Magnetostrictive properties of amorphous SmCo thin films with imprinted anisotropy

We examine the magnetostriction in amorphous SmCo thin films with a composition in the range 4–27 at. % Sm. The magnetostriction increases significantly with increasing Sm content but is small compared to terbium-based ferromagnetic compounds, despite the large imprinted anisotropy. The magnetostriction and anisotropy both increase approximately linearly as the temperature is reduced. The magnetoelastic energy is found to be far smaller than the anisotropy energy so the magnetoelastic atomic displacements during growth cannot be the origin of the imprinted anisotropy. The anisotropy is only slightly altered by the application of large tensile stresses, indicating that the local strain fields involved in magnetostriction are not equivalent to the global strain produced by mechanical bending.

Figure 1

(a) The relationship between magnetostriction and curvature. An applied field $H$, parallel to the hard axis, induces a biaxial strain which results in a biaxial curvature. The grid of laser beams, used to measure the curvature, is aligned parallel to the hard and easy axes ($x$ and $y$, respectively). (b) The grid of laser beams reflected off the surface of a curved sample in a magnetic field $H$. The spacing of the reflected beams $d_{\mathrm{r}}$ differs from the spacing of the incident beams $d_{\mathrm{i}}$, by an amount dependent on the curvature. (c) The MOKE laser reflected off a sample, curved by the applied force $F$, in an applied field $H$.

Figure 2

The curvature and stress of the SmCo/Si samples as a function of applied magnetic field parallel to the hard axis (the $x$ direction). The stress at zero applied field is defined as zero. The curvature is measured simultaneously along two perpendicular directions: (a) parallel to the hard axis and (b) parallel to the easy axis. The three different SmCo compositions are shown and the arrows indicate the field sweep direction for the Sm${}_{27}$Co${}_{73}$ sample. The size of the magnetostriction is strongly dependent on the Sm content of the film.

Figure 3

(a) The curvature and stress of the Sm${}_4$Co${}_{96}$ sample as a function of magnetic field, measured over a range of temperatures. (b) The temperature dependence of the magnetoelastic coupling coefficient $b^{\gamma ,2}$ and the saturation field $H_{\mathrm{sat}}$. Both $b^{\gamma ,2}$ and $H_{\mathrm{sat}}$ increase with decreasing temperature.

Figure 4

Magnetic response of the Sm${}_4$Co${}_{96}$ film under a mechanically applied stress. (a) The magnetization $M$ along the easy axis, normalized by the saturation magnetization $M_{\mathrm{s}}$, for 0 and 8 GPa of applied stress. (b) The coercive field $H_{\mathrm{c}}$ along the easy axis as a function of stress. (c) The magnetization along the hard axis for 0 and 8 GPa of applied stress. (d) The saturation field $H_{\mathrm{sat}}$ along the hard axis as a function of applied stress.