The magnetomechanical damping Q mag−1 of high purity iron is measured as function of the amplitude of an alternating shear strain γ (frequency ≈ 1 Hz) in presence of a longitudinal magnetic field H. Successively the influence of a dc magnetic field and of ac magnetic fields of frequencies either equal (synchronous field) or very great (50 Hz) compared to that of the mechanical oscillations are considered. The variation of Q mag−1 = f(γ) exhibits a maximum; the evolution of this maximum is followed as function of the intensity of H. At small values of H and for the three magnetic fields, first an increase is noted of the intensity of the maximum followed by a decrease at higher values of H. In order to explain the experimental results, the energetic conditions of jumps of 90° magnetic domain walls are studied, in the field of force in which they are moving. The applied magnetic field plays two opposed parts: one favouring the other stabilizing the irreversible jumps which are induced by the mechanical alternating stress. The dc field and ac 50 Hz field have these two roles whereas the synchronous field exerts nearly only an initiating role.
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