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The giant magneto-impedance effect ͑GMI͒ is studied as a function of the structural modification induced in an Fe 73.5 Si 13.5 B 9 Cu 1 Nb 3 amorphous alloy wire by annealing. The values of GMI are correlated to those structural changes and with the corresponding variation of the magnetic properties and intrinsic resistivity. Excellent soft magnetic properties, associated with low resistivity values, make this nanostructured material as one of the most promising for future applications of the GMI effect. The tailoring of the structure which can be induced by adequate thermal treatments easily allows one to obtain excellent combinations of circumferential permeability and resistivity during different devitrification stages, in order to produce materials for specific aims. Maximum GMI ratios of 200% are found after annealing the wires in the range 550-600°C, where an optimum compromise between and is found. A simple model is developed correlating the fundamental physical properties of the soft magnetic wires with the measured values of both components of the impedance, allowing the prediction of experimental GMI ratios and an easy visualization of the phenomenon.

The effective anisotropy of hard-soft magnetic nanostructures is analyzed using the concept of the exchange correlation length of both phases. The dependence of coercivity on volume fraction, fluctuation length, temperature, and magnetic properties of the components is derived from the degree of magnetic coupling, defined through an effective interphase exchange constant. Coercivity and remanence measurements carried out on devitrified FeZrBCu amorphous alloys point out the transition from an uncoupled to a coupled regime by increasing the temperature in a very diluted system, according to the predictions of the analysis.

The magnetization reversal in ordered arrays of Co nanowires with tailored hcp-phase texture, controlled by pH synthesis and nanowires length, has been investigated. The angular dependence of coercivity has been experimentally determined for different crystal textures, and the corresponding magnetization reversal mode is interpreted by analytical modelling. The results show that reversal takes place by propagation of a transverse-like domain wall mode. The fitting of experimental and calculated data allows us the quantitative evaluation of the magnetocrystalline anisotropy constant strength whose magnetization easy direction evolves from parallel to the wires toward in-plane orientation with the change of hcp-phase texture. The simple geometry and high aspect ratios of arrays of magnetic nanowires make them a model system for the study of magnetic phenomena in uniaxial nanomagnets for modern devices applications.1,2 While ultrasoft magnetic nanowires (i.e., permalloy) have been exhaustively investigated, Co nanowires form a particularly interesting system as it is a hard magnetic material which magnetic properties strongly depend on their crystal structure (i.e., phases, texture, and grain size). The control of the orientation of hcp-c axis (i.e., magnetization easy axis of magnetocristalline anisotropy, K mc ) results in the control of the Co nanowires effective anisotropy. A strong longitudinal magnetic anisotropy is achieved when the c axis is oriented parallel to the nanowires, so reinforcing the shape anisotropy. Since the magnetization reversal process is determined by the strength and orientation of the effective magnetic anisotropy, its detailed control and understanding will benefit advances in those applications.In Co nanowires prepared by electrochemical route, crystalline structure can be tuned by adjusting fabrication parameters as current density, plating time, pH, pore diameter, or annealing and deposition under external magnetic fields.3-7 Particularly, it has been shown that pH-controlled electroplating enables the switching between fcc and hcp-Co phases, which modifies the magnetization easy axis from parallel to perpendicular to the wires. 6,7 This is typically qualitatively concluded from the differences in the hysteresis loops shape between parallel and perpendicular applied magnetic field configurations.To achieve full understanding of the magnetization reversal defined by a given anisotropy, the study of coercivity and, specifically of its angular dependence, is an useful tool. Different reversal modes can be induced by suitable modification of the K mc parameter. This feature is relevant for the design of hybrid systems, as multilayers of different Co crystallographic structures, and consequently different controlled reversal modes, which is of interest in spintronic and microwaves devices. Previous works on Co/Cu multilayer nanowires 8 show that competing anisotropies can be present in nanowires. The control of magnetic anisotropy in these nanostructured systems is very important both fo...

Cobalt nanowires, 40 nm in diameter and several micrometers long, have been grown by controlled electrodeposition into ordered anodic alumina templates. The hcp crystal symmetry is tuned by a suitable choice of the electrolyte pH (between 3.5 and 6.0) during growth. Systematic high resolution transmission electron microscopy imaging and analysis of the electron diffraction patterns reveals a dependence of crystal orientation from electrolyte pH. The tailored modification of the crystalline signature results in the reorientation of the magnetocrystalline anisotropy and increasing experimental coercivity and squareness with decreasing polar angle of the 'c' growth axis. Micromagnetic modeling of the demagnetization process and its angular dependence is in agreement with the experiment and allows us to establish the change in the character of the magnetization reversal: from quasi-curling to vortex domain wall propagation modes when the crystal 'c' axis tilts more than 75° in respect to the nanowire axis.

Ferromagnetic resonance (FMR) in a single thin conducting ferromagnetic wire is investigated from theoretical and experimental points of view. It is shown that the wire radius, the symmetry of microwave magnetic field at the sample surface, and the skin depth (magnetic and nonmagnetic) should be considered as a whole for a correct interpretation of the microwave absorption. As a consequence, various resonance modes can be excited in metallic wires. The resonance fields of bulk samples satisfy the Kittel's resonance condition for a thin planar plate (FMR 0). However, as the wire radius decreases below the nonmagnetic skin depth a weak resonance peak separates from the main resonance and moves to the field fulfilling the Kittel's resonance condition for an axially magnetized cylinder (FMR 1). Theoretical predictions show that this "insulator" resonance mode should be the dominant one for a nanowire, where the radius is much smaller than the minimum magnetic skin depth. The existence of the two resonance modes is supported by experimental results on thin (down to 1.5-μm thick) amorphous microwires.

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