J.L. Porteseil

Influence of the grain size on the magnetic and magnetomechanical properties of high-purity iron

Abstract The following quantities were measured on high-purity polycrystalline iron: Rayleigh parameters, coercive field, remanent induction and residual stress. The dependences of those quantities on the grain size are presented.

Irreversible evolution in the topology of magnetic domains (invited)

Hysteresis does not vanish in materials essentially free from defects and domain wall pinning. It arises from the existence of many geometrically different domain structures separated by intrinsic energy barriers. Although generally metastable, these states can nevertheless be explored by suitable excursion in the manifold of domain structures, along which irreducible irreversibility is present. This idea is illustrated by experiments performed on garnet layers with uniaxial anisotropy (bubble material). Cooling the layer from T>Tc in a field H normal to it results in various states: parallel stripes (H=0), bubbles of increasing diameters and mixtures of bubbles and stripes (H<52 Oe), and mazes of convoluted stripes with defects (forks, dead ends). Cycling the applied field at constant temperature also causes evolution of the structure. For example, starting from parallel stripes the structure evolves toward a maze with intermediate steps corresponding to stripe folding, and then there is the onset of def...

Predictions of the random potential energy models of domain wall motion: An experimental investigation on high-purity iron

Abstract In several phenomenological models of magnetic domain wall motion, the wall-defect interactions are pictured by a random potential energy V ( x ). These models predict that the following quantities have to remain practically constant: aH c , bH 2 c , bH c / a , bM s / a 2 , M 0 ( H c : coercive field, a and b : parameters of the Rayleigh law, M s : saturated magnetization, M 0 : magnetization at the upper limit of the Rayleigh region). These predictions were tested on polycrystalline specimens of high-purity iron in which the densities of lattice defects (grain boundaries and dislocations) could be varied. The experimental data showed that the above-mentioned quantities, instead of remaining constant, can change by a factor of 3 or more. In view of these results, the statistical assumptions which underly the models of random potential energy are discussed.

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.

Topological hysteresis in stripe domain structures

Abstract Irreversible evolution in stripe domain structures in a bubble material was investigated. Equilibrium parallel stripe structure folds progressively up to the labyrinth one under normal magnetic field cycles. This topological hysteresis, not related to wall-pinning by defects but to nucleation and collapse field problems, shows memory effects of upper fields encountered.

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.

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.

Role of material purity on the structural and magnetic properties of rapidly solidified FeSi 6.5 wt% alloys

Abstract A study has been made on the structural and magnetic properties of rapidly quenched FeSi 6.5 wt% alloys, obtained from high purity (HP) and commercial purity (CP) master alloys and characterised upon a range of annealing treatments. Superior magnetic behavior is exhibited by the HP alloys after annealing, which correlates with enhanced growth of a favourable cube-on-face crystallographic texture. As suggested by metallographic analysis, a surface energy mechanism appears to control the evolution of such texture, with a critical role being played by solute oxygen. Mechanical strength and ductility also appear improved in the HP samples. Substantial loss of Si and weakening of the texture are however observed after high temperature annealing ( T > 1200°C), with ensuing deterioration of the material soft magnetic properties. It is concluded that optimum dc and high frequency magnetic behavior of HP and CP alloy ribbons requires annealing treatments at temperatures around 1100–1150°C, where the achieved material structure provides minimum losses and maximum permeability and the high Si concentration of the as-quenched material is retained.

Magnetic hysteresis near the Curie temperature of Gd

Abstract We measured the temperature dependences of H c , M R and of Barkhausen noise power on a Gd polycrystal. We observed a crossover from Bloch to Ising-type walls and a decrease of the domain size near T c . The magnetizations acquired by alternating and thermal demagnetizations under a bias field were compared.

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.