We report on the observation of quantized surface spin waves in periodic arrays of magnetic Ni 81 Fe 19 wires by means of Brillouin light scattering spectroscopy. At small wavevectors (q || ≅ 0 -0.9·10 5 cm -1 ) several discrete, dispersionless modes with a frequency splitting of up to 0.9 GHz were observed for the wavevector oriented perpendicular to the wires. From the frequencies of the modes and the wavevector interval, where each mode is observed, the modes are identified as dipole-exchange surface spin wave modes of the film with quantized wavevector values determined by the boundary conditions at the lateral edges of the wires. With increasing wavevector the separation of the modes becomes smaller, and the frequencies of the discrete modes converge to the dispersion of the dipole-exchange surface mode of a continuous film.Patterned magnetic films are attracting increasing interest due to their potential applications in magnetic storage devices and sensors. Although static properties and coupling phenomena in magnetic films patterned on the micron scale have been studied extensively [1,2,3,4], high-frequency dynamic properties of such films have been rarely investigated. On the other hand, the study of spin wave properties in conventional finite size systems is well established, such as the investigation of so-called Walker-modes in magnetic spheroids [5], and of dipolar-dominated surface modes (Damon-Eshbach modes) in finite-thickness slabs with infinite lateral dimensions [6]. For periodic, micron-sized magnetic structures such a study has been still lacking, likely due to the high requirements both concerning the sample quality and the performance of the Brillouin light scattering experiment to detect the rather weak spin wave signals. In this Letter we report on the observation of quantization of spin waves in an array of magnetic wires. The quantization effects are identified as being due to the finite width of the wires. The evolution of the Damon-Eshbach mode of a continuous film from the discrete eigenmode spectrum of the wires with increasing wavevector, i.e., with diminishing influence of the finite size effect, is demonstrated and quantitatively described by a model based on quantized dipoleexchange modes. We show that both the frequency values and the wavevector intervals, where these modes are observed, are in a good agreement with our proposed model. The samples are made of a 200 Å thick permalloy (Ni 81 Fe 19 ) film deposited in UHV onto a Si(111) substrate by means of e-beam evaporation. Patterning was performed using X-ray lithography. The patterning masks were fabricated by means of a JEOL 5D2U nanopattern generator at 50 keV. X-ray exposure was performed at the super-ACO facility (LURE, Orsay, France) using a negative resist with a lift-off process with Al coating and ion milling. Two samples with periodic arrays of wires with a wire width w=1.8 µm and distances between the centers of the wires, Λ, of 2.5 µm and 4 µm (i.e., wire separations of 0.7 µm and 2.2 µm) were prepared for the inv...

An experimental study of spin-wave quantization in arrays of micron-size magnetic Ni 80 Fe 20 wires by means of Brillouin light-scattering spectroscopy is reported. Dipolar-dominated Damon-Eshbach spin-wave modes laterally quantized in a single wire with quantized wave vector values determined by the width of the wire are studied. The frequency splitting between quantized modes, which decreases with increasing mode number, depends on the wire sizes and is up to 1.5 GHz. The transferred wave vector interval, where each mode is observed, is calculated using a light-scattering theory for confined geometries. The frequencies of the modes are calculated, taking into account finite-size effects. The results of the calculations are in a good agreement with the experimental data.

Static magnetic and spin wave properties of square lattices of permalloy micron dots with thicknesses of 500 Å and 1000 Å and with varying dot separations have been investigated. A magnetic fourfold anisotropy was found for the lattice with dot diameters of 1 µm and a dot separation of 0.1 µm. The anisotropy is attributed to an anisotropic dipole-dipole interaction between magnetically unsaturated parts of the dots. The anisotropy strength (order of 10 5 erg/cm 3 ) decreases with increasing in-plane applied magnetic field.The physical properties of magnetic particles on the micro-and nanometer scale have attracted much interest in the last few years especially because of the promising prospects in future data storage and sensor applications [1][2][3][4]. One of the many unsolved problems is the origin and strength of magnetic interaction between the particles as a function of parameters like the dot separation and the aspect ratio (diameter to height ratio). In this Letter we report on first observations of an anisotropic in-plane coupling between magnetic dots arranged in a square lattice, which is correlated to inherent inhomogeneities of the dot magnetizations.Well separated dots of circular shape made of polycrystalline permalloy (Ni 80 Fe 20 ) will not exhibit any in-plane anisotropy.For dots with small enough separation any observed in-plane anisotropy is therefore a fingerprint for a magnetic coupling between the dots. Measurements of coupling induced anisotropies allow for a quantitative study of magnetic interdot coupling.The method of choice is Brillouin light scattering (BLS) from dipolar dominated spin wave modes (DamonEshbach modes) propagating within the dots [5]. The spin wave frequencies depend on the demagnetization factor of each dot, on the value and distribution of the magnetization vector, on anisotropies, and to a weak degree on the volume exchange constant. A study of the mode dispersion and of the dependence on the in-plane direction of the external field allows to determine the interdot coupling strength.We have studied regular square lattices of circular shaped dots of permalloy of 1 µm and 2 µm diameter and 500 Å and 1000 Å thickness. The films were prepared in ultrahigh vacuum onto Si/SiO 2 10x10 mm 2 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 at LURE, Orsay, using ion beam etching to transfer the patterns into the permalloy films.A subtractive process was used as described elsewhere [7]. Each sample consists of circular dots arranged in a 1x1 mm 2 square lattice with a diameter/periodicity of 1/1.1, 1/2, 2/2.2 and 2/4 µm, respectively. For comparison, and in order to check for modifications of the magnetic bulk properties during the patterning process, the remaining parts of the samples were not patterned , i.e. consist of a continuous film. Depth profile measurements show that the etching process cuts 200 Å deep into the substrate between the do...

