IMAGING OF MAGNETIC MICROSTRUCTURES AT SURFACES

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IMAGING OF MAGNETIC MICROSTRUCTURES AT

SURFACES

H. Oepen, J. Kirschner

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Supplement au no 12, Tome 49, decembre 1988

IMAGING OF MAGNETIC MICROSTRUCTURES AT SURFACES

H. P. Oepen ( I ) and J. Kirschner (Iv2)

( I ) Institut f i r GrenzfEiichenforschung und Vakuumphysik, Kernforschl~ngsanlage, Jiilich 0 - 5 1 70 Jiilich, F. R. G. (2) Institut fir Atom- und Festkorperphysik, FU Berlin, Arnirnallee 14, 0-1000 Berlin 33, F.R.G.

Abstract. - We present an experimental analysis of domain wall fine structure at the surface of a semi-infinite ferromagnetic single crystal. It is found on Fe(100) that the domain walls in the surface show a NBel-like structure. This means that at the surface the rotation of the magnetization vector from one domain to the other occurs only within the surface. This is at variance with the commonly believed wall behaviour. For the 180°-walls we find a surface wall width of 220 nm f 15 nm, while the value for the 90'-wall is 150 nm f 30 nm.

Introduction

In 1976 Chrobok and Hofmann [l] discovered that secondary electrons from ferromagnetic material are spinpolarized, even if exited with unpolarized primary electrons. The polarization is antiparallel to the magnetization of the sample [2-51. This effect has been used in a new generation of microscopes for imaging magnetic structures at surfaces. Their basic idea is to combine a scanning electron microscope with a spinpolarization analyser for the secondary electrons. The orientation of the polarization vector corresponds to that of the magnetization of the sample within a small spot given by the size of the probe. Scanning the primary beam across the sample provides the magnetization distribution at the sample surface, e.g. the surface domain structure. The essential quantity is the polarization vector orientation, while its absolute value is of minor interest. This kind of microscope was first re- alized by Koike and Hayakawa [6], followed about one year later by Unguris et al. [7]. In the first case a classical Mott detector is used for spinpolarization analysis

[7], while in the other microscope the spinpolarization

is measured via diffuse scattering from evaporated Au films [8].

We built a similar microscope with a LEED-detector

[9] for spinpolarization analysis (Low Energy Electron _-

_-

Diffraction). In this paper we report on recent results

-

with this microscope, demonstrating the power of this new technique. In particular, we focus on magnetic structures with very small lateral dimensions, the domain walls. First we give a brief description of the experimental setup and its special features, and discuss some advantages. Results taken from Fe(100) single crystal surfaces are presented with emphasis on high resolution line scans across different kinds of domain walls. From these findings surface wall widths and surface wall configurations are deduced.

Experimental aspects

The microscope consists of two main parts: a scanning electron microscope column and the spinpolarization detector with its associated electron optics. The SEM column has a field emission source, which yields high resolution even at low primary energies, e.g.

5 20 nrn a t 1000 eV, because of the small emis-

sion spot and high brightness of this kind of source. Its advantage is the flexibility of selecting working con- ditions with high secondary electron yields, mostly at low primary energies, at sufficient resolution. The base resolution of the column is 3 nm at 25 keV.

The secondary electrons from the sample are col- lected and focused into the suinuolarization detector

_{- -}

by a special electron transport optics. At the detector crystal, d W(100)-single crystal, the electrons are scattered and the intensities of four equivalent diffraction beams are measured. One pair of oppositely mounted multipliers is used for determining the polarization component perpendicular t o the scattering plane, spanned by the incoming and diffracted beams. Measuring the intensities in all four (20) diffraction beams yields simultaneously two polarization components perpendicular to each other. A side view of the analyser is shown in figure 1. The whole detector is mounted on a 150 mm ID flange. The detector crystal, located on the vertical axis, is surrounded by four electron multipliers which detect the four (20) diffraction beams. In front of each multiplier, within the hous- ing (not visible in Fig. I), a grid assembly is installed to suppress inelastically scattered electrons. The assembly of W-crystal and multipliers is fixed relative to each other, but can be rotated about the vertical axis normal t o the W(100) surface. This is the axis along which the electrons enter the detector. We found this feature of the analyser quite useful and advantageous. It allows to align the polarization sensitive axes i f the detector with preferential orientations of the sample or

