Applied physics

Strong magnets by self-assembly

Newly developed nanomaterials are proving useful in many fields, but materials that make strong permanent magnets are difficult to devise. Progress has been made using a self-assembled mixture of nanoparticles.

Controlled structuring of materials at the nanoscale can enhance some of their properties and widen their range of applications. Magnetic materials, such as recording media, field sensors and memory devices, are advancing rapidly in terms of their miniaturization, sensitivity and other figures of merit. But progress in producing permanent magnets has been limited by the difficulty of finding new compounds with the necessary properties. On page 395 of this issue, Zeng et al.1 show how self-assembly of mixtures of magnetic nanoparticles makes it possible to fabricate materials with excellent magnetic properties. The process of chemical synthesis and the resulting nanostructures, with magnetic coupling between nanoscale grains in the composite material, are impressive.

The figure of merit by which permanent-magnet materials are judged is the energy product — a measure of the maximum magnetostatic energy that would be stored in free space between the pole pieces of a magnet made from the material in question2. The energy product depends on the area of the 'hysteresis loop' (Fig. 1). A typical hysteresis loop arises from plotting the magnetization of the material as the applied magnetic field is varied — the response of the materials follows two distinct paths on magnetization and demagnetization. As well as the saturation point (maximum magnetization), the hysteresis loop is characterized by the 'coercivity' of the material, which is the reverse-field strength needed to reduce the flux density to zero. To obtain a large energy product requires large magnetization and large coercivity.

Figure 1: Typical hysteresis loops for a permanent magnet.
figure1

The response of a magnet to an applied field is shown in terms of its magnetization (a) and the magnetic induction or flux density (b). If the applied field strength is increased until the magnetization saturates, and then reduced, the variation of the magnetization and the flux density traces a hysteresis loop. The coercivity of a magnet is defined as shown in a, and is a factor in the figure of merit of permanent magnets, known as the energy product (shaded area in b).

The largest room-temperature magnetization for a material — a Fe65Co35 alloy — corresponds to an internal flux density or magnetic induction of about 24 kilogauss. Theoretically, the upper bound on the energy product for the material (proportional to the saturation magnetization squared) is 144 MGOe, for induction measured in gauss (G) and field in oersted (Oe). But in practice, the Fe–Co alloy does not have such a large energy product because it is a soft magnet — that is, its coercivity is actually fairly low and a small reverse field is sufficient to reduce the magnetization to zero.

The largest energy product observed in nature, for the compound Nd2Fe14B, is 56.7 MGOe (ref. 3). Nd2Fe14B has a complex structure containing 68 atoms in its unit cell. The Nd–Fe–B class of magnets owes its position as the highest-energy magnet to its high magnetization (mainly from Fe ions) and to its tetragonal structure and Nd ions, which together produce a high degree of magnetic anisotropy. This anisotropy in turn causes high coercivity.

Is there hope of raising the maximum energy product observed to values approaching the upper bound of 144 MGOe? Despite strenuous efforts, a compound with the necessary properties has eluded researchers. But, about a decade ago, an idea emerged that brought new hope. This is the concept of 'exchange coupling' between a hard (high-coercivity) material and a soft (low-coercivity) material with a large magnetization. In a two-phase mixture of such materials, exchange forces between the phases mean that the resulting magnetization and coercivity of the material will be some average of the properties of the two constituent phases4. But for the exchange coupling to be effective, the relative sizes of the grains of the two materials must be chosen carefully: Kneller and Hawig5 showed that the characteristic dimensions of the soft phase cannot exceed about twice the wall thickness of magnetic domains in the hard phase. Typically, this limits the soft phase to grains of about 10-nm diameter. Similarly, the hard phase must have dimensions of this order or the volume fraction of the soft phase will be rather low, thus limiting the magnetization of the composite.

Skomski and Coey4 showed that in an ideal exchange-coupled magnet, consisting of aligned grains of Sm2Fe17N3 and Fe65Co35, the theoretical upper bound on the energy product is about 125 MGOe. But in the real world, attempts to fabricate two-phase nanostructures with a high energy product have had only limited success. Bulk magnets produced by subjecting the two-phase material to ultra-fast cooling and then annealing, or by mechanical milling, have not achieved energy products beyond about 20 MGOe (ref. 6). There are several difficulties to be overcome: controlling the material structure at the nanoscale, especially to create uniform grain sizes of about 10 nm; aligning the hard grains sufficiently; and ensuring effective exchange coupling between the two phases in all grains through a homogeneous distribution.

In fact, higher energy products have been obtained in thin-film materials7 based on iron–platinum compounds. In these, perpendicular anisotropy (and hence high coercivity) is achieved by first preparing nanoscale Fe–Pt multilayers, and then applying a specific thermal processing technique. Unfortunately, however, this technique is not appropriate for making practical bulk magnets. All in all, the work so far has shown the nanostructuring challenges to be formidable.

But now Zeng et al.1 have devised a method of chemical synthesis with the potential for making three-dimensional magnets with high-energy product. When FePt and Fe3O4 nanoparticles of similar sizes (about 4 nm) are mixed and allowed to self-organize, they form structures which, when heated and chemically reduced, form 5-nm-scale homogeneous mixtures of a hard tetragonal FePt phase and a high-magnetization soft Fe3Pt phase. The admixture of Pt in the Fe3Pt nanograins results from sintering at a temperature of 650 °C, which is used to induce the phase transition from disordered FePt to ordered tetragonal FePt. The energy product of the two-phase material is 20.1 MGOe — considerably higher than the value expected for isotropic FePt particles alone (13 MGOe).

There are still many practical challenges to be faced if energy products approaching the magic number of 144 MGOe are to be achieved. For instance, ways of compressing the two-phase material into a high-density compact must be explored, as well as improved alignment of the axes of the hard grains to exploit the full magnetic potential of the nanocomposite system. Nevertheless, the work of Zeng et al.1 is an exciting development that shows the way to making strong magnets for practical applications.

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Correspondence to David J. Sellmyer.

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Sellmyer, D. Strong magnets by self-assembly. Nature 420, 374–375 (2002). https://doi.org/10.1038/420374a

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