Thin-Film Microsusceptometer With Integrated Nanoloop

• Published in: IEEE transactions on applied superconductivity Vol. 24; no. 4; pp. 1 - 6
• Main Authors: Drung, Dietmar, Storm, Jan-Hendrik, Ruede, Frank, Kirste, Alexander, Regin, Marcel, Schurig, Thomas, Repolles, Ana M., Sese, Javier, Luis, Fernando
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.08.2014; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Coils; Couplings; Focused ion beam (FIB); Inductance; Ion beams; Magnetic flux; Magnetic tunneling; Magnetism; microsusceptometer; nanoloop; Noise; Permalloy; SQUIDs; superconducting quantum interference device (SQUID); superconducting quantum interference device (SQUID); nanoloop; Permalloy; Focused ion beam (FIB); microsusceptometer
• ISSN: 1051-8223; 1558-2515
• DOI: 10.1109/TASC.2014.2318322

We report the design and performance of thin-film microsusceptometers intended for magnetic measurements on samples at variable temperature down to the low milliKelvin range and excitation frequencies of up to about 1 MHz. The devices are realized as first-order gradiometers with two circular loops of 60- or 30-μm average diameter resulting in a total inductance of 360 or 250 pH, respectively. An integrated excitation coil generates a magnetic field with a transfer coefficient of 0.1 T/A at the sample position, whereas the Josephson junctions are located in a field-reduced area. The susceptometers are fabricated by a conventional Nb/AlOx/Nb trilayer process. In order to enhance the sensitivity to the level required for the measurement of submicrometer samples, an extra detection loop of 450-nm inner diameter was integrated into one of the pickup loops by using a focused ion beam. We show that this device is able of detecting signals from very small Permalloy samples. An optimized susceptometer design with a predicted superconducting quantum interference device inductance of 12 pH is also presented, which can achieve an equivalent spin noise of 8 μ B /√Hz at 4.2 K (μ B is the Bohr magneton) when being equipped with a nanoloop of 100-nm line width and separation.