AVS 49th International Symposium
    Magnetic Interfaces and Nanostructures Wednesday Sessions
       Session MI-WeA

Paper MI-WeA6
Dynamical Investigation of Transient Magnetic Anisotropy in Ni@sub 80@Fe@sub 20@

Wednesday, November 6, 2002, 3:40 pm, Room C-205

Session: Magnetization Dynamics
Presenter: R. Lopusnik, National Institute of Standards and Technology
Authors: R. Lopusnik, National Institute of Standards and Technology
J.P. Nibarger, National Institute of Standards and Technology
T.J. Silva, National Institute of Standards and Technology
Z. Celinski, National Institute of Standards and Technology
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The values of static and dynamic uniaxial anisotropy in thin permalloy films are anomalously different by a factor of 2. The dynamic response of different thickness films are measured with a pulse inductive microwave magnetometer. The time-resolved precessional response was measured as a function of the applied bias field varying from 0 to 8 kA/m. The frequency range varies from 700 MHz to 3 GHz. Spectroscopic analysis of the data yields quantitative information about the intrinsic gyromagnetic properties of the films. The observed dependence can be fitted to high precision with the Kittel formula for ferromagnetic resonance to extract anisotropy field H@sub k@, the spectroscopic factor g etc. The static anisotropy field value was obtained by a quasi-static measurement of a hysteresis loop along the magnetic hard axis. In this case, the saturation field corresponds to the anisotropy field of the sample. To understand this effect, variable angle measurements were performed for several different orientations of the uniaxial anisotropy with respect to the applied bias field. For each angle the value of the dynamical anisotropy was obtained. The angular dependence of the anisotropy can be fitted to a cosine function, but with an additional angle-independent offset field of ~400 A/m. The modulation amplitude of the fitted cosine function is equal to the static anisotropy field value of ~320 A/m. Both the cosine amplitude and angle-independent offset are found to be independent of film thickness below 100 nm. We interpret the constant offset field as a transient component of the magnetic anisotropy that only affects dynamical response at time scales below 10 ns. Similar behavior has been observed using magneto-optical methods.@footnote 1@ In these recently reported studies, an initial fast response of the magnetization was followed by very slow increase over a much longer time scale. We will discuss possible explanations, including non-linear dynamics and eddy currents. @FootnoteText@ @footnote 1@ M. Bauer, R. Lopusnik, J. Fassbender, B. Hillebrands, J. Bangert, and J. Wecker, J. Appl. Phys. 91, 543 (2002); M. Pufall and T. Silva, IEEE Trans. Mag. 38, 129 (2002)