EPR study, in the normal and superconducting states, of GdBa 2Cu 3O 7 single crystal before and after grinding

Abstract

Electron paramagnetic resonance (EPR) at various temperatures (300–4 K) is reported for the superconducting single crystal ( T c≅90 K), before and after grinding. In the normal state ( T> T c), the EPR line from the single crystal is isotropic and asymmetrical due the skin effect. On the contrary in the superconducting state ( T< T c), characterised by a non-resonant signal just below T c and by the noise on the spectrum, the EPR line is symmetrical and anisotropic. The grinding of crystal allowed us to avoid some of these complications. After grinding the skin effect is eliminated and the line becomes symmetrical. The temperature dependence of the EPR line width shows a Korringa-type interaction between Gd 3+ moments and the conduction electrons in the normal state, and an important broadening below 20 K announcing a magnetic order of the Gd 3+ at low temperature; but the origin of the line width narrowing appeared between 90 and 20 K is not clear. The line intensity varies with temperature as the Curie–Weiss law in the normal state. Below T c it decreases, when the temperature is lowered from T c, as result of the reducing of the volume producing the EPR signal.