Permanent & Electro-Magnetic Product FAQs - Miscellaneous

1. Cup or holding assemblies: Magnets in a cup made of 17-4 stainless seem to have a higher holding force when modeled, compared to the same assembly in 316L stainless. Is this true and why are there differences?
2. Define terminology commonly used in sputtering, e.g. magnetron, cathode, plasma, anode, targets, etc.
3. Do permanent magnet materials have the ability to temporarily carry electric current?
4. What is a "bucking" dipole? Why would we want to use one?
5. Why is it sometimes magnets can't be shipped via airfreight?

1. Cup or holding assemblies: Magnets in a cup made of 17-4 stainless seem to have a higher holding force when modeled, compared to the same assembly in 316L stainless. Is this true and why are there differences?
   Encore. 17-4 stainless is known as a "Precipitation-Hardening Stainless Steel", and alloy 17-4 belongs to the martenisitic family of these types of steels. This alloy isn't as strong as pure iron or cold rolled steel, but it can boost performance in certain applications. In cup or holding assemblies, it helps prevent leakage flux and can help improve the operating point of the magnet in the assembly.
   
   316L is an austenitic stainless steel and is nonmagnetic. All austenitic stainless steels (the 300 series) are nonmagnetic unless they are mechanically worked.
      
2. Define terminology commonly used in sputtering, e.g. magnetron, cathode, plasma, anode, targets, etc.
   A sputtering magnetron is a device that provides a magnetic field to confine and control electron trajectories, and therefore the plasma location and intensity.
     
   A cathode is a feature from which electrons are emitted, giving it a negative charge. The entire target assembly is referred to as the cathode.
     
   A plasma is a "cloud" of charged particles, negative electrons and positive ions. Because it contains approximately the same number of negative and positive particles, it is often referred to as "neutral". Like a cloud, a plasma is dynamic, constantly forming and dissipating. The visible spectrum of a plasma is the result of photon release when electrons and ions recombine.
     
   An anode is the part of the system on which electrons collect, so it is said to have a positive charge. In many systems everything except the cathode assembly is part of the anode. The negatively charged target repels gyrating negative electrons back into the plasma and attracts the much heavier positive ions. Ion bombardment of the target material frees target atoms to coat the substrate.
        
3. Do permanent magnet materials have the ability to temporarily carry electric current?
   All of the metal-based magnets are conductive. Elements such as aluminum, iron, cobalt, and nickel are metals. Ceramic magnets do have some very limited conductivity, though not much. The epoxy binder in bonded magnets typically insulates metal magnet particles. Although a high enough voltage could cause this to break down and let electricity flow. If used as a conductor, as current density increases, local hot spots could alter the chemistry of some portion of the volume, and thus degrade performance of a magnet.
     
   Useful conductors of electricity are generally limited to those metals with fine grain structures, with intimate contact between molecules and minimal grain boundaries. In the case of large grain materials, like cast Alnico, resistivity may be a poor indicator of actual performance as a conductor because the conducting path through the material can vary considerably from sample to sample. A high resistivity may be falsely indicated by a high resistance reading simply because the high conductivity grains are effectively separated by the grain boundaries that help Alnico materials resist demagnetization. Conversely, a very low resistivity indication may be the result of point contact between grains of the material, and it will never perform well as a conductor.
      
   Sintered Alnico products have a smaller, more homogenous, grain structure so they would perform better as conductors than cast products would. However, it is generally preferable to plate the surface of a magnet with a conducting material if it must be used as an electrical circuit element.
       
4. What is a "bucking" dipole? Why would we want to use one?
   A bucking dipole is arranged so that like poles face and repel each other across the air gap. There are several reasons why a customer might like this. A bucking arrangement increase the "throw" of a magnetic field, and such a device can be used to project the field into an area that cannot contain a magnet for whatever reason.
      
   A simple example of a bucking dipole is two horseshoe magnets facing each other in repulsion. In this case the magnetic flux intensity increases as the magnets approach each other. In this situation, flux that normally arches out is compressed into a "flat field" between the poles. This would be desirable for orienting the magnetic particles on magnetic recording tape. The wet coating would be pulled through the bucking dipole while it is being dried such that the drying particles saw the desired field direction as they "set". Two "rare earth" magnet assemblies of the type shown in Dexter's patent US 5,865,970, when placed in opposition, can provide a very high "in plane" magnetic field.
     
   Bucking dipoles can be used to generate high magnetic gradients. These high gradients are useful for separation devices. In these applications, a high gradient is required across the air gap. In the air gap of a bucking dipole, the field is reduced to zero at the center of the gap, and the field at the pole faces is high, creating the required gradient.
      
   Care should be taken in designing bucking magnet dipoles, as the fields at pole edges can be high enough to demagnetize a portion of low coercivity materials and / or magnet shapes with low permeance coefficients. A high coercivity material is advisable for designs of this type.
        
5. Why is it sometimes magnets can't be shipped via airfreight?
   The Federal Aviation Administration (FAA) and the International Airline Association (IAA) have VERY strict regulations on how materials are shipped by airplanes. Dexter Magnetic Technologies complies fully with these guidelines.
     
   To ship a magnet by air, the magnet must be packaged and tested to determine the amount of magnetic flux that is 'leaking' from that package. The permissible amount of leakage is relatively small, and an extra sensitive instrument known as a magnetometer is needed for testing. If the leakage field is below a permissible level at specified distances, the box can be safely shipped.
     
   If leakage is excessive, the magnet can be repackaged in a shielded container and retested. Packages of this sort that pass the leakage tests can be shipped by air, but must be marked as dangerous goods. Such packages are very expensive to ship and can significantly increase the price of the final product.
     
   Some magnets and magnetic assemblies are so powerful that they fail leakage tests even in shielded containers. These magnets must be shipped via ground or ship transport as FAA and IAA regulations eliminate airfreight as an option.