Wiley Encyclopedia of Electrical and Electronics Engineering

A magnetic material is considered "soft" when its coercive fieldstrength is of the order of or lower than the earth's magnetic field (about 40 A/m).A soft magnetic material can be employed as an efficient flux multiplier in a large variety of devices, including transformers, generators, motors, to be used in the generation, distribution, and conversion of electrical energy, and a wide array of apparatus, from household appliances to scientific equipment.With a market around 20 billion in the year 2015 and annual growth rate around 5 %, soft magnetic materials (SMMs) are today an ever-important industrial product, offering challenging issues in properties understanding, preparation and characterization.An overview of the whole market of magnetic materials and the relative contributions of the different types of soft magnets is given in Fig. 1.SMMs were at the core of the development of the early industrial applications of electricity.The steel practice at the turn of the 19 th century was sufficiently developed to satisfy the increasing need of mild steel for the electrical machine cores.In 1900 Hadfield, Barrett, and Brown proved that, by adding around 2% in weight Si to the conventional magnetic steels, one could increase the permeability and decrease the energy losses [1].Fe-Si alloys were more expensive and more difficult to produce and gained slow acceptance.In addition, the poor control of the C content was to mask the prospective performances of this product, compared with mild steels.It took more than two decades, characterized by a gradual improvement of the metallurgical processes, for Fe-Si to become the material of choice for transformers.An empirical attitude towards research in magnetic materials was prevalent at the time and applications came well before theoretical understanding.This is the case of the Goss process, developed in the early 1930s, by which the first grain-oriented Fe-Si laminations could be industrially produced [2].In the years 1915-1923 G.W Elmen and co-workers at the Bell Telephone Laboratories systematically investigated alloys made of Fe and Ni, discovering the excellent soft magnetic properties of the permalloys (78% Ni) [3].J.L. Snoek and co-workers are credited for the successful industrial development of ferrites in the 1940's [4], following attempts dating back to the first decade of the century.The discovery in 1967 of the soft magnetic amorphous alloys again occurred nearly by chance [5], but it provided a fertile field for technologists and theorists.It enriched the landscape of applicative magnetic materials, while straining existing theories on magnetic ordering.More recently, the need for increasingly high frequencies of operation in miniaturized devices and the appearance of novel phenomena of fundamental and applicative interest in lowdimensionality systems have propelled the investigation of the properties and the preparation techniques of soft magnetic thin films [6] [7].Of special interest in this respect are the magnetoresistive phenomena observed in multilayer structures, where different layers can display, by combination of exchange interaction and applied field, either parallel or antiparallel magnetization.Spin-polarized conduction electrons diffusing through the layers suffer a magnetization orientation dependent scattering, according to their spin-up or spin-down character, resulting in a giant magnetoresistance effect [8]. GENERAL PROPERTIES OF SOFT MAGNETS Magnetization curve and hysteresisThe behavior of a ferromagnetic material is summarized by the constitutive law J(H) (i.e., M(H)), the dependence of the polarization J (magnetization M) on the magnetic field H.In many instances one can usefully recur to the B(H) law, where the magnetic induction B, the quantity involved in the Faraday-Maxwell law, is related to M, J, and H by the relationship B = µ0H + µ0M = µ0H + J, (1) where µ0 = 4p×10 -7 NA -2 (H/m) is the magnetic constant (also called magnetic permeability of vacuum).The constitutive law (1) is the macroscopic outcome of an extremely complex sequence of microscopic processes, where, by combination of domain wall displacements, domain structure rearrangements, and rotations of the