Magnetization in electron- and Mn- doped SrTiO3

Mn- doped SrTiO3.0, when synthesized free of impurities, is a paramagnetic insulator with interesting dielectric properties. Since delocalized charge carriers are known to promote ferromagnetism in a large number of systems via diverse mechanisms, we have looked for the possibility of any intrinsic, spontaneous magnetization by simultaneous doping of Mn ions and electrons into SrTiO3 via oxygen vacancies, thereby forming SrTi1-xMnxO3-delta, to the extent of making the doped system metallic. We find an absence of any enhancement of the magnetization in the metallic sample when compared with a similarly prepared Mn doped, however, insulating sample. Our results, thus, are not in agreement with a recent observation of a weak ferromagnetism in metallic Mn doped SrTiO3 system.

Majorana fermions in superconducting wires: Effects of long-range hopping, broken time-reversal symmetry, and potential landscapes

We present a comprehensive study of two of the most experimentally relevant extensions of Kitaev's spinless model of a one-dimensional p-wave superconductor: those involving (i) longer-range hopping and superconductivity and (ii) inhomogeneous potentials. We commence with a pedagogical review of the spinless model and, as a means of characterizing topological phases exhibited by the systems studied here, we introduce bulk topological invariants as well as those derived from an explicit consideration of boundary modes. In time-reversal symmetric systems, we find that the longer range hopping leads to topological phases characterized by multiple Majorana modes. In particular, we investigate a spin model that respects a duality and maps to a fermionic model with multiple Majorana modes; we highlight the connection between these topological phases and the broken symmetry phases in the original spin model. In the presence of time-reversal symmetry breaking terms, we show that the topological phase diagram is characterized by an extended gapless regime. For the case of inhomogeneous potentials, we explore phase diagrams of periodic, quasiperiodic, and disordered systems. We present a detailed mapping between normal state localization properties of such systems and the topological phases of the corresponding superconducting systems. This powerful tool allows us to leverage the analyses of Hofstadter's butterfly and the vast literature on Anderson localization to the question of Majorana modes in superconducting quasiperiodic and disordered systems, respectively. We briefly touch upon the synergistic effects that can be expected in cases where long-range hopping and disorder are both present.

Spontaneous Breaking of Time-Reversal Symmetry in Strongly Interacting Two-Dimensional Electron Layers in Silicon and Germanium

We report experimental evidence of a remarkable spontaneous time-reversal symmetry breaking in two-dimensional electron systems formed by atomically confined doping of phosphorus (P) atoms inside bulk crystalline silicon (Si) and germanium (Ge). Weak localization corrections to the conductivity and the universal conductance fluctuations were both found to decrease rapidly with decreasing doping in the Si: P and Ge: P delta layers, suggesting an effect driven by Coulomb interactions. In-plane magnetotransport measurements indicate the presence of intrinsic local spin fluctuations at low doping, providing a microscopic mechanism for spontaneous lifting of the time-reversal symmetry. Our experiments suggest the emergence of a new many-body quantum state when two-dimensional electrons are confined to narrow half-filled impurity bands.

Magnetization-steps in Y2CoMnO6 double perovskite: The role of antisite disorder

Antisite disorder is observed to have significant impact on the magnetic properties of the double perovskite Y2CoMnO6 which has been recently identified as a multiferroic. A paramagnetic-ferromagnetic phase transition occurs in this material at T-c approximate to 75 K. At 2K, it displays a strong ferromagnetic hysteresis with a significant coercive field of H-c approximate to 15 kOe. Sharp steps are observed in the hysteresis curves recorded below 8K. In the temperature range 2K <= T <= 5K, the hysteresis loops are anomalous as the virgin curve lies outside the main loop. The field-cooling conditions as well as the rate of field-sweep are found to influence the steps. Quantitative analysis of the neutron diffraction data shows that at room temperature, Y2CoMnO6 consists of 62% of monoclinic P2(1)/n with nearly 70% antisite disorder and 38% Pnma. The bond valence sums indicate the presence of other valence states for Co and Mn which arise from disorder. We explain the origin of steps by using a model for pinning of magnetization at the antiphase boundaries created by antisite disorder. The steps in magnetization closely resemble the martensitic transformations found in intermetallics and display first-order characteristics as revealed in the Arrott's plots. (C) 2014 AIP Publishing LLC.

