We investigated the magnetodynamics in rectangular Permalloy platelets by means of time-resolved x-ray photoemission microscopy. 10 nm thick platelets of 16 32 m size were excited by an oscillatory field along the short side of the sample with a fundamental frequency of 500 MHz and considerable contributions of higher harmonics. Under the influence of the oscillatory field, the Néel wall in the initial classical Landau pattern shifts away from the center, corresponding to an induced magnetic moment perpendicular to the exciting field. This phenomenon is explained by a self-trapping effect of the dominating spin-wave mode when the system is excited just below the resonance frequency. The basic driving mechanism is the maximization of entropy. DOI: 10.1103/PhysRevLett.95.207201 PACS numbers: 75.40.Gb, 75.60.ÿd, 75.75.+a Excitations of magnetic moments in nanostructures have recently attracted considerable attention [1][2][3][4][5] due to their relevance to high-frequency applications of magnetic particles in data storage devices. Advanced magnetic recording technology pushes the switching time into the gyromagnetic regime. For mesoscopic elements, the high-frequency behavior is governed by confined spinwave eigenmodes as visible, e.g., in Brillouin light scattering [2,6]. So far, most studies have addressed the simplest case of elements in an almost monodomain state [6,7]. In larger microstructures, magnetic stray fields are minimized by the formation of multidomain configurations. Thin rectangular platelets of Permalloy often exhibit a Landau flux-closure structure comprising a Néel wall in the center [8].The magnetization dynamics is quantitatively described by the Landau-Lifshitz-Gilbert equation [9] that resembles the equation of motion for a spinning top, because the magnetic moment is accompanied by an angular momentum. In the ultrafast regime, the torque acting on the local magnetizationM becomes the dominant factor. This torque may initiate a precessional motion ofM, which can be observed, if the Fourier spectrum of the excitation by an external field comprises significant components of the precessional eigenfrequency of the system. In particular, M should not be affected, if the local torque disappears.In this Letter, we report on the striking phenomenon of a magnetic moment induced perpendicular to an exciting ac magnetic field. This phenomenon occurs for a Landau flux-closure structure excited slightly off resonance. We show that the induced perpendicular moment, which leads to a domain wall shift despite zero local torque, is caused by a self-trapping of an oscillating mode, thus maximizing the energy exhausted off the exciting field.The time dependent spatial distribution of the magnetization was measured using a photoemission electron microscope (PEEM) [10]. The PEEM measures the spatial distribution of the x-ray absorption via the electron yield of secondary electrons [11]. When the energy of circularly polarized (polarization vectorP) photons is tuned to the Ni-L 3 absorption edge, the electron...

The local modification of antiferromagnetic ͑AF͒ interlayer exchange coupling by focused ion-beam irradiation has been studied experimentally in the epitaxial Fe/Cr/Fe͑001͒ trilayer systems. Square ferromagnetic ͑FM͒ areas of 200ϫ200 m 2 were created in the initially AF trilayer by ion irradiation with a fluence of 10 15 ions/cm 2. It was found, that in the range of the external magnetic field of about Ϯ200 Oe, the change of magnetic properties at the boundaries separating FM and AF areas occurs within distances of less than 200 nm. This fact allows the use of the technique for magnetic patterning of antiferromagnetically coupled trilayers on the submicrometer scale.

We have studied ultrafast magnetodynamics in micropatterned spin-valve structures using time-resolved x-ray photoemission electron microscopy combined with x-ray magnetic circular dichroism. Exciting the system with ultrafast field pulses of 250 ps width, we find the dynamic response of the free layer to fall into two distinctly different contributions. On the one hand, it exhibits localized spin wave modes that strongly depend on the shape of the micropattern. A field pulse applied perpendicular to the exchange bias field along the diagonal of a square pattern leads to the excitation of a standing spin wave mode with two nodes along the field direction. This mode is strongly suppressed for a pattern of elliptical shape. On the other hand, the integrated response of the free layer roughly follows a single-spin model with a damping constant of ␣ = 0.025 independent of the shape and resembles the response of a critically damped forced oscillator. DOI: 10.1103/PhysRevB.76.134410 PACS number͑s͒: 75.40.Gb, 75.60.Ϫd, 75.75.ϩa A magnetic spin valve ͑SV͒ represents a very important functional structure in modern magnetism. SVs are extensively used as read heads in magnetic storage devices. Their functionality depends crucially on the interplay of magnetic coupling phenomena. In its simplest version, a SV is composed of two ferromagnetic ͑FM͒ layers separated by a nonmagnetic ͑NM͒ spacer layer mediating a usually antiferromagnetic indirect exchange coupling, 1,2 which determines the magnetic configuration of the layer stack. In a more refined approach, the magnetization in one of the FM layers ͑hard layer͒ is additionally stabilized by a strong coupling ͑exchange biasing͒ to an antiferromagnet. The orientation of the magnetization vector in the other-the free-FM layer is then sensed by the giant magnetoresistance ͑GMR͒ effect. 2,3 In more complex systems, further coupling mechanisms such as orange peel or edge coupling may take place. 4 Thus, micron-sized spin valves are extremely interesting structures from a fundamental point of view, as they provide a unique access to the interplay between different types of magnetic coupling in both static and dynamic experiments.Advanced magnetic recording schemes and spintronics push the switching time into the gyromagnetic regime. Ultrafast magnetization excitations in soft magnetic microstructures thus recently attracted particular attention. 5-10 New switching concepts involving the spin transfer torque 11,12 also rely on gyromagnetic processes. For microscopic elements with a small magnetic anisotropy and a welldefined shape, the high-frequency behavior is governed by confined spin wave eigenmodes. 5,6,13 Quantitatively, the magnetodynamic response may be described by thewith the magnetization M ជ , the gyromagnetic ratio ␥, the Gilbert damping parameter ␣, and the saturation magnetization M s . 14 The effective field H ជ ef f contains all coupling contributions and exerts a torque on M ជ , which initiates its precessional motion, if the Fourier spectrum of the exciting ext...

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