Proloy T. Das

Influence of La and Mn vacancies on the electronic and magnetic properties of LaMnO3 thin films grown by pulsed laser deposition

and magnetic properties. Specifically, we show that the Mn/La ratio can be systematically varied from 0.92 at 0.11 mbar to 1.09 at 0.30 mbar of oxygen. The cationic vacancies have markedly different effects that become most pronounced once the samples are fully oxygenated and thus strongly hole doped. All as-grown and thus slightly oxygen-deficient LMO films are ferromagnetic insulators with saturation moments in excess of 2.5 μB per Mn ion, their transport and optical properties can be understood in terms of trapped ferromagnetic polarons. Upon oxygen annealing, the most La-deficient films develop a metallic response with an even larger ferromagnetic saturation moment of 3.8 μB per Mn ion. In contrast, in the oxygenated Mn-deficient films, the ferromagnetic order is strongly suppressed to less than 0.5 μB per Mn ion, and the transport remains insulatorlike. We compare our results with the ones that were previously obtained on bulk samples and present an interpretation in terms of the much stronger disruption of the electronic and magnetic structure by the Mn vacancies as compared to the La vacancies. We also discuss the implications for the growth of LMO thin films with well-defined physical properties that are a prerequisite for the study of interface effects in multilayers.

Introducing Fractional-Order Dynamics to Sigma---Delta Modulators

The aim of the present paper is to investigate the performance of a fractional-order sigma–delta modulator wherein the integer-order integrator is replaced by a fractional integrator of order

{\hbox{Sm}}_{0.35} {\hbox{Pr}}_{0.15}{\hbox{Sr}}_{0.5}{\hbox{MnO}}_3

\alpha \,(1 <\alpha < 2)

Studies on dielectric, optical, magnetic, magnetic domain structure, and resistance switching characteristics of highly c-axis oriented NZFO thin films

α(1<α<2). A generalized approach to both linear frequency domain and non-linear time domain modeling and characterization of fractional-order sigma–delta modulator has been discussed. The performance of such modulator has been studied and compared with the corresponding integer-order modulators through simulation.

Structural, magnetic, and physical properties of La(1–x)MnO3±δ nano-manganite

In this paper, we report the exchange bias (EB) effect along with tunneling magnetoresistance (MR) in polycrystalline Sm0.35Pr0.15Sr0.5MnO3 manganite. Analogous to the shift in the magnetic hysteresis loop along the field (H)-axis, a shift is also observed in the MR-H curve when the sample is cooled in a static dc magnetic field at 5 K. The values of exchange fields (HE), coercivity (HC), remanence asymmetry (ME) and magnetic coercivity (MC) are found to strongly depend on cooling magnetic field and temperature. Magnetic training effect (TE) is supportive experimental evidence which describes the decrease of exchange bias field when sample is successively field-cycled at a particular temperature. We have also observed TE in the shift of the MR-H curve which has been interpreted by the spin relaxation model. We have observed the time dependent resistivity below the spin freezing temperature. The unusual MR behavior is interpreted in terms of the intragranular interface effect between short-range ferromagnetic clusters and spin-glass matrix giving rise to the EB effect, which is of special interest for potential application in multifunctional spintronic devices.

Emergence of ferromagnetism and metallicity in Pr0.5Sr0.5MnO3 nanoparticles

With the rapid development of new device miniaturization technology, there is invigorated interest in magnetic nanostructures for potential application in novel multifunctional devices. In continuation to our search for a suitable magnetic material having Curie temperature (Tc) well above room temperature for multifunctional applications, we have studied the dielectric, optical, magnetic, and resistance switching characteristics of Ni0.65Zn0.35Fe2O4 (NZFO) thin films. The observation of only (004) reflection in the X-ray diffraction patterns confirms the c-axis orientation and high quality growth of NZFO thin films. The presence of mixed valences of Fe2+/Fe3+ cations is probed by X-ray photon spectroscopy, which supports the cationic ordering-mediated large dielectric response. Our investigations reveal NZFO to be an indirect band gap material (∼1.8 eV) with a direct gap at ∼2.55 eV. These nanostructures exhibit high saturation magnetization and a low coercive field with a ferrimagnetic–paramagnetic phase transition of ∼713 K. Magnetic force microscopy studies revealed the stripe-like domain structure of the investigated thin films. In addition, these thin films exhibit reliable and repeatable unipolar resistive switching characteristics. The observed high dielectric permittivity with low loss tangent, large magnetization with soft magnetic behavior, striped magnetic domain structure and reliable resistance switching in NZFO thin films above room temperature suggest potential application in memory, spintronics, and multifunctional devices.

