In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

TOPICAL REVIEW: Nanomagnetics

Permanent magnet applications

Evolution of magnetic domain structures and coercivity in high-performance SmCo 2:17-type permanent magnets

High-performance permanent magnets (pms) are based on compounds with outstanding intrinsic magnetic properties as well as on optimized microstructures and alloy compositions. The most powerful pm materials at present are RE–TM intermetallic alloys which derive their exceptional magnetic properties from the favourable combination of rare earth metals (RE=Nd, Pr, Sm) with transition metals (TM=Fe, Co), in particular magnets based on (Nd,Pr)2Fe14B and Sm2(Co,Cu,Fe,Zr)17. Their development during the last 20 years has involved a dramatic improvement in their performance by a factor of >15 compared with conventional ferrite pms therefore contributing positively to the ever-increasing demand for pms in many (including new) application fields, to the extent that RE–TM pms now account for nearly half of the worldwide market. This review article first gives a brief introduction to the basics of ferromagnetism to confer an insight into the variety of (permanent) magnets, their manufacture and application fields. We then examine the rather complex relationship between the microstructure and the magnetic properties for the two highest-performance and most promising pm materials mentioned. By using numerical micromagnetic simulations on the basis of the Finite Element technique the correlation can be quantitatively predicted, thus providing a powerful tool for the further development of optimized high-performance pms.

High-performance permanent magnets

Sintered Sm2(Co,Fe,Cu,Zr)17 magnets with improved high temperature performance have been obtained by reducing the iron content in the magnet alloys. A record intrinsic coercive force, IHC, of 8.3 kOe at 400 °C was obtained when the iron content was decreased to 7 wt%. At 400–600 °C, 2:17 magnets with low iron content have demonstrated lower irreversible loss of magnetic flux, higher maximum energy product, and lower temperature coefficient of IHC. A temperature coefficient of IHC=−0.12%/°C (20–400 °C) was obtained for the low Fe magnets, compared to −0.23%/°C for commercial 2:17. Reducing iron content increases both Curie temperature and anisotropy field. Therefore, it is anticipated that new 2:17 magnet materials capable of operating at 400 °C or higher temperatures can be developed by reducing or eliminating the iron content and making other adjustments in composition and heat treatment.

G. Simon

Papers

Sm2(Co,Fe,Cu,Zr)17 magnets for use at temperature ⩾400 °C