S. N. Dolia

Study of defect-induced ferromagnetism in hydrogenated anatase TiO2:Co

Electronic and magnetic properties of Co-doped TiO2 polycrystalline pellets (Ti0.95Co0.05O2) have been investigated using x-ray diffraction, x-ray photoemission, magnetization, and resistance measurements. The as-synthesized and hydrogenated specimens crystallize in the anatase type tetragonal structure containing very small (∼4.4%) rutile phase. The dopant ions of Co are found to be divalent and well incorporated into TiO2 lattice, substituting the Ti site within the anatase phase, with no evidence of metallic Co or any other oxides of Co. The Co doping induces a weak ferromagnetic ordering in the diamagnetic TiO2 host matrix. Interestingly, when the Co-doped TiO2 is annealed in hydrogen atmosphere, it shows a giant enhancement in magnetization. However, an extended reheating in air causes this H-induced magnetization to vanish and the sample regains the as-prepared status. Our findings indicate that ferromagnetism originates from the doped matrix rather than any magnetic clusters and strongly correlated...

MAGNETIC BEHAVIOR OF FUNCTIONALLY COATED SUPER-PARAMAGNETIC Ni0.5Cu0.5Fe2O4-POLYPYRROLE NANO-COMPOSITES

This article has been retracted. A statement of retraction is published in Mod. Phys. Lett. B Volume 28, Issue 26 (2014) http://dx.doi.org/10.1142/S0217984914930014

Magnetization enhancement in nanocrystalline Co0.4Zn0.6Fe2O4 by 200 MeV Ag15+ ion irradiation

The effects of 200 MeV Ag15+ ion irradiation on the magnetic behavior of nano particles of Co0.4Zn0.6Fe2O4 have been studied using the field, temperature and time-dependent magnetization measurements. Rietveld profile refinement of the XRD patterns confirms the formation of a cubic spinel structure of the specimens retaining the spinel structure upon irradiation. The Ag15+ ion irradiation of the sample causes an appreciable enhancement in the saturation magnetization, coercivity, and remanent magnetization. These observations can be attributed to a slight increase in the particle size due to the heat evolved during the irradiation, cation redistribution, and irradiation-induced modifications on the surface states of the nanoparticles.

Influence of 50 MeV Li3+-ion irradation on M–H loop characteristics of Y3+-substituted YIG

The consequences of 50 MeV Li3+-ion irradiation on the magnetic hysteresis loop characteristics (H max=9 kOe, T=300 and 20 K) are investigated on polycrystalline samples of the Y3+x Fe5−x O12 (x=0.0, 0.2, 0.4 and 0.6) garnet system. The M–H loop characteristics reveal the co-existence of a ferrimagnetic component of the yttrium iron garnet phase and a weak ferromagnetic component of the yttrium orthoferrite phase. It is found that swift heavy ion irradiation (SHII) effectively reduces the unwanted YFeO3 phase. The results are explained in the light of a redistribution of cations and/or SHII-induced formation of paramagnetic centers in the irradiated samples.

Study of ferromagnetism in Mn doped ZnO dilute semiconductor system

The wide band gap semiconductor ZnO when doped with a very low percent of some transition metal ions can exhibit above room temperature ferromagnetism, transforming it into a unique compound for spin-electronic applications. In the present work we have compared the electronic structure of two polycrystalline Zn1-xMnxO pellets (for x=0.02 and 0.04), prepared by low temperature processing, and carefully characterized. The Rietveld refinement of the XRD patters established that the samples have the ZnO lattice with ZnS type Wurtzite hexagonal symmetry and no detectable impurities. The samples exhibit distinctly different magnetic properties. The pure ZnO pellet shows a diamagnetic behaviour, the 2% sample displayed a clear FM ordering at 300 K while the 4% sample did not show any ordering even upon cooling. Their electronic structure has been investigated using x-ray absorption and x-ray photoemission spectroscopy with an aim to find out how the changes in the electronic structure can correlate to the magnetic properties in such diluted magnetic semiconductor materials. The results show that most of the Mn ions of the ferromagnetic sample are in the divalent state. For the higher Mn percent nonmagnetic sample, a larger contribution of higher oxidation Mn states are dominant and the oxygen content also increases. The two factors can be correlated to the suppressed ferromagnetism, though it is hard to exactly predict that which of these two factors weighs more.