Electronic and magneto transport property of Pr0.7Sr0.3MnO3 system in nanometric gran size modulation

Abstract A detailed structural, magnetic and physics properties of La1−xMnO3±δ (LMO) nanomanganites were investigated to find out the role of cationic vacancies (La vacancy with Mn3+/Mn4+) in grain size modulation. Crystal structure and phase analysis of all samples were carried out by Rietveld refinement of high-resolution XRD and neutron diffraction data. We report here, the oxygen content in studied LMO compound decreases with increase in La vacancies in parent site and a parasitic Mn3O4 phase has been evolved in the range of 0.9 ≥ La/Mn ≥ 0.7. Para to ferro magnetic transition temperature (TC) of all nanometric samples (La/Mn < 0.9) was found at high temperature side (≥260 K) whereas, the same for bulk one (La/Mn ≥ 0.9) was around 160 K. The enhancement of TC (~70 K) with size reduction is attributed to broadening of bandwidth due to compaction of MnO6 octahedra in system unit cell. In bulk sample, a secondary cluster/spin glassy phase is found below 50 K, whereas the glassy phase has been suppressed in nanoscale. Field-dependent magneto-resistance measurements are also carried out for all samples at different temperatures to get a profound insight of magneto-transport dynamics of the present system.

Influence of samarium doping on electronic and magneto-transport properties of La0.9−xSmxSr0.1MnO3 (0.1≤x≤0.5) nanoparticles

In this paper, we have studied low-temperature magnetic properties and transport dynamics of half-doped Pr0.5Sr0.5MnO3 (PSMO) nano-manganite. Nanoparticles with different average grain sizes are synthesized using chemical pyrophoric reaction route and exhibit same crystal structure. It is reported that particle size reduction in half-doped PSMO manganite interestingly leads to ferromagnetic (FM) metallic behavior which is accompanied by coupled positive magnetoresistance (PMR) (up to 9%) at fields (H ≤ 1kOe at 50K) and negative magnetoresistance (NMR) (∼59% at 100K) at 80kOe fields. Interestingly, maximum PMR is appeared at much lower than the critical field (30kOe) for the melting of insulating -antiferromagnetic (AFM) ordered state. Moreover, it is found that low-temperature AFM-FM phase transition, TN ∼ 135K is smeared out with reduction of particle size; however, the transition is prominent in bulk material. A metastable magnetic state with a mixed fraction of FM and AFM phases is observed below TN. Superparamagnetic (SPM) behavior of PSMO nanoparticles has been revealed by probing ac-susceptibility measurements. Low-temperature transport dynamics has been investigated through four probe resistivity and magneto-transport measurements. A size-induced insulating-metallic phase transition is observed on nano dimension in the range of 125-155K under high magnetic fields; however, the bulk counterpart remains insulating over the entire temperature range. The non-trivial electronic- and magnetotransport properties are explained by the inter-grain spin-polarized hopping mechanism through double exchange (DE) interactions. These issues have been revisited and discussed in the framework of enhanced surface disorder, where surface spins plays the crucial role for inter-grain transport dynamics in nanometric dimension. We believe the present field dependent MR holds excellent potential for future spintronic devices and manifests the exotic properties of strongly correlated materials.In this paper, we have studied low-temperature magnetic properties and transport dynamics of half-doped Pr0.5Sr0.5MnO3 (PSMO) nano-manganite. Nanoparticles with different average grain sizes are synthesized using chemical pyrophoric reaction route and exhibit same crystal structure. It is reported that particle size reduction in half-doped PSMO manganite interestingly leads to ferromagnetic (FM) metallic behavior which is accompanied by coupled positive magnetoresistance (PMR) (up to 9%) at fields (H ≤ 1kOe at 50K) and negative magnetoresistance (NMR) (∼59% at 100K) at 80kOe fields. Interestingly, maximum PMR is appeared at much lower than the critical field (30kOe) for the melting of insulating -antiferromagnetic (AFM) ordered state. Moreover, it is found that low-temperature AFM-FM phase transition, TN ∼ 135K is smeared out with reduction of particle size; however, the transition is prominent in bulk material. A metastable magnetic state with a mixed fraction of FM and AFM phases is observed below TN. S...