Recent advances in spin-polarized scanning tunneling microscopy

Ultramicroscopy 42-44 (1992) 338-344 North-Holland

~

Y

Recent advances in spin-polarized scanning tunneling microscopy R. W i e s e n d a n g e r ~l, I.V. Shvets b, D. Biirgler ~', G. T a r r a c h ~', H.-J. G i i n t h e r o d t a and J . M . D . C o e y b a Department of Physics, Unit,ersity of Basel, Klingelbergstrasse 82, CH-4056 Basel Switzerland h Department of Physics, Unit,ersity o[ Dublin, Trinity College, Dublin-2, Ireland Received 12 August 1991

We present recent advances in the field of spin-polarized scanning tunneling microscopy (SPSTM) since our first report on the successful observation of vacuum tunneling of spin-polarized electrons between a half-metallic ferromagnetic CrO 2 sensor tip and an antiferromagnetic Cr(001) sample. We mainly focus on the in-situ preparation and testing of a variety of ferromagnetic and antiferromagnetic tips as well as on novel promising test samples such as Fe304(001) and Si(001). Atomic resolution in STM images obtained with a ferromagnetic tip and a ferromagnetic sample has been observed for the first time.

1. Introduction

The successful experimental observation of vacuum tunneling of spin-polarized electrons between a half-metallic ferromagnetic CrO 2 sensor tip and an antiferromagnetic Cr(001) sample [1] has led to the following conclusions: Firstly, the effects due to tunneling of spin-polarized electrons are tiny leading to stringent requirements for the choice of appropriate sensor tips and samples, their preparation as well as the experimental environment (UHV). Half-metallic ferromagnets [2] were found to be a very appropriate class of materials for spin-polarized scanning tunneling microscopy (SPSTM) experiments because they provide optimum spin filters even at room temperature. To exclude a possible influence of magnetic dipole forces acting between a ferromagnetic tip and a ferromagnetic sample at a distance of only a few fingstr6ms, at least one electrode (either tip or sample) should preferably not be chosen as a ferromagnet [3,4]. Finally, it has become clear that the measured local polarization P in SPSTM experiments is a joint polarization characteristic for the whole tunneling junction and that a bias-dependent measurement

of P can provide information about the local spin-dependent electronic structure [3]. Here, we report on the testing of novel magnetic sensor tips and samples which are promising candidates for SPSTM.

2. In-situ preparation and testing of S P S T M sensor tips

The ferromagnetic thin film CrO 2 sensor tips which were initially used for our SPSTM experiments offer sources of electrons with a high spin polarization [5], which is considered to be important for the ability to detect spin-polarized tunneling effects [1,3]. On the other hand, the preparation of the thin film CrO 2 tips is quite sophisticated [1,3]. More important, ferromagnetic CrO 2 sensor tips should not be used for SPSTM experiments if the sample would be a ferromagnet as well, due to possible disturbing magnetic force effects as mentioned above. Therefore, we have prepared and tested a variety of antiferromagnetic tips (e.g. Cr, NiMn, PtMn). Other ferromagnetic tips (e.g. Fe) have been tested as well. Since these materials easily oxidize when exposed to

0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

R. Wiesendanger et al. / Recent advances in spin-polarized scanning tunneling microscopy

air, we have developed a tip preparation technique which can be used in-situ in the U H V chamber (base pressure: < 1 × 10-11 mbar) and involves no high t e m p e r a t u r e treatment. A polycrystalline, 0.25 m m diameter, wire is electrochemically etched until a constriction of 20-100 /zm diameter is formed. One end of the wire is then fixed in the tip holder of the STM whereas the other end of the wire is fixed in another tip holder on top of a standard sample holder. The two ends are finally pulled apart in U H V by flipping the scan-head of our STM unit [6,7] back. As a result of this procedure, microtips are formed at the foremost end of the tips having a radius of curvature of 10 nm or even less. The cleanness of the obtained tips at the foremost end is only limited by the bulk impurity concentration. The atomic-resolution capability of such tips made, e.g., from Fe or Cr wire has been tested on the Si(111)7 × 7 [8,9] and Si(001)2 × 1 surfaces. For a more detailed description and illustration of this tip preparation technique as well as the experimental test results obtained with such tips, the reader is referred to refs. [8,9].

3. Novel test samples for SPSTM The main focus will now be on the discussion and presentation of initial experimental results on two novel test samples for SPSTM experiments, namely F%O4(001) and Si(001).