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C8 - 1854 JOURN.4L DE PHYSIQUE

Fig. 1. - The LEED-spinpolarization-andyser. It consists of a W(100)-single crystal (in the centre on the vertical axis) and four multipliers, mounted in housings (one opened). The orientation of the multipliers is along the direction of the (20)-diffraction beams. The base flange is a 150 mm ID flange on which the detector assembly is mounted. The support is equipped with a ball bearing for rotating the whole detector about the crystal surface normal.

its magnetic structure. This enables to maximize the polarization signal and in many cases simplifies the in- terpretation of results (e.g. images or line scans).

In designing the microscope special care was given to optimizing the transmission of the transport electron optics t o the detector, because all existing spinpolarization analyser suffer from strong intensity losses of about three orders of magnitude. The result is satisfying, which is best characterized by the obtain- able lateral resolution for magnetic microstructures of

<

40 nm (see below, Fig. 3). The polarization sensitiv- ity is high; for iron the polarization contrast between oppositely magnetized domains is typically f 24 % or above, depending on contamination

[lo].

Domain structures and domain walls

We have studied the magnetic microstructure of a Fe(100)-single crystal surface. The sample thickness was about 2 mm, which means that from the magnetic point of view the sample represents a semi-infinite sys- tem. A high resolution image of the domain structure is shown in figure 2. The two easy [loo] axes within the sample surface run parallel to the edges of the picture. Domains with different magnetization orientation are visualized by different colors. The arrows show the measured polarization orientation, which is antiparal-

Fig. 2. - Magnetic domain structure on a Fe(100) single crystal surface, showing several 90'-domain walls and a 180°-wall in the centred of the picture. It shows a stripe with magnetization perpendicular to the adjacent domains, but within the surface.

lel to the magnetization. Four domains can be seen, being all magnetized along the easy axes within the surface. The domains are separated by domain walls of two different types. Several 90'-walls separating domains with magnetization perpendicular to each other and one 180'-wall can be seen. Some roughening of the transitions is caused by statistical fluctuations. While the magnetic domain pattern on Fe(100) is well known, the domain wall configuration at the surface of semi- infinite crystals has not been investigated up to now. Even a realistic theoretical description for the wall structure incorporating the surface is missing. There- fore the most interesting part in figure 2 is the 180' transition, showing a yellow narrow stripe of about 200 nm width between the domains with opposite magnetization. The magnetization of the stripe is rotated by 90' relative t o the ones of the neighbowing domains and has a component in the surface, indicated by its color. This is quite an unexpected result, because the wall configuration at the surface is commonly believed to be similar t o that in the bulk, i.e. Bloch-like. If this was true, the 180'-wall should not have any magnetization component perpendicular to the wall within the (100)-surface. The opposite behavior is to be seen in figure 2. The discontinuity of the transition (Fig. 2) is an artifact caused by the color coding, which was chosen for reasons of clarity. In fact, the transition is continous as the following high resolution line scans show.

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El. P. Oepen et al. C8 - 1855

DISPLACEMENT ( n m j

Fig. 3. - High resolution line scan across a 180'-domain wall. The insert shows the orientation of polarization de- tection axes relative to the sample. The P+ component is corrected for the sample tilt. The lines are meant t o guide the eye. The horizontal step width is 20 nm. The vertical error bars give the 1 o statistical error. The non-zero component Po proves the NBel-like surface termination of a 180' (bulk) Bloch wall.

dependently of each other in the two polarization chan- nels of the detector. The step width, e.g. the spacing between adjacent points, is 20 nm. The

"+"

-signed component shows the polarization along the axis parallel t o the domain magnetization. Scanning the primary beam across the wall, this component changes sign as one would expect from the magnetization of the adjacent domains. The continuity of the transition clearly proves the stripe of figure 2 to be a wall, as was stated above, rather than a tiny domain. The second component ("on -signed) is aligned with the other easy axis in the surface and perpendicular to the wall. Within the domains this component shows zero polarization within the statistical uncertainty. Within the wall, however, where the

"+"

-component changes sign, a non-zero polarization is found. Its maximum, near the zero crossing of the

"+"

component, has the same value which is found inside domains with in-plane magnetization normal to the direction of the present 180'-wall. This demonstrates, that not only a component of the magnetization lies in the surface as one would suppose from figure 2, but that the full magnetization vector within the wall lies in the surface. (In view of the statistical uncertainty a slight tilting of

<

f 20' against the surface plane cannot be ruled out). This result contradicts the commonly accepted theoretical conjecture of a Bloch-like behaviour in the surface. This model (actually for the bulk wall), dat- ing back to the early beginnings of domain wall theory

[ll, 121 describes the transition to take place via a rotation of the magnetization vector parallel t o the wall. Such a wall configuration, if it encounters the (100) surface of iron, would create a magnetization pointing out of the surface, producing magnetic poles within the wall a t the surface. The Total Free Energy would be increased by the magnetic stray field caused by poles at the surface. From the results shown in figure 3 the surface wall configuration seems to prevent poles by the NBel-like rotation of the magnetization within the surface plane.

The question arising with this result is how the transition from the most likely Bloch-like wall in the bulk to the NBel-like wall at the surface looks like in detail. This question cannot be answered from the present results, because of the very small probing depth of the technique 1131. Possibly the structure is similar to the vortex-like domain wall configuration in thin films [14, 151.

From figure 3 the wall width a t the surface can be deduced. Following Lilley's [16] suggestions the wall width can be calculated to be N 220 n m f 15 nm

[lq.

This value is surprisingly close to the theoretical value for the bulk Bloch wall width of 225 nm [16], though the wall microstructure is totally different.

From the surprising result for the 180'-walls at the surface, one is led to ask what happens to 90'-walls encountering the surface? NBel [18] pointed out that going across a domain boundary (in the bulk) the component normal to the wall plane should be constant. This corresponds to the absence of free poles (which means div M = 0). For the 180'-walls discussed above this condition is trivially fulfilled, but for 90'-walls it determines the more complex magnetization rotation across the wall. The magnetization rotates such that the angle between wall-plane-normal and magnetization is constant. Generally speaking the magnetization vector rotates on a cone about the normal direction [16, 181. From this model for the bulk behaviour one could expect two different cases at the Fe(100)-surface, if the transition in the surface was identical with that in the bulk. First, if the wall-plane-normal lies in the surface (n = [110]), within the wall a magnetization component of 0.7 M, should arise perpendicular to the surface. (M, is the saturation magnetization at the surface.) Second, if the wall-plane-normal is tilted against the surface (n = [I, 1, 11, tilting angle 35, 26'), a smaller perpendicular component N 113 M, should

be expected. These values are easily measurable with the microscope and best checked by high resolution line scans.

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C8 - 1856 JOURNAL DE PHYSIQUE

DISPLACEMENT ( n m )

Fig. 4. - High resolution line scan across a 90'-domdn wall. Both components are aligned with easy axes of the Fe(100)-surface. The

"+"

component is corrected for the sample tilt. The lines are meant to guide the eye. The horizontal step width is 20 nm. The vertical error bars give the 1 o statistical error.

one direction [010] into the other one [OOl]. From this line scan alone it is difficult to deduce, whether there is any tilting against the surface or not, because this could only be done via a quantitative calculation of the projection in the surface plane using the two measured components. The easier and better way to clar- ify this issue is to measure the third magnetization vector component directly, i.e. along the surface normal. This has been done, and the result is shown in figure 5. The

