Frequency Modulation of ResonantγRays in Iron Foils: Influence of the State of Magnetization

The properties of sidebands in Mössbauer spectra in the presence of a transversal radio-frequency field have been investigated in iron slabs. The relative intensity of the sidebands is analyzed in terms of fast phonon relaxation, because the single-modulation-index model does not account for the observed sideband intensities. The results obtained in the presence of the external magnetic field $H_0$ show that the generation of sidebands is caused by the oscillatory motion of the bulk magnetization rather than the domain-wall motion. The mean-square value of the atomic displacement, $x^2$, as a function of $H_0$ shows an abrupt decrease near the anisotropy field $H_A$ in the hard direction (375 Oe) and drops to zero very rapidly. We point out that, concerning magnetostriction as a possible mechanism, the application of the formula $\Delta l=l_0{\Lambda }_s({cos}^2\theta -\frac{1}{3})$ is correct only for powdered samples where the displacement of atoms is expected to be a quasistatic coherent phenomenon. However, in the case of thin slabs, where the wavelength of ultrasonic phonons at ${\omega }_{\mathrm{rf}}$ is much smaller than the sample size, the application of the above formula does not account for the observed modulation. We have calculated $x^2$ by evaluating the average elastic energy density of the sample under rf irradiation. The strong modulation observed implies that the magneto-elastic coupling coefficient is much larger in the dynamic condition than in the static one. From our discussion it follows that magnetostriction could also be responsible for the published results on a low-magnetostriction material such as supermalloy.