FAQ – Limits of Magnet Materials

Are there new families of magnetic materials being developed?

Indeed there are. There has been much work on alloys based on a compound of Sm-Fe-N [aka Samarium-Iron Nitrides]. Although these materials do not as yet have the kinds of energy products that we are used to with Nd-Fe-B, they do have a much higher Curie temperature, which should mean that their services temperatures would be much higher.

In addition, there is much focus by magnetic materials scientists on nanocomposite materials. These materials combine the strong ferromagnetism of soft magnetic materials, with the ability to resist demagnetization found in hard magnetic materials.

What is the highest maximum energy product that can be achieved with today’s commercially available materials?

The current highest energy product for a commercially available permanent magnet material is 413.8 kJ/m3 for a Shin-Etsu Nd-Fe-B alloy. In August of 2002, scientists at Vacuumschmelze GmbH announced that they had produced a 442.4 kJ/m3 material in the laboratory.

What is the theoretical energy product limit for permanent magnet materials?

Maximum energy product is found by multiplying the values of Bd and Hd at points along the 2nd quadrant BH curve; BHmax is the highest value reached. The unit is kJ/m3 in SI units. As you move along the second quadrant curve, the value of Bd increases while the value of Hd decreases, and visa versa. Because the curve for rare earth materials is close to a straight line, the maximum energy product (BHmax) will be found close to the middle of the curve, where the values of Bd and Hd is Bd = Br/2 and Hd = Br/2µ0.

A line drawn from this point on the BH curve to where B and H are zero represents the optimum load line (PC) slope for the material. The value of the slope of this load line, or permeance coefficient, is close to 1.0, but it is usually more cost efficient to cause the magnet to work at a higher PC in its final magnetic circuit.

 Now, the value of Hc can mathematically never be greater than Br, but it gets very close in strong materials. If we assume that the ultimate material, Unobtanium, has an Hc value equal to 99.99% of Br, then there is little error in saying that µ0Hc = Br. Then the maximum energy product will be

 (BH) max = Br2/(4µ0).

The unit for Br is Tesla and the unit for (BH)max is J/m3.

The object of this exercise is to show that Br is the limiting factor. If we could make a permanent magnet from a material with the Permendur composition of 50/50 iron and cobalt, with a Br of 24 kG, then the maximum energy product would be

2.42/(4*4* *10-7)= 1145915.6 J/m3=1145.9 kJ/m3

Since the best magnet material today has a Br under 1.6 T, and we don’t know how to raise it much, the best material in the foreseeable future would have an energy product of less than 509.3 kJ/m3.