D. C. Joshi

The dielectric behavior of Zn1?xNixO/NiO two-phase composites

The effect of nickel content on the dielectric permittivity er and the ac-electrical conductivity of Zn1−xNixO/NiO (0 ≤ x ≤ 0.55) two-phase composites were investigated. The antiferro to the paramagnetic Neel temperature TN (~ 523 K) of the NiO associated with the structural phase transition from the rhombohedral to the cubic phase has been exploited to realize a dielectric anomaly across 523–541 K in the Zn1−xNixO/NiO composite system. Also, a giant dielectric peak across 410 °C in pure NiO was observed together with an anomaly across TN. The formation of tiny polar clusters due to the compositional heterogeneity for the samples with x ≥ 0.16 drove the system to exhibit a weakly coupled relaxor-like behavior with a locally varying maximum temperature of T* (~ 530 K at 106 Hz), obeying the Vogel–Fulcher law and the Uchino–Nomura criteria. The values of the diffuseness-exponent γ (1.91) and the shape-parameter δ (88 °C) were determined by using the empirical scaling relation (eA/er = 1 + 0.5 (T − TA)2/ δ2), which is often used to describe relaxor-like behavior. Our results provide strong evidence for the variable-range-hopping of charge carriers between the localized states. The effects of non-ohmic sample-electrode contact impedance and negative-capacitance on the global dielectric behavior of a Zn1−xNixO/NiO composite system are discussed.

Reentrant spin-glass behavior and bipolar exchange-bias effect in “Sn” substituted cobalt-orthotitanate

We report the co-existence of longitudinal ferrimagnetic behavior with Neel temperature TNu2009∼u200946.1u2009K and reentrant transverse spin-glass state at 44.05u2009K in Tin (Sn) doped cobalt-orthotitanate (Co2TiO4). The ferrimagnetic ordering is resulting from different magnetic moments of Co2+ on the A-sites (3.87u2009μB) and B-sites (5.069u2009μB). The magnetic compensation temperature (TCOMP) shifts from 31.74u2009K to 27.1u2009K when 40 at. % of “Sn4+” substitutes “Ti4+” at B-sites where the bulk-magnetization of two-sublattices balance each other. For Tu2009>u2009TN, the dc-magnetic susceptibility (χdcu2009=u2009M/Hdc) fits well with the Neels expression for the two-sublattice model with antiferromagnetic molecular field constants NBBu2009∼u200915.44, NABu2009∼u200932.01, and NAAu2009∼u200920.88. The frequency dependence of ac-magnetic susceptibility χac data follows the Vogel-Fulcher law, and the power-law of critical slowing-down with “zν”u2009=u20096.01 suggests the existence of spin-clusters (where “z” and “ν” being dynamic critical-exponent and correlation length of cri...

Low-temperature anomalous magnetic behavior of Co2TiO4 and Co2SnO4

We report the low-temperature anomalous magnetic behavior of ferrimagnetic spinels cobalt orthotitanate (Co2TiO4), which exhibits magnetic compensation behavior across 31.74u2009K, and cobalt orthostannate (Co2SnO4) exhibiting two sequential magnetic transitions, namely (i) ferrimagnetic to paramagnetic transition with Neel temperature TNu2009∼u200941u2009K and reentrant spin-glass behavior with glass transition temperature TSGu2009∼u200939u2009K. The Arrott plot (H/M versus M2) criterion has been used to extricate the order of sequential magnetic transitions occurring below TN. Negative slopes of the Arrott plots below 32u2009K, metamagnetic-like character of the M-H isotherms, anomalies in the specific-heat (CP T−1 versus T) below 15u2009K, and a zero-crossover of isothermal magnetic-entropy-change (ΔS) signify the presence of pseudo first-order discontinuous magnetic phase transition in the low-temperature regime 5u2009Ku2009≤u2009Tu2009≤u200932u2009K. The dc- and ac-susceptibilities of both Co2TiO4 and Co2SnO4 are interpreted in terms of frozen-spin-clusters, ...

Modulation of Peptide Based Nano-Assemblies with Electric and Magnetic Fields

Peptide based nano-assemblies with their self-organizing ability has shown lot of promise due to their high degree of thermal and chemical stability, for biomaterial fabrication. Developing an effective way to control the organization of these structures is important for fabricating application-oriented materials at the molecular level. The present study reports the impact of electric and magnetic field-mediated perturbation of the self-assembly phenomenon, upon the chemical and structural properties of diphenylalanine assembly. Our studies show that, electric field effectively arrests aggregation and self-assembly formation, while the molecule is allowed to anneal in the presence of applied electric fields of varying magnitudes, both AC and DC. The electric field exposure also modulated the morphology of the self-assembled structures without affecting the overall chemical constitution of the material. Our results on the modulatory effect of the electric field are in good agreement with theoretical studies based on molecular dynamics reported earlier on amyloid forming molecular systems. Furthermore, we demonstrate that the self-assemblies formed post electric-field exposure, showed difference in their crystal habit. Modulation of nano-level architecture of peptide based model systems with external stimulus, points to a potentially rewarding strategy to re-work proven nano-materials to expand their application spectrum.

