Paramagnetic Meissner effect in MgB2

Physica C 408–410 (2004) 109–110 www.elsevier.com/locate/physc

Paramagnetic Meissner effect in MgB2 H€ useyin S€ ozeri a

_ , Lev Dorosinskii a, U gur Topal a, Ismail Ercan

a,*

b

_ TUBITAK-UME, National Metrology Institute P.O. Box 54, TR-41470 Kocaeli, Turkey b ANAEM, Ankara N€ukleerArasßtırma ve E gitim Merkezi, TR-06100, Ankara, Turkey

Abstract We observed the paramagnetic Meissner effect (PME) in MgB2 pellets after cooling in low fields of less than 1 Oe. As external magnetic field increases, the paramagnetic response of the sample decreases and disappears completely at 1 Oe. Effect of gamma irradiation on magnetic properties of MgB2 has been investigated at low fields. In irradiated samples, the magnitude of the PME was found to be lower. To understand that whether this effect is of geometrical origin (so called flux compression model) or whether it is intrinsic to the material (e.g., p-junctions), measurements were performed with two samples having different geometries. Our results showed that paramagnetic response did not change as the geometry changes.
2004 Elsevier B.V. All rights reserved. Keywords: MgB2 ; Paramagnetic Meissner effect; Gamma irradiation

One of the special features of the superconductivity is the perfect diamagnetism, i.e. expulsion of magnetic field below transition temperature, Tc . This diamagnetic behavior is a result of surface screening currents which create exactly the same field in magnitude but opposite in direction. In this way, effect of external field is totally cancelled and materials exhibits perfect diamagnetism (if external field is below the first critical field). However, in case that defects are present in the sample, pinning occurs which results in decrease of diamagnetic response. Contrary to the above picture, it was observed that some superconductors may attract magnetic field when they are cooled in small fields (