The 20th century

neodymium magnetsTwo changes have gone hand-in-hand during the 20th century. First, applications were conceived that required new or greatly improved materials. Need being the mother of invention, materials were then developed to satisfy the applications. Permanent magnets are today used in devices that could only have been dreamed about 25 years ago. Who could have foreseen, for example, the advances in data storage that have occurred over the last 15 years? In 1984, hard disk drives of under 10 megabytes in size and with random access times of over 65 milliseconds were the norm. Today, the standard drive is well over 10 gigabytes, is smaller in size and costs less than a fourth of the old drives, with an access time of 10 milliseconds and more than 33 mbps burst throughput. “Novel” has two interpretations: it may refer to the magnetic material or it may refer to the application of a material. Several examples of each are presented. INTRODUCTION Until the late 1930’s, permanent magnets were predominantly steel compositions with low energy product and coercivity. Rather than utilizing permanent magnets, loudspeakers used an interaction of electromagnetic fields and motors were of the induction type. The invention of alnico allowed product size reduction through the use of permanent magnets in place of induction coils. Approximately every 12 years thereafter, a new magnetic material was discovered. Figure 1 shows how the maximum energy product has increased. It also illustrates that materials with lower energy product, specifically ferrite, can be commercially successful. Introduced in 1961, ferrite remains the largest selling permanent magnet material on a weight basis primarily because of its relatively low price. New materials have not obviated older ones: each has advantages and disadvantages. Alnico, though magnetically weaker than rare earth magnets, is much more temperature stable. Applications requiring stability over wide temperature ranges still rely on alnico. But the newer materials (ferrite, samarium cobalt, neodymium-iron-boron) all have a very important characteristic, a “square” second quadrant intrinsic curve, which allows use in applications which 1900 1920 1940 1960 1980 2000 0 10 20 30 40 50 60 0 40 80 120 160 200 240 280 320 360 400 440 480 YEAR BHmax — MGOe BHmax — kJ/cu meter KS STEEL MK STEEL ALNICO 5 COLUMNAR ALNICO FERRITE SmCo 1-5 and 2-17 Nd-Fe-B OTHER IMPORTANT CHARACTERISTICS FIELD TO MAGNETIZE THERMAL STABILITY MECHANICAL PROPERTIES CORROSION RESISTANCE MANUFACTURABILITY COST 1900 1920 1940 1960 1980 2000 0 10 20 30 40 50 60 0 40 80 120 160 200 240 280 320 360 400 440 480 YEAR BHmax — MGOe BHmax — kJ/cu meter KS STEEL MK STEEL ALNICO 5 COLUMNAR ALNICO FERRITE SmCo 1-5 and 2-17 Nd-Fe-B OTHER IMPORTANT CHARACTERISTICS FIELD TO MAGNETIZE THERMAL STABILITY MECHANICAL PROPERTIES CORROSION RESISTANCE MANUFACTURABILITY COST Figure I Development of Permanent Magnet Materials were not possible before. NOVEL MATERIALS The first question we must ask is: “What do we mean by novel?” It is proposed here that novel refers to a new magnetic material, a new application or design or a significant variation of an older design. To be considered, the material or application must also be commercially successful. Novel might also refer to the lack of general knowledge about a material or an application. Table I is a summary of commercially available materials and processing methods. Prior to the announcement of SmFeN for use in bonded magnets, the newest material in the table was NdFeB, which is 15 years old. Old materials are not necessarily stagnant. Improvements in composition and processing of NdFeB powders for bonded magnets, for example, have raised the maximum recommended use temperature from 110°C to 180°C. Neodymium-iron-boron is no longer a novel material in that it was commercially introduced in November 1984. However, the melt-spun isotropic powder for bonded magnets was not readily available until the late 1980’s.

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Since it represented a new family of materials with new magnetic properties and limitations, applications had to be designed from scratch, a process requiring two to four years. The first products to take advantage of this new material were microelectronics produced in the Far East. Bonded neodymium-iron-boron magnets have become widely used in the United States only in approximately the last five years. Bonded magnets represent a diverse set of capabilities and properties. Figure 2 shows the combinations of materials, binders and forming processes. The low processing temperatures allow mixing of heterogeneous materials within the binding matrix. Several combinations have been proposed or tried since the early 1970’s including ferrite with samarium cobalt or ferrite with neodymium-ironboron. Different grades of a single material family can also be mixed to achieve new properties, though with only an averaging effect. The advantage of ferrite over either of the rare earth compositions is that the ferrite has a Table I Commercially Available Permanent Magnet Materials BONDED MATERIAL CAST EXTRUDED OR ROLLED SINTERED FULLY DENSE INJECTION MOLDED COMPRESSION BONDED FLEXIBLE RIGID EXTRUDED ALNICO Y Y Y IRON-CHROMECOBALT Y CuNiFe Y SmCo Y Y Y SmFe(N,C) Y NdFeB Y Y Y Y Y FERRITE Y Y Y HYBRIDS Y Y Y BINDER: THERMOSET THERMOPLASTIC ELASTOMER Epoxy Polyamides Nitrile Rubber Acrylic Polyester Vinyl Phenolic PPS , PVC, LDPE Process Compression Injection Extrusion Calendering End Rigid Rigid Rigid Flexible Product Typical NdFeB NdFeB NdFeB NdFeB Magnetic SmCo SmCo Powders Ferrite Ferrite Ferrite Alnico Alnico Hybrids Hybrids Hybrids Figure 2 Diversity of Bonded Magnets positive temperature coefficient of coercivity while that of the rare earths is negative. There is a synergy in performance and in price. Another material is being prepared for commercial introduction: samarium-iron-nitride. Asahi Corporation received U.S. patents in 1987 for the material and processing method. Siemens, Hitachi and Sumitomo have all done extensive research on manufacturing SmFeN. Making the material has been challenging: when the nitride gas is forced into the crystal lattice of the samarium-iron base alloy, the alloy tends to decompose into samarium-nitride and alpha-iron. The nitrogen is interstitial, so it may be ejected from its position in the lattice causing decomposition. This is hastened when the temperature rises. Above about 450°C, decomposition is rapid. The claimed advantages include improved corrosion resistance over neodymium-iron-boron and improved temperature stability. Developments have also taken place in rolled alloys. These are malleable alloy compositions rolled in continuous strips of thin foils. Recently introduced materials are “semi-hard” with coercivity ranging from 20 to 100 oersteds. The primary use for these alloys is in EAS (electronic article surveillance or anti-theft tags). NOVEL APPLICATIONS With so little new in materials, we will focus on applications. Hard disk drives are sold by the tens of millions each year. Who here does not have at least one? What is novel, I propose, is the extent to which the technology has progressed: magnets have increased in energy output, the magnet voice coil assemblies have become smaller in size and the drives have gone from 8” (width form factor) to 3.5 inches. Portable computers use 2.5” drives. IBM has introduced an even
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smaller drive (smaller than a pack of cigarettes). Several years ago, Ted Davis, President of Ted Davis Manufacturing (now part of Vacuumschmelze), complimented the industry on advances in the permanent magnets used in drives and the miniaturization that new materials allowed. He went on to challenge the industry to improve the flux carrying ability of the soft magnetic components so their size could also be reduced. Figure 4 shows example components. Notice the relative thickness of the magnet and the steel to which it is mounted. Voice coil motors in hard drives are either rotary, such as in Figure 4, or