High spin-wave propagation length consistent with low damping in a metallic ferromagnet

Abstract

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...

Nonreciprocal Dzyaloshinskii–Moriya Magnetoacoustic Waves

“…here validates the potential of DMI magnetoacoustic waves for the realization of acoustic diodes or SAW valves. Promising routes towards more efficient nonreciprocal SAW devices are optimizing the magnetoelasticity and the DMI strength or lowering the SW damping constant, e.g., by employing low-damping magnetoelastic Co 25 Fe 75 [51,52]. The excellent accordance of theory and experiment demonstrates that MASW spectroscopy can be used to characterize thin magnetic films with regards to magnetic film anisotropies, magnetoelastic coupling constants, SW damping, and the average DMI strength also as a function of frequency and SW wave vector.…”

Advances in Magnetics Roadmap on Spin-Wave Computing

“…The data were experimentally obtained by Brillouin light scattering in the magnetostatic surface wave configuration in a Co25Fe75 magnonic conduit with a width of 5 µm and a thickness of 26 nm for an external magnetic field magnitude of 43 mT. The combination of high group velocity and record low intrinsic damping 𝛼 < 0.001 [65] results in experimentally observed spin-wave propagation lengths [70] Achievement of these high spin-wave propagation lengths requires suppression of two-magnon scattering and inhomogeneous relaxation effects [71].…”

Nonlinear losses in magnon transport due to four-magnon scattering

“…At 20μm distance to the antenna, the detected magnon intensities are attributed only to propagation, which was confirmed by phase-resolved BLS measurements in a previous investigation. 9 Nonetheless, Fig. 2f reveals that four-magnon scattering can still be evident at the highest excitation power, revealing the significant impact of four-magnon scattering on magnon propagation at high power levels.…”

“…As a consequence, studies of the propagation characteristics in Co 25 Fe 75 microstructures followed, comparing damping values obtained from FMR to those derived from magnon propagation lengths. [7][8][9] In Ref. 7, a 2.5 times higher damping is reported for magnon transport measurements than for FMR analysis of extended films, which is attributed to significant extrinsic contributions to the magnetic damping in the microstructured sample, such as local inhomogeneities and two-magnon scattering.…”

“…In a previous study, we reported an intrinsic damping for this metallic thin film of α 0 ≤ 3.18 × 10 4 in out-of-plane geometry. 9 In a second step, we patterned a Cr(5)/Au(70) film to a coplanar waveguide (CPW) with a 2.1μm wide center conductor, separated by 960nm from the 1.5μm wide ground planes. The magnon waveguide extends up to 38μm on one side of the CPW.…”