Collective electron ferromagnetism III. Nickel and nickel-copper alloys

Abstract

The collective electron treatment of ferromagnetism (Stoner 1938 a , 1939) is applied to the magnetic and thermal properties of nickel and nickel-copper alloys. In § 1 a brief description is given of the theoretical background, together with the necessary extensions of the basic treatment, in particular so as to cover the effects of the overlap of the electronic d and s energy bands. With simplifying assumptions regarding the effect of alloying on the electronic structure, and using a limited range of observational data, estimates are obtained of the variation of the specifying parameters, band width and interchange interaction, for nickel-copper alloys, over a wide range of composition (§ 2). A discussion is given in § 3 of the dependence of the electron distribution on temperature. Calculations are carried out of the variation of the number of holes in the d band, due to the temperature redistribution of electrons between the bands (transfer effect). Other temperature effects, such as the variation of the specifying parameters due to thermal expansion, are briefly discussed, though they are not included in the main treatment (§§ 1 and 4). The treatment is applied to the temperature variation of susceptibility above the Curie point (§4). For nickel-rich alloys satisfactory agreement is obtained with experimental results (figures 10, 11 and 12). For copper-rich alloys the observed high-temperature increase of susceptibility is well explained (figure 13), but no interpretation can be given of the observed low-temperature variation. It is suggested that this may be due to inhomogeneities in composition. A discussion is given in § 5 of the bearing of the treatment on the electronic heat at high temperatures. Comparison is made with the relevant experimental results for nickel above the Curie point. Application is also made to the low-temperature electronic heat of nickel-copper alloys (§5). The general character of the variation of the electronic heat with composition is covered satisfactorily in the nickel-rich regions, but in the copper-rich regions discrepancies occur, similar to those for the low-temperature susceptibility (§4). In §6 a brief account is given of the results of a preliminary analysis of the magnetic properties of nickel alloys other than those with copper.