M. Jaafar

Remanence of Ni nanowire arrays: Influence of size and labyrinth magnetic structure

The influence of the macroscopic size of the Ni nanowire array system on their remanence state has been investigated. A simple magnetic phenomenological model has been developed to obtain the remanence as a function of the magnetostatic interactions in the array. We observe that, due to the long range of the dipolar interactions between the wires, the size of the sample strongly influence the remanence of the array. On the other hand, the magnetic state of nanowires has been studied by variable field magnetic force microscopy for different remanent states. The distribution of nanowires with the magnetization in up or down directions and the subsequent remanent magnetization has been deduced from the magnetic images. The existence of two short-range magnetic orderings with similar energies can explain the typical labyrinth pattern observed in magnetic force microscopy images of the nanowire arrays.

Control of the chirality and polarity of magnetic vortices in triangular nanodots

Magnetic vortex dynamics in lithographically prepared nanodots is currently a subject of intensive research, particularly after recent demonstration that the vortex polarity can be controlled by in-plane magnetic field. This has stimulated the proposals of nonvolatile vortex magnetic random access memories. In this work, we demonstrate that triangular nanodots offer a real alternative where vortex chirality, in addition to polarity, can be controlled. In the static regime, we show that vortex chirality can be tailored by applying in-plane magnetic field, which is experimentally imaged by means of variable-field magnetic force microscopy. In addition, the polarity can be also controlled by applying a suitable out-of-plane magnetic field component. The experiment and simulations show that to control the vortex polarity, the out- of-plane field component, in this particular case, should be higher than the in-plane nucleation field. Micromagnetic simulations in the dynamical regime show that the magnetic vortex polarity can be changed with short-duration magnetic field pulses, while longer pulses change the vortex chirality.

Magnetic domain structure of nanohole arrays in Ni films

Nanohole arrays in Ni films have been prepared by a replica/antireplica method based on anodic alumina membranes. The nanohole arrays exhibited long range ordering with hexagonal symmetry, the hole distance was kept constant (105nm), and the hole diameter and the film thickness were varied between 50 and 70nm and 55 and 600nm, respectively. The magnetic domain structures of such samples have been studied by analyzing magnetic force microscopy images at remanent state. Different domain structures have been observed depending on the geometrical characteristics of the films. The experimental results have been interpreted with the help of micromagnetic simulations.

Quantitative magnetic force microscopy analysis of the magnetization process in nanowire arrays

Magnetic force microscopy (MFM) imaging is a useful technique to locally study the magnetic state of nanostructures. In this paper, we have used the MFM to characterize an ordered array of Ni nanowires embedded in porous membrane. Due to the large aspect ratio of the wires (30nm diameter and 1000nm length) they present an axial easy axis. Considering the nanowires as nearly single-domain structures and calculating the amount of wires pointing to each direction, we can obtain the average magnetization. An alternative method to analyze the MFM data is here introduced considering the distribution functions of magnetic contrast. By using this method, the magnetization process of the nanowire array is studied and the results are compared with major and minor hysteresis loops measured by superconducting quantum interference device magnetometer.

Preparation and Magnetic Characterization of Ni Membranes With Controlled Highly Ordered Nanohole Arrays

A replica/antireplica method is here proposed for the fabrication of metallic membranes reproducing the ordering of precursor nanoporous alumina membranes. Densely packed arrays of Ni nanoholes with long range ordering of hexagonal symmetry have been prepared. The study includes the magnetic properties of Ni nanohole arrays and those for a continuous Ni film. The magnetization process has been studied by analysis of in-plane and perpendicular hysteresis loops. In addition, combined magnetic force microscopy and atomic force microscopy have allowed us to observe triangular-shaped magnetic domains related to that hexagonal symmetry. An effective in-plane magnetic anisotropy is deduced that determines an in-plane magnetization process by wall displacement which is hindered by the nanoholes in the case of noncontinuous films

Nanostructured Co film on ordered polymer nanohills: a base for novel magnetic nanostructures

The preparation of a nanostructured polymer (poly(methyl methacrylate)--PMMA) replicating the order of an anodized aluminium (Al) template is first described. Nanohills at the PMMA surface are structured following the ordering of the template with tailored lattice parameter and hexagonal symmetry. A nanostructured Co magnetic film is then deposited by sputtering onto the polymer surface so keeping the high ordering induced by the precursor template. The magnetic properties depend on the polymer periodicity as well as on the sputtering conditions. In particular, single-domain or multidomain structures have been identified inside every Co/polymer hill depending on the lattice parameter. Moreover, an intrinsic distribution of the magnetic anisotropy can be deduced from the hysteresis loops.

Titanium nitride stamps replicating nanoporous anodic alumina films

Fabrication of nanostructured TiN films by magnetron sputtering using nanoporous anodic alumina films (NAAF) as substrates is reported. These hard nanostructured films could be used for pre-patterning aluminium foils and to obtain nanoporous films replicating the starting NAAF over a wide range of pore diameters and spacings. Pre-patterned Al foils are obtained by compression with pressures lower than those previously reported, then a new NAAF can be fabricated by means of only one anodization process. As an example, one of the TiN stamps was used for pre-patterning an Al foil at a pressure of 200?kg?cm?2 and then it was anodized in oxalic acid solution obtaining the corresponding replica of the starting NAAF.

Asymmetric magnetization reversal process in Co nanohill arrays

Co thin films deposited by sputtering onto nanostructured polymer [poly(methyl methacrylate)] were prepared following replica-antireplica process based on porous alumina membrane. In addition, different capping layers were deposited onto Co nanohills. Morphological and compositional analysis was performed by atomic force microscopy and x-ray photoemission spectroscopy techniques to obtain information about the surface characteristics. The observed asymmetry in the magnetization reversal process at low temperatures is ascribed to the exchange bias generated by the ferromagnetic-antiferromagnetic interface promoted by the presence of Co oxide detected in all the samples. Especially relevant is the case of the Cr capping, where an enhanced magnetic anisotropy in the Co/Cr interface is deduced.

Co nanostructure arrays in patterned polymeric template

Nanostructured polymer (polymethyl methacrylate) has been fabricated replicating the hexagonal symmetry ordering of an anodized Al template. A magnetic material, in this case Co, is then sputtered onto the polymer surface giving rise to a nanostructured surface thin layer keeping the high ordering induced by the precursor template. The study of this magnetic nanostructure allows one to conclude the presence of intrinsic distribution of magnetic anisotropy and the identification of single or multidomain structures inside Co depending on the controlled periodicity of the nanostructured Al template.

Magnetization process of Ni membranes with controlled highly ordered nanohole arrays

In this work, preparation and characterization of magnetic metallic membranes with well ordered nanoholes that gives rise to particular magnetization process is focused. This nanostructure has been achieved by controlled replica/antireplica processes starting from highly ordered nanoporous alumina membrane.