More classification
Common magnetic problems
Here is some general information on magnetism and magnetic physics. We hope that you find this useful.
• Q1:How does temperature affect the behavior of a permanent magnet?

Curie Temperature (Tc): This is the temperature at which a magnet material loses it's strength, permanently. Another useful number (if available) is Tmax, the recommended maximum operating temperature. Above Tmax (around 266 deg. F for most NdFeB magnets) a magnet will start ot lose its power, and at Tc all power is lost. If you need strong magnets that can be used at high temperatures, consider using Samarium Cobalt (SmCo) magnets.

Magnet Strength Measurements (B)--The units for measuring the field strength (flux density) of a magnet are Gauss or Tesla. 1 Tesla = 10,000 Gauss. The Earth's magnetic field is on the order of 1 Gauss. There are different ways to classify and measure field strength:
B (flux density): This is the measurement (in Gauss or Tesla) you get when you use a gaussmeter at the surface of a magnet. The reading is completely dependant on the distance from the surface, the shape of the magnet, the exact location measured, the thickness of the probe and of the magnet's plating. Steel behind a magnet will increase the measured 'B' significantly. Not a very good way to compare magnets, since B varies so much depending on measurement techniques.

Br (residual flux density): The maximum flux a magnet can produce, measured only in a closed magnetic circuit. Our figures for each magnet are provided to us by the magnet manufacturer. They are a good way to compare magnet strength...but keep in mind that a magnet in a closed magnetic circuit is not doing any good for anything except test measurements.

B-H Curve: Also called a "hysteresis loop," this graph shows how a magnetic material performs as it is brought to saturation, demagnetized, saturated in the opposite direction, then demagnetized again by an external field. The second quadrant of the graph is the most important in actual use--the point where the curve crosses the B axis is Br, and the point where it crosses the H axis is Hc (see below). The product of Br and Hc is BHmax. If we have these measurements available, they are provided to us by the magnet manufacturer--very complicated and expensive equipment is needed to plot a B-H curve.
A sample B-H curve:

Magnet Quality (BHmax): The quality of magnetic materials is best stated by the Maximum Energy Product (BHmax), measured in MegaGauss Oersted (MGOe). This is because the size and shape of a magnet and the material behind it (such as iron) have a large effect on the measured field strength at the surface, as does the exact location at which it measured. All of our Nickel-plated NdFeB magnets are grade N35 (BHmax=35 MGOe) and all of our Gold-plated NdFeB magnets are grade N45 (BHmax=45 MGOe). This gives about a 5% difference in strength, and a 150% difference in cost...it is wise to balance your magnet strength needs by cost too. Other magnets are measured the same way -- a grade 8 ferrite magnet (grade C8) has BHmax=8 MGOe.

Coercivity (Hc): This measures a magnet's resistance to demagnetization. It is the external magnetic field strength required to magnetize, de-magnetize or re-magnetize a material, also measured in Gauss or Tesla.

NdFeB magnets are a relatively new invention--they first became commercially available in 1984. General Electic held the patent, and some hard disc drive manufacturers were indeed sued over using unlicensed NdFeB magnets. Since then, the patent has been sold to Sumitomo, who is making the license easily available to magnet manufacturers worldwide. All NdFeB magnets are currently protected under international patent. ALL of our magnets are either new, legal, licensed material, or surplus from various industries.

• Q4:What is a magnetic field? What are magnetic field lines?

Magnetic fields are historically described in terms of their effect on electric charges. A moving electric charge, such as an electron, will accelerate in the presence of a magnetic field, causing it to change velocity and its direction of travel. This is, for example, the principle used in televisions, computer monitors, and other devices with CRTs (cathode-ray tubes). In a CRT, electrons are emitted from a hot filament. A voltage difference pulls these electrons from the filament to the picture screen. Electromagnets surrounding the tube cause these electrons to change direction, so they hit different locations on the screen.

This is how it works: An electrically charged particle moving in a magnetic field will experience a force (known as the Lorentz force) pushing it in a direction perpendicular to the magnetic field and the direction of motion:

As a result of this force, the charged particle accelerates in the direction of the force (this is Newton's second law). In the diagram above the particle's trajectory will curve upward.

