Ultra-low magnetic damping of a metallic ferromagnet

Abstract

Magnetic damping is of critical importance for devices that seek to exploit the electronic spin degree of freedom, as damping strongly a ects the energy required and speed at which a device can operate. However, theory has struggled to quantitatively predict the damping, even in common ferromagnetic materials 1-3 . This presents a challenge for a broad range of applications in spintronics 4 and spin-orbitronics that depend on materials and structures with ultra-low damping 5,6 . It is believed that achieving ultra-low damping in metallic ferromagnets is limited by the scattering of magnons by the conduction electrons. However, we report on a binary alloy of cobalt and iron that overcomes this obstacle and exhibits a damping parameter approaching 10 −4 , which is comparable to values reported only for ferrimagnetic insulators 7,8 . We explain this phenomenon by a unique feature of the band structure in this system: the density of states exhibits a sharp minimum at the Fermi level at the same alloy concentration at which the minimum in the magnetic damping is found. This discovery provides both a significant fundamental understanding of damping mechanisms and a test of the theoretical predictions proposed by Mankovsky and colleagues 3 .In recent decades, several theoretical approaches have attempted to quantitatively predict magnetic damping in metallic systems. One of the early promising theories was that of Kambersky, who introduced the so-called breathing Fermi-surface model 9-11 . More recently, Gilmore and Stiles 2 as well as Thonig et al. 12 demonstrated a generalized torque correlation model that includes both intraband (conductivity-like) and interband (resistivity-like) transitions. The use of scattering theory to describe damping was later applied by Brataas et al. 13 and Liu et al. 14 to describe damping in transition metals. A numerical realization of a linear response damping model was implemented by Mankovsky 3 for Ni-Co, Ni-Fe, Fe-V and Co-Fe alloys. For the Co-Fe alloy, these calculations predict a minimum intrinsic damping of α int ≈ 0.0005 at a Co-concentration of 10% to 20%, but was not experimentally observed 15 .Underlying this theoretical work is the goal of achieving new systems with ultra-low damping that are required in many magnonic and spin-orbitronics applications 7,8 . Ferrimagnetic insulators such as yttrium-iron-garnet (YIG) have long been the workhorse for these investigations, because YIG films as thin as 25 nm have experimental damping parameters as low as 0.9 × 10 −4 (ref. 16). Such ultra-low damping can be achieved in insulating ferrimagnets in part due to the absence of conduction electrons-and, therefore, the suppression of magnon-electron scattering. However, insulators cannot be used in most spintronic and spin-orbitronic applications, where a charge current through the magnetic material is required, nor is the requirement of growth on gadolinium gallium garnet templates compatible with spintronics and complementary metal-oxide semiconductor (CMOS) fabrication processes. One...

Magnetic properties in ultrathin 3d transition-metal binary alloys. II. Experimental verification of quantitative theories of damping and spin pumping

“…4(d), and are in the range of previously reported values for samples prepared under similar growth conditions [51][52][53][54][55]. Intermediate values of g [40]). The black squares are the intrinsic damping α int after correction for spin pumping and radiative contributions to the measured damping.…”

“…As previously reported, the dependence of α int on alloy composition in the Co x Fe 1−x alloys exhibits strongly nonmonotonic behavior, differing from the two previously discussed alloys [40]. α int displays a minimum at 25% Co concentration with a, for conducting ferromagnets unprecedented, low value of α int = (5 ± 1.8)×10…”

Tunable magnetization relaxation of Fe2Cr1−xCoxSi half-metallic Heusler alloys by band structure engineering

“…A practical criterion for engineering magnetic damping is α ∝ ξ 2 D(E F ) [12,13], where ξ is the SOC parameter and D(E F ) the density of states (DOS) at the Fermi level. Although Fe, Co and Ni render the window for tunable SOC in 3d-based materials formidably narrow, there have been encouraging developments through the correlation between damping and electronic structure, as demonstrated in FeV and CoFe binary alloys [14,15]. Still, reducing and tuning magnetic damping in metallic materials can be difficult due to magnon scattering by high density conduction electrons [10].…”

Low spin wave damping in the insulating chiral magnet Cu2OSeO3

“…3 (a). Note that Cu 2 OSeO 3 exhibiting 3.5 ×10 −4 outperforms the best metallic thinfilm magnet [44]. To correct for inhomogeneous broadening and determine the intrinsic Gilbert-type damping, we apply a linear fit to the linewidths ∆f in Fig.…”