3.1. Fe304(O01) Magnetite (Fe304) has also theoretically been predicted to be a half-metallic ferromagnet [10], similar to CrO 2 which has successfully been used in our earlier SPSTM experiments [1]. Due to its magnetic properties and spin structure, the Fe304(001) surface is expected to be a very appropriate ferromagnetic test sample for SPSTM. The cubic crystal structure as well as the spin structure of magnetite is shown in fig. la. The easy cleavage plane is (001) and the easy axis of magnetization is along the [001] direction below 130 K and along the [111] direction above 130 K. The anisotropy field at room t e m p e r a t u r e is about

339

500 Oe. We have studied natural single crystals of magnetite from different sources. The best crystals obtained originate from Austria (Zillertal). We have carefully analyzed the crystal structure by X-ray diffraction. No impurity phases were detectable in the best crystals. The stoichiometry was also checked by the occurrence of the Verwey transition in electrical resistivity measurements as a function of temperature [11,12]. Several different methods of surface preparation have been tried. Some samples were cut and polished followed by an in-situ annealing up to a temperature of about 1000 _+ 50 K. The polishing was achieved mechanically by using diamond paste down to 0.5 /xm grain size or by using a commercial suspension of amorphous SiO 2 (OP-S from Struers). Electrolytical polishing in a H 3 P O a - C r O 3 - H 2° solution was also tried. Other samples were cleaved in-situ in the U H V chamber at a background pressure in the 10-~° mbar range or ex-situ followed by an in-situ annealing up to 1000 K. The in-situ annealing up tO 1000 K was found to reproducibly remove all carbon from the surface as checked by Auger electron spectroscopy (fig. lb). However, the annealing sometimes caused bulk impurities such as K, N, S, Ca, etc. to diffuse to the surface. The cleanest surface was therefore obtained by in-situ cleavage of the sample. On the other hand, we have reproducibly obtained the sharpest L E E D spots of Fe304(001) surfaces which were mechanically polished with diamond paste followed by the in-situ annealing. The L E E D pattern obtained at 142 eV primary electron energy is shown in fig. lc. All L E E D patterns which we obtained by changing the primary energy of the electron b e a m are consistent with the cubic crystal structure of magnetite as shown in fig. la and give no indication of a surface reconstruction. To the best of our knowledge, no L E E D patterns on surfaces of bulk single crystals of Fe304 were published before. STM m e a s u r e m e n t s were p e r f o r m e d on Fe304(001) surfaces obtained by the different preparation procedures mentioned above. The best STM results were again obtained on mechanically polished and annealed Fe304(001) surfaces where steps of half the unit cell height (4.2 A, see fig. la) were reproducibly resolved by

340

R. Wiesendanger et al. / Recent adc'ances in spin-polarized scanning tunneling microscopy

using an electrochemically etched tungsten tip (fig. ld). Bias-dependent STM measurements have shown that a sample bias voltage of at least 1 V has to be applied in order to get a stable tunneling current of 1 nA. Interestingly, this behaviour of F % O 4 is similar to CrO 2 where a similar bias dependence was measured by STM [3] in agreement with photoemission data [5]. We have recently achieved atomic resolution on the Fe304(001) surface by using an in-situ prepared Fe tip (fig. 2a). To our knowledge, this

,/

is the first observation of atomic resolution in STM images obtained with a ferromagnetic tip and a ferromagnetic surface. The unit cell periodicity of 8.4 A could clearly be imaged (fig. 2b) indicating again the absence of a surface reconstruction. In addition, we have found an interesting novel one-dimensional superstructure on the Foe304(001) surface with a periodicity of about 30 A (fig. 2a). Its origin is not yet dear. The measured corrugation for the atomic lattice of about 1 ,a, with a tunneling current of 1 nA and a

t/

) Octahedral interstices ~t~ Tetrahcdral interstices ~,--~ 02 ions ','0~)

I!

2500! augep aC 1500 f u

I=e

500

t -500 s - 1 5 o o ~ - 2 5 0 0 , lO00 900 800

(b) 700 600 500 400 Kinetic Enepgy / eV

300

200

~00

Fig. 1. (a) Crystal structure and spin structure of Fe304. The height of the unit cell shown in this figure is 8,4 A. (b) Auger spectrum of a mechanically polished and annealed Fe304(001) surface. (c) LEED pattern of the mechanically polished and annealed Fe304(001) surface obtained at 142 eV electron energy. (d) Topographic STM image (155 ,~ × 100 ,~) of a mechanically polished and annealed F%O4(001) surface obtained with an electrochemically etched W tip showing steps of 4.2+0.1 A height. (Tunneling current: I = 1 nA, sample bias voltage: U = + 1.0 V.)

R. Wiesendanger et al. / Recent adL,ances in spin-polarized scanning tunneling microscopy

341

3.2. Si(O01)2 x 1

L I ". f i'

Fig. 2. (a) Atomic-resolution STM image (90 , ~ x 100 ,~) of the Fe304(001) surface obtained with a ferromagnetic Fe tip. The unit cell periodicity of 8.4 A is clearly observed as can be seen from a corresponding line section (b). A one-dimensional superstructure with a 30 ,~ period is also visible in (a). ( I = 1 nA, U = +3.0 V.)

sample bias voltage of + 3.0 V by using a ferromagnetic Fe tip is remarkably high indicating that additional effects such as spin-polarized tunneling a n d / o r magnetic exchange forces may contribute to the image contrast. The long-ranged magnetic dipole forces cannot contribute to an atomic-scale corrugation. The results obtained with the ferromagnetic Fe tip on the ferromagnetic Fe304(001) surface will be described and discussed in more detail elsewhere [13].