Magnetic fields are perhaps more easily understood in terms of magnetic field lines. Field lines, also known as lines of force, define the direction and strength of the magnetic field at any local in space. As explained later, magnetic fields have both a direction and strength (or "magnitude"). The direction of the field lines indicates the direction of the field, while the density of the field lines indicates the magnitude of the field. Thus at points where the field lines are closer together, the field is stronger. Field lines are described mathematically with a quantity known as flux.

Magnetic fields are commonly a result of magnetic dipoles. A simple example of a magnetic dipole is the bar magnet:

As you can see, the magnetic field lines always begin on the north pole of a magnet, and end on the south pole. This diagram illustrates the magnetic field lines of a typical magnetic dipole.

Magnetic dipoles always like to align themselves parallel to an external magnetic field, so the dipole's field matches the one applied to it. This is why bar magnets line up north-to-south. It also explains the behavior of a compass needle, which, being composed of Iron (a ferromagnet), behaves like a magnetic dipole.

• Q5:What is the north pole of a magnet and how can I identify it?

The north pole of a magnet is the pole that aligns itself with geographic north. As a result, the geographic north pole of the earth is actually very near the earth's magnetic south pole:

This is sometimes an issue of confusion, but we are stuck with it. What we call "magnetic north" is really magnetic south.
To identify the north pole of a magnet, you can make a compass out of it. Either hang it on a string or float it on water. The pole that faces geographic north is the north pole. Once you have one magnet with poles identified, it is easy to label others, as like poles repel and opposite poles attract.

• Q6:Why do magnets have poles?

This is a physical property of magnetic fields. Magnetic fields are vector quantities, meaning they have both a magnitude and a direction. Many measurable physical phenomena are described in terms of scalar quantities, which have only a magnitude. An example of a scalar quantity is temperature; temperature has a magnitude (which you can measure with a thermometer), but no direction.

On the other hand, magnetic fields are directional. In a magnet, the magnetic field vector always points from the north pole to the south pole. In the space around the magnet, the vectors vary in both direction and magnitude. This is the behavior you see when you dump iron filings around a bar magnet, for example. Vector quantities which vary in space are known as fields; thus we have the term magnetic field for the vector field surrounding a magnet.

• Q7:What is the differences between the coatings?

The coatings do not affect the magnetic strength or performance of the magnet. The preferred coating is dictated by preference or by the magnets intended application. Nickel is the most common choice for plating neodymium magnets, and is actually a triple plating of nickel-copper-nickel. It has a shiny silver finish and has good resistance to corrosion. Black nickel has a shiny, black/charcoal appearance and is slightly more corrosion resistant than regular nickel. It is also a triple plating of nickel-copper-black nickel. Zinc has a dull gray/bluish finish, that is more susceptible to corrosion than nickel. Zinc can leave a black residue on hands and other items. Epoxy is basically a plastic coating that is virtually 100% corrosion resistant as long as the coating is intact. From our experience, it is the least durable of the common coatings. Gold plating is applied over the top of nickel plating, so gold plated magnets have the same characteristics as nickel plated ones, but with a gold finish.

• Q8:How are magnets manufactured? Can I make them at home?

Manufacturing: NdFeB magnets are complicated to manufacture. The powdered NdFeB material is packed in molds, then sintered. The non-magnetized 'magnets' are then shaped to the correct size and plated. To magnetize them, they are placed in a very expensive machine that generates an extremely high-powered magnetic field for an instant, using high-voltage capacitor discharge and coils. The polarity of the finished magnet depends on how it was oriented in the magnetizing machine, and how the particles in the sintered mixture were oriented. So that makes home manufacture impossible. You CAN, however, make a simple steel magnet at home. Take a nail and stroke it with a strong NdFeB magnet 20 or 30 times, ALWAYS moving the magnet in only one direction on the nail. Presto, the nail will be magnetized, altough very weakly.

• Q9:Can I cut, drill or machine magnets to my own sizes and shapes?

