Magnetic states in low-pinning high-anisotropy material nanostructures suitable for dynamic imaging

We present magnetic domain states in a material configuration with high (perpendicular) magnetic anisotropy and particularly low magnetic pinning. This material, a B-doped Co/Pt multilayer configuration, exhibits a strong magnetic contrast in x-ray transmission experiments, making it apt for dynamic imaging with modern synchrotron techniques, providing high spatial and high temporal resolution simultaneously. By analyzing the static spin structures in nanodisks at variable external fields, we show that CoB/Pt multilayers exhibit low enough domain wall pinning to manipulate the domain pattern with weak stimuli and in particular to move domains and domain walls. We demonstrate in a proof-of-principle experiment using pump-probe x-ray holographic imaging that moderate magnetic fields can induce elastic and deterministic and hence repeatable small variations of the domain configuration in CoB/Pt multilayers, which is the key to perform high-resolution imaging of the domain wall motion to gain insight to the details of the local magnetization dynamics.

Figure 1

Evolution of the magnetic domain pattern in a $600\mathrm{nm}$ diameter Co/Pd disk during application of a uniform static external field of varying strength. White indicates magnetization pointing up (positive field direction) and black indicates magnetization pointing down. Between (b) and (c), the sample was saturated with a magnetic field of $-200\mathrm{mT}$.

Figure 2

(a)–(l) Selection of magnetization images of a $750\mathrm{nm}$ diameter CoB/Pt disk at various out-of-plane magnetic field steps, where white indicates magnetization pointing up (positive field direction) and black indicates magnetization pointing down. (a) shows the virgin state after deposition. Between (a) and (b), the sample experienced a field of $65\mathrm{mT}$. Between (e) and (f), between (g) and (h), and between (i) and (j), the sample was saturated. (k) visualizes the difference of (d) and (e), and (l) depicts the sum of (d) and (g), highlighting the different positions of the bubbles in these images. (m) shows an out-of-plane hysteresis loop of a continuous CoB/Pt film, in which the field values corresponding to images (a)–(l) are illustrated.

Figure 3

Pump-probe dynamic imaging of the excitation of a multibubble magnetic configuration in a $700\mathrm{nm}$ CoB/Pt multilayer disk with a static bias field of $140\mathrm{mT}$ in the out-of-plane direction. In (a) we see the topography of the sample, with the large circle defining the object hole in the opaque Au film. The area of magnetic material is highlighted by a lighter boundary and only the inner part is shown in (b) and (c). In (b) we show the magnetic state long after ($46\mathrm{ns}$) a $25\mathrm{mT}$ homogeneous magnetic field pulse of $4\mathrm{ns}$ duration, where black indicates magnetization pointing down, white indicates magnetization pointing up (positive field direction). This corresponds to the state without a field pulse. The black dotted lines mark the borders of bubblelike configurations of down-pointing magnetic moments. (c) depicts the domain configuration of the disk on the plateau of the pulse, and the border of the bubble domains off the pulse is sketched again with dotted lines to highlight the shrinking of the bubbles.