Using an optimally coupled nanometer-scale SQUID, we measure the magnetic flux originating from an individual ferromagnetic Ni nanotube attached to a Si cantilever. At the same time, we detect the nanotube's volume magnetization using torque magnetometry. We observe both the predicted reversible and irreversible reversal processes. A detailed comparison with micromagnetic simulations suggests that vortexlike states are formed in different segments of the individual nanotube. Such stray-field free states are interesting for memory applications and noninvasive sensing.

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The discovery of the Josephson effects 50 years ago initiated a revolution for electrical voltage metrology. This revolution started with single Josephson junctions delivering a few millivolt at most. Meanwhile, highly integrated series arrays containing more than 10 000 or even 300 000 junctions have been developed and fabricated for output voltages up to 10 V. Josephson voltage standards are nowadays used in many laboratories worldwide for dc applications. After their adoption for the representation of the unit of voltage in 1990, the focus of research shifted towards programmable Josephson series arrays. The increasing interest in highly precise ac voltages resulted in new developments for their metrological applications in the last 15 years. This paper summarizes the principal contributions from PTB to the present state of Josephson voltage standards with particular focus on developments and applications for ac standards in metrology and our proof-of-concept demonstrations. The presentation includes the two most promising versions of ac standards, being the programmable Josephson voltage standard based on binary divided series arrays and the Josephson arbitrary waveform synthesizer based on a pulse-driven series array.

We investigated, at temperature 4.2 K, electric transport, flux noise and resulting spin sensitivity of miniaturized Nb direct current superconducting quantum interference devices (SQUIDs) based on submicron Josephson junctions with HfTi barriers. The SQUIDs are either of the magnetometertype or gradiometric in layout. In the white noise regime, for the best magnetometer we obtain a flux noise S 1/2 Φ = 250 nΦ0/Hz 1/2 , corresponding to a spin sensitivity S 1/2 µ ≥ 29 µB/Hz 1/2 . For the gradiometer we find S 1/2 Φ = 300 nΦ0/Hz 1/2 and S 1/2 µ ≥ 44 µB/Hz 1/2 . The devices can still be optimized with respect to flux noise and coupling between a magnetic particle and the SQUID, leaving room for further improvement towards single spin resolution.