Probing Near‐Field EM Fluctuations in Superparamagnetic CoFeB With NV Quantum Dephasometry

ABSTRACT Superparamagnetism in nanoscale magnetic layers is a critical property for a wide range of spintronic‐based sensor and computing applications. While conventional magnetization measurements can detect superparamagnetic signatures, they often require the application of high perturbative fields and are difficult to implement for magnetic layers integrated within functional devices. In this study, we non‐invasively investigate the superparamagnetic spin dynamics of a nanoscale CoFeB layer of thickness 1.1 nm, deposited on a diamond substrate, by probing its low‐frequency near‐field electromagnetic (EM) fluctuations using nitrogen‐vacancy (NV) centers‐based quantum dephasometry. Our measurements reveal an unconventional, non‐monotonic temperature dependence of the dephasing time of NV centers, which we attribute to EM fluctuations produced by thermally driven superparamagnetic domain flipping in CoFeB. Our findings are further supported by the theoretical interpretation of the dephasing dynamics of NV centers and the complementary SQUID‐based magnetization characterizations. Additionally, we extract the spectral density of the EM fluctuations in CoFeB, which is used to isolate different EM fluctuations components acting on NV centers. These results provide critical insight into the magnetization dynamics and near‐field EM environment of nanoscale magnetic layers. It also has significant implications for the development of hybrid quantum spintronic devices and opto–magnetic applications.