200 MeV Ag+15 ion irradiation-induced modifications in structural, magnetic and dielectric properties of nanoparticles of Cu0.2Zn0.8Fe2O4 ferrite

The present investigation aims at studying the effect of swift heavy ion irradiation on the structural, magnetic and dielectric properties of the nanocrystalline Cu0.2Zn0.8Fe2O4 spinel ferrite. The sample was synthesised using the sol–gel technique and then irradiated with the 200 MeV Ag+15 ion beam. The Rietveld profile refinement of the X-ray diffraction patterns confirmed the cubic spinel structure of samples. The spherical morphology revealed through transmission electron microscopy images was consistent with the crystalline diameter. The overall magnetic behaviour pointed towards superparamagnetic relaxation at room temperature along with the significant increase in saturation magnetisation, coercivity and blocking temperature after irradiation. This could be attributed to the slight increase in the particle size and ion-induced modifications on the surface states of the nanoparticles. The enhancement in dielectric constant and loss tangent after irradiation could be attributed to the available Fe+2 ↔ Fe+3 and/or Zn+2 ↔ Zn+3 ion polarisation at the octahedral site, especially on grain boundaries of the sample.

Pr Substitution at Y and Ba sites in YBCO (123) System

We present structural, transport, and iodometric study of YBa2Cu3O7−δ system with Pr3+ substituting both the Y3+ and the Ba2+sites. The rate of the Tc depression in the case when with Pr3+ substitutes the Ba2+ is much higher than the case when Pr3+ is substituted at the Y3+ site. This is explained due to a composite effect of the depletion of itinerant holes due to the progressive depletion of the oxygen content, and the loss of orthorhombicity. Further studies on electronic structure and itinerant holes and their relation with superconductivity are underway.

Study of room temperature ferromagnetism for cobalt and manganese doped ZnO diluted magnetic semiconductor

We report the results of electronic and magnetic properties of poly-crystalline Zn1-xMxO (M = Co and Mn) pellets studied by XRD, VSM and XPS. The specimens were synthesized by solid state reaction method using high purity oxides. Samples exhibit Wurtzite hexagonal symmetry and show interesting magnetic properties. The Co (5%) doped sample prepared by heating in air shows a paramagnetic state in agreement with several claims. But the sample prepared by heating in Ar atmosphere depicts ferromagnetic ordering. Addition of 2% Mn in it further enhances its magnetic moment. The specimens ZnCoO (Ar heated) and ZnCoMnO were hydrogenated at 550°C that caused a significant change in their magnetization. The Co doped ZnO shows a robust increment in the magnetic moment but the Mn doping does not seem to respond to hydrogenation. O 1s XPS results show clear evidence of oxygen depletion for the samples prepared by heating in Ar atmosphere and a further depletion upon hydrogenation. Results suggest oxygen vacancies as the basic origin for room temperature ferromagnetism in doped ZnO.

ROOM TEMPERATURE FERROMAGNETISM IN Mn DOPED ZnO SEMICONDUCTOR

Following the theoretical prediction of ferromagnetism in Mn- and Co-doped ZnO, there has been an immense experimental search for dilute semiconductors that show ferromagnetic ordering above room temperature, and several workers have reported ferromagnetism in bulk samples as well as in thin films of these materials. Mn-doped ZnO is the key material in this regard, which has been, in the recent past, shown to exhibit such magnetic properties. Many more such attempts have either led to failure or to a much lower Tc, and there have been a lot of confronting reports casting considerable doubts on the magnetism in this system. In order to shed some light, we have prepared and characterized dilute Mn-doped (2 and 4%) ZnO pellets. SQUID measurements confirm that the 2% sample shows above room temperature ferromagnetic ordering, the ferromagnetic contribution coming mainly from the bulk. The ordering gets completely quenched for 4% Mn doping. Upon cooling down, the 2% Mn doped sample showed further enhancement in magnetic properties appreciably. On the other hand, the 4% sample did not show any ferromagnetic ordering, even down to 5 K, and has been found to retain the paramagnetic character.

Optical Band Gap Study Of Nanocrystalline NiCr0.8Fe1.2O4 Ferrite

Optical energy band gap of nanocrystalline NiCr0.8Fe1.2O4 ferrite have been studied. The nanocrystalline NiCr0.8Fe1.2O4 ferrite have been synthesized using sol‐gel technique. X‐ray diffraction pattern confirms the formation of spinel structure in single phase and the average particle size is 4 nm. The energy band gap measurements of nanocrystalline NiCr0.8Fe1.2O4 ferrite in pellet form have been carried out by absorption spectra using double beam spectrophotometer. A pellet of nanoparticle ferrite was made under a load of 10 tons. From the analysis of absorption spectra, nanocrystalline NiCr0.8Fe1.2O4 ferrite have been found to have energy band gap of 3.2 eV at room temperature.