NMR Spin-Echo Study of the Domain Wall Pinning in Cobalt Micropowders

The pinning force of domain walls was studied by the NMR two-pulse spin-echo method using the action of an additional magnetic video-pulse. It is shown that the pinning force value Ho is inversely proportional to the duration $\tau_{\mathrm{m}}$ of the magnetic video-pulse. In addition, the area of the magnetic video-pulse $A_{m}=H_{0}\ \tau_{m}$ turned out to be the constant for all threshold durations of the magnetic video-pulse. The dependence of the pinning force $H_{0}$ on the external magnetic field $H_{\mathrm{e}}$ is also studied. It is shown that up to 3 kOe, a linear dependence of $H_{0}$ on $H_{\mathrm{e}}$ is observed. This indicates that, as $H_{\mathrm{e}}$ increases, the domain walls are distributed over positions corresponding to the stronger pinning centers.