Olivier Geoffroy

Magneto-optical diffraction and transverse Kerr effect

Abstract The transverse non-specular magneto-optical Kerr effect was measured in diffraction from an inhomogeneous grating made of ferromagnetic Fe 0.75 Si 0.25 wires on a silicon substrate. The relative change in intensity of the diffracted beams when magnetisation is reversed (Δ I/I ) n shows a large enhancement (from 1.8 × 10 −2 up to 3 × 10 −1 ) for angles of incidence corresponding to minima of I n . The relative magnetic response, Δ D n , where D n is the diffraction efficiency ( I n /I inc ), reaches maximum values for non-zero diffracted orders and is higher than that obtained in specular conditions.

On the fractal nature of Barkhausen noise in magnetically soft materials

Abstract Concepts in fractal noises are applied to the ferromagnetic Barkhausen noise (BN). Various soft materials (amorphous alloy, 20–80 NiFe alloy, non-oriented FeSi) feature noise dimensions between 1.5 and 1.7. The scaling properties of the BN are examined (1⧸f power spectrum, Weierstrass distribution, wavelet expansion). It is argued that the hierarchical structure of the BN reflects scaling, avalanche-like irreversible processes.

Practical and Theoretical Investigations of a Rotating Coilless Actuator Using the Inverse Magnetostrictive Effect

In this paper, we present a novel rotating actuator based on the interaction of a permanent magnet with a magnetostrictive soft ribbon submitted to mechanical stress. The direction and magnitude of the stress generates a directional magnetic anisotropy (easy axis) in the soft ribbon. A nonzero angle between the polarizing magnet and the induced easy axis results in a torque between the two: hence the magnet being free to rotate aligns itself with the easy axis. A demonstrator of this principle was made using a Nd-Fe-B magnet above a disk cut from an iron-based amorphous ribbon glued between two unidirectional piezoelectric actuators. The analytical model takes into account the magnetic domains and assumes a magnetization mechanism of both coherent rotation and domain-wall displacement, with a uniaxial anisotropy induced by the applied stress.

Scaling properties of irreversible magnetization and their consequences on the macroscopic behaviour of soft materials

Abstract The scaling properties of irreversible magnetization in soft materials (FeSi, Permalloy, amorphous ribbon) have been investigated both in space and time. Barkhausen jumps tend to cluster into avalanches which exhibit a hierarchical character (self-similarity) over two orders of magnitude of time. Close fractal dimensions (1.5–1.7) for noises from various samples indicate a fairly universal behaviour. Barkhausen sequences are simulated by means of sandpile algorithms initiated by Bak et al. It is argued that magnetization processes in the neighbourhood of H c afford an example of self-organized critical regime. On the other hand, the local pinning force experienced by a wall in a FeSi crystal exhibits scaling properties in space. Consequently, simple analytical laws can be derived for the permeability and losses. The predictions of the model are compared with experiment in the frequency range 0.5–100 Hz.

Rotating sample magnetometer for precise, real time differential measurements

In this paper we present the development of a rotating sample magnetometer (RSM) for real time differential magnetic moment measurement. The high accuracy and simplicity of the described apparatus allow reliable and fast magnetic characterization of samples with small magnetic moment. This performances (about 10−10Am2 with our prototype), associated with the low cost of the apparatus, should make it a very promising tool for thin magnetic film characterization.

Wall pinning by nanocrystals in an amorphous matrix

Abstract The random potential energy of a Bloch wall inside a non-magnetostrictive, amorphous medium containing randomly distributed nanocrystals is estimated from simple statistical arguments. Expressions for the coercive field and coefficients of the Rayleigh law are derived and compared with experimental data from a Co-based amorphous ribbon.

An energy-based model for dynamic hysteresis

A new approach to face the problem of dynamic hysteresis and assess extra losses in soft magnetic materials is investigated. Thanks to a spatial averaging technique and a variational principle, a dedicated formulation is derived. It takes into account microscopic magnetization mechanisms and domain wall motion-induced Joule losses. Implementation within the finite element method is discussed, and calculations are carried out for a 2-D geometry. The model provides first-order low-pass filtering properties.

SANDPILE SIMULATION OF BARKHAUSEN NOISE IN SOFT MAGNETIC MATERIALS

Abstract Irreversible, avalanchelike magnetization jumps in soft materials are simulated by sandpile algorithms. Simulated noise sequences are compared with the experimental Barkhausen noise (BN). It is argued that magnetization processes in the neighbourhood of the coercive field are an example of self-organised critical regime. This point of view is supported by comparing the fractal dimensions of the experimental and simulated noises: D = 1.7. Simple improvements of the algorithm enable the rise and fall of the BN along a hysteresis branch to be reproduced in a straightforward manner. It is shown that the scaling properties of the BN influence the energy loss in soft conducting materials.

Modelization of Superferromagnetism in Soft Nanocrystalline Materials Based on an Accurate Description of Magnetostatic Interactions

At high temperature, Fe-Si nanograins obtained by partial crystallization of amorphous Fe-Si-Cu-Nb-B precursor are superparamagnetic, due to the disappearance of the magnetism of the residual amorphous matrix. At a transition temperature Ttr above the amorphous Curie temperature, a spontaneous polarization JSsf appears, despite a much lower blocking temperature. Dipolar interactions, or a residual ferromagnetic coupling between grains, have been invoked to explain this effect. We investigate here the dipolar hypothesis. The Lorentz field (L.f.) model is improved the idea being that the field acting on a nanograin is screened by a surrounding soft shell. The attenuation factor is calculated, leading to a decrease of Ttr compared with the L.f. approximation. Moreover, a description of the spontaneous magnetization curve JSsf (T)/JS(T) is obtained, with a slope near Ttr much sharper than predicted by the (Ttr - T)1/2 law associated to an invariant molecular field factor. Comparison with the experiments shows good agreement.

Contribution to quantitative interpretation of losses developed in soft materials

Abstract From the basic equations of motion for a domain wall, expressions for losses are given in a nonperfect metallic crystal. It is shown that two different cases have to be considered: discontinuous magnetization at low frequencies, and continuous motion of the wall at higher frequencies.