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NiO nanoparticles (NPs) were prepared by a sol-gel process using the citrate route. The sol-gel parameters were tuned to obtain samples with different average particle sizes, ranging from 12 to 70 nm. Magnetic characterization revealed an increase in the blocking temperature with the diameter of the NPs and an increase in the effective magnetic anisotropy (K(eff)) with decreasing particle size. The magnetic moment per particle was calculated for all samples using the susceptibility value at T = 300 K. The number of uncompensated spins per NP was found to be proportional to n (n(S)≡ total number of spins), indicating that they are randomly distributed on the NP surface. For small diameters (<30 nm) the surface anisotropy constant was estimated, using, for NiO NPs, a recent model describing the evolution of K(eff) with particle size. Hysteretic loops performed at low temperatures after field cooling displayed loop shifts (∼6.5 kOe in the field axis and ∼0.18 emu g(-1) vertically), coercive field enhancement (H(C)≈ 4.8 kOe) and training effects for the smaller NPs. The sample with NPs of larger diameters presented magnetic properties close to those of bulk NiO.

The role of the alumina barrier layer thickness (δ(b)) on the growth of Ni nanowires (NWs) in porous anodic alumina (PAA) has been revealed. By varying the final anodization voltage to form dendrites at the bottom of the nanoporous structure, we are able to optimize δ(b) (in the 2-16 nm range), allowing us to obtain a Ni pore filling percentage (f(p)) of almost 100% for δ(b) = 10 nm. However, deviations from this optimal δ(b)-value led to a strong decrease of f(p). Moreover, an increase of the electrodeposition efficiency (EE) and NW homogeneity was also verified for δ(b) up to 10 nm. Such increase in nominal δ(b) leads to a consistent growth rate in all pores and consequently a complete and uniform nanopore filling. On the other hand, the decrease in electrodeposition efficiency visible for δ(b) > 10 nm is related with hydrogen evolution and dielectric breakdown of the insulator layer due to the required high deposition voltages. Non-uniform NW growth is then visible, with the consequent decrease in f(p). The control of the pore filling and length homogeneity of the fabricated 1D metallic nanostructures, combined with the ability to adjust the pore dimensions of PAA, can bring novel approaches for the fabrication of nano-objects and thus exciting new applications.

Magnetic Resonance Imaging (MRI) is a powerful, noninvasive and nondestructive technique, capable of providing three-dimensional (3D) images of living organisms. The use of magnetic contrast agents has allowed clinical researchers and analysts to significantly increase the sensitivity and specificity of MRI, since these agents change the intrinsic properties of the tissues within a living organism, increasing the information present in the images. Advances in nanotechnology and materials science, as well as the research of new magnetic effects, have been the driving forces that are propelling forward the use of magnetic nanostructures as promising alternatives to commercial contrast agents used in MRI. This review discusses the principles associated with the use of contrast agents in MRI, as well as the most recent reports focused on nanostructured contrast agents. The potential applications of gadolinium- (Gd) and manganese- (Mn) based nanomaterials and iron oxide nanoparticles in this imaging technique are discussed as well, from their magnetic behavior to the commonly used materials and nanoarchitectures. Additionally, recent efforts to develop new types of contrast agents based on synthetic antiferromagnetic and high aspect ratio nanostructures are also addressed. Furthermore, the application of these materials in theragnosis, either as contrast agents and controlled drug release systems, contrast agents and thermal therapy materials or contrast agents and radiosensitizers, is also presented.

Ordered hexagonal arrays of Co nanowires (NWs) and nanotubes (NTs), with diameters between 40 and 65 nm, were prepared by potentiostatic electrodeposition into suitably modified nanoporous alumina templates. The geometrical parameters of the NW/NT arrays were tuned by the pore etching process and deposition conditions. The magnetic interactions between NWs/NTs with different diameters were studied using first-order reversal curves (FORCs). From a quantitative analysis of the FORC measurements, we are able to obtain the profiles of the magnetic interactions and the coercive field distributions. In both NW and NT arrays, the magnetic interactions were found to increase with the diameter of the NWs/NTs, exhibiting higher values for NW arrays. A comparative study of the magnetization reversal processes was also performed by analyzing the angular dependence of the coercivity and correlating the experimental data with theoretical calculations based on a simple analytical model. The magnetization in the NW arrays is found to reverse by the nucleation and propagation of a transverse-like domain wall; on the other hand, for the NT arrays a non-monotonic behavior occurs above a diameter of $50 nm, revealing a transition between the vortex and transverse reversal modes. V C 2013 American Institute of Physics. [http://dx