Brillouin light scattering investigations of structured permalloy films
B. Hillebrands, C. Mathieu, M. Bauer, S. O. Demokritov, B. Bartenlian, C. Chappert, D. Decanini, F. Rousseaux, F. Carcenac
1997
Journal of Applied Physics
The static and spin wave properties of regular square lattices of magnetic dots of 0.5-2 m dot diameter and 1-4 m periodicity patterned in permalloy films have been investigated by Brillouin light scattering. The samples have been structured using x-ray lithography and ion beam etching. The Brillouin light scattering spectra reveal both surface and bulk spin wave modes. The spin wave frequencies can be well described taking into account the demagnetization factor of each single dot. For the
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... les with smallest dot separation of 0.1 m a fourfold in-plane magnetic anisotropy with the easy axis directed along the pattern diagonal is observed, indicating anisotropic coupling between the dots. © 1997 American Institute of Physics. ͓S0021-8979͑97͒59808-2͔ Magnetic films patterned on the m and sub-m scale have received increased attention in the last few years due to their potential applications as future storage media and sensors. There remain many unresolved problems in the preparation and characterization of these structures, and therefore suitable preparation procedures and analysis tools are still under development. It is of particular importance to develop fabrication methods for magnetic islands with a very narrow distribution of the magnetic parameters and with well controlled coupling between the islands, which for many applications should be made negligibly small. In particular a precise control of magnetic anisotropies induced by the patterning process as well as by finite size effects is required. In this article we report on studies of regular square arrays of magnetic dots by Brillouin light scattering from spin wave excitations present in the dots. From the measured spin wave dispersion properties the uniformity of the magnetization distribution, demagnetizing effects, as well as coupling strengths between magnetic dots are obtained. We have prepared regular square arrays of circularly shaped dots of permalloy ͑Ni 80 Fe 20 ͒ of 0.5-2 m diameter and 500 and 1000 Å thickness. The films were prepared in ultrahigh vacuum onto Si/SiO 2 wafer substrates using an e-beam evaporator. The samples were patterned by synchrotron radiation based x-ray lithography performed at the L2M synchrotron station at the super ACO storage ring at LURE, Orsay, France using ion beam etching to transfer the patterns into the permalloy films. A subtractive process was used as described elsewhere. 1 It involved a lift-off of a 60-nm-thick aluminum film, which acts as a mask during the ion beam etching of the permalloy. On one film four arrays with dot diameters/lattice periods of 2/4, 2/2.2, 1/2, 1/1.1, and 0.5/1 m and an area of 1 mm 2 , and on a second film an array of 0.5/1 m and an area of 4 mm 2 have been patterned ͑the 0.5/1 m samples consists of square dots͒. The samples were characterized by electron microscopy. We have analyzed the dynamic properties of these structures, i.e., the spin wave excitation spectra, by Brillouin light scattering. 2 This approach offers high sensitivity and easy determination of magnetic anisotropies and the coupling strengths between magnetic dots. In particular we can study anisotropic coupling between magnetic dots as follows. The samples consist of circular shaped islands on a square lattice. Within each island no in-plane anisotropy is present, as tested independently on an unpatterned area of the sample and on dot arrays with large dot separations. Any observed in-plane anisotropy, which is of the same symmetry as the dot lattice, i.e., of fourfold symmetry, can only be induced by coupling between the islands, and it is thus a measure of the coupling strength between the dots. Furthermore, spin wave excitations sense the magnetic properties on a length scale comparable to the spin wave wavelength of Ϸ2000 Å, which is smaller than the characteristic pattern dimensions. The spin wave excitation of interest is the dipolar surface mode ͑Damon-Eshbach mode͒, which is sensitive to the saturation magnetization, to the external field, to anisotropy contributions, and in particular, to the demagnetizing factor. As noted above measurements of the anisotropy constants allow us to determine the coupling between the magnetic dots. The dispersion equation is given for zero anisotropy by 3 with ␥ the gyromagnetic ratio, M s the saturation magnetization, q ʈ the in-plane wave vector, and d the dot thickness. H i is the internal field, which in absence of anisotropy is given by: with N the demagnetizing factor of the magnetic dots. In Eq. ͑1͒ it is assumed, that the magnetization is saturated in the plane of the dots and that the product of dot diameter and q ʈ is large. This is fulfilled for the investigated samples. For nonzero anisotropy the dispersion relation can be obtained only numerically. 4 The Brillouin light scattering experiments have been performed in backscattering geometry using Ar ϩ -ion laser light of wavelength 514.5 nm with an incident power of 100 mW, and a laser focus size of Ϸ40 m in diameter, which is 4993
doi:10.1063/1.364881
fatcat:lhps3byn3re4zcr7ojcsjc6ale