Low temperature magnetoresistance and magnetization studies of iron encapsulated multiwall carbon nanotube/polyvinyl chloride composites

We present the experimental results of temperature dependent magnetoresistance (MR) and the magnetization studies of iron encapsulated multiwall carbon nanotube (MWCNT)/polyvinyl chloride (PVC) composites with different wt% of MWCNTs. Transmission electron microscopy characterization shows that MWCNTs are encapsulated with rod-shaped iron nanoparticles of aspect ratio of similar to 3. The MR behavior of 1.9 wt% MWCNT/PVC sample shows dominance of forward scattering and wave function shrinkage whereas, weak localization and electron-electron interactions explain the MR data of higher wt% samples (9.1, 16.6 and 44.4 wt%). The composites of 4.7 and 9.1 wt% exhibit ferromagnetic behavior at all temperatures with room temperature coercivities of similar to 1036 and 628 Oe, respectively. (C) 2014 Elsevier Ltd. All rights reserved.

Ferrimagnetism and magnetic phase separation in Nd1-xYxMnO3 studied by magnetization and high frequency electron paramagnetic resonance

Ferrimagnetism and metamagnetic features tunable by composition are observed in the magnetic response of Nd1-xYxMnO3, for x=0.1-0.5. For all values of x in the series, the compound crystallizes in orthorhombic Pbnm space group similar to NdMnO3. Magnetization studies reveal a phase transition of the Mn-sublattice below T-N(Mn) approximate to 80 K for all compositions, which, decreases up on diluting the Nd-site with Yttrium. For x=0.35, ferrimagnetism is observed. At 5 K, metamagnetic transition is observed for all compositions x < 0.4. The evolution of magnetic ground states and appearance of ferrimagnetism in Nd1-xYxMnO3 can be accounted for by invoking the scenario of magnetic phase separation. The high frequency electron paramagnetic resonance measurements on x=0.4 sample, which is close to the critical composition for phase separation, revealed complex temperature dependent lineshapes clearly supporting the assumption of magnetic phase separation. (C) 2014 Elsevier B.V. All rights reserved.

Preferential polarization and its reversal in polycrystalline BiFeO3/La0.5Sr0.5CoO3 heterostructures

Polycrystalline BiFeO3 thin films were grown on La0.5Sr0.5CoO3 buffered Pt (200)/TiO2/SiO2/Si substrates under different oxygen partial pressures (10, 25, 50 and 100 mTorr) by puked laser ablation. Piezo-response Force Microscopy and Piezo-Force Spectroscopy have shown that all the films are ferroelectric in nature with locally switchable domains. It has also revealed a preferential downward domain orientation in as-grown films grown under lower oxygen partial pressure (10 and 25 mTorr) with a reversal of preferential domain orientation as the oxygen partial pressure is increased to 100 mTorr during laser ablation. Such phenomena are atypical of multi-grained polycrystalline ferroelectric films and have been discussed On the basis of detect formation with changing growth conditions. For the 50 mTorr grown film, asymmetric domain stability and retention during write-read studies has been observed which is attributed to grain-size-related defect concentration, affecting pinning centres that inhibit domain wall motion. (C) 2015 Elsevier Ltd. All rights reserved.

