Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons1–4. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics5 and the flux limitations of ultrafast X-ray sources6,7. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2′-bipyridine)3]2+, where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy6–15. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2′-bipyridine)3]2+ on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

Solid-state quantum emitters that couple coherent optical transitions to long-lived spin states are essential for quantum networks. Here we report on the spin and optical properties of single tin-vacancy (SnV) centers in diamond nanostructures. Through magneto-optical spectroscopy at 4 K, we verify the inversion-symmetric electronic structure of the SnV, identify spin-conserving and spin-flipping transitions, characterize transition linewidths, and measure electron spin lifetimes. We find that the optical transitions are consistent with the radiative lifetime limit and that the spin lifetimes are longer than for other inversion-symmetric color centers under similar conditions. These properties indicate that the SnV is a promising candidate for quantum optics and quantum networking applications.A central goal of quantum information processing is the development of quantum networks consisting of stationary, long-lived matter qubits coupled to flying photonic qubits [1,2], with applications in quantum computing, provably secure cryptography, and quantumenhanced metrology [3]. Among matter qubits, quantum emitters in wide-bandgap semiconductors [4,5] have emerged as leading systems as their coherent, spinselective optical transitions act as an interface between quantum information stored in their spin degrees of freedom and emitted photons. While most work has so far focused on the nitrogen-vacancy (NV) center in diamond [6-8], its relatively poor optical properties, including a low percentage of emission into the coherent zero-phonon-line (ZPL) [9] and large spectral diffusion when located near surfaces [10,11], have fueled the investigation of alternative emitters. These include the group-IV color centers in diamond [12], comprising the silicon-vacancy (SiV) [13][14][15][16], germanium-vacancy (GeV) [17,18], and the recently observed lead-vacancy (PbV) [19] centers. These centers have a large fraction of emission into the ZPL and a crystallographic inversion symmetry that limits spectral diffusion and inhomogeneous broadening [20,21]. Unlike the NV center, however, the electronic spin coherence of SiV and GeV centers is limited by phonon scattering to an upperlying ground-state orbital [22,23], requiring operation at dilution-refrigerator temperatures (∼ 100 mK) [24,25], or controllably induced strain [26] to achieve long coherence times.The tin-vacancy (SnV) center in diamond [27, 28] is a group-IV color center that promises favorable optical properties and long spin coherence time at readily achievable temperatures (liquid helium, ∼ 4 K). DFT calculations predict that the SnV has the same symmetry as the SiV and GeV[9], while experimental measurement of a large ground-state orbital splitting indicates that single-phonon scattering, the dominant spin dephasing mechanism of SiV and GeV centers at liquid helium temperatures, should be suppressed significantly [27]. In this work, we report spectroscopic measurements that are consistent with the conjectured electronic structure of the SnV, demonstrate that its optical...

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...