We develop sensitive torque magnetometry to study particularly weak magnetic effects or magnetism in small samples. We combine such measurements with micro-magnetic simulations to reveal magnetization configurations, reversal behaviors, and magnetic phases. We have measured the hysteresis of individual nanomagnets, identified magnetic phase transitions in helimagnets, and found evidence for previously unknown magnetic states. The power of this technique lies in its ability to determine magnetic anisotropy and in its high sensitivity to tiny magnetizations. These properties, together with the sensitivity of magnetic torque to phase transitions, make it ideal for the measurement of 2D magnets.

We develop and apply scanning SQUIDs for high-resolution magnetic and thermal imaging. We are one of the few groups world-wide to implement a technique – first developed by the Zeldov group at the Weizmann institute – in which a nanometer-scale SQUID is integrated onto a scanning probe. We have used these SQUID-on-tip devices to study the behavior of nanomagnets, superconducting vortices, and current flowing in superconducting qubit devices [4]. We now coordinate a European FET-Open project, ‘Focused ion beam fabrication of superconducting scanning probes (FIBsuperProbes)’, aimed at further developing on-tip superconducting probes. In this context, we have developed a new SQUID-on-lever probe, which combines the magnetic and thermal imaging provided by a an on-tip SQUID with the tip-sample distance control and topographic contrast of a cantilever used in atomic force microscopy. These devices are now being used in imaging experiments on nanomagnets and 2D materials.

Their topologically protected spin-texture, which can be stable even at room temperature, their nanometer-scale size, and their manipulation via electric currents and fields make skyrmions a promising platform for information storage and processing. We have used sensitive torque magnetometry to measure the magnetic phase diagrams of nanostructured skyrmion hosts as well as small crystalline samples. We are now using scanning SQUID-on-tip probes to make real-space images of the magnetic field produced by skyrmions at the surface of bulk samples. This new application of scanning SQUID is starting to provide a microscopic spatial information on phase transitions, domains, domain walls, and the configuration of mixed magnetic phases in skyrmion hosting materials. It promises to bring new insight, which was – until now – experimentally inaccessible.

The proposal of magnetic resonance force microscopy (MRFM) and its subsequent realization combine the physics of magnetic resonance imaging (MRI) with the techniques of scanning probe microscopy. This pursuit is driven by the ultimate goal of imaging a single nuclear spin and the promise of a molecular structure microscope. The promise of this technique was demonstrated by a landmark experiment in 2004 in which an IBM group led by Dr. Dan Rugar measured the force of flipping a single isolated electron spin using a nanoscale cantilever. This achievement was not only among the first single-spin measurements, but also showed the versatility of nanoscale force sensors beyond conventional applications such as atomic force microscopy (AFM) and magnetic force microscopy (MFM). Progress continued with the report in 2009 of three-dimensional MRI of proton spins in a biological specimen with a spatial resolution under 5 nanometers.

We worked on improving MRFM detection sensitivity and on applying the method to nanometer-scale volumes of quadrupolar nuclear spins. Our demonstrated ability to detect nanometer-scale volumes of Ga and As nuclei using MRFM shows that — in principle — sub-surface, isotopically selective imaging of semiconductors nanostructures is possible. We also used MRFM to study the peculiar regime of nanometer-scale spin ensembles, in which statistical fluctuations in the nuclear polarization dominate the thermal polarization. More recently, our work on sensitive nanowire force sensors may eventually lead back to MRFM and to improvements in its sensitivity and resolution. As in any force microscopy, improvements in the force transducer are crucial for improving sensitivity and resolution.