S. K. Hasanain

Ferromagnetism in Li doped ZnO nanoparticles: The role of interstitial Li

ZnO nanoparticles doped with Li (Zn1−yLiyO, y ≤ 0.1) have been investigated with emphasis on the correlation between their magnetic, electronic, and structural properties. In particular, defects such as interstitial Li and Zn atoms, substitutional Li atoms, and oxygen vacancies have been identified by X-ray photoelectron spectroscopy (XPS) and their respective roles in stabilization of the magnetic moment are discussed. X-ray diffraction (XRD) and XPS give clear evidence of Li presence at both substitutional and interstitial sites. XPS studies further show that the amount of substitutional Li defects (Lizn) and interstitial Li defects (Lii) vary non-monotonically with the Li concentration, with the Lii defects being noticeably high for the y = 0.02, 0.08, and 0.10 concentrations, in agreement with the XRD results. Magnetization studies show room temperature ferromagnetism in these nanoparticles with the moment being largest for the particles with high concentration of interstitial lithium and vice versa. Both interstitial Zn (Zni) defects and Zn-O bonds were determined from the Zn LMM Auger peaks; however, the variation of these with Li concentrations was not large. Oxygen vacancies (Vo) concentrations are estimated to be relatively constant over the entire Li concentration range. We relate the Lii and Zni defects to the formation and stabilization of Zn vacancies and thus stabilizing the p-type ferromagnetism predicted for cation (zinc) vacancy in the ZnO type oxides.ZnO nanoparticles doped with Li (Zn1−yLiyO, y ≤ 0.1) have been investigated with emphasis on the correlation between their magnetic, electronic, and structural properties. In particular, defects such as interstitial Li and Zn atoms, substitutional Li atoms, and oxygen vacancies have been identified by X-ray photoelectron spectroscopy (XPS) and their respective roles in stabilization of the magnetic moment are discussed. X-ray diffraction (XRD) and XPS give clear evidence of Li presence at both substitutional and interstitial sites. XPS studies further show that the amount of substitutional Li defects (Lizn) and interstitial Li defects (Lii) vary non-monotonically with the Li concentration, with the Lii defects being noticeably high for the y = 0.02, 0.08, and 0.10 concentrations, in agreement with the XRD results. Magnetization studies show room temperature ferromagnetism in these nanoparticles with the moment being largest for the particles with high concentration of interstitial lithium and vice versa. ...

Magnetic response of core-shell cobalt ferrite nanoparticles at low temperature

Cobalt ferrite nanoparticles (size: 26±4nm) have been synthesized by coprecipitation route. The coercivity of nanoparticles follows a simple model of thermal activation of particle moments over the anisotropy barrier in the temperature range of 30–300K in accordance with Kneller’s law; however, at low temperatures (<30K), the coercivity shows some deviation from this law. The saturation magnetization follows the modified Bloch’s law in the temperature range of 10–300K. Exchange bias (Hex) studies of the samples show that Hex increases with decreasing temperature of the sample. A strong increase in the Hex values is found below 30K for the low applied field (±20kOe), while a smaller increase is found for the high applied field (±90kOe). The slow increase in the exchange bias at high applied field has been attributed to the high field effects on the surface (shell) spins. These shell spins align along the field direction that weakens the core-shell interface interactions leading to the reduction in the exch...

The magnetic behavior of iron oxide passivated iron nanoparticles

Iron oxide passivated iron nanoparticles were synthesized in an inert gas condensation system. The nanoparticles were single domain but not superparamagnetic. Varying amounts of oxide passivation of the nanoparticles were achieved. The oxide-passivated particles exhibit an exchange bias when cooled below a blocking temperature characteristic of the thickness of the oxide layer present. It was found that the exchange bias and blocking temperature both increase with oxide thickness with the blocking temperature in all cases being much lower than the Neel temperature for Fe oxides. We find that the oxide shell-core spin interaction leaves its imprint on the low field dc and ac magnetizations as well. Furthermore, below a characteristic freezing temperature a sharp increase in the field cooled magnetic moment of the samples is evidenced and is suggestive of a spin freezing process at the surface of the particles.

Effects of iron doping on the transport and magnetic behaviour in La0.65Ca0.35Mn1-yFeyO3

We report on the magnetic and transport measurements in an Fe-doped colossal magnetoresistance compound (La0.65Ca0.35MnO3). Increased spin disorder and a decrease of Tc with increasing Fe content are evident. We find that the resistivity data above Tp fits better to a variable range hopping model and the localization length decreases with Fe content. The variations in the critical temperature Tc, confinement length, magnetic moment and magnetoresistance show a rapid change at about 4-5% Fe. The maximum magnetoresistance is seen to increase consistently with the addition of Fe and increases up to 400% for 8% Fe. The effect of Fe is seen to be consistent with the disruption of the Mn-Mn exchange, possibly due to the formation of magnetic clusters.

Stabilization of surface spin glass behavior in core-shell Fe67Co33–CoFe2O4 nanoparticles

Magnetic properties of Co33Fe67–CoFe2O4 (core-shell) nanoparticles are presented. Both dc magnetization and ac susceptibility measurements indicate a spin glass (SG) like transition occurring at TF∼175 K. The SG nature of the transition is also confirmed by the field dependence of the freezing temperature (TF(H)) following the well known Almeida–Thouless line, δTF∼H2/3. Additionally, the particles exhibit a large exchange bias (HEB∼1357 Oe) arising from the core-shell (ferromagnetic-SG) coupling. The unusually high SG transition temperature and large exchange bias effects are attributed to a combination of several factors including the thickness of the amorphous oxide shell and large values of the exchange and anisotropy constants associated with the CoFe2O4 shell.

