Sensing chaotic vortex dynamics in Fe₃O₄ nanocubes with Nitrogen Vacancy (NV) centers

Magnetic nanoparticles (MNPs) are promising candidates for biocompatible, nanoscale heaters in cancer therapy through magnetic hyperthermia, where oscillating magnetic fields trigger heat generation via Néel and Brownian relaxation [Int. J. Hyperthermia 29, 715–729 (2013)]. While most studies have focused on single-domain particles, larger nanoparticles offer an underexplored mechanism: vortex-driven heating at lower, clinically safer magnetic fields [Sci. Rep. 8, 1224 (2018)]. Moreover, multi-domain nanocubes offer larger contact area and higher saturation magnetization, enhancing their biosensing capabilities [ACS Omega 2, 8010–8019 (2017)]. In this work, we investigate Fe₃O₄ nanocubes (80–100 nm) using micromagnetic simulations, which reveal that clusters in the 0.5–2 μm range support non-uniform vortex dynamics, including chaotic modes. Nanoscale magnetization dynamics of these highly anisotropic particles remain experimentally elusive owing to the low spatial resolution of traditional sensors. To probe these excitations, we use nitrogen-vacancy (NV) spin defect qubits in nanodiamonds, which serve as nT-scale quantum magnetometers and mK-scale quantum thermometers with spatial resolution of less than 50 nm [Nat. Comm. 7, 11584 (2016), npj Spintronics 1, 2 (2023), Acc. Chem. Res. 56, 95–105 (2023)]. These results are the first step in understanding chaotic vortex motion in multi-domain MNP clusters that can open new pathways for magnetic hyperthermia and biosensing.