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Ordered magnetic systems studied by nuclear orientation

University of British Columbia

The antiferromagnetic and spin-flop phases of MnCl‧4H₂O have been investigated by observing the nuclear orxentatxon of ⁵⁴Mn in that material. The sublattice magnetizations in the absence of an external field were found to lie in a direction between the a and c crystal axes at an angle of 11.5° ± 3.5° to the c axis. The field dependence of the spin configuration in the spin-flop state indicates that second order anisotropy is significant in this system. The molecular fields were determined by combining the results of this work with other measurements of the-critical fields (Rives and Benedict, 1975). The results are: the exchange field is 11.05 ± 0.21 KOe; the biaxial single ion anisotropy fields are 0.75 ± 0.22 KOe and 2.35 ± 0.23 KOe along the a* and b axes respectively; the second order anisotropy field is 1.45 ± 0.19 KOe and the anisotropic exchange field is 0.1 ± 0.3 KOe. The spin-flop transition region was found to be adequately described by a 'domain' structure in which regions of antiferromagnetic phase and regions of spin-flop phase co-exist in the crystal. Measurements were made of the temperature dependence of the spin-flop transition field and, contrary to the extrapolated results of Rives and Benedict (1975) , the spin-flop field was found to decrease with decreasing temperature from 0.3K to 0.15K. If there is a minimum in the transition field it must occur at lower temperatures. The cooling of the MnCl‧4H₂O crystal which was held in contact with a copper heat sink by Apiezon N grease was fitted to the relation Q = kA (T₁ⁿ-T₂ⁿ) where T₁ and T₂ are the temperatures of the crystal and copper heat sink respectively, and A is the contact area. For n = 4 the value obtained for the constant k is (8.2 ± 1.9) x 10³ ergK⁻⁴ sec⁻¹ cm⁻². Nuclear orientation experiments were also performed on the systems ¹⁰³Ru-Fe and ⁵⁹Fe-Fe. The gamma-ray anisotropies for these systems (at temperatures of 10 and 15 mK, respectively) were very small; however, it was possible to determine limits for the magnitudes of the nuclear magnetic moments of the active nuclei. The ¹⁰³Ru moment was found to be greater than 0.15uN and the ⁵⁹Fe moment was found to be less than 0.9uN. J.E. Rives and V. Benedict, Phys. Rev. B12, 1908 (1975).

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2010-02-17

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http://hdl.handle.net/2429/20338

Physics

University of British Columbia

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