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N. D. Browning

Publications and source records attributed to N. D. Browning.

10 recordsLinked to original sources
arXiv2013

Interfacial Ferromagnetism in LaNiO3/CaMnO3 Superlattices

We observe interfacial ferromagnetism in superlattices of the paramagnetic metal LaNiO3 and the antiferromagnetic insulator CaMnO3. LaNiO3 exhibits a thickness dependent metal-insulator transition and we find the emergence of ferromagnetism to be coincident with the conducting state of LaNiO3. That is, only superlattices in which the LaNiO3 layers are metallic exhibit ferromagnetism. Using several magnetic probes, we have determined that the ferromagnetism arises in a single unit cell of CaMnO3 at the interface. Together these results suggest that ferromagnetism can be attributed to a double exchange interaction among Mn ions mediated by the adjacent itinerant metal.

cond-mat.str-el↗
arXiv2006

Direct measurement of charge transfer phenomena at ferromagnetic/superconducting oxide interfaces

YBa2Cu3O7-x/La0.67Ca0.33MnO3 ferromagnetic/superconducting interfaces are analyzed by scanning transmission electron microscopy and electron energy loss spectroscopy with monolayer resolution. We demonstrate that extensive charge transfer occurs between the manganite and the superconductor, in a manner similar to modulation-doped semiconductors, which explains the reduced critical temperatures of heterostructures. This behavior is not seen with insulating PrBa2Cu3O7 layers. Furthermore, we confirm directly that holes in the YBa2Cu3O7-x are located on the CuO2 planes.

cond-mat.other↗
arXiv2006

Consequences of niobium doping for the ferromagnetism and microstructure of anatase Co: TiO2 films

It is shown that dilute niobium doping has significant effect on the ferromagnetism and microstructure of dilutely cobalt-doped anatase TiO2 films. Epitaxial films of anatase TiO2 with 3% Co, without and with 1% niobium doping were grown by pulsed-laser deposition at 875 C at different oxygen pressures. For growth at 10^{-5} Torr niobium doping suppresses the ferromagnetism, while it enhances the same in films grown at 10^{-4} Torr. High-resolution Z-contrast Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy show uniform surface segregation of cobalt-rich Ti_{1-x-y}Co_{x}Nb_{y}O_{2-d} phase, but without cobalt metal clusters.

cond-mat.mtrl-sci↗
arXiv2005

Percolative Ferromagnetism in Anatase Co:TiO2

We revisit the most widely investigated and controversial oxide diluted magnetic semiconductor (DMS), Co:TiO2, with a new high temperature film growth, and show that the corresponding material is not only an intrinsic DMS ferromagnet, but also supports a percolative mechanism of ferromagnetism. We establish the uniformity of dopant distribution across the film cross section by Z-contrast imaging via scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) at spatial resolution of 0.4 nm and the oxidized 2+ valence state of cobalt by x-ray absorption spectroscopy (XAS). The dependence of magnetic properties on cobalt concentration is consistent with the defect polaron percolation model. The peculiar increase in the transport activation energy above a specific cobalt concentration further emphasizes the polaron contribution to magnetic order.

cond-mat.mtrl-sci↗
arXiv2003

Co-doped LaLa1-xSrxTiO3-d : A Diluted Magnetic Oxide System with High Curie Temperature

Ferromagnetism is observed at and above room temperature in pulsed laser deposited epitaxial films of Co-doped Ti-based oxide perovskite (La1-xSrxTiO3-d). The system has the characteristics of an intrinsic diluted magnetic semiconductor (metal) at low concentrations (<~ 2 %), but develops inhomogeneity at higher cobalt concentrations. The films range from being opaque metallic to transparent semiconducting depending on the oxygen pressure during growth and are yet ferromagnetic.

cond-mat.str-el↗
arXiv2003

High Temperature Ferromagnetism with Giant Magnetic Moment in Transparent Co-doped SnO2-d

Occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn1-xCoxO2-d (x<0.3). Interestingly, films of Sn0.95Co0.05O2-d grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of about 7 Bohr-Magneton/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.

cond-mat↗
arXiv2002

Ferromagnetism in laser deposited anatase Ti$_{1-x}$Co$_{x}$O$_{2-$δ$}$ films

Pulsed laser deposited films of Co doped anatase TiO2 are examined for Co substitutionality, ferromagnetism, transport, magnetotransport and optical properties. Our results show limited solubility (up to ~ 2 %) of Co in the as-grown films and formation of Co clusters thereafter. For Ti0.93Co0.07O2-d sample, which exhibits a Curie temperature (Tc) over 1180 K, we find the presence of 20-50 nm Co clusters as well as a small concentration of Co incorporated into the remaining matrix. After being subjected to the high temperature anneal during the first magnetization measurement, the very same sample shows a Tc ~ 650 K and almost full matrix incorporation of Co. This Tc is close to that of as-grown Ti0.99Co0.01O2-d sample (~ 700 K). The transport, magnetotransport and optical studies also reveal interesting effects of the matrix incorporation of Co. These results are indicative of an intrinsic Ti1-xCoxO2-d diluted magnetic semiconductor with Tc of about 650-700 K.

cond-mat↗
arXiv2002

Observation of coherent oxide precipitates in polycrystalline MgB2

Here we describe the results of an atomic resolution study of oxygen incorporation into bulk MgB2. We find that ~20-100 nm sized precipitates are formed by ordered substitution of oxygen atoms onto boron lattice sites, while the basic bulk MgB2 crystal structure and orientation is preserved. The periodicity of the oxygen ordering is dictated by the oxygen concentration in the precipitates and primarily occurs in the (010) plane. The presence of these precipitates correlates well with an improved critical current density and superconducting transition behavior, implying that they act as pinning centers.

cond-mat.supr-con↗
arXiv2001

Direct observation of nm-scale Mg- and B-oxide phases at grain boundaries in MgB2

Here we describe the results of an atomic resolution study of the structure and composition of both the interior of the grains, and the grain boundaries in polycrystalline MgB2. We find that there is no oxygen within the bulk of the grains but significant oxygen enrichment at the grain boundaries. The majority of grain boundaries contain BOx phases smaller than the coherence length, while others contain larger areas of MgO sandwiched between BOx layers. Such results naturally explain the differences in connectivity between the grains observed by other techniques.

cond-mat.supr-con↗