The ability of a permanent magnet to support an external magnetic field results from small magnetic domains “locked” in position by crystal anisotropy within the magnet material. Once established by initial magnetization, these positions are held until acted upon by forces exceeding those that lock the domains. The energy required to disturb the magnetic field produced by a magnet varies for each type of material. Permanent magnets can be produced with extremely high coercive forces (Hc) that will maintain domain alignment in the presence of high external magnetic fields. Stability can be described as the repeated magnetic performance of a material under specific conditions over the life of the magnet. Factors affecting magnet stability include time, temperature, reluctance changes, adverse fields, radiation, shock, stress, and vibration.
Advances in materials science seem never ending. The ever changing requirements of science and industry necessitate more and more advanced materials. In turn, these advanced materials fuel research into more and more exciting applications for magnets. This symbiotic relationship has ensured end users a vast choice of permanent magnets materials. There are a great many factors to be considered when choosing a permanent magnets material for your application. At Dexter, we can provide you with detailed recommendations for your particular application. Our unsurpassed inventory and technical expertise will ensure that you use the ideal material for your applications and your business. Factors such as operating temperature, demagnetizing effects, environmental characteristics and available space all need to be considered; and to the magnetics novice, the decisions can seem overwhelming. One important consideration is that while material is key, the utilization of an optimized design is of primary importance. At Dexter, we can not only recommend materials, we can assist in design. Why use premium Sm-Co materials, when a ceramic ferrite magnet will do? Our driving force is to provide you with the best solution for your application. Our inventory and design expertise will get you to that point. There are 4 major families of permanent magnets – neodymium-iron-boron [Nd-Fe-B], samarium-cobalt [Sm-Co], alnico, and hard ferrite [ceramic]. Using these materials, as well as electromagnets, we can provide you with the unique solutions that you need. Although we cannot break the rules of physics, you will see that we can certainly bend them to your advantage!
Alnico was developed in the early 1930s. During WW2 it was used in military electronic applications. After the war it quickly spread into civilian versions of those applications and replaced magnet steel in many applications. High induction levels, with good resistance to demagnetization and stability, due to its low temperature coefficient (0.02% / °C), at a reasonable cost made Alnico the material of choice.
A high working temperature limit (550 °C / 1020 °F) makes Alnico especially well suited for sensitive automotive and aircraft sensor applications. Other popular Alnico applications include: Instruments, security sensors, magnetos, electronic distributors, separators, electron tubes, traveling wave tubes, radar, holding magnets, coin acceptors, generators and motors, clutches and brakes, relays, controls, receivers, telephones, microphones, bell ringers, guitar pickups, loudspeakers, security systems, cow magnets.
Alnico is produced in many grades to fit the requirements of these applications, from Alnico 1 to Alnico 12, but the most popular grades are 2, 5 and 8. By comparison to newer materials, like ceramic, NdFeB and SmCo the coercivity of Alnico is low, so they have replaced Alnico where cost and/or greater resistance to demagnetization are valued more than a high temperature limit and temperature stability.
MAGNET SELECTION
Magnet selection for all applications must consider the entire magnetic circuit and the environment. Where Alnico is appropriate, magnet size can be minimized if it can be magnetizing after assembly into the magnetic circuit. If used independent of other circuit components, as in security applications, the effective length to diameter ratio (related to the permeance coefficient) must be great enough to cause the magnet to work above the knee in its second quadrant demagnetization curve. For critical applications, Alnico magnets may be calibrated to an established reference flux density value.
A by-product of low coercivity is sensitivity to demagnetizing effects due to external magnetic fields, shock, and application temperatures. For critical applications, Alnico magnets can be temperature stabilized to minimize these effects.
ALNICO PRODUCTION
Alnico magnets material is made by alloying aluminum, nickel and cobalt with iron. Some grades also contain copper and/or titanium. The alloying process is casting or sintering. These constituents, the process and the heat treatment needed to optimize magnetic properties produces hard (Rc45) and brittle parts that are best shaped or finished by abrasive grinding. Cast parts are generally under 70 pounds and may be used as-is, but polar surfaces are usually ground flat and parallel. Sintering is confined to high volume parts in sizes under one cubic inch and an effective press length to diameter ratio under four.
MAGNETIZING
To minimize the volume of magnet material required by an application, the entire magnetic circuit must be considered. An optimized circuit design results in a circuit permeance coefficient that causes the magnet to operate above the knee of its demagnetization curve by a margin large enough to offset anticipated operating demagnetizing effects. Optimized steel components result in an effective magnetic length greater than the magnet itself, but this is only effective if the magnet can be magnetized after assembly into the circuit. The alternate is to design the magnet shape to produce a load line, on its own, that intersects the BH curve above its knee, so minimal flux is lost due to the self demagnetizing factor upon removal from the magnetizing fixture. In either case, a magnetizing force of 3.0 kOe must be applied to Alnico 5 magnets and 7.0 kOe for Alnico 8. When magnetized in a magnetic circuit, the magnetizing pulse must be wide enough to allow eddy currents in the steel to decay before dropping below these values.
Ferrite magnets, sometimes referred to as ceramic because of their production process, are the least expensive class of permanent magnet materials. This material became commercially available in the mid 1950s, and has since found its way into countless applications including arc shaped magnets for motors, magnetic chucks, and magnetic tools.
