Hard and soft magnetic materials
1 Hard magnets, also referred to as permanent magnets, are magnetic materials that retain their magnetism after being magnetized. It is believed that permanent magnets have been used for compasses by the Chinese since ~2500BC. However, it was only in the early twentieth century that high carbon steels and then tungsten / chromium containing steels replaced lodestone as the best available permanent magnet material. The present industrial permanent magnetic materials fall essentially in 3 groups: Alnico, Ferrite and Neodymium iron boron compounds.
2 Alnico alloys are based mainly on the elements nickel, cobalt and iron with smaller amounts of aluminium, copper and titanium (Typical weight%: Fe-35, Co-35, Ni-15, Al-7, Cu-4, Ti-4). The alloy composition and processing were developed over the years and they are used today as they have a high Curie temperature (~850°C), and as a result can operate at higher temperatures as well as having more stable properties around room temperature than some of the more modern alloys. However, their main disadvantage is that they have low intrinsic coercivity (~50kAm-1) and as a consequence must be made in the form of horseshoes or long thin cylinders, which cannot be exposed to significant demagnetizing fields.
3 Hard ferrites are ferrimagnetic and considering the proportion of iron within the material have quite a low remanence (~400mT). The coercivity of these magnets (~250kAm-1). The magnets could also be exposed to moderate demagnetizing fields and hence could be used for applications such as permanent magnet motors. Permanent magnetic materials have a wide range of applications which is illustrated in the following table:
Examples of applications for permanent magnetic materials
| Automotive: Starter motors, Anti-lock braking systems (ABS), Motor drives for wipers, Injection pumps, Fans and controls for windows, seats etc, Loudspeakers, Eddy current brakes, Alternators. Telecommunications: Loudspeakers, Microphones, Telephone ringers, Electro-acoustic pick-ups, Switches and relays. Data Processing: Disc drives and actuators, Stepping motors, Printers. Consumer Electronics: DC motors for showers, Washing machines, Drills, Low voltage DC drives for cordless appliances, Loudspeakers for TV and Audio, TV beam correction and focusing device, Compact-disc drives, Home computers, Video Recorders, Clocks. Electronic and Instrumentation: Sensors, Contactless switches, NMR spectrometer, Energy meter disc, Electro-mechanical transducers, Crossed field tubes, Flux-transfer trip device, Dampers. Industrial: DC motors for magnetic tools, Robotics, Magnetic separators for extracting metals and ores, Magnetic bearings, Servo-motor drives, Lifting apparatus, Brakes and clutches, Meters and measuring equipment. Astro and Aerospace: Frictionless bearings, Stepping motors, Couplings, Instrumentation, Travelling wave tubes, Auto-compass. Biosurgical: Dentures, Orthodontics, Orthopaedics, Wound closures, Stomach seals, Repulsion collars, Ferromagnetic probes, Cancer cell separators, Magnetomotive artificial hearts, NMR / MRI body scanner. |
4 Soft magnetic materials are those materials that are easily magnetized and demagnetized. They are used primarily to enhance and/or channel the flux produced by an electric current. The main parameter, often used as a figure of merit for soft magnetic materials, is the relative permeability ( mr, where mr = B/moH), which is a measure of how readily the material responds to the applied magnetic field. The other main parameters of interest are the coercivity, the saturation magnetization and the electrical conductivity. Soft materials are essentially composed of iron atoms and the industrial products include pure iron, the silicium-steels and iron-nickel alloys.
5 The types of applications for soft magnetic materials fall into two main categories: AC and DC. In DC applications the material is magnetized in order to perform an operation and then demagnetized at the conclusion of the operation, e.g. an electromagnet on a crane at a scrap yard will be switched on to attract the scrap steel and then switched off to drop the steel. In AC applications the material will be continuously cycled from being magnetized in one direction to the other, throughout the period of operation, e.g. a power supply transformer. A high permeability will be desirable for each type of application but the significance of the other properties varies.
6 For DC applications the main consideration for material selection is most likely to be the permeability. For AC applications the important consideration is how much energy is lost in the system as the material is cycled around its hysteresis loop.