Physics
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Item Surfactant assisted ball-milling of barium ferrite(IEEE Transactions on Magnetics, 1994) Jayasuriya, K.D.; Wu, E.; Kaczmarek, W.A.; Campbell, S.J.The effects of wet-milling barium ferrite in the presence of cationic or anionic surface active substances have been investigated by X-ray diffraction and Mossbauer spectroscopy measurements. Evidence for superparamagnetic relaxation of fine particles, both in the surfactant assisted ball-milled samples and in samples dry-milled in air is obtained from the room temperature and 4.2 K Mossbauer spectra. The similarity between samples milled at pH 3 with the cationic or anionic surfactants is due to electrostatic interactions of the surfactants' hydrocarbon groups with the positively charged barium ferrite surfaceItem A Mössbauer effect study of barium ferrite ball-milled in air(Hyperfine Interactions, 1994) Jayasuriya, K.D.; Kaczmarek, W.A.; Wu, E.; Campbell, S.J.The effects of dry-milling BaFe12O19 in air for periods of 190, 360, 590, 690 and 1000 h have been investigated by X-ray diffraction and M�ssbauer effect measurements. The sizes of the BaFe12O19 particles decrease on milling, as expected, although a partial decomposition of BaFe12O19 to ?-Fe2O3 is found to take place on extended milling (1000 h). The room temperature M�ssbauer spectra are consistent with superparamagnetic relaxation associated with the fine BaFe12O19 and ?-Fe2O3 particles. The X-ray diffraction patterns of all the milled samples exhibit features indicative of a disordered structural state, consistent with the nanoscale particles and a nanostructured state.Item A Mössbauer study of ball-milled Co?Fe?Si?B: II: with surfactants(Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1993) Jayasuriya, K.D.; Campbell, S.J.; Calka, A.; Jing, J.The use of a range of organic surface active substances in the ball milling of Co?Fe?Si?B has been studied by X-ray diffraction and M�ssbauer effect measurements. Effects characteristic of superparamagnetic relaxation of the magnetic moments are observed in the M�ssbauer spectra of samples milled with either an anionic or a cationic surfactant. This behaviour is linked with the decreased particle size and high shape homogeneity of surfactant-aided milled samples compared with the amorphous powders resulting from standard dry milling. Impurity iron-oxide phases are also found to occur in samples milled in the presence of surfactants.Item A Mössbauer study of ball-milled Co?Fe?Si?B: I: dry milling(Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1993) Jayasuriya, K.D.; Campbell, S.J.; Calka, A.; Jing, J.X-ray diffraction and M�ssbauer effect measurements of ball-milled Co?Fe?Si?B material which had first been re-crystallized from amorphous melt-spun ribbon are presented. Fully amorphous powders have been obtained after only 50 h milling with a ball-to-powder ratio of BPR ? 84. The amorphous powders exhibit chemical and structural inhomogeneities compared with the starting melt-spun amorphous ribbons.Item 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.Item 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%.Item 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.Item 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 -3.8Item 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 -3.4Item 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.