Experimental observations of field-dependent activation of core and surface spins in Ni-ferrite nanoparticles

The magnetic behavior of Ni-ferrite (NiFe2O4) nanoparticles synthesized in a solid state reaction process has been investigated. The cooling field, HCF, dependence of magnetization in a wide range of temperatures, from 5 to 300 K, has been examined for low and high field regimes. It has been observed that there is a transition region, ~3–4 T, between different mechanisms that controls the magnetization. At low fields, 4 T, starting below a well defined freezing temperature of 50 K. The HCF dependence of magnetic viscosity has shown that there is a significant jump in the relaxation rate of the particles around 4 T which appears as the boundary region for the temperature-dependent magnetization as well. These observations are interpreted as indicating that below the spin freezing temperature there is a boundary field (~4 T) where the strongly pinned surface spins are enabled to be thermally activated while below this field only core spins participate in the magnetic relaxation.

Competing Effects of Cu Ionic Charge and Oxygen Vacancies on the Ferromagnetism of (Zn, Co)O Nanoparticles

We report the effects on the ferromagnetism due to co-doping of ZnO with Co and Cu in the presence of variable numbers of oxygen vacancies. The co-doped nanoparticles Zn0.95−Co0.05CuyO (0.00≤y<0.009) were prepared via the chemical route with oxygen vacancies introduced via annealing in a reducing atmosphere for variable amounts of time. In addition to the magnetization, the particles were characterized by x-ray diffraction (XRD), x-ray photoemission spectroscopy (XPS) and x-ray absorption near edge spectroscopy (XANES). The Co ions were determined to be in the +2 state in a tetrahedral symmetry, with no evidence for metallic Co or Cu. However, the ionic state of Cu is found to change from +2 to +1 state with increasing Cu concentration, which appears to strongly decrease the concentration of oxygen vacancies. It is found that the ferromagnetic moment initially increases with the addition of Cu but decreases above a typical concentration that coincides with the appearance of the Cu+1 state and the decrease of O vacancy concentration. It is concluded that the effect of Cu in the very low range of concentrations, where it appears to go in as Cu+2, is to stabilize ferromagnetism indirectly via generation of O vacancies. The effects of O vacancy concentration on the ferromagnetism are interpreted in the light of the F-center exchange (FCE) model.

Magnetic control of relaxor features in BaZr0.5Ti0.5O3 and CoFe2O4 composite

We report the effect of magnetic field on the dielectric response in a relaxor ferroelectric and ferromagnetic composite (BaZr0.5Ti0.5O3)0.65-(CoFe2O4)0.35. Relaxor characteristics such as dielectric peak temperature and activation energy show a dependence on applied magnetic fields. This is explained in terms of increasing magnetic field induced frustration of the polar nanoregions comprising the relaxor. The results are also consistent with the mean field formalism of dipolar glasses. It is found that the variation of the spin glass order parameter q(T) is consistent with increased frustration and earlier blocking of nanopolar regions with increasing magnetic field.

Role of donor defects in stabilizing room temperature ferromagnetism in (Mn, Co) co-doped ZnO nanoparticles

We report the effects of co-doping ZnO with Co and Mn in an n-type environment on ferromagnetism (FM). Two sets of samples, Zn(0.95-x)Co(0.04)Mn(x)O (0.000 ≤ x ≤ 0.02) and Zn(0.95-y)Co(y)Mn(0.04)O (0.000 ≤ y ≤ 0.02), were synthesized by the chemical route with oxygen vacancies introduced via annealing in a forming gas (reducing the atmosphere). In addition to the magnetization, the particles were characterized by x-ray diffraction, diffuse reflectance spectroscopy and x-ray absorption near-edge emission spectroscopy. The Co and Mn ions were determined to be in the + 2 state in a tetrahedral symmetry, with no evidence of metallic Co or Mn. We find that while a purely Mn-doped sample exhibits weak FM at room temperature, the general effect of Mn as a co-dopant with Co, in an n-type environment, is to decrease the moment strongly. All of our results can be systematically explained within the context of defect mediated FM in these wide bandgap semiconductors, where the coincidence of the spin-split-impurity (defect) band states and the 3d states leads to the development of a net moment alongside the formation of spin polarons.

Disorder and weak localization effects in Co2MnxTi1−xAl Heusler alloy thin films

The effects of disorder on the structural, magnetic and transport properties of stoichiometric Co2MnxTi1?xAl (0???x???1) thin films are reported. All the compositions exhibited B2-type structure with the actual composition of each film, as determined by Rutherford backscattering spectroscopy (RBS), being close to the nominal value. The values of saturation magnetization increase with increasing Mn content in the films and are in general agreement with the values obtained using the Slater?Pauling rule and the actual (RBS determined) compositions. Electrical resistivity as a function of temperature shows a change from metallic-type behaviour for x???0.25 to a semiconducting type for x?>?0.25. At lower temperatures (T?<?25?K) metallic compositions show the presence of a resistivity minimum. The resistivity behaviour above low T upturn in metallic compositions follows a T2 dependence that is explained in terms of electron?electron scattering and one-magnon scattering. The presence of a resistivity minimum and the crossover from metallic to semiconducting type resistivity are explained in terms of localization effects originating mainly in the Co antisite disorder in these alloy films.