Rare Earth Magnets

Rare earth magnets are strong permanent magnets made from alloys of rare earth elements. Developed in the 1970s and 80s, rare-earth magnets are the strongest type of permanent magnets made, substantially stronger than ferrite or alnico magnets. The magnetic field typically produced by rare-earth magnets can be in excess of 1.4 tesla, whereas ferrite or ceramic magnets typically exhibit fields of 0.5 to 1 tesla. There are two types: neodymium magnets and samarium-cobalt magnets. Rare earth magnets are extremely brittle and also vulnerable to corrosion, so they are usually plated or coated to protect them from breaking and chipping.

The term "rare earth" is misleading; these metals are not particularly rare or precious, and as of 2007 rare earth magnets give the best cost/field ratios of any permanent magnet.[citation needed] Interest in rare earth compounds as permanent magnets began in 1966, when K. J. Strnat and G. Hoffer of the US Air Force Materials Laboratory discovered that YCo5 had by far the largest magnetic anisotropy constant of any material then known.

Neodymium magnets, invented in the 1980s, are the strongest and most affordable type of rare-earth magnet. Neodymium alloy (Nd2Fe14B) is made of neodymium, iron and boron. Neodymium magnets are typically used in most computer hard drives and a variety of audio speakers. They have the highest magnetic field strength, but are inferior to samarium-cobalt in resistance to oxidation and Curie temperature. Use of protective surface treatments such as gold, nickel, zinc and tin plating and epoxy resin coating can provide corrosion protection where required.

Traditionally, the high cost of these magnets has limited their use to applications requiring compactness together with high field strength. Both raw materials and patent licenses were expensive. Beginning in the 1990s, NIB magnets have become steadily less expensive and more popular in other applications such as children's magnetic building toys.

A good permanent magnet should produce a high magnetic field with a low mass, and should be stable against the influences which would demagnetize it. The desirable properties of such magnets are typically stated in terms of the remanence and coercivity of the magnet materials.

When a ferromagnetic material is magnetized in one direction, it will not relax back to zero magnetization when the imposed magnetizing field is removed. The amount of magnetization it retains at zero driving field is called its remanence. It must be driven back to zero by a field in the opposite direction; the amount of reverse driving field required to demagnetize it is called its coercivity. If an alternating magnetic field is applied to the material, its magnetization will trace out a loop called a hysteresis loop. The lack of retraceability of the magnetization curve is the property called hysteresis and it is related to the existence of magnetic domains in the material. Once the magnetic domains are reoriented, it takes some energy to turn them back again. This property of ferrromagnetic materials is useful as a magnetic "memory". Some compositions of ferromagnetic materials will retain an imposed magnetization indefinitely and are useful as "permanent magnets".

The table below contains some data about materials used as permanent magnets. Both the coercivity and remanence are quoted in Tesla, the basic unit for magnetic field B. The hysteresis loop above is plotted in the form of magnetization M as a function of driving magnetic field strength H. This practice is commonly followed because it shows the external driving influence (H) on the horizontal axis and the response of the material (M) on the vertical axis. Besides coercivity and remanence, a quality factor for permanent magnets is the quantity (BB0/μ0)max. A high value for this quantity implies that the required magnetic flux can be obtained with a smaller volume of the material, making the device lighter and more compact.

Data from Myers

The alloys from which permanent magnets are made are often very difficult to handle metallurgically. They are mechanically hard and brittle. They may be cast and then ground into shape, or even ground to a powder and formed. From powders, they may be mixed with resin binders and then compressed and heat treated. Maximum anisotropy of the material is desirable, so to that end the materials are often heat treated in the presence of a strong magnetic field.
The materials with high remanence and high coercivity from which permanent magnets are made are sometimes said to be "magnetically hard" to contrast them with the "magnetically soft" materials from which transformer cores and coils for electronics are made.

Centuries ago, it was discovered that certain types of mineral rock possessed unusual properties of attraction to the metal iron. One particular mineral, called lodestone, or magnetite, is found mentioned in very old historical records (about 2500 years ago in Europe, and much earlier in the Far East) as a subject of curiosity. Later, it was employed in the aid of navigation, as it was found that a piece of this unusual rock would tend to orient itself in a north-south direction if left free to rotate (suspended on a string or on a float in water). A scientific study undertaken in 1269 by Peter Peregrinus revealed that steel could be similarly "charged" with this unusual property after being rubbed against one of the "poles" of a piece of lodestone.

