The effect of change of temperature on the strength of permanent magnets, with special reference to modern magnet steels

Abstract

A survey is made of the literature and it is found that a magnet can be put into the steady condition, as regards magnetic strength, by mechanical shocks or by cyclic heating, and then it has a definite strength at each temperature, the connection between the two generally giving rise to a negative coefficient. The majority of the experimenters have assumed a linear relationship and have determined the strength/temperature coefficient from measurements at two temperatures only. A summary of published work shows that the coefficient depends upon the linear dimensions of the magnet and can be reduced by increase of the length/breadth ratio, by increase of the degree of drawing, by heat treatment, or, in the case of carbon steels, by increase of the carbon content. In the present paper a description is given of a rotating search-coil and ballistic-galvanometer arrangement for testing cobalt steel, aluminium-nickel, and aluminium-nickel-cobalt alloy magnets over the temperature range −60° C. to 100° C An electrically heated oil bath was used for the range 20° C. to 100° C, whilst solid carbon dioxide-dissolved in acetone wasemployed for the range 20° C. to −60° C. When the magnets had attained the reversible state it was found that the magnetic strength Hτ could be connected with the temperature τ(°C.) by an equation of the typeHτ = H0(1 + aτ + bτ2), where a and b were negative constants and were determined for each magnet. The total permanent loss of strength up to the attainment of the steady state was also measured. It was found that, in the case of the cobalt-steel magnets, the coefficient a was reduced by increase of the cobalt content; the total loss was similarly affected, but not to such a marked extent. In the case of 15% Co steels and the Al-Ni and Al-Ni-Co alloys, a and the loss were affected by the temperature at which magnetization was performed. The other cobalt magnets were magnetized at ordinary temperatures.