FACTORN

1 THE SELF-DEMAGNETIZATION FACTOR OF BONDED MAGNETS N.V. KHANH Quy nhon Pedagogical University, N.V. VUONG, M.M. TAN Institute of Materials Sc ience, NCST of Vietnam. The self-demagnetization effect in bonded magnets (BM), which are compacted from ferromagnetic particles embedded in a binder medium, was investigated by means of the numerical computation. It is shown that the self-demagnetization factor is dependent not only on the shape but also the granular structure of magnets. The calcu- lated factors N as functions of the magnet shape, mass density and particle orientation have been presented. The influence of the self-demagnetization effect on the characteri- zation of BMs is also discussed. I. INTRODUCTION Permanent magnets have become indispensa ble components in the modern tech- nology . They play an important role in many electromechanical and electronic devices used in domest ic and professional appliance [1]. The magnetization of a piece of ferromagnet ic mater ial has its or igin in the spin and or bi tal magnetic moment of the atomic electrons. Magnetized in a magnetizing f ield H ex , the ferromagnetic sample can be prepared in a metasta ble state where it retains some net magnet ization M and becomes a permanent magnet [2]. The magnetic f ield H of a magnet is more lik e a di pole f ield and may be est i- mated if the magnet is replaced by an equivalent surface distr ibution of magnet ic charges which act as sources or sink s of H. An induction B in a given mater ial is the secondary f ield which is induced in it by H. Outside the magnet volume the di pole f iel d is k nown as the stray f ield. In free space around the magnet, the magnetic induction B o and strengt h H o are parallel and proportional, B o = Q o H o . Within the magnet volume, the di pole f ield, H i, is k nown as the demagnet izing f ield since to reduce its M and the rela- tion between B i , H i and M is: B i = Q o (H i +M). (1) In a uniformly magnetized elli psoidal magnet , H i is uniform throughout the magnet volume and H i and M are related by the equation [3]: H i = -N M, (2) when M lies along the pr inci pal axis of the magnet , so B i = Q o M(1-N). (3) Here N is the self -demagnet ization factor and has a value between 0 and 1 that depends on the magnet shape. In all magnets other than elli psoids the concept of a self - demagnet izing f ield is only an approximation since H i is non-uniform. Nevert heless the self -demagnet izing inf luences encountered by a magnet can be esta blished by either analytically or by numer ical computation and expressed by the factor N as a f unction of a magnet shape.

FACTORN

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