Stripline ͑SL͒, vector network analyzer ͑VNA͒, and pulsed inductive microwave magnetometer ͑PIMM͒ techniques were used to measure the ferromagnetic resonance ͑FMR͒ linewidth for a series of Permalloy films with thicknesses of 50 and 100 nm. The SL-FMR measurements were made for fixed frequencies from 1.5 to 5.5 GHz. The VNA-FMR and PIMM measurements were made for fixed in-plane fields from 1.6 to 8 kA/ m ͑20-100 Oe͒. The results provide a confirmation, lacking until now, that the linewidths measured by these three methods are consistent and compatible. In the field format, the linewidths are a linear function of frequency, with a slope that corresponds to a nominal Landau-Lifshitz phenomenological damping parameter ␣ value of 0.007 and zero frequency intercepts in the 160-320 A / m ͑2-4 Oe͒ range. In the frequency format, the corresponding linewidth versus frequency response shows a weak upward curvature at the lowest measurement frequencies and a leveling off at high frequencies.

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...

We have measured spin-transfer-driven, large amplitude, current hysteretic, low frequency (< 500 MHz), narrowband oscillations in nanocontacts made to spin valve structures. The oscillations occur in zero field, persist up to 5 mT for in plane applied fields, and to beyond 400 mT for out of plane fields. Unlike previous measurements, the oscillation frequency is well below that for uniform-mode ferromagnetic resonance, is only a weak function of applied field, and is highly anharmonic. The oscillations are hysteretic with applied dc current, appearing at high currents but persisting to lower currents upon decrease of the current. We suggest that these observations are consistent with the dynamics of a nonuniform magnetic state in the vicinity of the contact nucleated by both the spin transfer torque and dc current-generated Oersted fields, with the dynamics driven by spin transfer. The electrical oscillation amplitudes are large and narrowband, with the largest amplitudes on the order of 1 mV and the narrowest linewidths below 1 MHz.Contribution of NIST, an agency of the U.S. government, not subject to copyright. -Pufall et al., Current-Hysteretic Low Frequency…Since the prediction that spin-polarized currents can exert significant torques in magnetic nanostructures, a wide variety of magnetization dynamics driven by spin transfer torques have been observed in a wide range of device geometries and experimental conditions.[1,2] The general characteristics of these observed dynamicsthe amplitude, the fundamental excitation frequency f 0 , the change of f 0 with current I and applied field µ 0 H app -are roughly understandable using theories that approximate the free layer dynamics as quasi-uniform large angle magnetization motion in the region of the device where current flows. [3,4] In the case of nanopillars, this region is the entirety of the free layer (possibly ignoring some region at the edge), and in nanocontacts consists of the region directly under the contact; in the latter case the mode remains centered (i.e., stationary) on the symmetry axis. Even this rough correspondence between theory and experiment is somewhat surprising, since one might expect excitations with nonuniform magnetization (on the scale of the contact) due to the large spatially-varying Oersted fields generated by the dc current itself. The effect of these fields, which approach 6.5 mT/mA (65 Oe/mA) at the edge of a 60 nm diameter contact, is an active area of computational magnetic research. [5] In this Communication we present measurements of large amplitude, narrowband signals from nanocontacts that are not easily explainable using such radially symmetric quasi-uniform mode approximations. The measurements were performed on nanocontacts nominally identical to those measured previously. The principal difference in the results reported here is that the in plane field magnitude is always less than 5 mT, whereas previously this magnitude was greater than 60 mT. We suggest that the observed dynamics may result from the generation and pert...

Clustered ferromagnetic Josephson junctions form ultralow energy synaptic elements.