K.D. Jayasuriya

Specific heat study of a holmium single crystal

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<log10 mod t mod <-1.2 below TN and -3.9<log10 mod t mod <-2.9 above TN. The authors find that the scaling law alpha += alpha - is valid and the condition TN+=TN- is obeyed in the critical regions. The value of alpha (=0.27) that they obtain is not close to the predictions of renormalisation group theory for magnetic systems with short-range exchange interactions. A cross-over from the critical region to a region with different behaviour is observed at log10 mod t mod =-2.9 above but not below TN. An unidentified anomaly is observed in the specific heat of holmium at 98K.

The critical specific heat of terbium

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<lg mod t mod <-1.5 below TN and -3.9<lg mod t mod <-2.2 above TN. The authors find that the relation TN+=TN- is obeyed and that the scaling law alpha += alpha - is valid in this region. In order for the condition B+=B-, predicted by theory, to be satisfied they have found it necessary to include the confluent singular term in E in the fit. The value of alpha (=+0.20) that the authors obtain is not close to the predictions of renormalisation group theory for magnetically collinear systems with short-range interactions. A cross-over from the critical region to a region with different behaviour is observed at lg mod t mod =-2.2 above TN, but not below it.

A Mössbauer effect study of barium ferrite ball-milled in air

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 Mössbauer effect measurements. The sizes of the BaFe12O19 particles decrease on milling, as expected, although a partial decomposition of BaFe12O19 to α-Fe2O3 is found to take place on extended milling (1000 h). The room temperature Mössbauer spectra are consistent with superparamagnetic relaxation associated with the fine BaFe12O19 and α-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.

Specific heat study of the transition from ferromagnetism to antiferromagnetism in terbium

Abstract The specific heat 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.

A specific heat study of natural haematite around the Morin transition and the effects of entrapped water

Abstract The Morin transition in samples of natural haemitite has been examined by specific heat and Mossbauer measurements. Specific heat 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 Morin transition was observed in the specific heat although the expected changes in the Mossbauer hyperfine parameters were observed for the different magnetic phases below and above the Morin transition. 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%.

The specific heat of samarium metal (80-400 K)

Abstract 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 T Nh ~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.

The specific heat capacity of GE varnish (200-400K)

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.