Study of magnetism in Fe-Ni Invar alloys by mechanically alloying

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Fe-Ni alloys with the Ni-concentration around 35 at.% are called the Invar alloys, which are well known with their very small thermal expansion coefficients. Besides those thermal expansion anomalies, Invar alloys show various anomalies in magnetic properties such as abnormal decrease of the magnetic moment from the Slater-Poling curve, anomalously large pressure effect of the Curie temperature and so on. Those anomalies are originated in the 3d-band electrons with a fcc-lattice structure, and therefore, the magnetic properties of Invar alloys are especially important for studies of magnetism in metals and alloys. It was pointed out that Fe-Ni Invar alloys always have a certain amount of concentration fluctuation, which makes some contribution to their magnetic properties. However, there have been made very few magnetic studies by artificially changing the concentration fluctuation. By adopting a method of mechanically alloying and successive annealing at high temperatures, we succeede … More d to make the concentration fluctuation much wider than those obtained by conventional alloying by melting. Measurements of the Curie temperature, magnetic susceptibility and magnetization curves were made on those mechanically alloyed Fe-Ni Invar alloys. It was found that the Curie temperature decreased first with increasing the range of the concentration fluctuation, taking a minimum, and then, increased largely. The saturation magnetization also decreased first, taking a minimum, and then increased with increasing the fluctuation width. The high-field susceptibility decreased first with increasing the fluctuation width, taking a maximum, then, decreased. Those properties could be explained by considering a gaussian distribution for the concentration fluctuation. By using an accererator, high-energy heavy ion irradiation were made on Fe-Ni Invar alloys. It was found that the Curie temperature of the irradiated part increased largely, as large as 100 K with the dose of 10^<14> cm^<-2>. This effect has a possibility for application of nano-scale magnetic memory device. Less