Many strong permanent magnets have been made from alloys of aluminum, nickel and cobalt with iron. Alnico and alcomax are two of the trade names of such alloys. Small amounts of Cu, Ti and Nb may also be used, and the number of variants is large. Even the same composition may be given different names in different countries.

Chronic low back pain is one of the most prevalent and costly medical conditions in the United States. Permanent magnets have become a popular treatment for various musculoskeletal conditions, including low back pain, despite little scientific support for therapeutic benefit. OBJECTIVE: To compare the effectiveness of 1 type of therapeutic magnet, a bipolar permanent magnet, with a matching placebo device for patients with chronic low back pain. DESIGN: Randomized, double-blind, placebo-controlled, crossover pilot study conducted from February 1998 to May 1999. SETTING: An ambulatory care physical medicine and rehabilitation clinic at a Veterans Affairs hospital. PATIENTS: Nineteen men and 1 woman with stable low back pain of a mean of 19 years' duration, with no past use of magnet therapy for low back pain. Twenty patients were determined to provide 80% power in the study at P<.05 to detect a difference of 2 points (the difference believed to be clinically significant) on a visual analog scale (VAS). INTERVENTIONS: For each patient, real and sham bipolar permanent magnets were applied, on alternate weeks, for 6 hours per day, 3 days per week for 1 week, with a 1-week washout period between the 2 treatment weeks. MAIN OUTCOME MEASURES: Pretreatment and posttreatment pain intensity on a VAS; sensory and affective components of pain on the Pain Rating Index (PRI) of the McGill Pain Questionnaire; and range of motion (ROM) measurements of the lumbosacral spine, compared by real vs sham treatment. RESULTS: Mean VAS scores declined by 0.49 (SD, 0.96) points for real magnet treatment and by 0.44 (SD, 1.4) points for sham treatment (P = .90). No statistically significant differences were noted in the effect between real and sham magnets with any of the other outcome measures (ROM, P = .66; PRI, P = .55). CONCLUSIONS: Application of 1 variety of permanent magnet had no effect on our small group of subjects with chronic low back pain.

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 B_r and H_ci. 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:

Rare Earth Magnets are used in many devices that we use daily but there are few who know what exactly Rare Earth Magnets are. So here is an article that discusses the origin, types and uses of Rare Earth Magnets.
Rare earth Magnets are strong magnets, which are small in size but massive in strength. They are permanent magnets that consist of alloys of rare earth elements. They are names after these elements, which are found in the Rare Earth or Lanthanides part of the periodic table. Lanthanides are elements, which have an f-shell that is not fully filled. Electrons in such cases are strongly confined and restricted which makes them retain their magnetic properties and attributes them the potential to create a paramagnetic surrounding around them.

These magnets as stated earlier are stronger (almost fifteen times stronger) than any other ferrite or alnico magnets. These rare earth magnets are so strong that they can lift four hundred times their own weight. They are used in expensive electronic gadgets such as cell phones, computer hard drives and the like; and since they are expensive, to save them from chipping and breaking, these rare earth magnets are coated with nickel. This nickel coating also protects them from rust, makes them shiny and stain resistant as well.

Types of Rare Earth Magnets

Rare Earth Magnets broadly fall into two categories which are as follows:

Neodymium (Nd-Fe-B) magnets- Neodymium magnets consist of neodymium, iron, boron and some of the transition metals. These magnets are the strongest and are also quite affordable amongst all the rare earth magnets. These are the types that are used in the hard drives of computer and speakers as they have the strongest magnetic field. These magnets are expensive as the cost of raw materials required is quite high and also because of the licensing of the patents. These magnets are also not very resistant to oxidation and temperature, thus they have to be coated with nickel or gold. Neodymium magnets are also corrosive but the main advantage is they give high-energy output compared to their size. These magnets can be used in microphones and computer printers and speakers.

