A. Hernando

A soft magnetic wire for sensor applications

The main magnetic characteristics regarding the domain structure and magnetization processes (axial, circular and Matteucci and inverse Wiedemann effects) of amorphous wires and glass-coated microwires are analysed. Magnetic bistability, spontaneously observed in samples with large enough ratio magneto-elastic anisotropy with axial easy axis to shape anisotropy, is the main source for a number of sensor applications in pulse generators, position and field sensors, encoded security tags, rotational counters, magnetostrictive delay lines, and so on. The relevant perspectives of the novel giant magneto-impedance effect recently reported and observed in non-magnetostrictive samples are also introduced.

Magnetic properties and spin disorder in nanocrystalline materials

The more interesting features in magnetism of systems of nanoparticles are reviewed. Tailoring of soft and hard magnetic materials as well as basic studies on magnetic interactions are discussed. Particular emphasis is given to the magnetic properties of the particle shell and grain boundaries, generally different to these of the core, and responsible for phenomena such as interphase exchange penetration, Curie temperature enhancement and magnetic coupling. The magnetic behaviour of different nanocrystalline systems has been described. Spin disorder has been found to be a general trend for the magnetic ground state of the outer shell of magnetic particles. Disorder at the surface can be due to competing interactions with different signs originating from broken bonds or topological disorder (grain boundaries), random surface anisotropy, surface magnetostriction, compositional gradients and in general to the enhanced gradient of different properties at the surface. The spin-glass-like ground state of the surface only affects the macroscopic properties in nanocrystalline samples for which the ratio between the number of atoms at the interface and the number of atoms in the core can be enormous, actually as large as 30%.

Applications of amorphous and nanocrystalline magnetic materials

A review of the main applications of amorphous and nanocrystalline soft magnetic materials is presented in this work. A good control of magnetic properties as anisotropy field, magnetostriction constant, initial susceptibility or switching field is very important in order to find suitable materials for applications. A wide spectrum of magnetostrictive and non-magnetostrictive soft magnetic amorphous materials applications is shown here. Each application is based on a specific property of the sample i.e. magnetic bistability in wires and magnetoelastic resonance in ribbons of magnetoelastic materials, giant magnetoimpedance and ferromagnetic resonance in non-magnetostrictive materials.

A position sensor based on magnetoimpedance

A magnetic‐field sensor based on the giant magnetoimpedance phenomenon is presented. It is shown that a low, negative magnetostriction CoFeBSi amorphous wire can be used to detect the presence or passage of moving pieces or vehicles, simply by pasting a small permanent magnet on the vehicles/pieces. The detection is observed as a decrease in the ac voltage on the wire’s ends. A system of such devices can be used to monitor and control a number of industrial processes.

Effects of bias field and driving current on the equivalent circuit response of magnetoimpedance in amorphous wires

Magnetoimpedance in as-cast, non-magnetostrictive CoFeBSi amorphous ferromagnetic wires, submitted to AC electric current, irms in the 0.1-20 mA range and frequencies between 100 Hz and 100 kHz, is analysed in terms of equivalent circuits. The effects of the bias longitudinal field, Hdc, up to 3600 A m-1 are also investigated. It is shown that the equivalent circuit representing the wire frequency behaviour can be approximated by a series RsLs arrangement, in series with a parallel LpRp arm. Ls and Lp inductor elements are associated with the rotational and domain wall contributions to circumferential permeability, respectively. Rp is related to wall damping and Rs accounts for all the resistances in the circuit (the wire itself, contacts and so on). The circumferential permeability associated with domain wall movements exhibits a maximum for irms=5 mA (that is, a circumferential field Hrmsphi =12 A m-1), similar to the classical behaviour of wall permeability. The increase in bias field has the effect of strongly decreasing the Lp value; for Hdc=3600 A m-1, the series circuit along accounts for the frequency response of the wire. The association of the circuit elements with basic magnetization processes is discussed. Results are interpreted in terms of the influence of both fields (DC bias, Hdcr,and AC circumferential, Hrmsphi fields) on the inner-core-outer-shell magnetic structure of the wire.

