American University
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posted on 2023-08-05, 07:34 authored by James H. Ferrick

For alloys containing a magnetic constituent, theory predicts that conventional magnetic ordering will vanish only in the limit of zero concentration of the magnetic component. This has been found to be untrue in neutron scattering and magnetization experiments on RE(,x)Sc(,1-x) (RE = Gd, Tb, Ho, and Er) alloys, and more recently in Mossbauer experiments on DySc alloys. Long-range order disappears at rather high rare earth concentrations (Gd - 15%, Tb - 25%, Ho - 18%, Er - 39%, and Dy - > 25%). When the ordering temperatures are plotted against the two-thirds power of the effective spin parameter (de Gennes factor), substantial deviations from linearity exist for RE - Sc alloys, while intra-rare earth alloys yield excellent agreement with the phenomenologically determined dependence. The Mossbauer studies indicated that for Dy concentrations of 25% or less, the magnetic hyperfine field was essentially constant, independent of either concentration or temperature, up to a temperature at which it abruptly vanished. For Dy concentrations of 50% or more, a conventional Brillouin function temperature dependence of the hyperfine field was found. This work was pursued as a complement to the Mossbauer studies, using the same source materials. Bulk magnetization measurements were made on Dy(,.25)Sc(,.75) and Dy(,.75)Sc(,.25) alloys. The Dy(,.25)Sc(,.75) alloy has the characteristics of a spin-glass material. When the sample is cooled in zero field the magnetization increases with temperature up to a maximum value at T(,sg), then decreases in the paramagnetic region when measured in low to moderate fields (0.1 to 10 kOe). When cooled in an applied field, the magnetization versus temperature behavior exactly reverses the warming curve down to the temperature at which the maximum magnetization was observed; the magnetization remains nearly constant below this temperature. The spin-glass transition temperature is found to be 27 K for this 25% Dy alloy. The Dy(,.75)Sc(,.25) alloy undergoes an antiferromagnetic transition at T(,N) = 118 K, and it behaves paramagnetically above this temperature. No spontaneous ferromagnetic ordering was observed down to 4.2 K; attempts to induce ferromagnetic ordering resulting in determinations of the critical field necessary to effect a ferromagnetic transition. The minimum field in which a ferromagnetic transition was observed was 13.5 kOe. The critical field increases as temperature decreases, in contrast to conventional behavior, and exhibits a further anomalous rise below 29 K. The helical spin structure occurring in several of the heavy rare earth metals is apparently stabilized by the presence of moderate concentrations of scandium in RE - Sc alloys.



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Ph.D. American University 1981.


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