The use of magnets goes back deep into history with the early magnetic stones being varieties of magnetite (Fe3O4). Practical magnets however came much later and were based on quenched carbon steels. At this stage, it is best to introduce the “hysteresis loop”, the basic tool of the magnetician. Ferro-magnetic materials – cobalt, nickel, iron – are characterised by having unbalanced electron spins.
The electrons circuiting the nucleus spin, hence being charged, generate a field in one or another direction. If even numbers spun in opposite directions, the net result/atom would be nil. However, in Co, Ni, Fe, these spins are unbalanced and a net field results.
The material would still be non-magnetic as each atom is producing a randomly orientated net field. Applying an external field can persuade these internal fields to line up (in fact small patches – domains of similarly orientated atoms line up). The hysteresis loop describes the relationship between the magnetising force H compared to the magnetic flux B generated by the magnetic material. A typical curve is shown below.
Figure 1 – Hysteresis loop of ferromagnetic material
Br: Remanence Hc: Coercive forceSlope OA: Initial permeability
S: Saturation Slope CD: Recoil permeability Units: H-Oested, B - Gauss
Initially, the plot of BH follows OS to a point were saturation occurs (saturation level). On reversing the field, hysteresis occurs and the curve hits the B axis at a point Br. This is now a permanent magnet and Br is the remanence level. In practice, a magnet exerts a demagnetising force and a magnet will exist on the next part of the curve where H is reversed until B reaches zero. This force Hc is the coercive force (coercivity).
Here we now have the most important measurements of a magnet, hard or soft:
a) Remanence Br – Flux density
b) Saturation level – S
c) Coercivity – Hc
d) Not shown, but B x H in the demagnetising quadrant reaches a maximum at some point (Bhmax)
In a good magnetic circuit, the permanent magnet used will operate at this point, thus generating the largest field possible for its size.
Cobalt is vital in the magnet industry in that is the only element which when added to iron actually increases the value of S (saturation magnetisation) and it also has one UNIQUE feature. Temperature destroys permanent magnetism as the atomic activity increases and removes the order required. The temperature at which this occurs is the Curie Point. Cobalt has a Curie Point of 1121°C (the highest known).
There are common names for two classes of magnetic substances – hard magnets/high coercivity and soft/high saturation, minimum coercivity. The origin of these terms is in the second generation of magnets, the quench hardened steels. Here coercivity is raised by the internal strain caused by martensitic transformation, hence coercivity was indirectly related to hardness.
Soft ferromagnetic materials are characterised by their inability to retain the magnetism induced by a field when it is removed.
The main applications are in rotating machines – generators, motors and in static transformers. Cobalt does not enter into most soft magnet compositions with Fe/Si being used for the largest application – transformer laminations – as core losses are low with this material.
The important series of alloys from cobalt’s point of view are the Co/Fe series typified by Permendur – this alloy being modified by vanadium additions to improve ductility. The reason for the use of cobalt is that this alloy benefits from the maximum saturation known, 23,500 gauss (2.35 Tesla) with a square shaped hysteresis loop, and also from the high Curie Point.
These alloys because of their higher strength also find application in rotating equipment but their main use is in top performance electrical machines where weight and size are at a premium and cost is of lower importance. The properties which make them unique are:
- A high saturation induction, the highest known
- Good permeability in fields > 16,000 Gauss compared to other materials such as Fe and Fe/Si alloys
- A Curie Point of 950/980°C so that magnetic properties remain little changed up to 500°C. In modern tightly packed systems this is a great advantage.
A new series of materials is also available, cobalt-based but with metalloid additions – e.g. boron. These are amorphous alloys and the series based on cobalt Vitrovac 6000 demonstrate two unique features: The lowest to date realised coercivity and magneto-striction (i.e. dimensional change under a field) close to zero.
Hard Magnetic Material
As we have seen earlier, the term hard magnets originates from hard steels with magnetic properties. It has come to mean any magnetic material, which can be permanently magnetised by applying a magnetic field.
A typical loop for a hard magnet would show a square curve with Br-remanence as high as possible, coupled with a high value of Hc-cercivity. Such a combination of properties would ensure a high value of the factor (B x H)max – the energy product. Essentially, the higher the (BH)max is, the smaller the magnet needed to generate the magnetic flux required in any application. Unfortunately, one cannot simply get Br x Hc to be infinite values at will and one often has to sacrifice one at the expense of the other. Hard magnets have developed over many years and up until earlier this century, only steels were available.
In 1932, a new series of Al-Ni-Fe (25%Ni, 10%Al, balance Fe) with coercivities 9 times that of magnet steel became available. Research led to cobalt additions to the alloys to enhance their properties. These alloys became known as AlNiCo alloys.
Since then, not only has a series of alloys been developed, but other processing changes have further improved properties.
In the 1960s, Rare-Earth magnets became more available and by 1970, dense Co5Sm magnets had been produced by sintering. The properties of Rare-Earth magnets are a quantum leap from Alnico, just as Alnico was from steel.
For example, Supermagloy 1, a sintered magnet, has a Br value of 8,000 Gauss like Alnico but its coercivity Hc is 8,000 Oersted (Alnico is 600) and Bhmax is 15-17 Megagauss Oersted as compared to about 5.6.
These massive changes have led to a revolution in instrumentation, telephones, electronics and even motors.
However, in recent years Samarium cobalt has faded in importance due to its high cost and has been eclipsed by a more powerful, less costly and more versatile sister rare-earth material invented in 1983, Neodymium-iron-boron (NdFeB).
The development of NdFeB magnets since 1983 has resulted in energy products climbing from 28 Megagauss Oersted (MGOe) to the current commercial standard 45 MGOe. This ten-fold increase in available magnetic energy has opened doors to the designers of such varied devices as disc drives, magnetic resonance imaging, high efficiency dc motors, etc.
Early versions of these magnets had two weaknesses – thermal instability and poor corrosion resistance. Significant advances have been made during the past few years to overcome these deleterious characteristics by the use of coating techniques and the development of alloys with higher corrosion resistance. The latter development has often been achieved by the addition of cobalt to the alloys. Nevertheless, the cobalt contents of these magnets is low in comparison to Alnicos – typically between 1 and 5% although some melt spun magnets do contain up to 16% cobalt.