D. Navas

Magnetic properties of densely packed arrays of Ni nanowires as a function of their diameter and lattice parameter

High-quality densely packed hexagonal arrays of Ni nanowires have been prepared by filling self-ordered nanopores in alumina membranes. Nanowires with different diameter d (18–83 nm) and lattice parameter D (65 and 105 nm) have been studied by atomic force, high resolution scanning electron microscopies, Rutherford backscattering, and vibrating sample magnetometer techniques. Axial loops coercivity and remanence decrease with increasing ratio diameter to lattice parameter, r, until nanowires start to interconnect locally. Additionally, hysteresis of in-plane loops increases with packing factor. In order to interpret the experimental results, multipolar magnetostatic interactions among nanowires with increasing ratio r are considered.

Magneto-optical properties of nickel nanowire arrays

We report on the magneto-optical properties of hexagonally arranged Ni nanowires embedded in anodic alumina templates. Due to the nanowire geometry, these samples show different response depending on the polarization orientation of the incident light, which leads to a high anisotropy of both their optical and magneto-optical properties. A strong increase of the magneto-optical activity is clearly observed with respect to the Ni bulk material. We associate this behavior to plasmon resonance of the Ni nanowires.

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.

Magnetic and structural properties of fcc/hcp bi-crystalline multilayer Co nanowire arrays prepared by controlled electroplating

We report on the structural and magnetic properties of crystalline bi-phase Co nanowires, electrodeposited into the pores of anodized alumina membranes, as a function of their length. Co nanowires present two different coexistent crystalline structures (fcc and hcp) that can be controlled by the time of pulsed electrodeposition. The fcc crystalline phase grows at the early stage and is present at the bottom of all the nanowires, strongly influencing their magnetic behavior. Both structural and magnetic characterizations indicate that the length of the fcc phase is constant at around 260–270 nm. X-ray diffraction measurements revealed a strong preferential orientation (texture) in the (1 0–1 0) direction for the hcp phase, which increases the nanowire length as well as crystalline grain size, degree of orientation, and volume fraction of oriented material. The first-order reversal curve (FORC) method was used to infer both qualitatively and quantitatively the complex magnetization reversal of the nanowires...

Ordered Ni nanohole arrays with engineered geometrical aspects and magnetic anisotropy

Ni nanohole arrays are prepared by a replication process involving sputtering, polymer molding pressing, and electroplating techniques, using anodic alumina membranes as templates. Nanohole diameter to interhole distance ratio is engineered by suitable template processing. From the analysis of the magnetization curves for increasing nanohole diameter, it is concluded that coercivity increases due to the pinning of domain walls to nanoholes, while in-plane anisotropy decreases owing to local shape anisotropy effects.

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.

Plasmon-enhanced magneto-optical activity in ferromagnetic membranes

Experimental and theoretical evidence of plasmon-enhanced Kerr rotation in purely ferromagnetic membranes with sufficiently small dimensions to be out of extraordinary optical transmission conditions (45 nm pore diameter, 90nm lattice constant), is reported in this work. It is shown that the spectral location of the enhanced Kerr rotation region varies as the refractive index of the material inside the pore is modified. A similar behavior is obtained if the pore radius changes while keeping the pore concentration unchanged. Those are clear signatures indicating that localized surface plasmon resonances propagating along the pores govern the magneto-optical response of the membrane.

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

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.