Magnetization dynamics in microscopic spin-valve elements: Shortcomings of the macrospin picture

Abstract

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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