1,720,973 research outputs found
Influence of domain walls on the singular point detection of energy losses in hard magnetic materials
No full textThe imaginary part of the second harmonic of the reversible parallel susceptibility (RPS) has been recently demonstrated to be a sensitive probe for determining the anisotropy fields in polycrystalline hard magnetic materials, by means of the singular point detection technique (SPD). A modified SPD theory is depicted here considering the presence of domain walls in the multi-domain crystallites, on the basis of Néel's phase theory. Measurements performed on Ba-ferrite random powder show that the energy losses related to domain wall motion are likely at the origin of the observed effect
Micromagnetic analysis of the exchange-coupled hard-soft planar nano-composites
No full textA complete magnetic phase diagram for exchange-coupled planar hard-soft nanocomposites has been obtained in the frame of a one-dimensional micromagnetic model describing the dependence of the properties along the growth direction. The phase diagram in terms of layer thicknesses provides information on the type of demagnetization processes and the critical fields at which nucleation and reversal take place. The basic criterion to this purpose is the analytical expression we have obtained of the critical susceptibility at the nucleation field. The phase diagram is divided into three regions: the exchange-spring magnet (ES), the rigid composite magnet (RM), and the decoupled magnet (DM). The main boundary line is an U-shaped line corresponding to divergence of the critical susceptibility. The diagram also reports the isocritical field lines both for the nucleation and the reversal field. These lines bifurcate along the RM boundary line. The essential characteristics of the phase diagram are directly connected with the intrinsic properties of the chosen soft and hard materials. With increasing ratio between the anisotropy constants of soft to hard phases the ES region is reduced until it disappears at a critical value. The model includes as limiting cases the classical problems of the planar soft inclusion in a bulk magnet and of the domain-wall depinning at the hard-soft interface
A Vibrating Wire Susceptometer with a special electronic control for fast measurements at high temperatures
No full textThe measurement of the temperature behaviour of initial magnetic susceptibility is a powerful method for the thermomagnetic analysis of ferromagnetic materials. However, its application to nanostructured materials with technical relevance, particularly in the case of metastable systems, is made difficult by several conflicting conditions: the necessity to employ low magnetic fields, the required high sensitivity and the need for rapid scans in the high-temperature range. The vibrating wire susceptometer, an instrument belonging to the class of alternating gradient force magnetometers, has, in theory, the right characteristics to make such measurements. However, management of the instrument when carrying out rapid scans is intrinsically complex and requires a special electronic controller described here in detail. A combination of two phase-locked loop blocks is needed to provide the correct phase shift to ensure the locking of the resonance frequency while the instrument is working. A new measurement procedure that keeps the oscillation amplitude constant has also been implemented and it has proved to be very useful for rapid overview of the sample magnetic properties. The limitations of the controller performance due to the presence of noise are discussed. Extensive test measurements were carried out and analysed
Flux reversal in hard-soft composite magnets
No full textWe analyze the reversal process and calculate the critical fields in the relevant case of a hard bulk enclosing a soft layer of varying thickness. The system behaves like a rigid magnet, an exchange spring magnet or a decoupled magnet in well-defined intervals of the layer thickness. The analysis is performed by considering a one-dimensional micromagnetic model of the hard-soft multilayer magnet. In particular, an analytical expression of the domain wall depinning field is obtained. In the case of the NdFeB/Fe system, the depinning field is about 10% of the hard-phase anisotropy field
Exchange coupling in nano-metric Fe/Co multilayers
No full textFe/Co multilayers were e-beam evaporated in uhv. Iron thickness was comparable to the domain wall width of bulk iron, and three times that of cobalt. A single-phase magnetic behavior was observed with saturation magnetization comparable to that of iron and coercive force just below that of cobalt film. The squareness of hysteresis loop indicates high homogeneity and low defectivity
Hard-soft composite magnets (invited)
No full textA thorough micromagnetic analysis of the exchange-spring problem is reported with special emphasis on multilayers constituted by hard-soft exchange-coupled phases. The developed one-dimensional micromagnetic model leads to a complete magnetic phase diagram in terms of layer thicknesses. Both perpendicular and parallel configurations are considered. The phase diagram provides information on the type of demagnetization processes and the critical fields at which nucleation and reversal take place, depending on the intrinsic properties of the chosen soft and hard materials. The model has been applied to a variety of hard magnetic phases (e.g., FePt, CoPt, SmCo and NdFeB), coupled to different soft materials (e.g., Fe, FeCo, FeRh or permalloy) both in the form of bilayers and multilayers. The most significant results will be highlighted. (C) 2007 Elsevier B.V. All rights reserved
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