Tracking Temperature-Dependent Relaxation Times of Ferritin Nanomagnets with a Wideband Quantum Spectrometer

Abstract

We demonstrate the tracking of the spin dynamics of ensemble and individual magnetic ferritin proteins from cryogenic up to room temperature using the nitrogen-vacancy color center in diamond as magnetic sensor. We employ different detection protocols to probe the influence of the ferritin nanomagnets on the longitudinal and transverse relaxation of the nitrogen-vacancy center, which enables magnetic sensing over a wide frequency range from Hz to GHz. The temperature dependence of the observed spectral features can be well understood by the thermally induced magnetization reversals of the ferritin and enables the determination of the anisotropy barrier of single ferritin molecules. The study of magnetic fluctuations -time-dependent deviations of a magnetic system from equilibrium -is of high intrinsic interest and provides a powerful tool to gain insight into magnetic coupling mechanisms [1, 2]. Experimentally, however, access to fluctuations is frequently challenging, owing to two reasons: Firstly, fluctuations can extend over an excessively large range of frequencies. Secondly, many relevant magnetic systems are small entities with dimensions in the nanometer range, such as single molecules, clusters, or magnetic domains. Hence, the study of magnetic fluctuations requires a measurement technique featuring simultaneously nanoscale resolution and a wide frequency bandwidth.Widely used methods to study ensembles of magnetic nanosystems are SQUID magnetometry [3,4], Mößbauer spectroscopy [5], electron spin resonance [6], neutron scattering [7], or X-ray magnetic circular dichroism [8]. Furthermore, individual magnetic nano-objects have been investigated by scanning probe microscopy [9, 10], micro-SQUIDs [11,12] or scanning X-ray microscopy [13]. In general these techniques have a limited detection bandwidth, such that the experimental observation of magnetic fluctuations in a wide frequency range relies on stitched measurements combining complementary techniques in different frequency domains.Here we show that the Nitrogen-Vacancy (NV) center in diamond employed as a magnetic field sensor [14,15] simultaneously provides access to both nanoscale objects and a frequency bandwidth spanning ten orders of magnitude. We use the unique properties of the NV to monitor thermal magnetization reversals of single biological nano-magnets from cryogenic to room temperature where these fluctuations accelerate from the sub-Hz to the GHz range.We study ferritin protein complexes adsorbed onto a diamond surface (Fig. 1a). Each of these proteins en- closes a cluster of up to 4500 Fe atoms wich net magnetic moment exhibits strong thermally activated fluctuations. We detect the magnetic stray field of these clusters with NV defect centers embedded 5-10 nm below the diamond surface. This center enables the precise determination of the local magnetic field via the Zeeman shift of its spin sublevels, which can be measured by optically detected magnetic resonance (ODMR) techniques. Due to its atomic size, an individual NV center can be pl...

Relaxometry and Dephasing Imaging of Superparamagnetic Magnetite Nanoparticles Using a Single Qubit

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High-efficiency resonant amplification of weak magnetic fields for single spin magnetometry at room temperature

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