Magnetic microstructure and range of critical magnetic correlations in nanocrystalline gadolinium

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We will study the influence of internal interfaces (grain and phase boundaries) on the Curie transition and on the magnetic microstructure of nanocrystalline bulk gadolinium and nanocrystalline gadolinium alloys. High-quality nanocrystalline bulk gadolinium samples can now be prepared by the inert-gas condensation technique, and their magnetic properties will be investigated by means of magnetometry, specific-heat measurements, neutron diffraction, and in particular by measurements using magnetic small-angle neutron scattering on the low-capturing isotope 160gadolinium. Magneticfield dependent small-angle neutron scattering in conjunction with the theory of micromagnetics is a novel approach which allows one to determine the set of intrinsic interaction parameters of a ferromagnet such as exchange constant, anisotropy field, stray field, and a characteristic length of spin misalignment. By alloying nanocrystalline gadolinium with the highly anisotropic rare-earth metal terbium, we aim at tailoring increasing magnetic anisotropy and study its effect on the magnetic interactions. Furthermore, we propose to study the effect of grain-boundary chemistry on the magnetic behavior by alloying nanocrystalline gadolinium with the nonmagnetic element tantalum which exhibits a strong tendency to interfacial segregation. In particular, by varying the tantalum interfacial excess concentration, we are able to study the mechanism of exchange coupling across grain boundaries.