Magnetization dynamics of cobalt grown on graphene

Abstract

Ferromagnetic resonance (FMR) spin pumping is a rapidly growing field which has demonstrated promising results in a variety of material systems. This technique utilizes the resonant precession of magnetization in a ferromagnet to inject spin into an adjacent non-magnetic material. Spin pumping into graphene is attractive on account of its exceptional spin transport properties. This article reports on FMR characterization of cobalt grown on CVD graphene and examines the validity of linewidth broadening as an indicator of spin pumping. In comparison to cobalt samples without graphene, direct contact cobalt-on-graphene exhibits increased FMR linewidth-an often used signature of spin pumping. Similar results are obtained in Co/MgO/graphene structures, where a 1nm MgO layer acts as a tunnel barrier. However, SQUID, MFM, and Kerr microscopy measurements demonstrate increased magnetic disorder in cobalt grown on graphene, perhaps due to changes in the growth process and an increase in defects. This magnetic disorder may account for the observed linewidth enhancement due to effects such as two-magnon scattering or mosaicity. As such, it is not possible to conclude successful spin injection into graphene from FMR linewidth measurements alone.Microwave-driven FMR has been shown to be a viable means for injecting spins into non-magnetic materials such as Pt, Au, GaAs, Si, and possibly graphene 1-7 . This is accomplished by placing a ferromagnet (FM) in contact with the non-magnetic material (NM), and driving the FM magnetization into precession. The free carriers of the NM absorb some angular momentum from the precessing FM magnetization, thus becoming spin polarized. This angular momentum transfer will enhance the damping of the FM. Spin pumping serves as an alternative to electrical spin injection schemes in which a bias current carries spin-polarized charges across the FM/NM interface 8,9 . The spin pumping effect was first observed by monitoring the FMR linewidth, a measure of the total damping of the FM magnetization by several available mechanisms 10,11 . However, the magnetization dynamics of a thin film FM are also extremely sensitive to sample defects and disorder. Graphene's low surface energy and high surface diffusion cause the deposited metal atoms to form clusters and make growth of smooth layers difficult 8,12 . As a consequence, we anticipate that the ferromagnetic properties of thin film cobalt and possibly other ferromagnets grown on graphene will be affected. In this report, we reproduce the linewidth broadening of cobalt FMR spectra due to underlying graphene 6 . Additional magnetostatic and dynamic characterizations make evident that substantial changes have been made to the cobalt films by growth on graphene.Single layer graphene is synthesized by low pressure chemical vapor deposition (CVD) on Cu foil using a Cu enclosure 13 . The resulting large area graphene (∼ 5 mm × 5 mm) is then transferred to SiO 2 (300 nm)/n-Si substrate by etching the Cu foil while using poly-methyl

Suppression of spin pumping between Ni80Fe20 and Cu by a graphene interlayer

“…Unfortunately, no independent measurements of M s or H ⊥ k were reported in [30] or [14]. Further evidence of the need for caution comes from a study of 14 nm thick Co films deposited by evaporation onto Gr/SiO 2 and SiO 2 substrates [15]. Careful characterization by FMR, magnetometry, magnetic force microscopy, and Kerr microscopy revealed such clear magnetic differences between the samples that the authors, commendably, declined to draw any conclusions about spin pumping into Gr.…”

“…We discuss these limitations in detail, drawing on our results and on other recently published work [15]. Finally, we attribute the suppression of spin pumping in Py/Gr/Cu to the large contact resistance of the Gr/Cu interface [16].…”

Dynamical spin injection at a quasi-one-dimensional ferromagnet-graphene interface

“…Estimates of the spin-mixing conductance obtained from the broadening of the FMR peaks resulted in surprisingly high values (e.g.,   g 5.2610 19 m -2 from Py/Gr = Py/Gr -Py = 110 -2 ), comparable to systems with high spin-orbit coupling 7 (such as heavy metals Pt and Pd). In addition, the direct deposition of the ferromagnetic Permalloy (Py) film on top of the graphene layer could cause magneto-structural 16 changes in the Py surface and a subsequent increase in straight fields altering the spin dynamics and accumulation in the semiconductor 17 , responsible for the observed change in damping when comparing with films without graphene, i.e., deposited on the bare wafer. A direct measurement of the spin Hall angle in FM/Gr interfaces has not been reported yet.…”

“…Most likely, the observed FMR broadening is due to changes in the magnetic response of the Py due to surface changes induced by the graphene, such as an enhancement of two-magnon scattering processes. It has been recently shown that the deposition of Co films on graphene results in magnetic variations and enhanced magnetic coercivity 16 . In our case, for example, we observe a slight change (<10%) in the magnetization saturation when graphene is present (not shown here).…”

Correlating spin transport and electrode magnetization in a graphene spin valve: Simultaneous magnetic microscopy and non-local measurements

“…4(c)). Growth of Co on graphene can result in enhanced grain and domain formation of Co due to graphene's high surface diffusion 18,24,25 , and domains could be pinned above the graphene by these grain boundaries.…”