Self-Trapping of Magnetic Oscillation Modes in Landau Flux-Closure Structures

Abstract

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...

Ferromagnetic resonance study of thin film antidot arrays: Experiment and micromagnetic simulations

“…Besides the static and quasistatic properties, also the magnetization dynamics in nanopatterned particles has recently attracted considerable interest. [3][4][5] In addition to numerous studies of the influence of nanopatterning on the magnetization in mesoscopic magnetic dots, several studies have also addressed the issue for antidot arrays. While the static magnetic properties of antidot arrays [6][7][8][9] have been studied to a good extent, not much is known about their dynamic properties up to now.…”

Influence of topography and Co domain walls on the magnetization reversal of the FeNi layer inFeNi∕Al2O3∕CoRomanens1, Vogel2, Kuch3 et al. 2006Phys. Rev. B8081View full textAdd to dashboardCite

“…A similar behavior has been observed for Co/Pt systems 25 . Oscillations due to precessional-like magnetization reversal have been observed in several micrometer-sized thin film structures 26,27,28,29 . In these structures, the zero-field ground state is a flux-closure domain state due to finite-size and demagnetizing field effects, which also have an important influence on the magnetization dynamics.…”

Microwave Assisted Switching of Single DomainNi80Fe20Elements

“…Nembach et al also recently observed a microwave induced reduction of the coercivity for much larger multidomain elliptical Ni 80 Fe 20 elements (160 m 80 m 2 ) [12]. This reduction was explained by an enhanced domain wall nucleation in a microwave field [13] and preferential entropy-driven domain growth in a transverse microwave field [14]. In this Letter we show that for micron-sized single domain magnetic elements the microwave assisted switching is actually based on a coherent motion of the magnetization.…”