NdFeB magnets are by nature very hard and brittle. Although they can be cut, drilled and machined, it should ONLY be done by folks who are experienced with ceramics. If the magnets get over about 300 deg F, they will lose their magnetism permanently. They are flammable, and it is not difficult while grinding or machining to get them (or the chips and dusts from cutting) so hot they ignite. If they do ignite, the fumes are toxic and the material burns very fast and hot, like Magnesium! In our experience any machining of these magnets should be done with diamond tools under lots of coolant with good ventilation and the risk of fire in mind.

• Q10:How can you ship magnets safely? Don't they affect airplane compasses?

We take great care when packing orders to see that any magnetic fields are well contained within the box we send them in. We pack very carefully so the external magnetic fields cancel out, and we use steel box liners as needed to insure that every box is safe and non-magnetic to comply with national and international postal regulations. We also test each package before it goes out to be sure it complies with all regulations.

• Q11:If I have a Neo magnet with a Br of 12,300 Gauss, should I be able to measure 12,300 Gauss on its su

No. The Br value is measured under closed circuit conditions. A closed circuit magnet is not of much use. In practice, you will measure a field that is less than 12,300 Gauss close to the surface of the magnet. The actual measurement will depend on whether the magnet has any steel attached to it, how far away from the surface you make the measurement, and the size of the magnet (assuming that the measurement is being made at room temperature). For example, a 1" diameter Grade 35 Neo magnet that is 1/4"long, will measure approximately 2,500 Gauss 1/16" away from the surface, and 2,200 Gauss 1/8" away from the surface.

• Q12:Can a particular pole be identified?

Yes, the North or South Pole of a magnet can be marked if specified.

• Q13:What are the differences between the different magnet formulations you sell?

NdFeB (Neodymium-Iron-Boron) -- The most powerful 'rare-earth' permanent magnet composition known to mankind, and our specialty. This formulation is relatively modern, and first became commercially available in 1984. NdFeB magnets have the highest B, Br, and BHmax of any magnet formula, and also have very high Hc (see below for definitions). They are however very brittle, hard to machine, and sensitive to corrosion and high temperatures. Useful in the home, workshop, pickup truck, laboratory, wind turbine, starship and more. We carry both new and surplus stock in many sizes and shapes.In almost all magnet applications, NdFeB are the best choice for incredible strength and coercivity at a reasonable price! In power generation applications, NdFeB magnets can be expected to give 4-5 times the power output of ceramic magnets.

Ferrite (Ceramic) -- Also known as 'hard ceramic' magnets, this material is made from Strontium or Barium Ferrite. It was developed in the 1960s as a low-cost and more powerful alternative to AlNiCo and steel magnets. Less expensive than NdFeB magnets, but still very powerful and resistant to demagnetization. Useful everywhere. We carry both new and surplus in multiple shapes and sizes. Ferrite magnets are lower in power (B, Br, BHmax) compared to other formulations, and are very brittle. However, they have very high Hc and good Tc (see below), and are quite corrosion-resistant. A very cost-effective choice.

AlNiCo (Aluminum-Nickel-Cobalt) for medium strength and excellent machinability. Developed in the 1940s and still in use today. They perform much better than plain steel, but are much weaker in strength (lower B, Br and BHmax) and must be carefully stored since they are prone to demagnetization. Contact with a NdFeB magnet can easily reverse or destroy the field of an AlNiCo magnet.

SmCo (Samarium Cobalt)-- for high power and resistance to high temperatures and corrosion. Developed in the 1970s, these were the first so-called 'rare earth' magnets. They are almost as powerful as NdFeB magnets, and far more powerful than all the others (high B and Br). They are the most expensive magnet formulation, and usually only used where resistance to high temperatures (high Tc) and corrosion are needed. Also very brittle and hard to machine.

Bonded (flexible)-- magnets are a rubberized formulation often seen on refrigerators and magnetic signs. Though they may be manufactured from any magnet formulation when powerdered and mixed with rubberizer, the result is always less powerful than a traditional sintered magnet of any formula. Used only where unusal and difficult shapes are needed.

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Shanghai ROCO Magnetics Co.,Ltd.