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.