Cobalt-Platinum Alloy Magnets
Cobalt-Platinum Alloy Magnets
Effect of High Temperatures on Magnetic Properties
The magnetic properties of the 50 atomic per cent cobalt-platinum alloy are quite exceptional; it is one of the most powerful permanent magnetic materials available, and it is readily fabricated. It thus has its major applications in miniaturised circuits where a high flux is required in a limited space and where a small magnet can be made from strip or wire. Engineering data to assist in the design of magnetic circuits employing Platinax II – the Johnson Matthey cobalt-platinum alloy – were provided in a paper by the present writer (1) two years ago.
In common with other magnetic materials, however, cobalt-platinum loses performance on heating due to a loss of magnetisation. Part of the loss is often recovered on cooling, but when for example continuous operation at high temperature makes this impracticable, some allowance should be made for the probable deterioration. This is rarely straightforward and can, under arduous service conditions, severely limit the choice of material. Detailed information relating magnetic behaviour with specific thermal conditions is therefore very valuable.
Work has recently been reported by Dr Hermann Dietrich of the Research Institute of Deutsche Edelstahlwerke, Krefeld, in which a study was made of changes in the magnetic state of the more stable permanent magnets at temperatures up to their Curie points (2). Measurements were made of both short-term stability in tests of 30 minutes duration, and long-term stability over periods of up to 10,000 hours, the results of which were analysed to determine the maximum temperature at which the materials could be used. Although interest was focused chiefly on the effects produced by irreversible structural changes, the better known changes in magnetisation, both reversible and irreversible, were also considered and some interesting comparisons made.
They show for example in Fig. 1, that structural ageing in a 50 atomic per cent cobalt-platinum alloy held for 30 minutes at 500°C accounts for only a small proportion of the total magnetic loss. The remaining loss is due to magnetic ageing which, at this temperature, is substantially irreversible. However, as temperatures recede from the Curie point at 530°C, the losses rapidly decrease in magnitude, and after 30 minutes at 450°C magnetic ageing is reduced sufficiently to enable more than 80 per cent recovery on cooling (Fig. 2). At still lower working temperatures the recovery on cooling is almost complete, since at 350°C, the total irreversible loss is only 6 per cent, consisting of 1 per cent structural ageing and 5 per cent magnetic ageing.
These losses are considerably less than values obtained in the Johnson Matthey laboratories on Platinax II cobalt-platinum (1). However, since these previous tests were designed to study the temperature characteristics of magnets working under severe conditions, i.e. short magnets having an 1/d ratio of 0.6 and operating in open-circuit, the differences do not seem unreasonable. In comparing these results it is of interest to note the difference between the magnetising fields employed. Whereas the Platinax II magnets were saturated in a field of at least 20,000 oersteds, the present tests employed only 13,500 oersteds. This is considerably less than the value generally considered necessary to saturate cobalt-platinum and suggests that maximum properties were not initially achieved. If so, it is possible that subsequent estimations of stability might be adversely affected.
The examination of long-term stabilities by Dr Dietrich was confined to three materials whose properties were considered sufficient to warrant investigation. Of these, cobalt-platinum was found to be structurally stable for short periods up to 600°C, suggesting that on the grounds of structure alone long-term stable working at 450°C should be possible. Unfortunately, this is incompatible with a Curie temperature of 530°C, and for continuous operation a somewhat lower temperature must be accepted. In attempting to define this, it is concluded that the low temperature magnetisation changes would produce a practical temperature limit not much higher than 200°C. At this point, it is stated, the magnetisation and therefore the coercive force of cobalt-platinum have decreased so much that these magnets become practically useless.
Whether this conclusion is entirely justified might be questioned on the grounds that in practice other related properties, such as coercive force, must often be considered. If sufficient emphasis is to be placed on these, higher working temperatures with correspondingly higher losses might be tolerated.
For example, Fig. 1 shows that cobalt-platinum magnets lose approximately 10 per cent of their magnetisation at 200°C. Initial coercivity is, however, between five and ten times greater than other highly stable magnets such as the Alnico and Vicalloy types, and this advantage should be maintained with magnetisation losses of this order, or even higher. Moreover, their high coercive force allows short cobalt-platinum magnets to be used in applications where the limitations imposed by poor geometry would override the effects of stability at temperature. In these circumstances, the arbitrary fixing of a maximum utilisation temperature at 200°C would almost certainly give a pessimistically low figure.
L. A. Ford, Platinum Metals Rev., 1964, 8, 82
H. Dietrich, Cobalt, 1966, No. 30, (March), 3