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Permanent Magnets: History, Current Research, and Outlook

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Novel Functional Magnetic Materials

Part of the book series: Springer Series in Materials Science ((SSMATERIALS,volume 231))

Abstract

Recent developments in permanent magnetism are summarized, considering both intrinsic and extrinsic properties. After a general introduction to permanent magnetism, several classes of materials are discussed in the light of future improvements. Emphasis is on magnets rich in Fe, Co, and Mn. The search for new magnetic compounds with improved magnetization, Curie temperature, and anisotropy is accompanied by the need to realize a microstructure that ensures high coercivity. This need refers to both bulk magnets, where hcp Co and tetragonal FeNi are briefly discussed as negative and positive examples, respectively, and to aligned hard–soft nanocomposites. A very recent concept is imaginary magnetic hardness, which reflects easy-plane magnetism and may be exploited in some ferromagnetic compounds. In aligned two-phase nanostructures, soft-in-hard geometries are better than hard-in-soft geometries, and different shapes behave different in the first and second quadrants of the hysteresis loops. Both intrinsically and extrinsically, the most important task is to maximize the hard phase anisotropy while maintaining a high magnetization. Anisotropy field and magnetic hardness can be maximized by choosing a small magnetization, but this strategy is detrimental to the energy product. The last section deals with the behavior of permanent magnets above room temperature, with emphasis on nanoscale effects. Throughout the chapter, current research trends are critically evaluated, and several common misconceptions are dispelled.

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Acknowledgment

This chapter is partially based on original research supported by DOE BES (DE-FG02-04ER46152, Sect. 3), ARO (Nr. WF911NF-10-2-0099, Sect. 4), ARPA-E (PNNL/Maryland and Argonne/Delaware), DREaM (Ames), HCC, and NCMN. It has also benefitted from discussions and collaborations with B. Balamurugan, R. Choudhary, J. M. D. Coey, S. Constantinides, J. Cui, B. Das, A. Enders, G. C. Hadjipanayis, S. Hirosawa, Y. Jin, A. Kashyap, L.-Q. Ke, M. J. Kramer, L. H. Lewis, S.-H. Liou, J. P. Liu, Y. Liu, P. Kumar, P. Manchanda, R. W. McCallum, F. Pinkerton, T. Rana, S. G. Sankar, J. E. Shield, D. J. Sellmyer, S. Valloppilly, V. Sharma, I. Takeuchi, and W.-Y. Zhang.

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    R. Skomski

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Appendix: Units in Magnetism

Appendix: Units in Magnetism

It is generally recommended to use the international or SI system or transparent units differing by multiples of 10, such as Å = 100 pm. Some researchers, most notably in the USA and China, continue to us the cgs system, which was developed by Carl Friedrich Gauß around 1830. The British Association for the Advancement of Science officially endorsed and widely popularized the Gaussian system in 1874 but replaced it in 1889 by the MKS predecessor of the SI system.

In strict sense, today’s Gaussian system is a “reduced” or dimensionless system as far as magnetism is concerned. The situation is similar to the atomic unit (a.u.) system, where all physical quantities are made dimensionless by division, using combinations of quantities such as Bohr’s hydrogen radius ao = 0.529 Å. Similar to “a.u.,” “emu” is not a unit but a reminder that the moment is measured in a variant of the cgs system. The expression “emu/cm3” is also such a reminder, albeit slightly differently structured by involving cm, which is a well-defined length unit. The Gaussian system exhibits some oddities that can never happen in a physically meaningful unit system. For example, multiplication of the magnetization by the dimensionless number 4π changes the units from emu/cm3 to kG. In the SI, this problem does not occur, because the corresponding quantities are connected through the permeability of free space, μo = 4π × 10−7 N/A2. (N/A2 can be written in a variety of equivalent SI forms, notably H/m, T . m/A, Wb/A . m and V . s/A . m.) Note that electrostatic units (esu) are rarely used today, and few solid-state scientists can even recall the electron charge in esu units (e = 4.803 . 1010 esu).

As far as permanent magnetism is concerned, the only shortcoming of the SI system is that the magnetization is measured in A/m. This feature dates back to the nineteenth century, when scientists believed that the magnetization was caused by microscopic currents. We now know that this is incorrect: currents, or orbital moments, are largely quenched in materials like Fe and Co , where most of the magnetization is caused by the spin. Explaining the spin by local currents implies that the electron’s charge distribution moves with a velocity larger than the velocity of light, which is not a meaningful physical concept. The role of μo in the conversion between A/m and T may be compared to the role of kB in the conversion between temperature (K) and energy (J): a typical dust particle, of radius 1 μm and one millimeter above the ground, has a potential energy of about 10−16 J. There is nothing wrong with quoting this energy as a temperature, about 107 K, unless one believes that this temperature is actually the temperature of the dust particle.

The situation in permanent magnetism would be much easier if B, M, and H had the same unit (T). A seeming counterargument is that H and the flux density B are physically different and should therefore have different units , but the example of energy and torque, both measured in Nm, proves that different physical quantities do not need different units. J = μoH is sometimes used, but J also denotes exchange and the total angular momentum, which creates a messy situation in some contexts. Expressions such as Br = μoMr are common, but they obscure the situation as far as physics is concerned. A compromise, used in the present chapter, is to consider the magnetization μoM and the magnetic field μoH, both measured in tesla (T). Here are some informal conversion rules for cgs and A/m aficionados: 1 T = 10 kG = 10 kOe, 1 T = 10/4π MA/m ≈ 800 kA/m, 1 emu/cm3 = 1 kA/m, 1000 kA/m = 4π/10 T ≈ 1.25 T, 1 kA/m = 4π Oe ≈ 12.5 Oe, 1 MGOe = 100/4π kJ/m3 ≈ 8 kJ/m3, 1 kJ/m3 = 4π/100 MGOe ≈ 0.125 MGOe, 1 kJ/m3 = 1 kPa, 100 MGOe = 1 T2, 1 kOe = 1000/4π kA/m ≈ 80 kA/m.

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Skomski, R. (2016). Permanent Magnets: History, Current Research, and Outlook. In: Zhukov, A. (eds) Novel Functional Magnetic Materials. Springer Series in Materials Science, vol 231. Springer, Cham. https://doi.org/10.1007/978-3-319-26106-5_9

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