An alternative approach to giant magnetoimpedance phenomena in amorphous ferromagnetic wires

Magnetoimpedance in as‐cast, nonmagnetostrictive CoFeBSi amorphous ferromagnetic wires, submitted to an ac electric current of 5 mA in the frequency range 100 Hz–100 kHz, is analyzed in terms of equivalent circuits. It is shown that the equivalent circuit representing the wire frequency behavior can be approximated by a parallel LpRp arrangement with elements Rs and Ls in series; Lp and Rp are associated with circumferential domain wall permeability and wall damping, respectively; Ls is related to the circumferential rotational permeability of the wire, and Rs accounts for the dc resistance in the circuit. When the wire is submitted to a longitudinal dc field high enough to approach saturation (Hdc=3600 A/m), the circuit becomes simply a series RsLs circuit. The various contributions from basic magnetization processes to magnetoimpedance are discussed, as well as deviations from this idealized model.

Transverse demagnetizing factors of long rectangular bars: I. Analytical expressions for extreme values of susceptibility

The demagnetizing problem should be studied numerically in nonellipsoids with nonzero susceptibility χ, except for a few limiting cases where analytical treatment turns out to be practical. In this work, for an infinitely long bar with rectangular cross section 2a×2b and χ=∞ and −1 in a uniform transverse applied field Ha along dimension a or b, analytical expressions for the surface pole or current distributions and the fluxmetric and magnetometric demagnetizing factors Nf,m are derived using a technique of conformal transformation. From the new as well as the existing formulas, Nf,m(χ=∞,0,−1) are evaluated, plotted, and tabulated as functions of a/b.

sp magnetism in clusters of gold thiolates

Using first-principles calculations, we consider the bond between thiolate and small Au clusters, with particular emphasis on the resulting magnetic moment. The moment of pure gold clusters is 1µB for clusters with an odd number of Au atoms and zero for those with an even number. The addition of the thiolate, having an odd number of electrons itself, shifts the phase of the odd-even oscillations so that particles with an even number of Au atoms now have unit moment. Surprisingly, gold thiolate exhibits a dramatic and non- intuitive distribution of charge and spin moment. Our results show that the S-Au bond is such that sulfur does not get charge and an electron is transferred to the Au cluster. This extra electron is mainly sp in character and resides in an electronic shell below the Au surface. The calculations suggest that any thiolate- induced magnetism occurs in the gold nanoparticle and not the thiolate, and can be controlled by modifying the thiolate coverage.

Metallic Magnetic Nanoparticles

In this paper, we reviewed some relevant aspects of the magnetic properties of metallic nanoparticles with small size (below 4 nm), covering the size effects in nanoparticles of magnetic materials, as well as the appearance of magnetism at the nanoscale in materials that are nonferromagnetic in bulk. These results are distributed along the text that has been organized around three important items: fundamental magnetic properties, different fabrication procedures, and characterization techniques. A general introduction and some experimental results recently obtained in Pd and Au nanoparticles have also been included. Finally, the more promising applications of magnetic nanoparticles in biomedicine are indicated. Special care was taken to complete the literature available on the subject.

Thermally activated demagnetization in Co/Ni multilayers involving discrete identifiable stages

We present data, obtained from measured Co/Ni multilayers, that indicate that the magnetic relaxation processes take place through a nucleation-coalescence sequence, and thus crucially implicates the existance of exchange and dipolar interactions. Depending on the layer thickness the relaxation proceeds either through a single global stage or through several partial ones. In both cases the relaxation stages involve a time lapse during which the macroscopic state of the system evolves very slowly, followed by a sharp variation of the total moment of the sample. Such behavior clearly implies a significant departure from the conventionally accepted Arrhenius relaxation kinetics.