Details of the Superconducting Phase Transition in Nb Showing the Paramagnetic Meissner Effect

✍ Scribed by L. Půst; L. E. Wenger; M. R. Koblischka

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 114 KB
• Volume: 212
• Category: Article
• ISSN: 0370-1972
• DOI: 10.1002/(sici)1521-3951(199904)212:2<r13::aid-pssb999913>3.0.co;2-b

✦ Synopsis

The paramagnetic Meissner effect (PME) or Wohlleben effect, first observed in melt-processed Bi 2 Sr 2 CaCu 2 O 8d ceramics [1,2], has also been observed in Nb disks by several groups [3,4]. In order to clarify the origin of this paramagnetic moment in Nb, which may serve as a model system, we have performed a detailed study of the magnetic properties of a Nb disk in its superconducting transition region. Specifically, field-cooled cooling (FCC) and field-cooled warming (FCW) magnetization measurements were done in various applied magnetic fields B e ranging from 5 mT to 200 mT.

In our SQUID magnetometer system [5], the sample is kept in a stationary position with the field being generated by a copper solenoid for fields less than 1 mT and a superconducting magnet for higher fields. This magnet was warmed before measuring in fields below 1 mT to avoid any residual magnetic flux. The change in the magnetic moment mT is then measured as the temperature is swept at a rate of 26 mK/min. The Nb sample was punched from a 0.127 mm thick Nb sheet (99.8% pure) yielding a disk with a diameter 2r 6.4 mm. Fig. 1a illustrates several characteristic features of the transition into the superconducting state of a Nb disk exhibiting the PME. Just below the onset of a diamagnetic signal in the FCC curves (or the disappearance of the signal in the FCW curves) at the superconducting transition temperature T c , a fairly distinct minimum in the diamagnetic response is observed in both FCC and FCW measurements at a temperature we define as T 1 . This signifies the temperature where the paramagnetic contribution first occurs during cooling or vanishes during warming. At slightly lower temperatures, a maximum is observed at T p which is paramagnetic in all FCC measurements.