Magnetic field assisted stem cell differentiation - role of substrate magnetization in osteogenesis

Among the multiple modulatory physical cues explored to regulate cellular processes, the potential of magneto-responsive substrates in magnetic field stimulated stem cell differentiation is still unperceived. In this regard, the present work demonstrates how an external magnetic field can be applied to direct stem cell differentiation towards osteogenic commitment. A new culture methodology involving periodic delivery of 100 mT static magnetic field (SMF) in combination with HA-Fe3O4 magnetic substrates possessing a varying degree of substrate magnetization was designed for the study. The results demonstrate that an appropriate combination of weakly ferromagnetic substrates and SMF exposure enhanced cell viability, DNA synthesis and caused an early switchover to osteogenic lineage as supported by Runx2 immunocytochemistry and ALP expression. However, the mRNA expression profile of early osteogenic markers (Runx2, ALP, Col IA) was comparable despite varying substrate magnetic properties (diamagnetic to ferromagnetic). On the contrary, a remarkable upregulation of late bone development markers (OCN and OPN) was explicitly detected on weak and strongly ferromagnetic substrates. Furthermore, SMF induced matrix mineralization with elevated calcium deposition on similar substrates, even in the absence of osteogenic supplements. More specifically, the role of SMF in increasing intracellular calcium levels and in inducing cell cycle arrest at G0/G1 phase was elucidated as the major molecular event triggering osteogenic differentiation. Taken together, the above results demonstrate the competence of magnetic stimuli in combination with magneto-responsive biomaterials as a potential strategy for stem cell based bone tissue engineering.

Disappearance of electron-hole asymmetry in nanoparticles of Nd1-xCaxMnO3(x=0.6, 0.4): magnetization and electron paramagnetic resonance evidence

We study and compare magnetic and electron paramagnetic resonance behaviors of bulk and nanoparticles of Nd1-xCaxMnO3 in hole doped (x = 0.4; NCMOH) and electron doped (x = 0.6; NCMOE) samples. NCMOH in bulk form shows a complex temperature dependence of magnetization M(T), with a charge ordering transition at similar to 250 K, an antiferromagnetic (AFM) transition at similar to 150 K, and a transition to a canted AFM phase/mixed phase at similar to 80 K. Bulk NCMOE behaves quite differently with just a charge ordering transition at similar to 280 K, thus providing a striking example of the so called electron-hole asymmetry. While our magnetization data on bulk samples are consistent with the earlier reports, the new results on the nanoparticles bring out drastic effects of size reduction. They show that M(T) behaviors of the two nanosamples are essentially similar in addition to the absence of the charge order in them thus providing strong evidence for vanishing of the electron-hole asymmetry in nanomanganites. This conclusion is further corroborated by electron paramagnetic resonance studies which show that the large difference in the ``g'' values and their temperature dependences found for the two bulk samples disappears as they approach a common behavior in the corresponding nanosamples. (C) 2015 AIP Publishing LLC.

Comparative study of magnetic ordering in bulk and nanoparticles of Sm0.65Ca0.35MnO3: Magnetization and electron magnetic resonance measurements

To explore the effect of size reduction to nanoscale on the hole doped Sm0.65Ca0.35MnO3 compound, dc magnetic measurements and electron magnetic resonance (EMR) were done on bulk and nanoparticle samples in the temperature range 10 <= T <= 300 K. Magnetization measurement showed that the bulk sample undergoes a charge ordering transition at 240K and shows a mixed magnetic phase at low temperature. However, the nanosample underwent a ferromagnetic transition at 75 K, and the charge ordered state was destabilized on size reduction down to nanoscale. The low-temperature ferromagnetic component is found to be enhanced in nanoparticles as compared to their bulk counterpart. Interestingly around room temperature, bulk particles show higher magnetization where as at low temperature nanoparticles show higher magnetization. Ferromagnetism in the bulk is due to super exchange where as ferromagnetism in nanoparticles is due to uncompensated spins of the surface layer. Temperature variation of EMR parameters correlates well with the results of magnetic measurements. The magnetic behaviour of the nanoparticles is understood in terms of the core shell scenario. (C) 2015 AIP Publishing LLC.

