Nanometer-scale Magnetometry

The synthesis and investigation of ferromagnetic nanostructures has been motivated both by a large number of potential applications and by fundamental questions about the physics of nanometer-scale magnetism. Magnetic nanoparticles have potential biological and biomedical applications, applications in high-resolution magnetic imaging, as magnetic sensors, and as dense magnetic storage media. At the same time, the low-dimensionality of these structures results in magnetic configurations not present in macroscopic magnets. Until recently, due to their small size and inherently weak magnetic moment, experimental investigations have often been conducted on large ensembles of nanomagnetics structures. The results are often difficult to interpret due to stray-field interactions and the distribution in size and orientation of the individual nanomagnets. We are developing and applying techniques to measure the both the magnetization and stray field of of individual nanomagnets.

On the one hand, we use sensitive dynamic-mode cantilever magnetometry to investigate the volume magnetization of single nanomagnets. Our approach has allowed us, for example, to measure the moment, anisotropy, and switching behavior of single ferromagnetic nanotubes and individual skyrmion-containing nanowires as a function of applied magnetic field and orientation [1-3]. The technique requires that the nanomagnet under investigation be attached to an ultrasensitive cantilever. The cantilever is then mounted in a vacuum chamber at cryogenic temperatures and in an variable applied magnetic field. Due to its high sensitivity, cantilever magnetometry is well-suited for the detection of the weak magnetic response of a variety of nanometer-scale systems. In particular other groups have used the technique to measure persistent currents in normal metal rings [4], the magnetization of superconducting nanostructures [5], and magnetization reversal in a single iron-filled carbon nanotube [6] and a single Ni nanorod [7].

On the other hand, using an optimally coupled nanometer-scale superconducting quantum interference device (nanoSQUID), we can also measure the magnetic flux originating from an individual nanomagnets [8-10]. With the nanomagnet to be investigated affixed to the end of an ultrasensitive cantilever, it can be positioned in the proximity of a nanoSQUID with the aid of piezo-electric positioners. Once optimally coupled to the nanoSQUID, stray field hysteresis loops can be measured. Simultaneously we can monitor the cantilever resonance frequency to infer the magnetization state of the nanomagnet.

This dual functionality provides two independent and complementary measurements: one of local stray magnetic flux and the other of volume magnetization. Using this method we can gain microscopic insight into the reversal mechanism of an individual nanomagnets. In particular, we have applied this technique to ferromagnetic Ni and Py nanotubes [9,10]. Ferromagnetic nanotubes are particularly interesting because, unlike nanowires, they support corefree magnetic states. Such configurations avoid the magnetic point singularity along the axis of the structure, thereby resulting in a fast and controllable reversal process. In addition, previously unforeseen dynamic effects are possible in nanotubes. Domain walls moving in nanotubes are predicted to avoid a Walker breakdown and give rise to Cherenkov-like spin wave emission. Both numerical simulations and analytical calculations show that the tubular geometry favors two main in-plane states: a uniform axial state (UAS) with the magnetic moments pointing along the tube axis and a global vortex state (GVS) with moments pointing circumferentially around the tube. Due to their flux-closure configuration, vortex states produce much lower stray fields than uniform states; as a result, magneto-static interactions between nanomagnets could be reduced resulting in densely packed magnetic memories.

 

[1] D. P. Weber et al., Nano Lett. 12, 6139 (2012).
[2] B. Gross et al., Phys. Rev. B XX, xxxxxx (2016).
[3] A. Mehlin et al., Nano Lett. 15, 4839 (2015).
[4] A. C. Bleszynski-Jayich et al., Science 326, 272 (2009).
[5] J. Jang et al., Science 331, 186 (2011).
[6] P. Banerjee et al., Appl. Phys. Lett. 96, 252505 (2010).
[7] S. Lee et al., J. Appl. Phys. 111 083911 (2012).
[8] J. Nagel et al., Phys. Rev. B 88, 064425 (2013).
[9] A. Buchter et al., Phys. Rev. Lett. 111, 067202 (2013).
[10] A. Buchter et al., Phys. Rev. B 92, 214432 (2015).