Magnetometry based on the nitrogen-vacancy (NV) sensor spin in diamond is emerging as a unique tool for probing condensed-matter physics. It is operable from cryogenic temperatures to above room temperature, has a dynamic range of DC to GHz, and allows sensor-sample distances down to the few-nanometer scale. As such, NV magnetometry provides access to magnetic phenomena with high spatial resolution and sensitivity to spin fluctuations with nanoscale correlation lengths. In this talk, I will describe the application of NV magnetometry to probing collective spin dynamics in ferromagnets [1]. I will focus on a recent breakthrough result: the ability to locally characterize a spin chemical potential [2], which is a key quantity governing spin transport. Exploring spin waves in a magnetic insulator, we discovered a new way to tune the spin chemical potential that reveals the coupling between the ferromagnetic resonance and the thermal spin-wave bath. A key advantage of NV sensing is the in-situ access to auxiliary physical quantities such as applied magnetic fields and temperature. I will illustrate the power of these capabilities by analyzing the spin chemical potential under electrically-controlled spin injection. These results open the way for nanoscale control and imaging of spin transport in mesoscopic spin systems.

[1]        T. van der Sar^*, F. Casola^*, et al., Nanometre-scale probing of spin waves using single electron spins. Nat. Commun. 6, 7886 (2015).

[2]        C. Du^*, T. van der Sar^*, T. Zhou^*, et al., Control and local measurement of the spin chemical potential in a magnetic insulator. Submitted (2016).