We report the manipulation of nitrogen vacancy (NV) spins in diamond when nearby ferrimagnetic insulator, yttrium iron garnet, is driven into precession. The change in NV spin polarization, as measured by changes in photoluminescence, is comparable in magnitude to that from conventional optically detected magnetic resonance, but relies on a distinct mechanism as it occurs at a microwave frequency far removed from the magnetic resonance frequency of the NV spin. This observation presents a new approach to transferring ferromagnetic spin information into a paramagnet and then transducing the response into a robust optical signal. It also opens new avenues for studying ferromagnetism and spin transport at the nanoscale.

Development of sensitive local probes of magnon dynamics is essential to further understand the physical processes that govern magnon generation, propagation, scattering, and relaxation. Quantum spin sensors like the NV center in diamond have long spin lifetimes and their relaxation can be used to sense magnetic field noise at gigahertz frequencies. Thus far, NV sensing of ferromagnetic dynamics has been constrained to the case where the NV spin is resonant with a magnon mode in the sample meaning that the NV frequency provides an upper bound to detection. In this work we demonstrate ensemble NV detection of spinwaves generated via a nonlinear instability process where spinwaves of nonzero wavevector are parametrically driven by a high amplitude microwave field. NV relaxation caused by these driven spinwaves can be divided into two regimes; one- and multi-magnon NV relaxometry. In the one-magnon NV relaxometry regime the driven spinwave frequency is below the NV frequencies. The driven spinwave undergoes four-magnon scattering resulting in an increase in the population of magnons which are frequency matched to the NVs. The dipole magnetic fields of the NV-resonant magnons couple to and relax nearby NV spins. The amplitude of the NV relaxation increases with the wavevector of the driven spinwave mode which we are able to vary up to 3 × 106 m−1, well into the part of the spinwave spectrum dominated by the exchange interaction. Increasing the strength of the applied magnetic field brings all spinwave modes to higher frequencies than the NV frequencies. We find that the NVs are relaxed by the driven spinwave instability despite the absence of any individual NV-resonant magnons, suggesting that multiple magnons participate in creating magnetic field noise below the ferromagnetic gap frequency which causes NV spin relaxation.

We demonstrate optical detection of a broad spectrum of ferromagnetic excitations using nitrogenvacancy (NV) centers in an ensemble of nanodiamonds. Our recently developed approach exploits a straightforward CW detection scheme using readily available diamond detectors, making it easily implementable. The NV center is a local detector, giving the technique spatial resolution, which here is defined by our laser spot, but in principle can be extended far into the nanoscale. Among the excitations we observe are propagating dipolar and dipolar-exchange spinwaves, as well as dynamics associated with the multi-domain state of the ferromagnet at low fields. These results offer an approach, distinct from commonly used ODMR techniques, for spatially resolved spectroscopic study of magnetization dynamics at the nanoscale.PACS numbers: 07.79.-v, 72.25.-b, 85.75.-d Spintronic [1,2] and magnonic devices [3][4][5] are receiving intense scientific attention due to their promise to deliver new technologies that can revolutionize computing and provide greater energy efficiency. In particular, tools for understanding phenomena such as angular momentum transfer across interfaces [6][7][8][9][10], spin wave propagation in low dimensional and nanoscale systems [11,12], domain wall motion [13][14][15], microwaveassisted switching [16], and relaxation and damping in small structures [17] are needed. There is current interest in materials with more novel magnetic textures than simple ferromagnets, such as skyrmions [18]. Electrical detection has been widely used for studying domain wall motion, but does not have imaging capabilities. Optical techniques such as Brillouin light scattering (BLS) [12] and the magneto-optic Kerr effect (MOKE) [19] are also widely used but are ultimately limited by the optical diffraction limit. Scanned probe techniques can provide high spatial resolution but can be perturbative and may require a more challenging set-up such as vacuum and cryogenic environment to achieve high sensitivity.Nitrogen-vacancy (NV) centers in diamond have emerged as an attractive tool to study magnetic phenomena at the nanoscale, and they offer a way to convert magnonic signals into optical signals. NV centers offer a powerful magnetometry tool due to a potent combination of optical and magnetic properties that make the intensity of their photoluminescence (PL) dependent on their spin state. This has allowed detection of just a few resonant nuclear spins and nuclear magnetic resonance imaging with resolutions of tens of nanometers, all under ambient conditions and at room temperature [20][21][22]. NV centers have also been used to study domain wall hopping [23], the helical phase in FeGe [24], * hammel@physics.osu.edu* † bhallamudi.1@osu.edu* and spinwave modes in permalloy [25]. High sensitivity to detect dynamic fields has been achieved by finding optimal NV centers with long lifetimes and manipulating them (and sometimes the target spins) with intricate microwave and optical pulse sequences.We have recently demonstrated a ne...