Samarium- Cobalt (Sm Co) magnets-Samarium cobalt magnets consist of samarium, cobalt and iron. These magnets are not as commonly found the Neodymium magnet since the latter is easy to produce compared to the former and also gives a better magnetic field in terms of strength though Samarium Cobalt magnets are resistant to demagnetization and also stable under high temperature unlike Neodymium magnets. These magnets are also resistant to oxidation but shaped magnets are prone to breaking and chipping when they are exposed to thermal shocks. These magnets can be used in satellite systems and traveling wave tubes.

Rare Earth Magnets are very strong, thus they have to be handled carefully. It has to be remembered that small children should not be allowed to play with such magnets. These magnets should be not be put in to the mouth or consumed. Since these rare earth magnets are very strong their magnetic fields are also large and powerful, therefore people who have pacemakers should not come in close proximity of these magnets. They should also be kept away from credit cards, computers, cell phones and any other electronic equipment that can be influenced by the strong magnetic field of these rare earth magnets.

Rare Earth Magnets are strong permanent magnets. They are made from alloys of rare earth elements. They are much stronger than alnico or ferrite magnets, and the magnetic field produced bythemcan exceed 1200 milliteslas. Ceramic and ferrite magnets produce a field of 50 to 100 milliteslas, and are not as strong as rare earth magnets. Because they are very brittle, they are sometimes coated in nickel, to make them more durable and protect them from chipping and breaking, and they may be plated or coated to prevent corrosion.
Some common applications for them can include:
•    Audio speakers
•    Fishing reel brakes
•    Bicycle dynmos
•    Computer hard drives
Although these types of magnets have rare in their name, they are not particularly precious or rare, and interest in these compounds began in 1966 when it was discovered that these compounds had the largest magnetic anisotropy constant of any material then known.
Rare earth elements are metals which are ferromagnetic, which means they can be magnetized like iron, but in pure form their magnetism only appears at low temperatures.
They have a distinct advantage over other magnets because they have a high magnetic anisotropy. They are very easy to magnetize in one particular direction and resist being magnetized in any other direction.
There are hazards associated with these magnets which are not seen with magnets of other types.  Rare earth magnets larger than a couple of centimeters can be strong enough to cause injuries to body parts pinched between two magnets, or a magnet and a metal surface. The stronger magnetic fields can also be hazardous and may erase magnetic media data from devices such as credit cards and hard drives.
Rare earth magnets have a high magnetic anisotropy. Magnetic anisotropy is the direction of a materials magnetic properties. Magnetic isotropic material has no preferential direction, while a magnetically anisotropic material will align itself to an easy axis.

Rare Earth Magnets are strong permanent magnets. They are made from alloys of rare earth elements. They are much stronger than alnico or ferrite magnets, and the magnetic field produced bythemcan exceed 1200 milliteslas. Ceramic and ferrite magnets produce a field of 50 to 100 milliteslas, and are not as strong as rare earth magnets. Because they are very brittle, they are sometimes coated in nickel, to make them more durable and protect them from chipping and breaking, and they may be plated or coated to prevent corrosion.
Some common applications for them can include:
•    Audio speakers
•    Fishing reel brakes
•    Bicycle dynmos
•    Computer hard drives
Although these types of magnets have rare in their name, they are not particularly precious or rare, and interest in these compounds began in 1966 when it was discovered that these compounds had the largest magnetic anisotropy constant of any material then known.
Rare earth elements are metals which are ferromagnetic, which means they can be magnetized like iron, but in pure form their magnetism only appears at low temperatures.
They have a distinct advantage over other magnets because they have a high magnetic anisotropy. They are very easy to magnetize in one particular direction and resist being magnetized in any other direction.
There are hazards associated with these magnets which are not seen with magnets of other types.  Rare earth magnets larger than a couple of centimeters can be strong enough to cause injuries to body parts pinched between two magnets, or a magnet and a metal surface. The stronger magnetic fields can also be hazardous and may erase magnetic media data from devices such as credit cards and hard drives.
Rare earth magnets have a high magnetic anisotropy. Magnetic anisotropy is the direction of a materials magnetic properties. Magnetic isotropic material has no preferential direction, while a magnetically anisotropic material will align itself to an easy axis.

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!