Journal of Alloys and Compounds 530 (2012) 91–96 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Measurements of ultra-low DC fields by high-T c superconducting cores: The effect of calcination temperature Ugur Topal a,∗ , Fatma Alıkma a,b , Cengiz Birlikseven a , Orhan Uzun c , U˘ gur Kölemen c a TÜB ˙ ITAK-UME (National Metrology Institute) P.K. 54, 41470 Gebze-Kocaeli, Turkey b Marmara University, Physics Department, ˙ Istanbul, Turkey c Department of Physics, Faculty of Science and Arts, Gaziosmanpas ¸ a University, 60240 Tokat, Turkey article info Article history: Received 28 February 2012 Received in revised form 16 March 2012 Accepted 19 March 2012 Available online 28 March 2012 PACS: 74.25.Ha 74.25.Op 74.62.Bf 74.81.Bd Keywords: 2nd harmonic signal High-Tc superconductor DC-magnetometer Ultra-low DC field measurements Magnetic field sensor abstract In this study, the effects of weak links between grains of the ceramic superconductors on the sensitivity of a DC magnetic field sensor were examined. The evolution of 2nd harmonic signal strength, which determines the magnetometer sensitivity, with critical current density J c was especially analyzed. The J c values were adjusted by synthesis of Y-123 superconductors at different calcination temperatures. It was observed that the strength of 2f signal increases as the calcination temperature and so, the J c values increases. This is contrary to the common expectations. Our experiments have shown that the 2f signal changes quite linearly with DC field in a wide dynamic range (10 -4 to 10 -9 T) on the sample calcined at 950 ◦ C. The ways of measuring DC fields as low as 1 nT are described in the paper. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The magnetic field sensors have a very diverse range of appli- cations from locating submarines, detecting unexploded ordinance to archaeology, planetary exploration and medicine to many more different applications. For most of these applications, sensitivity and reliability have crucial importance. For instance, monitor- ing of submarine activities requires the detection of magnetic field strength as low as 1 nT (1 nT ∼ 8 × 10 -4 A/m). Together with these applications, rapidly growing interests and developments in nanotechnology have forced metrologists to search new mea- surement techniques for ultralow physical quantities, such as DC field, by supplying traceability to primary standards. From the mag- netic metrology side, constitution of standards for the creation of ultralow DC magnetic fields is relatively easy, but the measurement of such fields is quite difficult. The only known sensitive technique for measuring ultralow DC fields has been fluxgate magnetometry by which DC fields down to several tens of nT can be measured. On ∗ Corresponding author. Tel.: +90 262 6795000; fax: +90 262 6795001. E-mail address: ugur.topal@ume.tubitak.gov.tr (U. Topal). the other hand, verification or comparison of the data obtained by such unique methods with other accepted techniques plays a vital role in metrology science; providing confidence in measurements. A novel form of ultralow field magnetometer, which is based on nonlinear magnetization of polycrystalline type II superconductors, was reported before [1,2]. Many studies have shown that harmon- ics of reference signal have been generated due to such nonlinear magnetization in case a superconductor is both in AC and DC fields and if the amplitude of total applied field exceeds a threshold value [3–8]. The principle of proposed magnetometer is especially based on the perfect linear variation and the strong low DC field depen- dency of the second harmonic signal for much small H DC /H AC ≪ 1 ratios. In literature, the magnetization of a polycrystalline supercon- ductor is known to be due to the summation of intergranular and intragranular contributions of circulating supercurrents [9]. At low fields intergranular regions dominate the sample’s magnetization (i.e. the flux first enters to the specimen through interbrain regions rather than intracranial regions). Therefore, the links between grains are desired to be weak in order to make quite low flux entries possible and so, to increase DC field sensitivity. The amplitude of the second harmonic signal has been expressed with the formula 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.03.085