How Strong is a Magnet?
Release time:2017-04-12 11:08 Author:Nature
Keywords: strong magnet
What measures a magnet's strength?
There are two measurements that count with magnets. The first is the ability of an alloy to be magnetized,
which determines the attractive force. We measure this in Gauss per cubic inch of material at saturation
magnetism, a measurement of the strength of the magnetic flux. The second important feature is the
permanence of the attractive force, measured in Oersteds. In the world of permanent magnets it is not
particularly useful to have a strong magnet that rapidly weakens. The Oersted is a measurement of the
amount of coercion required to completely neutralize a magnet. It is usually referred to as the "coercivity"
of the magnet. But today neither of these measurements is commonly used; they are multiplied by each
other to get a "Maximum energy product" measured in mega (one million) gauss (x) oersteds or MGOe.
What does saturation magnetism mean?
Saturation magnetism is attained when every polarized molecule in the material has the same magnetic
orientation – all norths pointing north. This is as good as a magnet gets. Sometimes, however, bad things
happen to good magnets, which can cause them to lose their magnetic minds (referred to as Irreversible Loss).
Heating magnets beyond their operational temperature, striking them, exposing them to strong magnetic
fields, or just old age can all cause Irreversible Loss. Despite the fatal-sounding name for the condition, the
losses are recoverable by remagnetization of the magnetic materials. Resistance to demagnetization is called
coercivity, for which rare-earth magnets are the champions!
Iron, cobalt and nickel are the only elements that are ferromagnetic at room temperature. Rare earths are alloyed
with these materials to increase their coercivity.
What shape of magnet works best?
Disc magnets provide the highest usable surface area to mass ratio; this shape generally provides the greatest
usable magnetic force for the money.
Materials Used to Make Magnets
The rare-earth magnets currently available at the retail level are made of neodymium iron boron (NdFeB). Only
three elements are ferromagnetic at room temperature; these are iron, cobalt and nickel. Virtually all other elements
increase permanence (coercivity), but any magnet must contain one of the base three to work. The four main
magnet types used today are ceramic, alnico, neodymium, and samarium cobalt.
An alnico magnet is made of aluminum, nickel and cobalt. These can be cast (melted, then shaped in a mold) or
sintered (fused together by heat and pressure). A magnet that is cast has better magnetic properties than one that
is sintered. Although this material can lose its magnetic properties if dropped or struck, the advantage of an alnico
magnet is that it can endure temperatures up to 550 °C.
Ceramic magnets are quite hard and brittle. They are made of strontium ferrite and iron oxide, mixed into a ceramic
base. For applications under 300 °C, these have lower energy than the other types of magnets, but resist corrosion
and demagnetization. Their main advantage is that they are inexpensive.
Neodymium has one of the highest magnetic properties of any magnetic alloy. Although it is the magnet to use for
high-strength applications, one of its drawbacks is that it cannot be used where it will be exposed to temperatures
higher than 150 °C, or it will demagnetize.
For high-temperature applications, magnets made of samarium cobalt are used. Even though samarium cobalt is
not quite as strong as neodymium, this member of the rare-earth family can withstand temperatures up to 300 °C.
The different types of materials used increase the versatility of magnets. The characteristics of each make it possible
to find a magnet suitable for just about any application, from keeping a calendar posted in the shop to ensuring there
are no nuts and bolts in industrially processed food.