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Dynamic processes in magnetic thin films. Domain wall motion and ferromagnetic resonance

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Caltech Theses and Dissertations
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NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The objective of the present investigation has been threefold: (1) To characterize domain wall motion in thin ferromagnetic films experimentally and to determine what film properties influence wall mobility. (2) To investigate ferromagnetic resonance relaxation in thin films over a wide range of temperature, frequency, and thickness and to determine what physical relaxation processes contribute to the resonance linewidth. (3) To correlate the losses for wall motion with relaxation processes for ferromagnetic resonance. Domain wall mobility for Ni-Fe alloy films has been measured as a function of film thickness from 300 to 1650 [angstroms]. Between 300 and 800 [angstroms] the mobility decreases with increasing film thickness, ranging from 8 x 10[superscript 3] cm/sec-0e at 300 [angstroms] to 3 x 10[superscript 3] cm/sec-0e at 800 [angstroms]. Between 900 and 1000 [angstroms], the mobility increases rapidly with increasing film thickness to about 7 x 10[superscript 3] cm/sec-0e. Above 1000 [angstroms], the mobility increases slowly with film thickness. Predictions based on Lorentz microscopy static wall shape measurements are in good agreement with the data for a constant value of the Landau-Lifshitz damping parameter [alpha] = 0.014. Eddy-current losses are negligible. The crosstie and Bloch line structures associated with domain walls in thin films do not appear to influence the mobility. The sharp increase in mobility between 900 and 1000 [angstroms] is associated with a wall structure transition in this region. Ferromagnetic resonance linewidth measurements have been made for films 150 to 3200 [angstroms] thick at frequencies from 1 to 9 Gc/sec and temperature from 2[degrees]K to 300[degrees]K with the static field in the film plane. Linewidths between 3 0e (1 Gc/sec) and 50 0e (9 Gc/sec) were observed. For fixed thickness, the 300[degrees]K linewidth increases monotonically with anisotropy dispersion. To eliminate dispersion, samples with the smallest linewidth [...] were selected for each thickness. For thickness less than a critical thickness [...]. [...] is independent of thickness, but increases with thickness for D > D[...]. The data are in good agreement with predictions based on two-magnon scattering between the uniform mode and degenerate magnons. Eddy-current losses are not important. The phenomenological damping varies from 0.005 (D = 400[angstroms] to 0.009 (D = 3200[angstroms]) for the 300[degrees]K data. As a function of temperature, the linewidth exhibits a maximum at about 80[degrees]K which is generally larger in thinner films. The amplitude of the peak (as high as 15 0e) is independent of frequency and the peak shifts to slightly higher temperatures with increasing frequency. Two annealing treatments at 150[degrees]C, one in a vacuum and one in hydrogen or oxygen, indicate that the temperature dependence is associated with a surface oxide layer. Two mechanisms, valence exchange and exchange anisotropy, may be important. Even though phenomenological damping parameters for the two processes, wall motion and resonance, are quite different (at 300[degrees]K), there is a definite connection between the losses. Changes in the wall mobility between 300[degrees]K and 77[degrees]K have been measured for films exhibiting, to varying degrees, the above linewidth effect. From these mobility and linewidth data, the losses for wall motion were found to be directly related to the losses for resonance from 300[degrees]K to 77[degrees]K. There is a definite connection between the relaxation processes which are important for wall motion and those involved in resonance.

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