Anisotropy-patterned magnetic films using focused ion beam
Thin magnetic films of rare-earth transition-metal (RE-TM) alloys have a perpendicular magnetic anisotropy (PMA)
that is used in magneto-optical devices like the minidisc to store data in the form as magnetic domains.
The magnetism in these films organizes itself in a pattern of socalled magnetic domains;
regions in the film that have their local magnetic moments oriented in the same direction (either up or down) as sketched in Fig. X1.
Focused ion beam irradiation (FIB) provides a novel means to engineer the anisotropy in these kind of films on length scales comparable to
the domain wall width (~10-100 nm).
In the spin science group we studied the effect of anisotropy patterns on the magnetic domain structure of GdFe and GdTbFe films.
Square arrays of 50-nm sized dots with reduced anisotropy were created in these films with a focused Ga ion beam.
Their magnetic domain structures were then followed in magnetic field with x-ray resonant microscopy (XRM), x-ray resonant magnetic scattering
(XRMS) and magnetic force microscopy (MFM).
This website shows some of our recent results.
Magnetic domain pinning in an anisotropy-engineered GdTbFe thin film
Stan Konings, Jorge Miguel, Julio Camarero, Jan Vogel and Jeroen Goedkoop
J. Appl. Phys. 100, 033904 (2006)
[pdf]
Paper: Lock in of magnetic stripe domains to pinning lattices produced by focused ion-beam patterning
Stan Konings, Jorge Miguel, Jeroen Luigjes, Hugo Schlatter, Huib Luigjes, Jeroen Goedkoop, Vishwas Gadgil
J. Appl. Phys. 98, 054306 (2005)
[pdf]
Poster: Magnetic domain engineering with Focused Ion Beam Irradiation
[pdf] (653 kB)
Text and images by Stan Konings:
GdFe
FIG. X1: Simple sketch of magnetic domains. The arrows show the direction of the local magnetization. The exact magnetic
structure is more complicated and includes
domain walls and closure domains. Domain walls are the regions between two domains in which the magnetic moments gradually change direction.
Closure domains are regions at the interfaces of the film in which the moments lie in the plane of the surface
that reduces the energetically unfavourable stray field setup by the sample.
FIG. 1: MFM images of patterned Gd16.7Fe83.3 showing the dependence of the interdot spacing
[(a) 150 nm; (b) 200, and (c) 250 nm] to the lock-in. The right sides are pristine areas.
FIG. 2: MFM image of patterned Gd16.7Fe83.3 (spacing 250 nm, fluence 5x1015 ions/cm2).
An in situ perpendicular magnetic field of 25 kA/m is switched on halfway the scanning (lower part of the image).
The upper part of the image shows the remanent state. Down domains are displayed in light grey.
FIG. 3. MFM images of patterned Gd16.7Fe83.3 with overlay of dots (in white) extracted from the corresponding AFM image.
At remanence (150 nm spacing) the dots pin the domain walls (a) whereas in a 25 kA/m perpendicular field (250 nm spacing)
the down domains are positioned at the dots (b).
FIG. 4: X-ray resonant microscopy image of Gd16.7Fe83.3 at the edge of the anisotropy-pattern.
Click here
to see movies (animated GIFs) of the magnetization reversal of pristine and patterned Gd16.7Fe83.3
as measured with X-ray resonant microscopy at the
Advanced Light Source (Berkeley, CA).
Be prepared to wait a while: 2.6MB.
FIG. 5: Scanning electron microscopy image of the dots made by the DUAL beam FIB system. [image by P. Alkemade]
FIG. 6: Out-of-plane and in-plane hysteresis loops of pristine Gd16.7Fe83.3 measured with VSM.
FIG. 7: Before the FIB sputters atoms from the sample, there exist a bulge at the dot position.
At higher focused ion beam fluences per dot, atoms are sputtered from the sample and a small hole arises at the dot.
FIG. 8:The change from bulges at the dots to holes seems to be a coherent process.
The regular array of bulges collapse coherently to one bigger blob although in other regions the bulges are still intact.
FIG. 9: The coherent collapse of the bulges to one bigger blob is even better visible in the 3D view of this AFM image.
The height of the bulges are about 7-9 nm.
GdTbFe
FIG. 1: Hysteresis loops of the pristine Gd11.3Tb3.7Fe85 thin film.
Out-of-plane loop measured with polar VSM and in-plane loop measured with longitudinal SQUID.
