Sudhish Kumar

Role of electronic structure and oxygen defects in driving ferromagnetism in nondoped bulk CeO2

This article reports on reversible manipulation of room temperature ferromagnetism (RTFM) in nondoped bulk CeO2. The magnetization measurements establish that paramagnetic CeO2 is driven to a ferromagnetic state, without change in structure, when vacuum annealed at 600 °C. The Ce ions transform from 4+ to 3+ state, accompanied by evolution of oxygen vacancies (VO) during the RTFM transition, as determined by x-ray photoemission. The F+ centers (the electrons in singly occupied oxygen vacancies) play key role in the exchange mechanism. The transition shows complete reversibility where the RTFM is removed by removing the vacancies through re-heating the vacuum-annealed CeO2 in air.

Evidence of defect-induced ferromagnetism and its “switch” action in pristine bulk TiO2

In this work, we demonstrate that room temperature ferromagnetism can be induced in pristine anatase TiO2 paramagnetic bulk powder through extended hydrogenation. Defect complexes, Ti3+–VO (Ti3+ ions accompanied by oxygen vacancies) are clearly identified in hydrogenated TiO2 by combining x-ray diffraction and photoemission spectroscopy. The observed ferromagnetic ordering is reversible that can be switched between “on” and “off” by inducing or removing, respectively, these defect complex. We convincingly elucidate that the factors (i) Ti 3d–O 2p hybridization (iii) F+ centers (the electrons in singly occupied oxygen vacancies), and (iii) oxygen vacancy assisted fragmentation of grains, compositely contribute to the ferromagnetic ordering.

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...

Study of electronic structure and magnetization correlations in hydrogenated and vacuum annealed Ni doped ZnO

The influence of hydrogen and vacuum annealing on the structural, electronic, and magnetic properties of Ni-doped ZnO pellets viz. Zn0.95Ni0.05O has been investigated using x-ray diffraction (XRD), magnetization, resistance measurements, and x-ray photoelectron spectroscopy (XPS). Rietveld refinement of the XRD patterns confirms that all the polycrystalline samples possess wurtzite type hexagonal structure with no evidence of secondary phases. The superconducting quantum interference device magnetometry measurements exhibit a paramagnetic state for the as-synthesized Zn0.95Ni0.05O. However, the post annealing in H2 as well as in vacuum, drive it to a ferromagnetic state at 300 K. The obtained values of coercivity are 177 and 270 Oe, and the saturation magnetizations are 1.67 and 1.91 emu/g, respectively, for the H2 and vacuum annealed samples. Temperature dependent magnetization results show that the Curie temperatures are nearly the same (∼550 K) for both the annealed samples. Resistance of the annealed ...

Magnetic structure of (Fe0.97Cr0.03)2P

Magnetic behaviour of di-metal iron phosphide with a small substitution of iron by chromium, (Fe0.97Cr0.03)2P, has been studied using SQUID magnetometry and powder neutron diffraction. It is paramagnetic at temperatures above ∼180 K with persisting short range ferromagnetic (FM) order. At lower temperatures three different regions of magnetic behaviour are identified. FM order evolves in the region 180 K-120 K but much more slowly and with much less magnetic moments than in Fe2P. In the region 120 K-50 K negative exchange interactions gain some importance leading to a loss of FM order. Below 50 K FM interactions again dominate. Pinning centres influence the behaviour at low temperature up to ∼100 K.

Magnetic behaviour of nano-particles of Fe2.9Zn0.1O4

DC magnetization measurements are reported in the temperature range 20–100 K on a poly-disperse nano-particle sample of the spinel ferrite Fe2.9Zn0.1O4 with a log-normal size distribution of median diameter 43.6 Å and standard deviation 0.58. Outside a core of ordered spins, moments in surface layer are disordered. Results also show some similarities with conventional spin glasses. Blocking temperature exhibits a near linear variation with two-third power of the applied magnetic field and magnetizationM evolves nearly linearly with logarithm of timet. Magnetic anisotropy has been estimated by analysing theM-logt curve. Anisotropy values show a large increase over that of bulk particle samples. Major contribution to this enhancement comes from the disordered surface spins.

On nature of magnetism in ferromagnetic alloys (Fe1−xCox)2P

Inverse magnetic susceptibility χ−1 has been measured as a function of temperature for five alloys in the series (Fe1−xCox)2P. Short range magnetic order above the Curie temperature in the studied alloys was observed. Paramagnetic moments have been obtained using χ−1–T curves. These are much larger than saturation moments showing dominance of the itinerant nature of magnetism. The degree of itinerancy is smaller in the orthorhombic phase than in the hexagonal phase. Substitution of Co causes magnetization to monotonically decrease. Anisotropy drops in the hexagonal phase with increasing Co but in the orthorhombic phase it again progressively increases.

Structural and magnetic properties of (Fe0.93Ni0.07)2P

Structural and magnetic properties of (Fe0.93Ni0.07)2P have been investigated by means of powder x-ray and neutron diffraction, magnetization and paramagnetic susceptibility measurements over a temperature range of 10–500 K. The system crystallizes in the Fe2P type hexagonal structure ( space group, Z = 3) in which the Ni atoms occupy the tetrahedral MI sites with total preference. Refined values of the cell parameters and bond distances are found slightly higher than those for Fe2P and the atomic positional parameters are found quite close to those reported for the parent compound Fe2P. The temperature dependence of the magnetization shows a sharp magnetic phase transition around 298(5) K. However there is difference of the zero-field cooled and field cooled modes of the magnetization, which is indicative of the formation of ferromagnetic clusters. The ferromagnetic to paramagnetic transition shifts towards the higher temperature side with increase in the applied magnetic field, which indicates that the phase transition is a field induced type first-order magnetic phase transition. There is no crystallographic structural transition associated with the magnetic phase transition. The transition is caused by the change in the c/a ratio. The alloy retains the ferromagnetic order of Fe2P with the moments orienting along the [001] direction. The Rhodes–Wohlfarth ratio (µp/µs) of (Fe0.93Ni0.07)2P is found to be 1.53 (>1), showing itinerant magnetism in this compound. At 297 K the magnetic moment at the MI site is found negligible but at the MII site it is ~0.51 µB. The observed non-linearity above Tc in the χ−1–T curve gives clear evidence of the presence of short range magnetic order above Tc. The moments at the MII sites in the paramagnetic state and the exchange interactions are responsible for the short range one-dimensional ferromagnetic chains along [001] well above the Tc in (Fe0.93Ni0.07)2P.

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.