arXiv subjects

M. K. Dalai

Publications and source records attributed to M. K. Dalai.

7 recordsLinked to original sources

arXiv2013

Effect of Thermal Annealing on Boron Diffusion, Micro-structural, Electrical and Magnetic properties of Laser Ablated CoFeB Thin Films

We report on Boron diffusion and subsequent crystallization of Co$_{40}$Fe$_{40}$B$_{20}$ (CoFeB) thin films on SiO$_2$/Si(001) substrate using pulsed laser deposition. Secondary ion mass spectroscopy reveals Boron diffusion at the interface in both amorphous and crystalline phase of CoFeB. High-resolution transmission electron microscopy reveals a small fraction of nano-crystallites embedded in the amorphous matrix of CoFeB. However, annealing at 400$^\circ$C results in crystallization of CoFe with \textit{bcc} structure along (110) orientation. As-deposited films are non-metallic in nature with the coercivity (H$_c$) of 5Oe while the films annealed at 400$^\circ$C are metallic with a H$_c$ of 135Oe.

cond-mat.mtrl-sci↗

arXiv2012

Rare-earth free yellow-green emitting NaZnPO4:Mn phosphor for lighting applications

Manganese doped sodium zinc phosphate phosphor with exceptional features having ultra-violet (UV) to visible absorption (300-470 nm), yellow-green (~543 nm) broad-band photoluminescence (PL) and appreciable color co-ordinates (x=0.39, y=0.58) is reported. It has a crystal structure consists of discrete PO4 tetrahedra linked by ZnO4 and NaO4 distorted tetrahedral such that three tetrahedra, one of each kind, share one corner. The presence of UV sensitive Zn-O-Zn bonds efficient energy transfer to Mn2+ ions resulted in brightest PL and external quantum yield of 63% at 418 nm. Our experiment demonstrated the possibility of producing inexpensive white-light emitting devices for future.

cond-mat.mtrl-sci↗

arXiv2012

Probing the structure, morphology and multifold blue absorption of a new red-emitting nanophosphor for LEDs

There has been a stringent demand for blue (~450-470 nm) absorbing and red (~611 nm) emitting material system in phosphor converted white light emitting diodes (WLEDs) available in the market. Conventionally used red-emitting Y2O3:Eu3+phosphor has negligible absorption for blue light produced by GaInN based LED chip. To address this issue, a new red-emitting Gd2CaZnO5:Eu3+ (GCZO:Eu3+) nanophosphor system having exceptionally strong absorption for blue (~465 nm) and significant red (~611 nm) photoluminescence (PL) is presented. This is attributed to a dominant f-f transition (5D0\rightarrow7F2) of Eu3+ ions, aroused due to an efficient energy transfer from the Gd3+ sites of the host lattice to Eu3+ ions. X-ray diffraction and microscopy observations revealed the nanocrystalline nature and a bit elongated morphology of the sample respectively. While the energy dispersive x-ray analysis identified the chemical constituents of the GCZO:Eu3+ nanophosphor, the color overlay imaging confirmed the substitution of Eu3+ for Gd3+ ions. It is highly anticipated that the multifold absorption at ~465 nm would certainly improve the color rendering properties of existing WLEDs.

cond-mat.mes-hall↗

arXiv2011

Pseudogap behavior in Pr$_{0.5}$Sr$_{0.5}$MnO$_3$: A photoemission study

The valence band electronic structure of Pr$_{0.5}$Sr$_{0.5}$MnO$_3$ has been investigated across its paramagnetic metallic (PMM) - ferromagnetic metallic (FMM) - antiferromagnetic insulator (AFMI) transition. Using surface sensitive high resolution photoemission we have conclusively demonstrated the presence of a pseudogap of magnitude 80 meV in the near Fermi level electronic spectrum in the PMM and FMM phases and finite intensity at the Fermi level in the charge ordering (CO)-AFMI phase. The pseudogap behavior is explained in terms of the strong electron-phonon interaction and the formation of Jahn Teller (JT) polarons, indicating the charge localizations. The finite intensity at the Fermi level in the insulating phase showed a lack of charge ordering in the surface of the Pr$_{0.5}$Sr$_{0.5}$MnO$_3$ samples.

cond-mat.mtrl-sci↗

arXiv2008

Near Fermi level Electronic Structure of Pr$_{1-x}$Sr$_x$MnO$_3$: Photoemission Study

In this study, we report the observation of a pseudogap associated with the insulator-metal transition in compositions of Pr$_{1-x}$Sr$_x$MnO$_3$ system with no charge ordering. Our valence band photoemission study shows that the observed shifts in the near Fermi level density of states are abrupt at the Curie transtion and occur over an energy scale of $\sim$ 1 eV, strongly suggesting that the charge-ordering gap observed earlier in other manganites and the pseudogap observed here may indeed have same origin. These results could be understood within the framework of models based on electronic phase separation where it has been shown that the pseudogap is a generic feature of the mixed-phase compositions. Also, our band structure calculations on Pr$_{0.75}$Sr$_{0.25}$MnO$_3$ show the possible existence of half-metallicity in this system.

cond-mat.str-el↗

arXiv2008

Pseudogap behavior of phase-separated Sm$_{1-x}$Ca$_x$MnO$_3$ : A comparative photoemission study with double exchange

Using valence band photoemission we have demonstrated the presence of a pseudogap in the near Fermi level electronic spectrum of some of the mixed phase compositions of Sm$_{1-x}$Ca$_x$MnO$_3$ system. The pseudogap was found to grow in size over a large region of the phase diagram of this system, finally leading to a metal-insulator transition. We have made a study comparing the near Fermi level behaviors of this system to those of a canonical double exchange system, namely, La$_{1-x}$Sr$_x$MnO$_3$. This study intends to highlight one of the important differences between the phase separated and double exchange colossal magnetoresistance systems in the nature of their energy gaps across the metal-insulator transitions. These differences could be ascribed to the distortions in the MnO$_6$ octahedra of their structures that regulate the localization of charge carriers. We have discussed our results from the point of view of models based on the idea of phase separation.

cond-mat.mtrl-sci↗

arXiv2006

Electronic structure of Pr$_{0.67}$Ca$_{0.33}$MnO$_3$ near the Fermi level studied by ultraviolet photoelectron and x-ray absorption spectroscopy

We have investigated the temperature-dependent changes in the near-$E$$_F$ occupied and unoccupied states of Pr$_{0.67}$Ca$_{0.33}$MnO$_3$ which shows the presence of ferromagnetic and antiferromagnetic phases. The temperature-dependent changes in the charge and orbital degrees of freedom and associated changes in the Mn 3$d$ - O 2$p$ hybridization result in varied O 2$p$ contributions to the valence band. A quantitative estimate of the charge transfer energy ($E$$_{CT}$) shows a larger value compared to the earlier reported estimates. The charge localization causing the large $E$$_{CT}$ is discussed in terms of different models including the electronic phase separation.

cond-mat.str-el↗