The Influence of Grain-Boundary Structural Disorder on the Magnetic Properties of Nanocrystalline Nickel
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ABSTRACT The influence of grain boundaries on the magnetic properties of nanocrystalline nickel, in particular the E = 3 and E = 5 special grain boundaries and the extreme case of a purely amorphous sample, has been investigated. Our calculations reveal that the magnetic moment is rather insensitive to the structural disorder varying by at most 15 %. These results correlate extremely well with the recent observation in electrodeposited nickel that the magnetic moment depends very little on grain size, down to about 10 nm. INTRODUCTION The development of new magnetic materials, such as nanocrystalline materials, has been the object of intense efforts in recent years. Characteristic of these new materials is the presence of "extended defects", such as grain boundaries. The contribution of these defects to the magnetic properties are most probably negligible in materials with large grains, since the volume of grain boundaries is only a small fraction of the total volume. However in nanocrystalline materials, the fraction of atoms at grain boundaries can be as high as 50 %
[1, 2].
Indeed, a reduction in the saturation magnetization by 40 % for iron with 6 nm grain size has been reported, compared to only 2% for metallic iron glasses [1]. Moreover, following a study of the magnetic microstructure of nanocrystalline iron (7 nm grain size) by small angle neutron scattering, Wagner et al. [3] proposed that the material consists of ferromagnetic grains separated by a non-magnetic (or weakly magnetic) interface, with a density of about 40 % that of the crystallites. Finally, other authors [4, 5, 6] have observed a decrease in the saturation magnetization with decreasing grain size which they attributed to oxide layers on the ultrafine particles. In contrast to these results, we have recently shown that the saturation magnetization of nanocrystalline Ni produced by electrodeposition does not strongly depend on grain size [7]: the saturation magnetization, we have found, decreases by less than 10 % upon decreasing the grain size from - 100 pm (conventional material) to ,-10 nm (nanocrystals). M~ssbauer spectroscopy on nanocrystalline iron [8] has revealed the existence of two contributions to the total spectrum: one from the nanocrystals themselves at small values of the hyperfine field, and the other from the interfaces for larger values of the hyperfine field. While the interfacial component decreases rapidly with increasing temperature, becoming almost negligible at room temperature, the crystalline component is much less sensitive to temperature variations. This suggests a strong temperature dependence of the magnetization saturation of nanocrystals as low temperatures are approached, in contradiction with recent observations [9] that the magnetization in nanocrystalline and crystalline nickel behave in essentially the same manner at the same low temperatures, the saturation magnetization being always smaller in the former than in the latter. 477
Mat. Res. Soc. Symp. Proc. Vol. 318. @1994 Materials Research Society
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