Sintered neodymium-iron-boron (Nd-Fe-B) magnets are the most powerful commercialized permanent magnets available today, with maximum energy product ranging from 26 MGOe to 52 MGOe. Nd-Fe-B is the third generation of permanent magnet developed in the 1980s. It has a combination of very high remanence and coercivity, and comes with a wide range of grades, sizes and shapes. With its excellent magnetic characteristics, abundant raw material and relatively low prices, Nd-Fe-B offers more flexibility in designing of new or replace the traditional magnet materials such as ceramic, Alnico and Sm-Co to achieve high efficiency, low cost and more compact devices.
A powder metallurgy process is used in producing sintered Nd-Fe-B magnets. Although sintered Nd-Fe-B is mechanically stronger than Sm-Co magnets and less brittle than other magnets, it should not be used as structural component. Selection of Nd-Fe-B is limited by temperature due to its irreversible loss and moderately high reversible temperature coefficient of Br and Hci. The maximum application temperature is 200 °C for high coercivity grades. Nd-Fe-B magnets are more prone to oxidation than any other magnet alloys. If Nd-Fe-B magnet is to be exposed to humidity, chemically aggressive media such as acids, alkaline solutions salts and harmful gases, coating is recommended. It is not recommended in a hydrogen atmosphere.
Flexible plastic mangets are commonly used for refrigerator magnets, combine ceramic ferrite magnet powder with a flexible thermoplastic binder. The manufacturing process involves injection molding, which is well suited to high volume applications. The flexible nature of the material enables forming into intricate, tight-tolerance shapes. However, since the material is an alloy of ceramic ferrite material, the magnetic strength is weaker than a solid ceramic magnet. Still, the versatility of the flexible sheet enables its use in many applications.
Flexible sheet is often magnetized in a multi-polar arrangement: N-S-N-S. The North and South poles are spaced close together, anywhere from 2 poles per inch to 60 poles per inch and up. This is useful in holding applications because a higher pole density results in higher holding forces. For a sensing application, a high pole density allows tighter resolution.
Flexible sheets come in various thicknesses and widths. Thicknesses vary between 0.020′ and 0.375′. Widths vary from 0.187′ up to 24′. The material can be sold in strips up to 100 ft. long. Also, some come with an adhesive backing, which can simplify assembly if needed. The magnetic orientation is normally through the thickness.
One of the most fun things to do with your miniatures is to allow for customization by magnetizing limbs and weapons. This allows you to swap out different features to face different enemies.
We sell strong rare earth magnets that you can use for almost any purpose, including magnetizing your miniatures.
These magnets are incredibly strong and permanent, and will retain their strength for years to come.
How to choose the right magnet
I remember the first time I ever tried to buy magnets. It wasn’t from a war gaming website. I had the hardest time knowing which size to get.
With that experience in mind, we have created this short guide to choosing the right magnets:
| Magnet Size |
Uses |
| 1/16″ x 1/32″ Disc | Infantry size Arms (e.g. Space Marine troops, Tyranid Termagaunts) |
| 1/8″ x 1/16″ Disc | Infantry size miniatures (e.g. Space Marine troops, Tyranid Termagaunts) |
| 3/16″ x 1/16″ Disc | Monsterous creatures limbs (e.g. Tyranid Carnifex) |
| 1/4″ x 1/16″ Disc | Customizing Vehicles, Gargantuan Creatures (e.g. Space Marine Rhino, Tyranid Bio-titan) |
| 3/8″ x 1/16″ Disc | Customizing Vehicles, Gargantuan Creatures (larger magnet) |
| 1″ x 1/8″ Disc | Huge Titans, and just for fun |
Permanent magnets can lose their magnetism if they are dropped or banged on enough to bump their domains out of alignment. Can you turn something back into a magnet by banging on it in a specific way? How do you build a powerful electromagnet that will attract a large metal object from a distance of four inches away? Is the number of windings, voltage or current the most important factor in an electromagnet?
It’s pretty unlikely, but not impossible, that you could bump a piece of iron and make it a magnet. To bump a piece of iron and turn it into a magnet you would have to bump it in such a way that a perfect vibration travels through the material. The reason that would be hard to bump a piece of iron and make it magnetic is because of the way vibrations propagate in the material. Vibrations radiate out from the point of impact and are going at different angles relative to a straight line – the line you would like the domains to line up with. Non-uniformities that exist in all materials also change the flow of the vibrations in a material.
There are some metal forming operations that can align the material and make a magnet. Usually, stretching a piece of iron will do this. This can happen when the metal is cold-formed or bent. Usually stainless steel is not magnetic, but if you sniff around a piece of bent stainless steel, you might find that it is lightly magnetic around the bends.
Field strength is linear with the current in magnets until the magnet saturates. That means that if you double the current, the field strength will double. After reaching the current that puts as much magnetism as the magnet’s core can handle, adding more current just makes the magnet hotter. The amount of windings also are linear in relation to field strength. That means if you double the amount of winding, the magnetic field doubles. At some point, the windings get so far away from the magnet’s core that their effect on the core becomes less and less. Changing voltage has a small affect on field strength. We have some of the most powerful magnets in the world at Jefferson Lab and they all operate at fairly low voltage, on the order of 10′s of volts, but a few of them might go up to as much as 5,000 amperes of current!
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