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Nuclear orientation studies at low temperatures Daly, Patrick William

Abstract

The interaction between the nuclear magnetic moment and the hyperfine field in ferromagnetic and antiferromagnetic materials has been used to orient radioactive nuclei at low temperatures. The measurement of the resulting angular distribution of the gamma radiation has yielded a variety of information in both nuclear and solid state physics. The ⁵⁹Fe nucleus in an iron sample has been studied in an attempt to verify a report by Tschanz and Sapp (1970) who measure an unexpectedly large effect in the gamma ray distribution from that nucleus oriented in double nitrate salts. Since in their experiment assumptions have to be made regarding the fraction of iron atoms in lattice sites, the well-defined environment of the iron lattice was chosen for the present experiment in order to permit an unambiguous interpretation of the results. In contradiction to predictions based on the work of Tschanz and Sapp, a null result was obtained at a temperature of 15 mK, indicating an upper limit for the magnetic moment of ⁵⁹Fe of |µ| < 0.9 µ[sub N. The anisotropy of gamma radiation emitted in the decay of ¹⁰³Ru oriented in ferromagnetic iron was measured to determine the magnetic moment of the isotope and to resolve mixing ratios in the decay scheme. The small effect observed at 11 mK permitted a lower limit to be placed on the magnetic moment: |µ| > 0.15 µ[sub N]. The sign of the E2/M1 mixing ratio for the 497 keV gamma transition was determined for the first time to be found negative. The techniques of nuclear orientation have also been used to observe the spin-flop transition in antiferromagnetic MnCl₂H₂O. By observing the gamma ray anisotropy in the decay of radioactive ⁵⁴Mn we find that at 60 mK the transition occurs for a critical field of H[sub c] = 6.50 ± 0.05 kG applied along the crystallographic c-axis. In the spin-flop phase, the electronic spins point at an angle of 71° to this axis. These data allow values of the molecular exchange and anisotropy fields to be determined yielding respectively H[sub E] = 11.7 ± 0.6 kG and H[sub A] = 2.0 ± 0.1 kG in agreement with the values reported by Miedema et al. (1965). This experiment demonstrates that nuclear orientation is a viable technique for studying spin-flop transitions and moreover allows the measurement of the directions of electronic spin magnetization in the spin-flop phase. The nuclear orientation of ²⁰⁷Bi in cobalt was studied to determine the hyperfine field. An unexpectedly small gamma ray anisotropy was observed. However, saturation of the effect occurred at a relatively high temperature. These results indicate that only a fraction (~12%) of the Bi nuclei felt a large hyperfine field (~600 kG). Presumably this was the fraction of atoms in lattice sites. Comparison of the anisotropics of the 1.064 MeV and the 1.761 MeV gamma rays shows that the former effect is enhanced, possibly because of the long half-life of the 1.633 MeV intermediate state.

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