Magnets & Magnetic Properties

Understanding magnets and their use with Reed Switches

Using permanent magnets with a Reed Switch in a sensing application is not always straight forward so it is helpful to review various types of rare earth magnets and their magnetic properties to gain the full understanding the Reed Switch and magnet relationship.

What is a Magnet?

  • The magnetic effect is created at the sub-atomic level
  • An atom has a nucleus composed of protons and neutrons.  Electrons encircle the nucleus

  • Two things occur in an atom that produce a magnetic field
  • Both by the negatively charged electrons

What is Magnetic?

  • A dipole is in essence a tiny magnet.  In most elements the dipoles align themselves with equal and opposite dipole, effectively canceling out any net over effect as shown in the figure below.


  • In many metals this is not the case.  They are not canceled out within the atom itself.
  • However, when the atoms are grouped with themselves many billions of times over as with a piece of iron, the net effect is canceled within its bulk material.

  • However there are few of the elements, where the dipoles are free to move when brought into a magnetic field, and these dipoles will align themselves and will become 'locked in place' in the crystal's lattice structure.
  • This is what we now call a permanent magnet.

What are some magnet materials and types?

  • Iron (FE), Nickel (Ni), and Cobalt (Co) are the most common materials used for permanent magnets in reed sensors.
  • Less common materials are Chromium (Cr) and Manganese (Mn)
  • Among the types of magnets, rare earth magnets offer a very strong or high magnetic field and the most popular are the(NdFeB)  Neodymium rare earth magnets and Samarium Cobalt (SmCo)
  • There are also a large number of magnets which combine several elements such as in the AlNiCo

Which magnet properties are important in Reed Product applications?

  • All magnets have different properties.
  • The properties of each magnet allow us to selectively use them in different applications
  • There are some key magnet properties to carefully consider when using a permanent magnet with a Reed Switch in a Reed Sensor or Reed Relay application such as:

Ability to Magnetize and Demagnetize Easily

  • Many times particularly for a Form B (Normally Closed) or latching reed relay it is very helpful to be able to magnetize and demagnetize a given magnet to adjust the precise activate and deactivate points.
  • For the most stable results, the magnet should be fully magnetized and then demagnetized to the best operating point.
  • The AlNiCo series magnets usually provides the best results for this
  • Sintered magnets are also very good as well

High and Low Temperature Stability

  • When magnets are used above 150 degrees Celsius care should be taken to select magnets that are more stable at high temperatures
  • Probably the most stable and best magnets at high temperatures are the AlNiCo series and for the rare earth samarium cobalt (SmCo)
  • Most magnets are relatively stable at temperatures 0 degrees Celsius and below.

Magnetic Strength

  • As shown in the table below AlNiCo magnets are the most stable in high and low temperature applications
 Magnet Type Low Temp
 High Temperature
 Comments
 SmCo Magnets Stable to 4 degrees K
 Stable to 250 degrees C
 Below 20 degrees C magnetic
   strength will rise slightly
 NdFeB Magnets Stable to 15 degrees K
 Stable to 160 degrees C
 Below 20 degrees C magnetic
   strength will rise slightly
 AlNiCo Magnets Stable to near 0 degrees K
 Stable to 550 degrees C
Most stable of all magnetic materials

 Ferrite Magnets
 Stable to -10 degrees C
 Stable to 250 degrees C
 At -20 degrees C they suffer a
   permanent loss of magnetism

  • Magnetic strength is often an important selection that determines the distance in which a sensor will open and close.
  • If there are other magnetic fields or ferromagnetic materials in the area, they will have to be taken into consideration as well.

Magnet Size

  • Size will also determine the operate points of a reed sensor.
  • Potentially the greatest sensing distance is achieved when matching up size and strength of the magnet.

Stability

  • Depending upon the application, stability can be an important parameter in a permanent magnet if the sensing distances for a given sensor need to be very accurate.
  • Careful evaluation of the magnet specification needs to be considered.

    Shock

  • Strong shock can change the magnetic strength for a given magnet.  If an application calls for an environment involving shock, care must be taken in selecting the correct magnet.
  • Shock can become a factor in a Form B or latching relay where improper relay handling by a customer can cause enough shock to alter the operate points of the reed switch.

Curie Temperature

  • The Curie temperature of a magnet is the temperature when the magnetic properties of a magnet are lost.
  • The temperatures are usually quite high; however they can and are reached in several sensor applications.
  • Combining different elements and bringing them to the liquidus state; then rapidly cooling them produce a large assortment of magnets, all of which will have a varied set of properties and Curie temperatures.
 
 Material Curie
Temp. (K)
  Curie
Temp. (C)		  Curie
Temp. (F)
Co 1388 1115 2039
Fe 1043 770 1418
FeOFe2O3 858 858 1085
NiOFe2O3 858 585 1085
CuOFe2O3 728 455 851
MgOFe2O3 713 440 824
MnBi 630 357 674
Ni 627 354 669
MnSb 567 314 597
MnOFe2O3 573 300 571
Y3Fe5O12 560 287 548
CrO2 386 113 235
MnAs
 318 45 113
Gd 292 19 18
Dy 88 -185 -301
EuO 69 -204 -335