Hard Nanocrystalline Materials

3.1. An Introduction

Permanent magnets are devices capable of producing, during a time of the order of several years and without any on-service energy input, a magnetic field whose magnitude is at least in the order of the tens of T. The devices, based on fields produced by permanent magnets, are ubiquitous in modern technology; a good example of this point is that in a modern car, it is typically possible to identify up to 30 different magnets [87]. In most of the cases (the exception are those in which the magnets are used to produce a field gradient), the figure of merit characterizing the device is the so-called energy product of the permanent magnet [88]. If the permanent magnet is used as a part of a magnetic circuit in which the magnetization is essentially uniform, the energy product gives the magnetic energy density, stored in a thin air gap introduced in the circuit and limited by surfaces (the poles) perpendicular to the average magnetization (the energy product also is proportional to the square of the induction created in the gap). In addition to a sufficiently high energy product, a permanent magnet should exhibit (especially those used in the automotive industry) a weak decrease of the coercive force with the increase of the temperature and good corrosion resistance properties.

Modern permanent magnets are based on the so-called hard magnetic phases, that is on phases difficult to demagnetize. A hard phase should present, at the temperatures of interest from the standpoint of the applications [89], (i) a coercive force large enough to preserve the remanence from either on-service or spurious demagnetizing effects (it is possible to show that, in a uniform magnetization circuit, the stability of the remanence is granted by a coercivity larger than half the magnitude of the remanence), (ii) a remanence as large as possible (in a polycrystalline hard material, the remanence value results from the saturation magnetization value and from the degree of macroscopic texture of the grain orientation distribution), and (iii) a magnetic transition temperature high enough so as to be compatible with the temperature increases occurring during the use of the magnet.

The optimization of this set of properties can, first of all, be correlated to an adequate choice of the structure and intrinsic properties of the hard phase. The induction of a large coercivity is linked to the occurrence of a large magnetic anisotropy. For the coercivity values required by modern applications, the anisotropy can only be of a magne-tocrystalline origin [90]. The magnetocrystalline anisotropy results from the electrostatic interaction between the charge distribution of the atoms bearing the localized magnetic moments responsible for the macroscopic magnetization and the crystalline electric field created by the ions surrounding those magnetic atoms. Requisite for the observation of large magnetocrystalline anisotropy is the existence of a large spin-orbit coupling in the atoms having magnetic moments [91]. This large spin-orbit coupling has, as a consequence, the fact that any modification of the orientation of the moments is linked to a rotation of the charge linked to the orbital part of the total moment and, consequentially, to a variation of the energy of interaction between that charge and the ions in its neighborhood (the relative orientation(s) resulting in a minimum of this interaction energy are called easy axes and those corresponding to maxima, hard axes). Considering this initial requisite, it is possible to identify the complete Rare-Earth series as a group of elements with a potential to either be used as or to form hard phases [92] (the rare-earth elements have large spin-orbit interactions and the Rare-Earth ions exhibit, with the only exception of Gd, large asphericities which make the crystalline field interactions highly dependent on the charge orientation).1

The achievement of a large saturation magnetization is linked to the identification of a highly packed atomic structure in an element or compound formed by atoms bearing the larger possible magnetic moments and having the smallest possible atomic volume. Due to their large atomic volumes, which is not compensated by their often large atomic moments, the rare-earths are, in respect to the elevated

1 The occurrence of uniaxial anisotropies (corresponding to hexagonal, tetragonal, or rhombohedral crystal structures) is an additional requisite for the achievement of elevated coercivities.

magnetization, in a clear disadvantage with respect to the transition metal magnetic elements and, specially, in comparison with Co and Fe (which, in turn, have, due to the very small spin-orbit coupling, a reduced magnetocrystalline anisotropy).

The third requisite, that corresponding to the elevated order temperature, also excludes the rare-earths since the small magnitude of the exchange interactions between the elements of the series results in the fact that the rare-earth with the larger Curie temperature is Gd, the most spherically symmetric rare-earth ion, which goes paramagnetic at ca. 300 K, an order temperature incompatible with any practical application. It is thus possible from this discussion to discard any magnetically pure element as a potential hard magnetic phase.

A hard phase must, consequently, at least be a binary compound joining in a single structure high anisotropy, magnetization, and order temperature [93]. The intermetallic transition metal-rare earth alloys are thus clear candidates for a hard magnetic behavior and, in fact, hexagonal phases of the SmCo system exhibit coercivities in single-phase, poly-crystalline samples of up to 4 T at room temperature (these phases can bear remanences of the order of 1 T, have reasonably good corrosion properties, and can be used up to ca. 650 K without a large deterioration of their hysteretic properties). The main problem with the extensive use of SmCo magnets is related to the limited availability of Co and its high (and highly fluctuant) price.

Phases alternative to the SmCo ones and not containing Co are the ordered tetragonal FePt (exhibiting coercivities smaller than those obtained in SmCo) and, more importantly, the ternary tetragonal NdFeB, the phase having the better hard properties achieved up to the moment.

In this section, we will review the basic characteristics and behaviors of these hard phases, the links between the demagnetization properties and the phase distribution and morphology, the influence of the demagnetization mode in the optimization of the preparation methods, and will finish with the analysis of the way of overcoming the limitations of the hysteretic properties related to the values taken by the intrinsic quantities: the induction of different types of nanostructures.

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