Ranjan K. Sahu

Ruthenium-induced enhanced magnetization and metal-insulator transition in two-dimensional layered manganites

A strong correlation between magnetism and transport behavior is observed in ruthenium (Ru) substituted two-dimensional layered manganites, La1.2Ca1.8Mn2O7, which show an increase in the Curie temperature and the metal–insulator transition (MIT) by 15–20 K up to x=0.2, due to a dominant double-exchange mediated transport, aided by the presence of a Ru (V) ion, which is metrically and chemically similar to the Mn (IV) ion. Beyond x=0.225, Tc and the metal–insulator transition decrease marginally, predominantly controlled by (i) hole carrier concentration and (ii) spin-wave excitation with a T2 dependence. The magnetoresistance ratio is not affected significantly by Ru doping in the vicinity of the MIT, indicating the double-exchange interaction between Ru and Mn.

Magnetic pair-making double exchange coupling in Ru substituted orthomanganites, La0.7A0.3Mn0.9Ru0.1O3

A detailed study of Ru substitution at the Mn site in La0.7A0.3Mn0.9Ru0.1O3 (A=Ca, Sr, Pb, Ba) polycrystalline samples shows an unusual magnetic pair between Ru+4/Ru+5 and Mn+4/Mn+3 redox couple. As a result, in the entire 10 at. % Ru-substituted compositions, the TC varies only by 10–30 K. The similarity of Mn4+/Ru4+:eg parentage facilitates a redox interaction between Mn and Ru ions, Ru+4/+5/Mn+3/+4. Ruthenium (IV) low spin state has a magnetic moment of 2.7–2.9μB, and with an extended 4d orbital, it enhances exchange coupling between Mn and Ru sites.

Microwave synthesis of magnetoresistive La0.7Ba0.3MnO3 using inorganic precursors

We report here the use of inorganic precursors as good microwave absorbers, by virtue of its polarity and high dielectric constant, to synthesize high temperature stable rare earth manganite, La0.7Ba0.3MnO3. Compared to other wet chemical methods, the oxides prepared by microwave assisted route give fine particle oxides (<30 nm) with effective BET surface area of ∼25 m2/g. Two factors contribute to the rapid synthesis of these high temperature phases. Firstly, the dielectric constant of the precursors employed increases the microwave power loss in the material and this leads to a local heating effect. Secondly decomposition of these precursors, lead to formation of finely divided oxides often accompanied by an exothermic reaction which provides the needed energy to effect the formation of the product. This method offers a new approach to employ inorganic precursors as starting materials to realize fast and effective reaction in microwave assisted material synthesis.

A Zener pair effect in lanthanum rutheno manganite

A strong correlation between the Curie temperature and the metal–insulator transition is displayed in Ru doped La0.6Pb0.4Mn1−xRuxO3, where an equiatomic ratio of Ru and Mn is present in the Mn–O–Ru sublattice. The magnetic pair making effect of Mn with Ru is facilitated by the variable valency of Ru (IV)/Ru (V) which is similar in ionic size to the Zener pair Mn (III)/Mn (IV). Further Ru, being a 4d metal with itinerant t2g electrons, facilitates the exchange coupling interaction. The spin-wave excitation and the hole carrier density dominate the magnetization and the electron transport for T<0.5 TC for a higher Ru doped composition.

Intergrain tunneling and magnetotransport properties in CrO2–RuO2/TiO2

We report an enhanced negative magnetoresistance ratio of about 16% at 4.2 K at 1 T for CrO2/TiO2 composites, compared to CrO2 powder compacts and CrO2/RuO2 composite. The magnetoresistance ratio of 60% at 4.2 K is noted when TiO2 acts as tunneling barrier. The absence of substantial barrier heights with RuO2 grains does not alter the magnetoresistance compared to the CrO2 powder compacts. The insulating TiO2 grains increases the grain boundary barrier for the spin polarized tunneling, thereby enhancing the low field magnetoresistance.

