Magnetic Properties of Nanocomposite Permanent Magnets
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Permanent magnets (PMs) are one of the key materials that support modern electronic technologies and the society that is built upon them. The basic function of a PM is to provide a static magnetic field in a limited space in which electric energy is converted into mechanical energy or vice versa. The quest for PM materials that can generate a larger magnetic flux with a given volume, or that can generate a given magnetic flux with a smaller volume, still exists even after the advent of the Nd–Fe–B anisotropic magnets. The maximum magnetic energy product (BH)max exceeding 450 kJ/m3, a magnificent record that was only a dream 20 years before, has been achieved in the Nd–Fe–B magnets prepared by controlled powder-metallurgical route. An ideal permanent magnet having a rectangular hysteresis loop with the remanence value of Br will have a (BH)max value of 1/4μ0 × Br2 provided that the intrinsic coercivity is greater than one half of Br/μ0. As a remanence cannot exceed the saturation polarization (Js) of the PM material, the maximal value of Br is μ0Js. Therefore when one reminds that B = J + μ0H and that H < 0 for the case under consideration, the theoretical limit of (BH)max is 1/4 × μ0Js2, namely, a product of 1/2 × μ0Js and 1/2 × Js. Therefore a search for a more powerful PM involves, as its principal element, a search for ferromagnetic materials that have Js larger than that of Nd2Fe14B, the main hard magnetic component of the Nd–Fe–B magnets.
Nanocomposite PM (NPM) is a PM consisting of a combination of a hard magnetic phase (HP) and a relatively soft magnetic phase (SP) having a large Js. The concept of NPM has attracted considerable attention because of the possibility that a larger Br than the Js value of the currently most powerful PM (Nd–Fe–B) can be realized by combining an SP of a larger Js than that of Nd2Fe14B (1.6 T at room temperature) as was pointed out by Skomski and Coey.
The first NPM was probably Nd4Fe81B19 ribbons obtained by rapid solidification and a subsequent annealing treatment. This material consisted of Fe3B, Nd2Fe14B, and a small amount of α-Fe. Its Br was approximately 1.2 T at room temperature. Because of the isotropic nature of the material, i.e., no orientation of crystallographic axes was present, this value of Br is smaller than the Js of both Fe3B and Nd2Fe14B phases. However, it is significantly larger than the expected value, which is one half of Js, for noninteracting ensemble of hard magnetic particles of uniaxial magnetic anisotropy. This type of materials, which we classify as the Fe3B/Nd2Fe14B-type hereafter, have been constantly improved with respect to their magnetic performance by several research groups including ourselves, and form a class of commercial products that are used as a hard magnetic component of resin-bonded PM. A variety of combinations of hard magnetic and soft magnetic phases have been tried, and mechanisms of formation of nanostructures have been investigated in the past years as presented in this article.