Broadband electron paramagnetic resonance spectroscopy in diverse field conditions using optically detected nitrogen-vacancy centers in diamond

Abstract

Paramagnetic magnetic resonance, a powerful technique for characterizing and identifying chemical targets, is increasingly used for imaging; however, low spin polarization at room temperature and moderate magnetic fields poses challenges for detecting small numbers of spins. In this work, we use fluorescence from nitrogen-vacancy (NV) centers in diamond to detect the electron paramagnetic resonance (EPR) spectrum of optically inactive target spins under various conditions of field magnitude and orientation. The protocol requires neither direct microwave manipulation of the NV spins nor spectral overlap between NV and target spin resonances, thus enabling broadband detection. This unexpected non-resonant coupling is attributable to a two-phonon process that relaxes NV spins proximate to the fluctuating dipole moment of the target spin, suggesting that the sensitivity is determined by the dipoledipole coupling strength. This approach holds promise for sensitive EPR detection, particularly in settings where control over the diamond-crystal orientation is difficult. This is notably the case for biological sensing applications, where nanodiamonds are being pursued for their bright, stable fluorescence and biocompatibility.

Broadband multi-magnon relaxometry using a quantum spin sensor for high frequency ferromagnetic dynamics sensing

“…NV centers have a spin-dependent fluorescence intensity and long spin lifetime [7][8][9] , which can be used to sensitively measure magnetic field noise at the NV resonance frequency which causes NV spin relaxation. Noise sensing with NVs, also called relaxometry, is a widely popular technique, which has been used to detect electronphonon instability in graphene 10 , spin labels [11][12][13][14][15][16] , driven electron paramagnetic resonance 17,18 , and ferromagnetic dynamics [19][20][21][22][23][24][25][26] , among others. Early NV relaxometry studies of ferromagnetic resonance (FMR) provide a clear picture of the sensing scheme: 23 microwave drive excites a mode in a ferromagnetic film, the excited mode undergoes incoherent four-magnon scattering processes leading to a redistribution of magnon population throughout the magnon spectrum.…”

Fast, Broad-Band Magnetic Resonance Spectroscopy with Diamond Widefield Relaxometry

“…Copper II ions in solution (dissolved in water in acidic conditions) measured using standard EPR technology show two g-factors of g ∥ Cu = 2.400 and g ⊥ Cu = 2.099 (15). Typically, using standard EPR technology, in a solution at ambient temperature, a motion average g factor is expected to appear around 2.23 (484 G) for copper II ions (1).…”

“…11 Further it is possible to image EPR signals 12,13 or to perform spectroscopy. 14,15 This found various applications in chemistry, in monitoring reactions, 16,17 and biology, 18,19 in detecting free radicals in living cells. 20,21 Inherited from conventional magnetic resonance techniques, several measuring modes can be used when working with NVcenters.…”

Observation of the magneto-optic Voigt effect in a paramagnetic diamond membrane

“…P1 resonances are directly detectable by the N-V − fluorescence contrast, which occur here between 600 and 850 MHz. These P1 electron spin resonances are likely detectable due to a Raman-based mechanism of cross-relaxation between the N-V − and P1 electron spins, as discussed in [10], and consist of nine peaks based on the hyperfine coupling of the P1's electron (S=1/2) and quadrupolar nuclear (I=1) spins: five peaks originating from allowed |m I , −1/2 ↔|m I , 1/2 transitions, and four low amplitude peaks related to nuclear spin flip-flop and forbidden transitions (∆m I = {0, 1}) [15].…”

“…Diamond-based spin ensembles, in particular N-V − ensembles, are compelling systems for the development of biologically compatible magnetometers [5], masers [6], and the exploration of novel collective quantum phases [7]. Studying the magneto-optical aspects and origins of such ensembles is therefore pertinent for their continued development, and the observation of MOVE in such paramagnetic diamond systems is noteworthy due to the distinct optical spin-polarisation mechanism of the N-V − spin system [8,9], and in turn, other optically un-addressable magnetic spins that couple to the N-V − spins [10,11]. While the observation of optical birefringence and dichroism in natural diamonds with high defect concentrations is well documented e.g.…”