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The spin chemical potential characterizes the tendency of spins to diffuse. Probing this quantity could provide insight into materials such as magnetic insulators and spin liquids and aid optimization of spintronic devices. Here we introduce single-spin magnetometry as a generic platform for nonperturbative, nanoscale characterization of spin chemical potentials. We experimentally realize this platform using diamond nitrogen-vacancy centers and use it to investigate magnons in a magnetic insulator, finding that the magnon chemical potential can be controlled by driving the system's ferromagnetic resonance. We introduce a symmetry-based two-fluid theory describing the underlying magnon processes, measure the local thermomagnonic torque, and illustrate the detection sensitivity using electrically controlled spin injection. Our results pave the way for nanoscale control and imaging of spin transport in mesoscopic systems.

We observe highly efficient dynamic spin injection from Y 3 Fe 5 O 12 (YIG) into NiO, an antiferromagnetic (AF) insulator, via strong coupling, and robust spin propagation in NiO up to 100-nm thickness mediated by its AF spin correlations. Strikingly, the insertion of a thin NiO layer between YIG and Pt significantly enhances the spin currents driven into Pt, suggesting exceptionally high spin transfer efficiency at both YIG/NiO and NiO/Pt interfaces. This offers a powerful platform for studying AF spin pumping and AF dynamics as well as for exploration of spin manipulation in tailored structures comprising metallic and insulating ferromagnets, antiferromagnets and nonmagnetic materials.

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We report a systematic study of spin transport in antiferromagnetic (AF) insulators having a wide range of ordering temperatures. Spin current is dynamically injected from Y 3 Fe 5 O 12 (YIG) into various AF insulators in Pt/insulator/YIG trilayers. Robust, long-distance spin transport in the AF insulators is observed, which shows strong correlation with the AF ordering temperatures.We find a striking linear relationship between the spin decay length in the AFs and the damping enhancement in YIG, suggesting the critical role of magnetic correlations in the AF insulators as well as at the AF/YIG interfaces for spin transport in magnetic insulators.

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There are two common methods in generating magnetic resonance in FMs for spin pumping, cavity FMR and microstrip waveguides [3,7,8,12,19]. FMR cavities produce modest-strength, uniform rf fields over a relatively large space (cm-scale); while microstrip waveguides produce rf fields typically in micron to sub-mm scale, and when made very close to the FMs, can generate fairly large h rf [12,19]. Since the magnitude of rf field determines the excitation strength for spin pumping and only a few reports on microstrip spin pumping presented values of h rf [12,19], in this letter, we mainly compare our results with previous reports of spin pumping using cavity FMR. respectively, which reach the resolution limit of conventional high-resolution XRD systems, demonstrating excellent crystalline quality. In this letter, we focus on two 20-nm YIG films (YIG-1 and YIG-2) for FMR and spin pumping measurements.Room-temperature FMR measurements of the YIG films are carried out in a cavity at a microwave frequency f = 9.65 GHz and power P rf = 0.2 mW. Figure 3e shows the angular dependence of V ISHE for Pt/YIG-1 and W/YIG-2 normalized by the maximum magnitude of V ISHE at θ H = 90°.The clear sinusoidal shape is characteristic of ISHE since [15]thus confirming that the observed ISHE voltage arises from FMR spin pumping. The spin pumping signals we observed in insulating YIG cannot be explained by artifacts due to thermoelectric or magnetoelectric effects, such as anisotropic magnetoresistance (AMR) or anomalous Hall effect (AHE) [13,16,32,34,35].While a spin current is generated by transfer of angular momentum from YIG to metal, simultaneously, the coupling between YIG and metal exerts an additional damping to the magnetization precession in YIG, resulting in increased linewidths [10,12], as shown in Fig. 5 4 for the three samples before (∆H 0 ) and after (∆H 1 ) the deposition of where G r , γ, and B are the real part of spin mixing conductance, the gyromagnetic ratio, factor and Bohr magnetron, respectively. Using Eq. (2), we obtain = 4.56× 10 14 and 2.30× 10 14 Ω -1 m -2 for Pt/YIG-2 and W/YIG-2, respectively, which agree with the theoretical calculations [36] and are among the highest of reported experimental values [3,5,8,9].Previously, spin pumping of Pt/YIG excited by similar cavity FMR gave ISHE voltages in the µV range [1,9,11,16]. The large spin pumping signals and high spin mixing conductance observed in our YIG films may be attributed to two possible reasons. First, the small thickness (20 nm) of our films compared to LPE films (100 nm or larger) may play an important role, as suggested by a recent report [7] that a 200-nm YIG film shows much higher spin pumping efficiency than 1-µm and 3-µm films excited by a microstrip waveguide. [7,12]. Further investigation of spin pumping in these thin YIG films using microstrip waveguides will access larger dynamic range of spin pumping. In addition, the mV-level ISHE voltages reported here using a moderate h rf will allow miniaturization of spin pumping structures while m...

Direct current voltage generated in metallic layers by spin pumping J. Appl. Phys. 109, 07C910 (2011); 10.1063/1.3549582 Electric detection of the spin-Seebeck effect in ferromagnetic metals (invited) J. Appl. Phys. 107, 09A951 (2010); 10.1063/1.3357413 Inverse spin-Hall effect and spin pumping in metallic films (invited)