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We have used time resolved scanning Kerr microscopy to image collective spin wave modes within a 2D array of magnetic nanoelements. Long wavelength spin waves are confined within the array as if it was a continuous element of the same size but with effective material properties determined by the structure of the array and its constituent nanoelements. The array is an example of a magnonic metamaterial, the demonstration of which provides new opportunities within the emerging field of magnonics.

The picosecond magnetization dynamics of arrays of square Ni 88 Fe 12 /Co 80 Fe 20 bilayer nanoelements were studied by optical pump-probe measurements. Experimentally observed modes were found to fall upon two branches, with a crossover from the high-to low-frequency regime as the element size was reduced to less than 220 nm. Micromagnetic simulations revealed that the branches are associated with center and edge modes. The edge mode is found to dominate as the element size is reduced so that the magnetic response to a pulsed field becomes less spatially uniform. DOI: 10.1103/PhysRevB.71.220409 PACS number͑s͒: 75.40.Gb, 75.30.Ds, 75.50.Ss, 78.47.ϩp Increased storage densities in magnetic data storage technology require the use of nanoscale magnetic elements.1 A full understanding of the spin-wave ͑SW͒ spectrum is also essential if higher data rates are to be achieved, for example, by the implementation of ultrafast precessional switching. 2Spatial confinement leads to quantized modes with frequency and spatial character that have a complicated dependence upon the exchange interaction and nonuniform demagnetizing field within the element.3-8 Interest in switching processes has led to the development of experimental techniques that allow SW excitations to be observed within the time domain.9,10 Although a number of studies have been performed upon microscale elements, nanoscale elements have not yet been extensively explored. Continued progress therefore requires the study of high-quality arrays of elements of identical shape and size. 3,5,8,11 In this paper, time-resolved scanning Kerr microscopy ͑TRSKM͒ measurements 9 are used to investigate magnetization dynamics in arrays of magnetic elements with size ranging from 64 to 630 nm. Specifically, we study elements with a composition similar to that used in the free layer of a spin or tunnel valve recording sensor or a magnetic random access memory element. Using the TRSKM as a probe of the magnetization dynamics at the center of an array, we record its time-dependent response to a pulsed magnetic field. We show that the measured precession undergoes a crossover to a lower-frequency regime as the element size is reduced below a certain value. Numerical simulations performed with the object oriented micromagnetic framework ͑OOMMF͒ ͑Ref. 12͒ reproduce the observed variations in the mode frequencies and lead to the surprising conclusion that the magnetic response of the smallest nanoscale elements is less, rather than more, spatially uniform.A Ta͑50 Å͒ /Co 80 Fe 20 ͑10 Å͒ /Ni 88 Fe 12 ͑27 Å͒ /Ta͑100 Å͒ film was sputtered onto a Si substrate and patterned, using a combination of electron-beam lithography and ion milling, into square arrays of square elements. The element lengths ͑edge-to-edge separations͒ were 630 ͑37.5͒, 425 ͑21.9͒, 220 ͑95͒, 120 ͑37.5͒, and 64 ͑48.4͒ nm, while the length of each array was about 4 m. Scanning electron microscope images 13 showed that the corners of the 64 and 120 nm elements were slightly rounded. The compositions of the Ni 88 Fe...

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We study domain wall dynamics in Permalloy nanowires excited by alternating spin-polarized current applied perpendicular to the nanowire. Spin torque ferromagnetic resonance measurements reveal that domain wall oscillations at a pinning site in the nanowire can be excited with velocities as high as 800 m/s at current densities below 10{7} A/cm{2}.