Double-exchange enhanced magnetoresistance in SrRu0.5Mn0.5O3

Magnetoresistance ratio (MR), Δρ/ρ0, in SrRu0.5Mn0.5O3 is enhanced by 20% near the Curie temperature (Tc=125 K) compared to the parent compound SrRuO3 (Tc=165 K) which is a super exchange mediated ferromagnetic metal. The enhancement in MR ratio in the equiatomic B site composition is due to the onset of double-exchange mediated ferromagnetic interaction. The Mn, Ru L2,3 edge of the x-ray absorption spectra of the SrRu0.5Mn0.5O3 phase shows the coexistence of ionic pairs involving Mn+3/Ru+5↔Mn+4/Ru+4. Mn+3 (t2g3eg1) recognizes both Mn+4/Ru+5 (t2g3eg0) as magnetically equivalent sites for eg electron hopping and for maintaining the eg hole carrier concentration.

Magnetic disorder induced enhanced magneto-resistance in La0.5Sr0.5Co1−xRuxO3

Abstract We observe a sharp increase in negative magneto-resistance ratio up to 40% for x =0.1, in La 0.5 Sr 0.5 Co 1− x Ru x O 3 which is due to the magnetic disorder induced by an anti-ferromagnetic interaction between Co and Ru ions. We also observe a metal to insulator and a ferromagnetic to anti-ferromagnetic transition for 0≤ x ≤0.3. Ruthenium (IV) ion disrupts an intermediate spin state of cobalt (Co 3+ :t 2g 5 e g 1 ), forcing a double exchange mediated ferromagnetic state to an anti-ferromagnetic spin state for x ≥0.2.

Powder neutron diffraction evidence for enhanced inter plane magnetic coupling in La1.2Sr1.8Mn2−xRuxO7 layered manganites

Rotation of Mn spin moments along the c axis enhances interplane magnetic coupling, leading to a steep increase in the Curie temperature (TC) and metal to insulator transition (TMIT) in Ru substituted two dimensional layered manganites, La1.2Sr1.8Mn2−xRuxO7. The temperature dependent neutron diffraction data show the absence of (004) reflection, indicating that the Mn spin moments are not aligned in the a-b plane, and rather the increase in intensity of (110) reflection at 12K shows that the MnO2 layers are stacked ferromagnetically along the c axis. The presence of Ru at the Mn site in pentavalent state assures the required hole carrier density for a sustained double-exchange interaction. The cumulative effect of rotation of Mn spin moments and the variable valence states of Ru lead to a pronounced increase in TC up to ∼50K, which is not observed hitherto in any of the B-site substituted manganites.

Enhanced magnetic and metal-insulator transition in Ru-doped layered manganites

Double exchange mediated magnetism and transport properties are exemplified in two-dimensional layered manganites, La1.2Ca1.8Mn2O7. We demonstrate here the role of ruthenium in showing increased metal–insulator transition temperature, TP and Curie temperature Tc, by about 15 K when Ru is substituted up to 10% (x=0.2) at the Mn site. We discuss these unique features based on the existence of a redox ionic pair involving Mn3+/Mn4+↔Ru4+/Ru5+ up to x∼0.2. Beyond the critical level (x∼0.2), Ru exists mostly in Ru4+ oxidation state leading to a weak double exchange interaction. We document here, an enhanced Tc and TP in the layered manganites despite the disorder induced at the B site.

Single Step Synthesis Of Ni Wire, Sponge And Flower, And Their Magnetic Properties

We reported a single step process for the synthesis of Ni having wire, sponge and flower type microstructures that are obtained via a microwave‐assisted decomposition of Ni‐TEA (triethanolamine) complex precursor with and without amino acid. The products formed at different stages of the growth process were characterized using scanning electron microscope (SEM), transmission electron microscope (TEM) and magnetometer. Morphological modification enhanced the coercivity from 40 Oe for the Ni‐wire to 120 Oe for the Ni‐flower with exchange bias effect, which mechanistically arises due to the presence of NiO phase.