An observation of self-focusing of dipolar spin waves in garnet film media is reported. In particular, we show that the quasistationary diffraction of a finite-aperture spin-wave beam in a focusing medium leads to the concentration of the wave power in one focal point rather than along a certain line ͑channel͒. The obtained results demonstrate the wide applicability of nonlinear spin-wave media to study nonlinear wave phenomena using an advanced combined microwave-Brillouin-light-scattering technique for a two-dimensional mapping of the spin-wave amplitudes.

Magnetic anisotropies of molecular-beam-epitaxy-grown fcc Co͑110͒ films on Cu͑110͒ single-crystal substrates have been determined by using Brillouin light scattering and have been correlated with the structural properties determined by low-energy electron diffraction and scanning tunneling microscopy ͑STM͒. Three regimes of film growth and associated anisotropy behavior are identified: coherent growth in the Co film thickness regime of up to 13 Å, in-plane anisotropic strain relaxation between 13 and about 50 Å and in-plane isotropic strain relaxation above 50 Å. The structural origin of the transition between anisotropic and isotropic strain relaxation was studied using STM. In the regime of anisotropic strain relaxation long Co stripes with a preferential ͓110͔-orientation are observed, which in the isotropic strain relaxation regime are interrupted in the perpendicular in-plane direction to form isotropic islands. In the Co film thickness regime below 50 Å an unexpected suppression of the magnetocrystalline anisotropy contribution is observed. Symmetry reflections based on a crystal-field formalism and discussed within the context of band theory, which explicitly takes tetragonal misfit strains into account, reproduce the experimentally observed anomalies despite the fact that the thick Co films are quite rough. ͓S0163-1829͑98͒03810-7͔

We report on measurements of the twodimensional intensity distribution of linear and non-linear spin wave excitations in a LuBiFeO film. The spin wave intensity was detected with a high-resolution Brillouin light scattering spectroscopy setup. The observed snake-like structure of the spin wave intensity distribution is understood as a mode beating between modes with different lateral spin wave intensity distri-butions. The theoretical treatment of the linear regime is performed analytically, whereas the propagation of non-linear spin waves is simulated by a numerical solution of a non-linear Schrödinger equation with suitable boundary conditions. Index Terms-Brillouin light scattering spectroscopy, micro-waves, non-linear spin-wave excitation

͓S0021-8979͑98͒01704-6͔The exchange bias effect, which results from exchange coupling between adjacent ferromagnetic ͑F͒ and antiferromagnetic ͑AF͒ layers, which are either deposited in a magnetic field or cooled down in a magnetic field after heating above the Néel temperature, is a well-established phenomenon. 1 It manifests itself in a shift of the hysteresis loop along the axis of the applied field, the so-called exchange bias field, H eb , and it is often described as an inplane unidirectional anisotropy. Although exchange bias systems have been extensively studied, 2-5 the microscopic origin of the exchange bias effect still remains unclear. The most advanced models propose the formation of magnetic domains in the AF layer causing a macroscopic exchange coupling strength of the experimentally observed size, 6,7 which is two orders of magnitude smaller than the atomic F-AF exchange coupling strength. Key issues to be tested experimentally are, ͑i͒ to prove the assumption of large local spatial variations of the F-AF coupling, and ͑ii͒ apart from the detection of the unidirectional anisotropy, to search for possible additional anisotropy contributions of higher order induced by the exchange coupling mechanism.We have chosen to study the ͑110͒-oriented system of Ni 80 Fe 20 /Fe 50 Mn 50 bilayers, since the lowest order in-plane anisotropy contributions, which are of unidirectional, uniaxial, and fourfold symmetry, can be separately determined for simple symmetry reasons. An unexpected large uniaxial anisotropy contribution caused by the FeMn coverage has already been reported for this orientation. 5,6 We have used Brillouin light scattering ͑BLS͒ from dipolar spin waves ͑Damon-Eshbach modes͒ propagating in the F film parallel to the surface to extract the anisotropy constants. 8 Two molecular beam epitaxy ͑MBE͒ grown staircaseshaped samples have been prepared on Cu͑110͒ single crystal substrates. The samples consist of Ni 80 Fe 20 films with thicknesses of 27, 34, 50, and 70 Å for the first, and 18, 24, 37, and 90 Å for the second staircase-shaped sample. Half of each sample was covered by an 80 Å thick Fe 50 Mn 50 layer. At this thickness the exchange bias effect is saturated. 4 Finally, both samples were covered with a 20 and 30 Å thick protective Au layer, respectively. The samples were characterized using low energy electron diffraction, and their chemical cleanliness was checked by Auger electron spectroscopy. An analysis of the process parameters revealed that the residual oxygen content in the gas used for sputter cleaning was significantly larger during preparation of the second sample compared to the first. Evaporation on a slightly contaminated surface can therefore not be excluded for the second sample. During growth a magnetic field of Ϸ250 Oe was applied in the film plane along the ͓110͔ direction to induce exchange biasing. We note that the ͑110͒-oriented Fe 50 Mn 50 surface is an uncompensated plane with a resultant in-plane magnetization along the Ϯ͓001͔ directions. 4 The Brillouin light scatt...

This letter reports a new approach for microwave-assisted magnetization reversal (MAMR) measurements and the demonstration of MAMR in a Fe65Co35 thin film with this approach. The approach uses a microstrip line to deliver microwaves to and measure the ferromagnetic resonance (FMR) of the sample. The switching field is determined through the measurements of the FMR absorption as a function of static field. In the presence of microwaves, a reduction in switching field by 50% was observed in a Fe65Co35 film. This reduction was independent of the duration of microwave pulses. This indicates the reduction is not a heating effect.