Characteristics of Alnico Magnets

This section provides information of the physical properties of Alnico magnets.

As already discussed in previous sections, the Alnico magnet has various grades. The grades have their own magnetic properties but generally share similar physical properties. Below is an overview of these properties:-

Summary of Physical Properties of Alnico magnets

Characteristic

Symbol

Unit

Value

Density

D

g/cc

6.9 – 7.3

Vickers Hardness

Hv

D.P.N

520 - 700

Compression Strength

C.S

N/mm2

300 - 400

Coefficient of Thermal Expansion

C//

10-6/°C

11.5 - 13

 

C^

10-6/°C

11.5 - 13

Electrical Resistivity

r

micro Ω.cm

45 - 70

Tensile Strength

σUTS, or SU

kg/mm2

20 – 450 (37 ACA44)

Curie Temperature

Tc

°C

810 - 860

Structural use of Alnico magnets

There is a risk of chipping or breaking the magnets because all magnets are inherently brittle. It is advised to not put magnets in conditions of mechanical stress e.g. in load bearing situations.

Alnico does however have a low coercive force, Hc. This means that it is easily demagnetized by external demagnetising fields but it is also very easy to magnetise to saturation. It is therefore possible to magnetise the magnet when part of an assembly (providing the means to magnetise is possible). But, it also means that Alnico may also self demagnetize if the design is poor (it depends on the total magnetic circuit). An example may be removing an Alnico magnet from a motor assembly for inspection and then placing it back and discovering the magnetic output has fallen – putting the magnet in open circuit may be enough to cause self demagnetisation (the working point is taken beyond the ‘knee’ of the Intrinsic curve, causing demagnetisation).; the working point changes if the magnetic circuit changes. Self demagnetisation occurs because the magnetic field from the magnet can pass back through the magnet itself (the magnetic path is more preferable than the air surrounding the magnet), which is thus an external demagnetising field applied by the magnet to itself (all magnets in free space will apply a self –demagnetising field but only magnets with very low working point and/or a low Hci values will show demagnetisation). Rare Earth magnets (NdFeB and SmCo) and ferrite magnets, in normal operation, do not demagnetise by self demagnetisation.

An advantage of this possible sensitivity to external magnetic fields is that Alnico is easy to “tune”. In some applications (e.g. spectrometers and other specialist testing equipment), the material being tested gives uniquely identifiable results but only if the magnetic field being applied to it is within a narrow precise range. This is where the low Hci of Alnico is extremely useful. It is possible to build a magnetic assembly containing Alnico, magnetise it to full saturation (with a pulse of sufficient strength, up to 1Tesla usually, and longevity to overcome any eddy currents in magnetic steel components), then deliberately weaken it magnetically (by demagnetisation using a controlled external magnetic field) until the assembly offers the required field strength at the desired air gap location. This method of tuning is sometimes also known as “knocking-back” – you should always fully magnetise then weaken rather than just weakly magnetise because knocking back demagnetises the zones (domains) of the magnet that have the worst Hci values leaving the domains that are harder to demagnetise leaving a more stable magnet which is better at resisting being demagnetised accidentally (which is why knocking back is also known as “stabilizing”). Some designs will deliberately request a stabilization to provide a designed-in lower magnet performance but with an improved resistance to demagnetisation by external magnetic fields.

Alnico is better at resisting self demagnetisation by being longer in the direction of magnetisation than in the dimensions of the pole face (usually termed Length to Diameter, L/D, ratio). A higher L/D ratio increases the working point on the Intrinsic curve. This is why Alnico magnets are very rarely short in length (they are often seen in bar, rod or horseshoe shapes) and a L/D ration of at least 4:1 is ideally required to get good performance from an Alnico magnet or an Alnico magnetic assembly (having Alnico is a ferromagnetic assembly and magnetising in-situ boosts the L/D ratio of Alnico creating a higher magnetic output). If the design is correct with Alnico, it can outperform many magnets at ambient temperature and all other magnets at high temperatures. To get the most out of an Alnico magnet, the entire magnetic circuit (magnet, ferrous parts nearby, external magnetic fields, etc) needs to be considered.

Designing with Alnico magnets

When designing with Alnico it is important to consider the shape of the magnet (and hence the Permeance coefficient, Pc), any external demagnetizing forces and the intended use of the magnet. Because Alnico has a lower Hc than the other magnets, it is prone to being easily demagnetized if designed incorrectly. But it can, and is, used in motors and it can offer higher field strengths than the rare earth magnets (by virtue of utilizing the high Br available).

It is a hard yet brittle magnet which is very difficult to machine or drill and is not recommended for structural use.

Also, there are some additional design improvements that can minimize losses from Alnico magnets, such as tapering (making the magnetic poles narrower) and use of steel pole tips (to concentrate the magnetic field to boost performance further and to limit field leakage). In designs where the assembly is a C-shape such that there is a small gap where field is required and the magnetic circuit structure is magnet/steelwork, you will get better performance from Alnico if it is placed either side of the air gap (if Alnico is put in the “back-iron” section further away from the air gap, the air gap field will be much weaker).

Unless the magnet has a very good Pc (Permeance coefficient) it is best to avoid pushing Alnico magnets into each other in repulsion. Because Alnico has low Hc (coercive force) and Hci (Intrinsic Coercive force) values, Alnico can demagnetise itself. This is why many Alnico magnet have “keeper plates” of mild steel – they short-circuit the magnetic circuit to increase the Pc making it harder o demagnetise the magnet whilst in storage.

With correct designing, Alnico can be a very powerful magnet with a superb temperature characteristic that no other magnet can match. It can also be used for applications where fine tuning is advantageous.

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