J. M. Vargas

Structural and magnetic properties of chemically synthesized Fe doped ZnO

We report on the synthesis of Fe-doped ZnO with nominal composition of Zn0.99Fe0.01O by using a coprecipitation method. X-ray diffraction and selective area electron diffraction studies reveal a single phase wurtzite crystal structure without any secondary phase. Field emission transmission electron microscopy measurements infer that Zn0.99Fe0.01O have nanorod-type microstructures. Magnetic hysteresis measurement performed at different temperatures show that Zn0.99Fe0.01O exhibits a weak ferromagnetic behavior at room temperature. A detailed investigation of the electronic and local structure using O K-, Fe L3,2 near edge x-ray absorption fine structure suggests that Fe is substituting Zn in ZnO matrix and is in Fe3+ state.

The nature and enhancement of magnetic surface contribution in model NiO nanoparticles

We report an alternative synthesis method and novel magnetic properties of Ni-oxide nanoparticles (NPs). The NPs were prepared by thermal decomposition of nickel phosphine complexes in a high-boiling-point organic solvent. These particles exhibit an interesting morphology constituted by a crystalline core and a broad disordered superficial shell. Our results suggest that the magnetic behavior is mainly dominated by strong surface effects at low temperature, which become evident through the observation of shifted hysteresis loops (approximately 2.2 kOe), coercivity enhancement (approximately 10.2 kOe) and high field irreversibility (>or=50 kOe). Both an exchange bias and a vertical shift in magnetization can be observed in this system below 35 K after field cooling. Additionally, the exchange bias field shows a linear dependence on the magnetization shift values, which elucidate the role of pinned spins on the exchange fields. The experimental data are analyzed in terms of the interplay between the interface exchange coupling and the antiferromagnetically ordered structure of the core.

Dipolar interaction and size effects in powder samples of colloidal iron oxide nanoparticles

Dipole–dipole interactions in nanostructured materials deeply affect their magnetic properties, and detailed studies are still required to fully understand them. In this work, the dependence of magnetic properties on particle size has been evaluated in powder samples of Fe oxide nanoparticles produced by colloidal methods. Zero-field-cooled and field-cooled magnetization curves and magnetization versus applied field data have been analysed by taking into account dipolar interactions through a correction to the classical superparamagnetic model. Morphological and magnetic data were in very good agreement, which has allowed us to quantify relevant physical parameters, such as the anisotropy constant, magnetic moment, and interacting volume for our system.

Single-step chemical synthesis of ferrite hollow nanospheres

We report a single-step chemical synthesis of iron oxide hollow nanospheres with 9.3 nm in diameter. The sample presents a narrow particle diameter distribution and chemical homogeneity. The hollow nature of the particles is confirmed by HRTEM and HAADF STEM analysis. Electron and x-ray diffraction show that the outer material component is constituted by 2 nm ferrite crystals. Mössbauer data provide further evidence for the formation of iron oxide with high structural disorder, magnetically ordered at 4.2 K and superparamagnetism at room temperature. An unusual magnetic behavior under an applied field is reported, which can be explained by the large fraction of atoms existing at both inner and outer surfaces.

Synthesis and tuning the exchange bias in Ni–NiO nanoparticulate systems

We report studies on exchange bias effects in Ni–NiO nanoparticles with different particle diameters/distributions and concentration of metallic nickel, which vary from 0% to 32%. The exchange bias field, Hex, depends strongly upon both particle size and the concentration of metallic Ni, being maximum (∼2.2 kOe) at 5 K for the sample with almost negligible concentration of metallic Ni, whereas the corresponding value for the sample with highest concentration of metallic Ni (∼32±5.0%) is about 0.07 kOe. The structural features of the samples have been investigated thoroughly by using the Reitveld refinement of x-ray diffraction data and high resolution transmission electron microscopy, where as the magnetic properties using superconducting quantum interference device magnetometer.

Annealing effects on the magnetization of Co–Ni–B amorphous nanoparticles

Abstract Chemically prepared (Co x Ni 1− x ) 1− y B y ( x =0.5, 0.75, 1; y ≈0.4) amorphous fine particles were characterized by X-ray diffraction, DTA and TGA, and in situ magnetic measurement as a function of annealing temperature in an inert atmosphere. Magnetic measurement performed in as-prepared and ∼150°C annealed samples shows an increase of the saturation magnetization and magnetic moment after thermal treatment. Room temperature magnetization increases by factors of 3.5, 1.8, and 1.5, for x =0.5, 0.75, and 1, respectively. These measurements may indicate a local re-ordering of the amorphous phase at temperatures much lower than the full crystallization temperature.

Experimental evidence for off-center rattling of Yb3+ in the skutterudite compounds of Ce1−xYbxFe4P12

Electron Spin Resonance (ESR) experiments in the filled Cei1-xYbxFe4P12 (x 0.0023) skutterudite compounds of Th cubic symmetry (Im3) reveal the coexistence of two distinct Yb3+ sites with large difference in their site occupation, hyperfine interaction and temperature dependence of the ESR parameters. These results suggest a scenario where the oversized (Fe2P3)4-cages of the Ce1-xYbxFe4P12 compounds are occupied with small number of on-center-Yb3+ ions and a distribution of highly occupied off-center-Yb3+ions, rattling at ~1 GHz and above ~20 GHz, respectively.

Finite Size Effect on the Resonant Microwave Absorption of Er^{3+} Doped Ag Nanoparticles

The in∞uence of flnite sample size efiects and structural defects on the ground state of Er 3+ in diluted Ag:Er alloys is studied. The choused metallic systems were, nanoparticles (5{ 10nm) of the alloy Ag1ixErx (x = 0:04) and the bulk alloy of Ag1ixErx (x = 0:001) which was used as a reference. The Ag:Er alloy nanoparticles were prepared by chemical synthesis that gives an excellent morphological and crystalline homogeneity and the bulk alloy of Ag:Er by a conventional arc-melting. The nanoparticles resonant microwave absorption (Electron Spin Resonance) at X (9.5GHz)-band of Er 3+ obtained at low-T (4{20K) show a T-independent g-value of 6.74(4) and linewidth of 50(5)Oe. However, above T … 20K an exponential T- dependence of the linewidth is observed. A preliminary interpretation of these results suggests that: i) the exchange interaction, JfsS.s, between the Er 3+ localized magnetic moment and the hosts conduction-electrons has been quenched; and ii) at high-T the spin-lattice relaxation is mainly due to the spin-orbit coupling via lattice phonons involving Er 3+ crystal fleld exited levels.

Morphological characterization by HRTEM and STEM of Fe3O4 hollow nano-spheres

Morphology, surface and finite size effects in magnetic nanoparticles have been the subject of growing interest in recent years from both experimental and theoretical point of view [1]. The magnetic properties are strongly associated with the morphological and structural homogeneity of the nanoparticles [2]. Interparticle interactions also play an important role in the magnetic behaviour of an ensemble of nanoparticles, which differs from that of non-interacting systems [3].