Thin-Film Microsusceptometer With Integrated Nanoloop

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- $\mu\hbox{m}$ 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 $\mu_{\rm B}/\!\surd\hbox{Hz}$ at 4.2 K ( $\mu_{\rm B}$ is the Bohr magneton) when being equipped with a nanoloop of 100-nm line width and separation.