We report quantitative measurements of optically detected ferromagnetic resonance (ODFMR) of ferromagnetic thin films that use nitrogen-vacancy (NV) centers in diamonds to transduce FMR into a fluorescence intensity variation. To uncover the mechanism responsible for these signals, we study ODFMR as we 1) vary the separation of the NV centers from the ferromagnet (FM), 2) record the NV center longitudinal relaxation time T1 during FMR, and 3) vary the material properties of the FM. Based on the results, we propose the following mechanism for ODFMR. Decay and scattering of the driven, uniform FMR mode results in spinwaves that produce fluctuating dipolar fields in a spectrum of frequencies. When the spinwave spectrum overlaps the NV center ground-state spin resonance frequencies, the dipolar fields from these resonant spinwaves relax the NV center spins, resulting in an ODFMR signal. These results lay the foundation for an approach to NV center spin relaxometry to study FM dynamics without the constraint of directly matching the NV center spin-transition frequency to the magnetic system of interest, thus enabling an alternate modality for scanned-probe magnetic microscopy that can sense ferromagnetic resonance with nanoscale resolution.Understanding magnetic dynamics in future storage and information processing technologies will be a key to their development [1, 2]. In particular, it will be necessary to measure and understand relaxation [3,4], angular momentum transfer [5][6][7][8] and spinwave propagation [9][10][11], not only in extended magnetic films, but also in nanoscale devices [12]. In addition, establishing new mechanisms for imaging magnetization dynamics in confined structures will aid in improving current magnetic technologies [13][14][15][16] and enhance them using emerging materials such as those featuring magnetic textures [17][18][19].The nitrogen-vacancy (NV) center in diamond has emerged as a flexible and sensitive platform for nanoscale magnetic sensing [20][21][22] due to its atomic-scale size and its spin-sensitive fluorescence, enabling optical detection of magnetic dynamics [23][24][25]. NV-based magnetometry aimed at dynamic magnetic fields have typically required either spin-echo protocols [26], which are constrained to frequencies that are quasi-static compared to FMR (e.g. ∼ MHz or below), or it requires direct resonance with an NV center spin transition [27,28].In contrast, we have recently demonstrated an alternate modality [29,30] for detecting ferromagnetic resonance with diamond NV centers placed in nanoscale proximity to Yttrium Iron Garnet (YIG) that uses a simple, continuous wave (CW) protocol. A surprising observa-* M. R. Page and F. Guo contributed equally to this work. Figure 1. A schematic of the experiment. The sample is a 20 nm Py ferromagnetic film deposited on a single crystal diamond with an implanted layer of NV centers 25 nm -100 nm from the surface. In order to apply microwave magnetic fields to the sample, a microwire (5 nm Ti/ 300 nm Ag) is patterned on an insulati...

We observe a dependence of the damping of a confined mode of precessing ferromagnetic magnetization on the size of the mode. The micron-scale mode is created within an extended, unpatterned YIG film by means of the intense local dipolar field of a micromagnetic tip. We find that damping of the confined mode scales like the surface-to-volume ratio of the mode, indicating an interfacial damping effect (similar to spin pumping) due to the transfer of angular momentum from the confined mode to the spin sink of ferromagnetic material in the surrounding film. Though unexpected for insulating systems, the measured intralayer spin-mixing conductance g ↑↓ = 5.3×10 19 m −2 demonstrates efficient intralayer angular momentum transfer.

Leveraging magnetoelastic coupling, we show a power efficient mechanism for exciting nitrogen-vacancy centers in diamond.

Magnetic field noise from magnons can reduce the lifetimes of proximate spins and degrade the performance of spin based technologies. However, spatial and temporal averaging over the area of typical field sensors makes measuring magnetic field noise challenging. Here, we use an ensemble of nitrogen-vacancy (NV) point-defects in diamond to measure the spectral profile of thermally excited spinwave noise at room temperature as a function of the distance away from a 20 nm thick Permalloy (Py) thin film. We systematically vary the separation between the NV and Py layers using a silicon-dioxide wedge and measure the longitudinal relaxation rate of the NV center ms = 0 state as a function of the separation. The measured spinwave-induced relaxation of an ensemble of NV centers is well described by a magnetostatic model of dipole fields from the spinwaves. We furthermore find that our all-optical, nonperturbative measurements of the spinwave noise can be used to extract information about the ferromagnetic source, such as magnetization, damping, and fluctuating amplitude. This technique is amenable to application with stand-off from ferromagnetic elements and from buried structures.

The availability of compact, low-cost magnetic resonance imaging instruments would further broaden the substantial impact of this technology. We report highly sensitive detection of magnetic resonance using low-stress silicon nitride (SiNx) membranes. We use these membranes as low-loss, high-frequency mechanical oscillators and find they are able to mechanically detect spindependent forces with high sensitivity enabling ultrasensitive magnetic resonance detection. The high force detection sensitivity stems from their high mechanical quality factor Q ∼ 10 6 [1, 2] combined with the low mass of the resonator. We use this excellent mechanical force sensitivity to detect the electron spin magnetic resonance using a SiNx membrane as a force detector. The demonstrated force sensitivity at 300 K is 4 fN/ √ Hz, indicating a potential low temperature (4 K) sensitivity of 25 aN/ √ Hz. Given their sensitivity, robust construction, large surface area and low cost, SiNx membranes can potentially serve as the central component of a compact room-temperature ESR and NMR instrument that has superior spatial resolution to conventional approaches.