature-processed Mn-Zn-O system.

The recent discovery of ferromagnetism above room temperature in low-temperature-processed MnO(2)-ZnO has generated significant interest. Using suitably designed bulk and thin-film studies, we demonstrate that the ferromagnetism in this system originates in a metastable phase rather than by carrier-induced interaction between separated Mn atoms in ZnO. The ferromagnetism persists up to approximately 980 K, and further heating transforms the metastable phase and kills the ferromagnetism. By studying the interface diffusion and reaction between thin-film bilayers of Mn and Zn oxides, we show that a uniform solution of Mn in ZnO does not form under low-temperature processing. Instead, a metastable ferromagnetic phase develops by Zn diffusion into the Mn oxide. Direct low-temperature film growth of Zn-incorporated Mn oxide by pulsed laser deposition shows ferromagnetism at low Zn concentration for an optimum oxygen growth pressure. Our results strongly suggest that the observed ferromagnetic phase is oxygen-vacancy-stabilized Mn(2-x)Zn(x)O(3-delta.).

Magnetics↗

PubMed2004

Self-assembled single-crystal ferromagnetic iron nanowires formed by decomposition.

Arrays of perpendicular ferromagnetic nanowires have recently attracted considerable interest for their potential use in many areas of advanced nanotechnology. We report a simple approach to create self-assembled nanowires of alpha-Fe through the decomposition of a suitably chosen perovskite. We illustrate the principle behind this approach using the reaction 2La(0.5)Sr(0.5)FeO(3) --> LaSrFeO(4) + Fe + O(2) that occurs during the deposition of La(0.5)Sr(0.5)FeO(3) under reducing conditions. This leads to the spontaneous formation of an array of single-crystalline alpha-Fe nanowires embedded in LaSrFeO(4) matrix, which grow perpendicular to the substrate and span the entire film thickness. The diameter and spacing of the nanowires are controlled directly by deposition temperature. The nanowires show uniaxial anisotropy normal to the film plane and magnetization close to that of bulk alpha-Fe. The high magnetization and sizable coercivity of the nanowires make them desirable for high-density data storage and other magnetic-device applications.

Crystallization↗

PubMed2004

2D-3D transformation of layered perovskites through metathesis: synthesis of new quadruple perovskites A2La2CuTi3O12 (A = Sr, Ca).

We describe the synthesis of two new quadruple perovskites, Sr(2)La(2)CuTi(3)O(12) (I) and Ca(2)La(2)CuTi(3)O(12) (II), by solid-state metathesis reaction between K(2)La(2)Ti(3)O(10) and A(2)CuO(2)Cl(2) (A = Sr, Ca). I is formed at 920 degrees C/12 h, and II, at 750 degrees C/24 h. Both the oxides crystallize in a tetragonal (P4/mmm) quadruple perovskite structure (a = 3.9098(2) and c = 15.794(1) A for I; a = 3.8729(5) and c = 15.689(2) A for II). We have determined the structures of I and II by Rietveld refinement of powder XRD data. The structure consists of perovskite-like octahedral CuO(4/2)O(2/2) sheets alternating with triple octahedral Ti(3)O(18/2) sheets along the c-direction. The refinement shows La/A disorder but no Cu/Ti disorder in the structure. The new cuprates show low magnetization (0.0065 micro(B) for I and 0.0033 micro(B) for II) suggesting that the Cu(II) spins are in an antiferromagnetically ordered state. Both I and II transform at high temperatures to 3D perovskites where La/Sr and Cu/Ti are disordered, suggesting that I and II are metastable phases having been formed in the low-temperature metathesis reaction. Interestingly, the reaction between K(2)La(2)Ti(3)O(10) and Ca(2)CuO(2)Cl(2) follows a different route at 650 degrees C, K(2)La(2)Ti(3)O(10) + Ca(2)CuO(2)Cl(2) --> CaLa(2)Ti(3)O(10) + CaCuO(2) + 2KCl, revealing multiple reaction pathways for metathesis reactions.

Journal Article↗

PubMed2004

Multiferroic BaTiO3-CoFe2O4 Nanostructures.

We report on the coupling between ferroelectric and magnetic order parameters in a nanostructured BaTiO3-CoFe2O4 ferroelectromagnet. This facilitates the interconversion of energies stored in electric and magnetic fields and plays an important role in many devices, including transducers, field sensors, etc. Such nanostructures were deposited on single-crystal SrTiO3 (001) substrates by pulsed laser deposition from a single Ba-Ti-Co-Fe-oxide target. The films are epitaxial in-plane as well as out-of-plane with self-assembled hexagonal arrays of CoFe2O4 nanopillars embedded in a BaTiO3 matrix. The CoFe2O4 nanopillars have uniform size and average spacing of 20 to 30 nanometers. Temperature-dependent magnetic measurements illustrate the coupling between the two order parameters, which is manifested as a change in magnetization at the ferroelectric Curie temperature. Thermodynamic analyses show that the magnetoelectric coupling in such a nanostructure can be understood on the basis of the strong elastic interactions between the two phases.

Journal Article↗

PubMed2003

High temperature ferromagnetism with a giant magnetic moment in transparent co-doped SnO(2-delta).

The occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn(1-x)Co(x)O(2-delta) (x<0.3). Interestingly, films of Sn(0.95)Co(0.05)O(2-delta) grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of 7.5+/-0.5 micro(B)/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.

Journal Article↗

PubMed2003

Identification of novel compositions of ferromagnetic shape-memory alloys using composition spreads.

Exploration of new ferroic (ferroelectric, ferromagnetic or ferroelastic) materials continues to be a central theme in condensed matter physics and to drive advances in key areas of technology. Here, using thin-film composition spreads, we have mapped the functional phase diagram of the Ni-Mn-Ga system whose Heusler composition Ni(2)MnGa is a well known ferromagnetic shape-memory alloy. A characterization technique that allows detection of martensitic transitions by visual inspection was combined with quantitative magnetization mapping using scanning SQUID (superconducting quantum interference device) microscopy. We find that a large, previously unexplored region outside the Heusler composition contains reversible martensites that are also ferromagnetic. A clear relationship between magnetization and the martensitic transition temperature is observed, revealing a strong thermodynamical coupling between magnetism and martensitic instability across a large fraction of the phase diagram.

Alloys↗