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Title: Magnetic properties of Gd-Y and Gd-Sc alloys
Author: Melville, Richard John
ISNI:       0000 0001 3393 2259
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1989
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The magnetic structures and behaviour of Gd-Y and Gd-Sc random alloys have been investigated by single crystal neutron diffraction, ultrasonic measurements of the crystal elastic constants and resistivity measurements. The composition of the alloys studied was chosen in order to observe the competition between the ferromagnetism favoured by Gd-rich alloys and helimagnetism favoured by dilution with Y or Sc. Neutron diffraction studies of Gd60Y40 and Gd66Y34 have been concerned principally with the helimagnetic/ferromagnetic transition. Of particular interest has been the behaviour of the modulation wavevector when the ferromagnetic phase is approached as the temperature is lowered. A novel approach to peak-fitting of the diffraction profiles for Gd66Y34 has enabled helical inter-layer turn angles of less than 1° to be measured, and no evidence has been observed for any discontinuity of the turn-angle as the transition occurs. This is unique behaviour among the rare-earths. Gd60Y40 exhibits differing behaviour as the helimagnetic/ferromagnetic transition is approached. This transition occurs at low temperature, but is unusual in that an apparently frozen-in' state of coexisting ferromagnetism and helimagnetism sets in below -50 K. The nature of the phase boundary is discussed concerning this behaviour together with the possible role of random field effects. Gd-Sc alloys of composition between 64% and 76.7% Gd were initially characterized over their full magnetically ordered temperature range by elastic constant measurements. Neutron diffraction was then used to make accurate structure determinations for the magnetic phases exhibited in this alloy system. Resistivity measurements have provided useful additional information regarding the phases, particularly with regard to the ferro-I phase observed in the alloy richest in Gd. The magnetic phase diagram for the Gd-Sc system has been observed to be quite dissimilar to that of Gd-Y in the region of 70% Gd. First, there is a conical helix phase separating the helical and ferromagnetic phases, which is absent in Gd-Y. Secondly, the ferro-I phase of Gd-Y is considerably less extensive in Gd-Sc than in Gd-Y. This is unusual in view of earlier belief that this phase should be enhanced by the substitution of Sc for Y. Also, preliminary elastic constant and resistivity measurements for Gd64Sc36 have revealed rich and interesting behaviour in an applied magnetic field.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics