Studying Nanoscale Magnetism and its Dynamics with Soft X-ray Microscopy Page: 4 of 6
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These layer resolved high resolution magnetic soft X-ray
microscopy images constitute the first direct imaging of
dipolar field induced domain replication in perpendicularly
Another fundamental question associated with the shrinking bit
size in high density storage media is whether the domain
nucleation during magnetization reversal process exhibits a
deterministic behavior . Repeatability of local domain
nucleation and deterministic switching behavior are basic and
essential factors for achieving high performance in high-
density magnetic recording . High spatial resolution
images down to the nanoscale in repeated images of the
magnetic domain structure can provide a definite answer.
Nanogranular CoCrPt alloy films have received significant
attention as a potential high-density perpendicular magnetic
recording media. Fig. 6 shows a typical image of the domain
configuration of (Coo s3Cro 17)s7Pt13 alloy film recorded at the
Co L3 absorption edge (777 eV). Since the XMCD signal
reflects the local magnetic moment of the Co constituent in
this system, integrating the intensity over the field of view
shown in Fig. 6a) provides a measure of the macroscopic
magnetization for the external magnetic applied during the
recording. The M(H) curve displayed in Fig. 6b) was obtained
by recording magnetic transmission soft X-ray microscopy
(MTXM) images throughout a full hysteresis scan. The
stochastic character can be addressed by analyzing high
resolution images taken in repeated M(H) cycles . The
obtained data show a nearly stochastic process in
- -- 400
-" . .1 - 200
- -2 -
- -400 b
*-3 2 -1 O 1 2 3
Fig. 6. Typical high resolution (Ar = 15nm) MTXM image of the
nanogranular CoCrPt layer showing the nucleation sites. The image was
recorded at an external field indicated by the position in the M(H) curve .
Instead of reversing magnetization by applying external
magnetic (Oersted) fields, the spin torque exerted by a spin
polarized current onto a non-linear domain configurations has
recently attracted not only significant scientific interest ,
, but also as novel pathways for memory  and logic
 devices. The current-driven domain wall motion in
magnetic nanowires, particularly the question how fast and
how reliable such a domain wall can be moved upon spin
current injection is of paramount importance. Numerous
experiment measuring e.g. transport properties in such systems
have been reported so far . The capability of MTXM to
image directly the magnetic domain wall can again contribute
significant and novel information.
Curved 60nm thin permalloy wires with a radius of 25pm
were prepared by e-beam lithography. Two contact pads were
added to the structure to inject Ins short current pulses. To
generate a domain wall in between the two contact pads an
external magnetic was applied to saturate the magnetization in
the wire and subsequently released. In Fig. 7 a) an b) the
domain wall before and after injecting a single Ins short pulse
with a current density of about 107-8 A/cm2 is schematically
shown. Experimental evidence for the current induced domain
wall motion is shown in Fig. 7 c) showing the MTXM image
obtained as a difference between the image before and after
the injected pulse. This is a direct proof to the current induced
domain wall motion. Interesting insight into the speed and the
stochastic character of the current induced domain wall motion
can be obtained . This is again possible, since a DW
moving with a speed of about 1 l0m/s as predicted from
micromagnetic simulations travels in inse only 110nm,
therefore high spatial resolution is a prerequisite for such
Fig. 7. Schematics of the position of the domain before (a) and after (b)
injection of a Ins short spin curent pulse into the nanowire. c) Difference
MTXM image between a) and b) showing the movement of the domain wall
upon spin injection.
B. Imaging sub-ns spin dynamics with high spatial
The capability to combine the high spatial resolution with soft
X-ray microscopy with sub-100ps time resolution is an
important step forward to understand spin dynamics on the
nanoscale. The groundstate configuration in PY nanoelements
are Landau patterns and their perfect repeatability and various
features (domain walls, vortices) makes them the ideal
candidates for numerous theoretical and experimental studies
Spin current induced spin dynamics in such elements is a
vastly unexplored area and gives access to an understanding of
basic parameters, such as the adiabatic and non-adiabatic
contribution to the modified Landau-Lifshitz-Gilbert (LLG)
equation of motion incorporating spin torque terms .
Rectangular PY elements with a size of 2x4pm2 were
lithographically prepared and placed in the gap in a waveguide
structure (see Fig. 4b). Injecting Ins short electric pulses
through the element generates a precessional motion of the
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Mccall, Monnikue M & Fischer, Peter. Studying Nanoscale Magnetism and its Dynamics with Soft X-ray Microscopy, article, May 1, 2008; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc896098/m1/4/: accessed September 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.