Browsing by Author "Stewart, A.M."

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A specific heat study of natural haematite around the Morin transition and the effects of entrapped water
(Journal of Physics and Chemistry of Solids, 1985) Jayasuriya, K.D.; Stewart, A.M.; Campbell, S.J.
The Morintransition in samples of natural haemitite has been examined by specificheat and M�ssbauer measurements. Specificheat measurements on as-received samples reveal an anomaly at ? 273.2 K and irregular behaviour at higher temperatures. However, heat treatment of samples demonstrated that these effects were due to water trapped inside the natural specimens. No anomaly due to the Morintransition was observed in the specificheat although the expected changes in the M�ssbauer hyperfine parameters were observed for the different magnetic phases below and above the Morintransition. The results are consistent with a coexistence of the two phases and a spread of temperatures ? 40 K over which the transitions take place throughout the material of purity 99.6 wt%.
Magnetic transitions in dysprosium: A specific-heat study
(Physical Review B, 1985) Jayasuriya, K.D.; Campbell, S.J.; Stewart, A.M.
The magnetic transitions of the heavy rare-earth metal dysprosium have been examined by specific-heat measurements using a small single crystal (0.6782 g) and a large polycrystalline sample (43.25 g). A value for the latent heat of 39.1�1.5 J mol-1 is obtained at the discontinuous transition from ferromagnetism to helical antiferromagnetism, and a temperature hysteresis of 1.2�0.4 K is observed at this transition. The data around the continuous transition from helical antiferromagnetism to paramagnetism at the N�el temperature TN=179.90�0.18 K have been analyzed in terms of the equation & &)+B�+D�t, where t=(T-TN)/TN. Agreement to within two standard errors was obtained for the critical parameters of the single-crystal and polycrystalline data. The value of the critical exponent ? obtained from the data of the single crystal is 0.24�0.02, whereas that obtained from the data of the polycrystalline sample is 0.14�0.05. We find that the scaling law ?+=?- is valid for the dysprosium critical data. In order for the condition B+=B-, predicted by theory, to be satisfied we found it necessary to include the confluent singular term in E in our fit. The values of ? that we obtain are not close to the predictions of renormalization-group theory for magnetic systems with short-range exchange interactions.
Specific heat study of a holmium single crystal
(Journal of Physics F: Metal Physics, 1985) Jayasuriya, K.D.; Campbell, S.J.; Stewart, A.M.
The magnetic transition at the Neel temperature, TN, of the heavy rare-earth metal holmium has been examined by specific heat measurements. The transition at TN=(132.245+or-0.080)K is found to be continuous and the data around TN have been analysed in terms of the equation C+or-=(A+or-/ alpha +or-) mod t mod - alpha +or-(1+E+or- mod t mod x+or-)+B+or-+D+or-t where t=(T-TN)/TN. The value of the critical exponent alpha is 0.27+or-0.02 for the data in the critical regions -3.8
Specific heat study of the transition from ferromagnetism to antiferromagnetism in terbium
(Journal of Magnetism and Magnetic Materials, 1984) Jayasuriya, K.D.; Campbell, S.J.; Stewart, A.M.; Gopal, E.S.R.
The specificheat of single crystal terbium has been measured over the range 200?400 K. A value for the latent heat of (13.6 � 0.6) J mol?1 is obtained at the first order antiferromagnetic to ferromagnetic transition temperature Tc = (221.45 � 0.03) K. A temperature hysteresis of (0.24 � 0.03) K is observed in the transition.
The critical specific heat of terbium
(Journal of Physics F: Metal Physics, 1983) Jayasuriya, K.D.; Gopal, E.S.R.; Campbell, S.J.; Stewart, A.M.
Specific heat measurements have been carried out on a high-purity single crystal of terbium of mass 3.734 g between temperatures of 200 and 400K. The data around the Neel temperature TN, have been analysed in terms of the equation C+or-=(A+or-/ alpha +or-) mod t mod - alpha +or-(1+E+or- mod t mod x+or-)+B+or-+D+or-t where t=(T-TN+or-)/TN+or-. The parameters obtained from our best fit are alpha += alpha =-0.20+or-0.03, A+/A-=0.58+or-0.34, B+=B-=21.68+or-13.93 J mol-1 K-1, E+/E-=-2.06+or-4.12, D+=D-=4.99 J mol-1 K-1 and TN+=TN-=229.9503+or-0.0035K. This fit is valid in the critical regions -3.4
The specific heat capacity of GE varnish (200-400K)
(Journal of Physics E: Scientific Instruments, 1982) Jayasuriya, K.D.; Campbell, S.J.; Stewart, A.M.
The specific heat capacity of GE varnish, a commonly used adhesive and insulator, is found to vary linearly with temperature over the range 200-330K. The deviation of the heat capacity above this temperature is associated with the approach to the curing temperature of this resin.
The specific heat of samarium metal (80-400 K)
(Materials Chemistry and Physics, 1986) Jayasuriya, K.D.; Stewart, A.M.; Campbell, S.J.
We have measured the specific heat of several high purity polycrystalline specimens of samarium metal over the temperature range 80 K to 400 K. A broad peak is observed at TNh ~106K, the temperature at which antiferromagnetic ordering first sets in with ordering of the moments on hexagonal sites. This behaviour contrasts with the sharp peak observed by previous workers on samarium samples of lower purity.