Applications based on spin currents strongly profit from the control and reduction of their effective damping and their transport properties. We here experimentally observe magnon mediated transport of spin (angular) momentum through a 13.4 nm thin yttrium iron garnet film with full control of the magnetic damping via spin-orbit torque. Above a critical spin-orbit torque, the fully compensated damping manifests itself as an increase of magnon conductivity by almost two orders of magnitude. We compare our results to theoretical expectations based on recently predicted current induced magnon condensates and discuss other possible origins of the observed critical behaviour. arXiv:1812.01334v3 [cond-mat.mtrl-sci]

We experimentally study the spin dynamics in a gadolinium iron garnet single crystal using broadband ferromagnetic resonance. Close to the ferrimagnetic compensation temperature, we observe ultrastrong coupling of clockwise and counterclockwise magnon modes. The magnonmagnon coupling strength reaches almost 40% of the mode frequency and can be tuned by varying the direction of the external magnetic field. We theoretically explain the observed mode-coupling as arising from the broken rotational symmetry due to a weak magnetocrystalline anisotropy. The effect of this anisotropy is exchange-enhanced around the ferrimagnetic compensation point. arXiv:1903.04330v2 [cond-mat.mtrl-sci] 23 Sep 2019The strong and ultrastrong interaction of light and matter is foundational for circuit quantum electrodynamics [1][2][3]. The realizations of strong spin-photon [4][5][6] and magnonphoton [7][8][9][10][11][12] coupling have established magnetic systems as viable platforms for frequency up-conversion [13,14] and quantum state storage [15]. Antiferromagnets and ferrimagnets further host multiple magnon modes. Their coupling allows for coherent control and engineering of spin dynamics for applications in magnonics [16,17] and antiferromagnetic spintronics [18,19].Recently, it has been shown [20][21][22] that the weak interlayer exchange interaction between two magnetic materials can cause magnon-magnon coupling. However, the much stronger intrinsic exchange has not yet been leveraged for coupling phenomena. While the THz-frequency dynamics in antiferromagnets is challenging to address experimentally [23], the sublattice magnetizations in compensated ferrimagnets can be tuned to achieve GHzfrequency quasi-antiferromagnetic dynamics. Here, we report the experimental observation of ultrastrong exchange-enhanced magnon-magnon coupling in a compensated ferrimagnet with the coupling rate reaching up to 37% of the characteristic magnon frequency. We furthermore demonstrate that the coupling strength can be continuously tuned from the ultrastrong to the weak regime.We investigate spin dynamics, or equivalently the magnon modes, in a compensated, effectively two-sublattice ferrimagnet in the collinear state. Around its compensation temperature, this system can be viewed as a "quasi-antiferromagnet" due to its nearly identical sublattice magnetizations M A M B . Figure 1 schematically depicts the typical spatially uniform spin dynamics eigenmodes of the system [25]. Within the classical description, these become clockwise (cw) and counterclockwise (ccw) precessing modes which correspond to spin-down and spin-up magnons, respectively, in the quantum picture. The key physics underlying our experiments is the tunable exchange-enhanced coupling, and the concomitant hybridization, between theses two modes. The essential ingredients -mode coupling and exchange-enhancement -are both intuitively understood within the quantum picture as follows. First, due to their opposite spins, a spin-up magnon can only be coupled to its spin-down counterpart by ...

We report ultra-low intrinsic magnetic damping in Co 25 Fe 75 heterostructures, reaching the low 10 −4 regime at room temperature. By using a broadband ferromagnetic resonance technique in out-of-plane geometry, we extracted the dynamic magnetic properties of several Co 25 Fe 75 -based heterostructures with varying ferromagnetic layer thickness. By measuring radiative damping and spin pumping effects, we found the intrinsic damping of a 26 nm thick sample to be α 0 3.18 × 10 −4 . Furthermore, using Brillouin light scattering microscopy we measured spin-wave propagation lengths of up to (21 ± 1) µm in a 26 nm thick Co 25 Fe 75 heterostructure at room temperature, which is in excellent agreement with the measured damping.Itinerant ferromagnets (FM) are advantageous for spintronic and magnonic devices. They benefit from, e.g., large magnetoresistive effects and current-induced spinorbit torques 1 . In many magneto-resistive technologies (e.g., anisotropic magnetoresistance, giant magnetoresistance, tunnel magnetoresistance) electronic conductivity is indispensable. Moreover, due to high saturation magnetization in metallic FMs, spin-wave (SW) group velocities are in general significantly higher than in insulating ferrimagnets 2-5 . High saturation magnetizations in general ease detection. Nevertheless, itinerant FMs typically have considerable magnetic damping 6,7 . This is unfavorable for many applications. For example, low damping is crucial for oscillators based on spin transfer torques and spin orbit torques as well as for achieving large spin-wave propagation lengths (SWPL) 8-10 . The need for thin film materials with low magnetic damping has triggered the interest in the insulating ferrimagnet yttrium-iron garnet (Y 3 Fe 5 O 12 , YIG) 11-13 . Although for YIG, very small total (Gilbert) damping parameters in the order of α G ≈ 10 −5 , and large SWPLs of a few tens of micrometers (up to ∼ 25 µm) in thin films (∼ 20 nm) have been reported 5,13,14 , its insulating properties and requirement for crystalline growth are challenges for large scale magnonic applications.Schoen et al. recently observed ultra-low intrinsic magnetic damping in Co 25 Fe 75 (CoFe) metallic thin films (α 0 = (5 ± 1.8) × 10 −4 ) 15 , and Krner et al. reported PLs of 5 µm − 8 µm in CoFe using time resolved scanning magneto-optical Kerr microscopy 4 . This motivated our study on sputter-deposited CoFe-based thin film heterostructures. We use broadband ferromagnetic resonance (BB-FMR) spectroscopy 16 in outa) Electronic of-plane (OOP) geometry and Brillouin light scattering (BLS) microscopy 17 and find intrinsic damping parameters in the lower 10 −4 regime as well as SWPLs of more than 20 µm. The damping is therefore comparable to YIG/heavy metal (HM) heterostructures 18 and the SWPL is comparable to that of state-of-the-art YIG thin films 5,13 . Thin film CoFe is a promising candidate for all-metal magnonic devices, as it combines low magnetic damping with good electrical conductivity and large saturation magnetization, while enabling easy fab...