Chiral magnets with topologically nontrivial spin order such as Skyrmions have generated enormous interest in both fundamental and applied sciences. We report broadband microwave spectroscopy performed on the insulating chiral ferrimagnet Cu2OSeO3. For the damping of magnetization dynamics we find a remarkably small Gilbert damping parameter of about 1 × 10 −4 at 5 K. This value is only a factor of 4 larger than the one reported for the best insulating ferrimagnet yttrium iron garnet. We detect a series of sharp resonances and attribute them to confined spin waves in the mm-sized samples. Considering the small damping, insulating chiral magnets turn out to be promising candidates when exploring non-collinear spin structures for high frequency applications. The development of future devices for microwave applications, spintronics and magnonics [1-3] requires materials with a low spin wave (magnon) damping. Insulating compounds are advantageous over metals for high-frequency applications as they avoid damping via spin wave scattering at free charge carriers and eddy currents [4,5]. Indeed, the ferrimagnetic insulator yttrium iron garnet (YIG) holds the benchmark with a Gilbert damping parameter α intr = 3 × 10 −5 at room temperature [6,7]. During the last years chiral magnets have attracted a lot of attention in fundamental research and stimulated new concepts for information technology [8,9]. This material class hosts non-collinear spin structures such as spin helices and Skyrmions below the critical temperature T c and critical field H c2 [10][11][12]. Additionally, Dzyaloshinskii-Moriya interaction (DMI) is present that induces both the Skyrmion lattice phase and nonreciprocal microwave characteristics [13]. Low damping magnets offering DMI would generate new prospects by particularly combining complex spin order with long-distance magnon transport in high-frequency applications and magnonics [14,15]. At low temperatures, they would further enrich the physics in magnonphoton cavities that call for materials with small α intr to achieve high-cooperative magnon-to-photon coupling in the quantum limit [16][17][18][19].In this work, we investigate the Gilbert damping in Cu 2 OSeO 3 , a prototypical insulator hosting Skyrmions [20][21][22][23]. This material is a local-moment ferrimagnet with T c = 58 K and magnetoelectric coupling [24] that gives rise to dichroism for microwaves [25][26][27]. The magnetization dynamics in Cu 2 OSeO 3 has already been explored [13,28,29]. A detailed investigation on the damping which is a key quality for magnonics and spintronics has not yet been presented however. To evaluate α intr we explore the field polarized state (FP) where the two spin sublattices attain the ferrimagnetic arrangement [21]. Using spectra obtained by two different coplanar waveguides (CPWs), we extract a minimum α intr =(9.9 ± 4.1)×10−5 at 5 K, i.e. only about four times higher than in YIG. We resolve numerous sharp resonances in our spectra and attribute them to modes that are confined modes across the macros...

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We present three designs for planar superconducting microwave resonators for electron spin resonance (ESR) experiments. We implement finite element simulations to calculate the resonance frequency and quality factors as well as the three-dimensional microwave magnetic field distribution of the resonators. One particular resonator design offers an increased homogeneity of the microwave magnetic field while the other two show a better confinement of the mode volume. We extend our model simulations to calculate the collective coupling rate between a spin ensemble and a microwave resonator in the presence of an inhomogeneous magnetic resonator field. Continuous-wave ESR experiments of phosphorus donors in nat Si demonstrate the feasibility of our resonators for magnetic resonance experiments. We extract the collective coupling rate and find a good agreement with our simulation results, corroborating our model approach. Finally, we discuss specific application cases for the different resonator designs.