Periodically Grafted Amphiphilic Copolymers: Effects of Steric Crowding and Reversal of Amphiphilicity

Two series of periodically clickable polyesters were prepared; one of them carries alkylene segments along its backbone, whereas the other carries poly(ethylene glycol) (PEG) segments. These polyesters were clicked with either MPEG-350 azide or docosyl (C22) azide to yield periodically grafted amphiphilic copolymers (PGACs) carrying either flexible hydrophilic or crystallizable hydrophobic backbone segments. The immiscibility between hydrocarbon and PEG segments causes both of these systems to fold in either a zigzag or hairpin-like conformation; the hairpin-like conformation appears to be preferred when flexible PEG segments are present in the backbone. The folded chains further reorganize in the solid state to develop a lamellar morphology that permits the collocation of the PEG and hydrocarbon (HC) segments within alternate domains; evidence for the self-segregation was gained from DSC, SAXS, and AFM studies. SAXS studies revealed the formation of an extended lamellar structure, whereas AFM images showed uniform layered morphology with layer heights that matched reasonably well with the interlamellar spacing obtained from the SAXS study. Labeling One representative PGAC, carrying crystallizable long alkylene segments in the backbone and pendant PEG-350 side chains, with a small mole fraction of pyrene fluorophore permitted the examination of the conformational transition that occurs upon going from a good to a poor solvent; this single-chain folded conformation, we postulate, is the intermediate that organizes into the lamellar morphology.

Exchange spring behaviour in SrFe12O19-CoFe2O4 nanocomposites

Nanocomposites of hard (SrFe12O19) and soft ferrite (CoFe2O4) are prepared by mixing individual ferrite components at appropriate weight ratio and subsequent heat treatment. The magnetization of the composites showed hysteresis loop that is characteristic of the exchange spring system. The variation of J(r)/J(r)(infinity) vs. J(d)/J(r)(infinity) for these nanocomposites are investigated to understand the presence of both the interacting field and the disorder in the system. This is further corroborated with the First Order Reversal Curve analysis (FORC) on the nanocomposites of 1:4 (Cobalt Ferrite: Strontium Ferrite) and 1:16 (Cobalt Ferrite: Strontium Ferrite). The FORC distribution reveals that the pinning mechanism is stronger in the nanocomposite of 1:4 compared to 1:16. However, the nanocomposite of 1:16 exhibit superior exchange coupling strength in contrast to 1:4. The asymmetric nature of the FORC distribution at H-c = 0 Oe for both the nanocomposites validates the intercoupling between the reversible and irreversible magnetization. (C) 2015 Author(s).

Curie Transition Of Nc Nickel By Mechanical Spectroscopy And Magnetization Study

Mechanical spectroscopy measurement is performed to study the internal friction of nanocrystalline ( NC) nickel with an average grain size of 23 nm from room temperature to 610 K. An internal friction peak is observed at about 550 K, which corresponds to the Curie transition process of the NC nickel according to the result of magnetization test. Moreover, the fact that the Curie temperature of NC nickel is lower than that of coarse-grained nickel is explained by an analytical model based on the weakening of cohesive energy.

A preliminary investigation into the effects of a sex-reversal androgen, methyltestosterone, on food utilization and growth of Sarotherodon niloticus (L) fry

Three groups of Sarotherodon niloticus fry were fed for 8 weeks on diets either treated with 17- & methyltestosterone (MT), alcohol (CA), or untreated (CO). Growth rate and food utilization in the different groups were compared. Results indicate that the best growth, Feed Conversion Ratio (FCR), Protein Efficiency Ratio (PER) and Mean Growth Rate (MGR) were obtained with the MT diet. There was no significant difference (P 0.05) in growth and food utilization of the CA and CO fry, nor in the mortality rate of the 3 treatments. The androgen, methyltestosterone promotes growth and protein anabolism without producing toxic effects in S. niloticus