"+"

-component again shows a polarization component within the surface plane, with similar shape as in figure 4. The "o" component, however, gives the perpendicular component of the polarization (magnetization). Within the statistical fluctuation no perpendicular component appears, though the lateral resolutions is about the same as in the case of the 180' wall. Thus, neither of the above expectations can be verified. According t o our findings within 90' domain walls as well as within 180' walls at the surface the magnetization vector rotates within the surface plane. In a similar way as for the 180' walls we determined the surface wall width of the 90" wall. The average over eight different walls, corrected for the tilt and az- imuthal angles, yields a surface wall width of 150 i

30 nm for the 90' domain walls. Comparing this result t o that from the 180'-wall and t o the theoretical approaches for the bulk wall widths, we arrive a t the following:

1) a t the surface the 90'-wall is twice as broad as in the bulk, while for the 180'-wall its width is about

200 400 600 800 DISPLACEMENT ( n m )

Fig. 5. - High resolution line scan across a SOo-domain wall, showing an in-plane component ("+" ) along (100) and the component normal to the surface ("0" ). Within the statistical error there is no component pointing out of the surface, demonstrating the N6el-like surface termination of 90'-domain walls.

the same (compared with the theoretical values from Lilley [16]);

2) the 90'-wall is smaller than the 180"-wall in the Fe(100)-surface by roughly a factor of 0.7 (about f i f 2 or cos 45").

Conclusions

We have shown that our microscope allows t o measure magnetic microstructures on a scale of 40 nm. The studies of domain walls a t the Fe(100) surface demon- strate the advantages of this type of microscope over classical techniques. For the first time the h e structure of domain walls a t the surface of semi-infinite sam- ples could be analysed. The main result is, that a t the surface the wall configuration avoids magnetic poles and stray fields by a N6el-like in-surface rotation of the magnetization vector. The wall widths of 90"- and 180'-walls were determined. A different behaviour of 90"- and 180"wall widths compared to their bulk values was found.

[I] Chrobok, G. and Hofmann, M., Phys. Lett. A 57 (1976) 257.

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H.

P. Oepen et al. C8 - 1857

[3] Hopster, H., Raue, R., Kisker, E., Giintherodt, G. and Campagna, M., Phys. Rev. Lett. 50 (1983) 70.

[4] Unguris, J., Pierce, D. T., Galejs, A. and Celotta,

R. J., Phys. Rev. Lett. 49 (1982) 72.

[5] Kirschner, J. and Suga, S., Solid State Commun.

64 (1987) 997.

[6] Koike, K. and Hayakawa, K., Jpn J. Appl. Phys. 23 (1984) L 187.

[7] Unguris, J., Hembree, G. G., Celotta, R. J. and Pierce, D. T., J. Microsc. 139 (1985) SRP1-2. [8] Hembree, G. G., Unguris, J., Celotta, R. J. and

Pierce, D. T., Scanning Microsc. Suppl. 1 (1987) 229.

[9] Kirschner, J., Polarized Electrons at Surfaces, Springer Tracts Mod. Phys. Vol. 106 (1985).

[lo] Allenspach, R., Taborelli, M. and Landolt, M., Phys. Rev. Lett. 55 (1985) 2599.

[ l l ] Bloch, F., 2. Phys. 74 (1932) 295.

[12] Landau, L. and Lifshitz, F., Phys. Sowjet. 8 (1935) 153.

[13] Kirschner, J., Proc. of 7th Course Int. School Electr. Microsc. Erice 1987 (NATO-AS1 series, Plenum Press) in press.

[14] La Bonte, A. E., J. Appl. Phys. 40 (1969) 2450. [15] Hubert, A., Phys. Status Solidi 32 (1969) 519. [16] Lilley, B. A., Philos. Mag. 41 (1950) 792. [17] Oepen, H. P. and Kirschner, J., (1988) submitted

for publication.