FIG. 2: Atomic force microscopy image of the patterned
Gd11.3Tb3.7Fe85 thin film (2x2 um2 scan, interdot spacing 400
nm, fluence 1x1015 ions/cm2). The arrows indicate the position of the height profile as given at the bottom.
FIG. 3: Atomic force microscopy (a) and corresponding magnetic force microscopy image (b) of an anisotropy dot array
in Gd11.3Tb3.7Fe85 in the remanent state (9x9 um2 scan, interdot spacing 200 nm,
ion fluence 1x1015 ions/cm2). Insets show the areas indicated by a white box magnified by a factor 2.
FIG. 4: Magnetic force microscopy images showing the magnetic domain structures of patterned Gd11.3Tb3.7Fe85
in in situ perpendicular fields (20x20 um2 scan, interdot spacing 400 nm, ion fluence 5x1015 ions/cm2).
The magnetic domain structure displays (a) an irregular structure at remanence, (b) a wormlike structure at 50 kA/m and
(c) bubble domains positioned on the dots at 100 kA/m (inset shows the area in the white box magnified by factor 3).
The considerable tip-induced field can be seen from the 20x20 um2 scan after measuring several 10x10 um2 scans at 100 kA/m (d).
FIG. 5: Atomic force microscopy (a) and corresponding magnetic force microscopy image (b) of an anisotropy dot array in
Gd11.3Tb3.7Fe85 in an in situ perpendicular field of 90 kA/m (5x5 um2 scan, interdot spacing 400 nm,
ion fluence 5x1015 ions/cm2).
FIG. 6: Sketch of the proposed domain structure at different magnetization states (a). At remanence (upper left),
at intermediate magnetization (upper right), at high magnetization (lower left) and at saturation field (lower right).
(b) shows the X-ray resonant magnetic scattering pattern at the Gd M5 edge of a patterned area at saturation just before the nucleation field.
(-24 kA/m, ion fluence 1x1015 ions/cm2, interdot spacing=225 nm ).
FIG. 7: X-ray resonant magnetic scattering patterns at 43 kA/m of a pristine area (a) and
the three patterned areas at high ion fluence [ion fluence=1x1015 ions/cm2 and interdot spacing=225 nm
for (b), 340 nm for (c) and 420 nm for (d)] . The origins q=(0, 0) are indicated with squares.
FIG. 8: The angularly-integrated scattered intensity I as a function of momentum transfer qr
of the pristine and nine patterned fields. The scattered intensities are normalized to the maximum scattered intensity from the unpinned domains.
The color of the line corresponds to the magnetic field as indicated by the out-of-plane hysteresis loop(upper right).
The loop was measured with XMCD and the saturation magnetization was obtained with VSM.
FIG. 9: Contour plots of the angularly-integrated scattered intensity I as a function of magnetic field H and
momentum transfer qr of the pristine and nine patterned areas.
The peak positions of the intensities scattered by the unpinned domains are indicated with yellow circles.
Movie: Scattering of Gd11.3Tb3.7Fe85
Here you find a movie that compares the resonant magnetic scattering of a FIB patterned (left) and a pristine (right)
GdTbFe thin film over the out-of-plane hysteresis loop.
The scattering experiment has been performed at the
ESRF (Grenoble, France).
Warning ! Large filesize: be prepared to wait
For the uncompressed AVI movie click
here [xrms.avi; 27 MB]
For the zipped AVI movie click
here [xrms.zip; 8 MB]
For animated gif click
here [xrms.html; 2 MB]
Movie: proposed mechanism of anisotropy reduction
This movie shows an animation of the proposed mechanism of FIB-induced anisotropy reduction in amorphous Rare earth Transition metal alloys.
GdFe has a sperimagnetic structure; the Gd (red) and Fe (yellow) sublattices have opposite spin just like in a ferrimagnet
but the spins are non-collinear. So they are dispersed over the perpendicular magnetic anisotropy axis due to
the intrinsic amorphous structure. Because Tb is just like Gd a rare-earth metal,
its sublattice has the same spin orientation (so opposite to Fe). The Ga+ ions from the focused ion beam are shown in blue.
Warning ! Large filesize: be prepared to wait
For the uncompressed AVI movie click
here [mechanism.avi; 18 MB]
For the zipped AVI movie click
here [mechanism.zip; 6 MB]
For an animated gif click
here [mechanism.gif; 7 MB]