Finite-size scaling and exchange-bias in SrRuO3/LaNiO3/SrRuO3 trilayers

We report a detailed magnetization study of the coherently strained trilayer heterostructures of metallic ferromagnet SrRuO3 (SRO)/paramagnetic-LaNiO3/SRO grown on a (001) oriented SrTiO3 single crystal substrate using the pulsed laser deposition technique. A tunable positive exchange bias (μoHEBu2009∼u20090.2u2009T, at 10u2009K) across the interface has been observed. Upon varying the SRO layer thickness (2u2009nmu2009≤u2009tSROu2009≤u200918u2009nm), the saturation magnetization (MSu2009∼u20090.46 μB/Ru) increases significantly owing to electronic-reconstruction across the interfaces and finite size effects. The magnitude of μoHEB increases continuously with increasing tSRO and saturates above 16u2009nm; however, the loop-asymmetry increases rapidly as the temperature is lowered below the ferromagnetic Curie temperature (TC) of SRO. Notable reductions in both TC (125.6u2009Ku2009≤u2009TCu2009≤u2009146.4u2009K) and blocking temperature TBu2009=u2009T* (90.6u2009Ku2009≤u2009T∗u2009≤u2009119.2u2009K) are observed due to surface effects that comply with the finite-size scaling law: TC(tSRO)u2009=u2009TC (∞) [1u2009−u2009(ξo/tSRO)...

Effects of Cu doping on the electronic structure and magnetic properties of MnCo2O4 nanostructures

Reported here are the results and their analysis from our detailed investigations of the effects of Cu doping ([Formula: see text]) on the electronic structure and magnetic properties of the spinel [Formula: see text]O4. A detailed comparison is given for the [Formula: see text] and [Formula: see text] cases for both the bulk-like samples and nanoparticles. The electronic structure determined from x-ray photoelectron spectroscopy and Rietveld analysis of x-ray diffraction patterns shows the structure to be: ([Formula: see text])A [Formula: see text] [Formula: see text] [Formula: see text]]B [Formula: see text] i.e. [Formula: see text] substitutes for [Formula: see text] on the octahedral B-sites. For the bulk samples, the ferrimagnetic [Formula: see text] K for [Formula: see text] is lowered to [Formula: see text] K for the [Formula: see text] sample, this decrease being due to the effect of Cu doping. For the nanosize [Formula: see text] ([Formula: see text]) sample, the lower [Formula: see text] K ([Formula: see text] K) is observed using [Formula: see text] analysis, this lowering being due to finite size effects. For [Formula: see text], fits of dc paramagnetic susceptibility data of [Formula: see text] versus T in nanosize samples to the Néel expression are used to determine the exchange interactions between the A and B sites with exchange constants: [Formula: see text] K (4.1u2009K), [Formula: see text] K (16.3u2009K) and [Formula: see text] K (13.8u2009K) for [Formula: see text]. The temperature dependence of ac susceptibilities [Formula: see text] and [Formula: see text] at different frequencies shows that in bulk samples of [Formula: see text] and [Formula: see text], the transition at T C is the normal second order transition. But for the nanosize [Formula: see text] and 0.2 samples, analysis of the ac susceptibilities shows that the ferrimagnetic transition at T C is followed by a re-entrant spin-glass transition at lower temperatures [Formula: see text] K (138u2009K) for [Formula: see text] ([Formula: see text]). Analysis of the ac susceptibilities, [Formula: see text] and [Formula: see text], versus T data is done in terms of two scaling laws: (i) Vogel-Fulcher law [Formula: see text] [Formula: see text]; and (ii) power law of critical slowing-down [Formula: see text]. These fits confirm the existence of glassy behavior below T SG with the parameters [Formula: see text] (8.91), [Formula: see text] (9.6u2009u2009×u2009u200910[Formula: see text]) and [Formula: see text] K (∼138u2009K) for the samples [Formula: see text] (0.2), with similar results obtained for other samples. The linear behavior of the peak maximum in [Formula: see text] versus [Formula: see text] (AT-line) further supports the existence of glassy states in nanosize samples. For [Formula: see text], the temperature and composition dependence of the hysteresis loop parameters are investigated; all the samples with xu2009u2009⩾u2009u20090.1 have the coercivity H C and remanence [Formula: see text]. Since the results reported here in these nanostructures are significantly different from those in bulk [Formula: see text] [Formula: see text], further investigations of their magnetic structures using neutron diffraction are warranted.