J Supercond Nov Magn (2013) 26:1155–1158 DOI 10.1007/s10948-012-1901-x ORIGINAL PAPER Magnetometer Design for Measuring Ultralow DC Magnetic Fields Using YBa 2 Cu 3 O 7−x Superconductors Fatma Alıkma · Ugur Topal Received: 6 November 2012 / Accepted: 30 November 2012 / Published online: 20 December 2012 © Springer Science+Business Media New York 2012 Abstract In the present work we examined a special sensor design using a YBa 2 Cu 3 O 7−x superconductor as a sensing element for the detection of ultralow DC magnetic fields. The experimental results have shown that the sensor sig- nal, which is the second harmonic signal generated by co- application of AC and DC fields to high-T c superconductors, varies linearly with the applied DC field even in the −0.5 Oe to 0.5 Oe range. The critical factors affecting the strength of the second harmonic signal, such as the frequency and amplitude of the AC field, the intergrain critical field H c1J , and the minimum field required for penetration through the entire specimen H ∗ , were optimized. In this way we could detect DC magnetic fields as low as 1 nT. Keywords 2nd harmonic signal · High-T c superconductor · DC-magnetometer · Ultra-low DC field measurements · Magnetic field sensor 1 Introduction Measurement of ultralow magnetic fields is important for various potential applications, such as biomedical research (neuro- and cardiomagnetism, etc.), geophysical surveying, and nondestructive testing of materials [1–3]. The use of high-T c superconductors (HTCSs) as a sensing core was reported as an efficient method for the detection of very F. Alıkma · U. Topal ( ) TÜB ˙ ITAK-UME (National Metrology Institute), P.K. 54, 41470 Gebze, Kocaeli, Turkey e-mail: ugur.topal@ume.tubitak.gov.tr F. Alıkma Physics Department, Marmara University, Istanbul, Turkey low magnetic fields [4, 5]. Polycrystalline HTCSs are com- posed of agglomerations of anisotropic grains separated by weakly superconducting regions, which act as Josephson weak links [6]. Since the intergrain first critical field H c1J is extremely low in HTCSs (H c1J is about 10 Oe or even less), quite small magnetic fields can easily penetrate into the specimen through the intergranular region. For alternat- ing fields the flux lines are assumed to sweep in and out of the material, lagging behind the driving field. This feature leads to nonlinear magnetization in these materials. In a pure AC field a symmetrical magnetization curve is expected. Such a symmetrical magnetization curve causes the genera- tion of odd harmonics of the fundamental signal. However, co-application of a DC field together with an AC field breaks the symmetry of the magnetization curve and leads to the ap- pearance of even harmonics along with the odd harmonics. The physics behind the harmonic generation in HTCSs was well analyzed by L. Ji et al. [7, 8]. In these studies it was shown that the largest even harmonic signal, which is the second harmonic signal, shows a quite linear dependence on the DC field, especially in the low DC field region. This interesting property of HTCSs can be employed in the mea- surements of ultralow DC magnetic fields as a magnetic field sensor. There are many critical factors affecting the strength of the second harmonic signal, such as sensor geometry, pick- up and exciting coils, AC and DC field amplitude, AC field frequency, and intergrain critical field H c1J . It is known that the AC and DC magnetic fields should be applied simulta- neously and the total magnetic field should exceed a cer- tain threshold value for the generation of second harmon- ics. According to basic superconductivity principles, the fre- quency of the AC field should have an effect on the noise level. Therefore, measurements have been done in a wide frequency range between 25 kHz and 200 kHz and at differ-