ESR studies on high-Tc superconductor MgB2

MgB2, a high-T-c superconductor, has been studied by electron spin resonance (ESR) techniques at the temperature range of 7-300 K. Polycrystalline powders consisting of MgB2, MgO and MgB4 phases were diluted and oriented in paraffin by applying an external magnetic field of 15 kG. A very narrow (2.5 G), strong, and isotropic signal that corresponded to almost free electron g-values was observed at all temperatures. Both the signal intensity and line width were observed to exhibit strong temperature dependence below T-c. The intensity of the ESR spectra, which corresponds to dc susceptibility, generally obeys the Curie law in this temperature range; However, some critical temperatures (approximately 215, 190, 150, and 39 K) were evident from both intensity and line width curves. While the ESR line is strongly broadened, the signal intensity significantly start to increase just below T = 39 K (corresponding to a transition temperature from normal to superconducting state), passes through a broad maximum around 30 K and then shows a sharp decrease as the temperature is decreased further. The origins of the minor changes both in the intensity and the line width curves at other (higher) critical temperatures are not clear yet. In fact, the change at 215 K was observed to be meta-stable. These minor changes might be taken as signs for changes of local crystalline field symmetry around weakly localized conduction electrons or holes, which are the sources of the ESR signal. (C) 2003 Elsevier Science B.V. All rights reserved.