In contrast to the Fe304(001) surface, the Si(001)2 × 1 surface is well known to surface scientists and has already been studied with STM by many research groups. It was found that the (2 x 1) unit cell of the dimer-type reconstructed surface appears different at opposite polarities. At negative sample bias, the (2 × 1) unit cell appears to consist of a single bean-shaped protrusion whereas at positive sample bias, two well-resolved protrusions are observed in each unit cell separated by a deep depression. It was also found that surface defects may play an important role for the appearance of the dimers in STM images. In defect-free regions of the surface the dimers were found to appear symmetric whereas near defects and steps the dimers often appear asymmetric. For a more detailed description and discussion of these STM observations on the Si(001)2 x 1 surface, the reader is referred to the literature [14-17]. Recently, the appearance of the dimers on the Si(001)2 × 1 surface in STM images has caused many discussions because discrepancies to experimental data obtained by other surface analytical techniques as well as to some electronic structure calculations have been recognized. Interestingly, a first spin-resolved non-parametrized calculation of the electronic structure of the Si(001)2 x 1 surface has become available [18] indicating that spin correlations are of great importance for this surface. The spin arrangement within the dimers has been found to be antiferromagnetic for both symmetric and asymmetric dimer models, i.e. the spin orientation is opposite for the two dangling bonds of the dimer. Therefore, the Si(001) surface is expected to be an ideal test surface for SPSTM studies on an atomic scale provided that individual dimers can be resolved both with nonmagnetic as well as with magnetic sensor tips. Consequently, we have started with a detailed investigation of the Si(001)2 x 1 surface by using non-magnetic W tips as well as magnetic tips aiming at the investigation of the degree of dimer asymmetry as a function of tip material. In fig. 3 we present STM images of the Si(001)2 x 1 surface obtained with a non-magnetic W tip indicat-

342

R. Wiesendanger et al. / Recent adt,ances in spin-polarized scanning tunneling microscopy

ing that the degree of dimer asymmetry can change drastically even for a given sample, a g i v e n b i a s v o l t a g e , a g i v e n tip m a t e r i a l a n d in surface areas with a given density of surface

d e f e c t s . O u r S T M d a t a i n d i c a t e t h a t t h e tip s t r u c ture might be very important for the appearance of the dimers. This dependence might result from the influence of the tip electronic structure which

Fig. 3. (a) Topographic STM image of the Si(001)2 × 1 surface obtained with a W tip showing dimer rows with almost symmetrical dimers. The measured corrugation is 0.3 A perpendicular and 0.05 ,~ along the dimer rows. ( I = 1 nA, U = + 1.5 V.) (b) Topographic STM image of the Si(001)2× 1 surface obtained with a W tip showing dimer rows with strongly asymmetric dimers. The measured corrugation is 0.2 ,~ perpendicular and 0.05 A along the dimer rows. (I - 1 nA, U = + 1.5 V.)

R. Wiesendanger et al. / Recent adcances in spin-polarized scanning tunneling microscopy

343

Future STM studies of the Si(001)2 × 1 surface will be performed with other ferromagnetic tips such as CrO 2 and Fe304 where a higher degree of spin-polarization compared with Fe is expected.

4. Outlook Besides the two test surfaces for SPSTM studies discussed above, the Fe304(001) and the Si(001)2 × 1 surfaces, several other magnetic samples are highly promising for SPSTM studies. A detailed overview of appropriate test samples for SPSTM experiments has recently been given [19]. We consider the choice of appropriate test samples as highly important for fundamental studies of spin-polarized tunneling which will determine the fields of application of the novel SPSTM technique.

Acknowledgments

Fig. 4. (a) STM image of the Si(001)2× 1 surface obtained with a ferromagnetic Fe tip. The dimer rows seen on the middle terrace show a varying degree of asymmetry as seen from a line section perpendicular to the dimer rows crossing a single dimer vacancy-type defect (b). The dimer rows are not visible on the upper and lower terraces due to the limited dynamic range of the grey scale. ( 1 = 1 hA, U = + 1.5 V.)

can change drastically if an atom is picked up from the surface or has left the foremost end of the tip. Very recently, atomic-resolution STM images of the Si(001)2 × 1 surface have also been obtained with in-situ prepared ferromagnetic Fe tips (fig. 4a). The initial experimental results do not indicate a strong dimer asymmetry (fig. 4b).

We would like to thank Prof. S. Gr~iser for kindly providing our best natural single crystals of magnetite as well as Wacker-Chemitronic, Burghausen, for providing silicon wafers. We also thank Prof. R.A. de Groot, Prof. G. Giintherodt, Prof. A. Hubert, Dr. V. Hoffmann, and Prof. J. Pollmann for useful discussions. Financial support from the Swiss National Science Foundation is gratefully acknowledged.

References [1] R. Wiesendanger, H.-J. Giintherodt, G. Giintherodt, R.J. Gambino and R. Ruf, Phys. Rev. Lett. 65 (1990) 247. [2] R.A. de Groot, F.M. Mueller, P.G. van Engen and K.H.J. Buschow, Phys. Rev. Lett. 50 (1983) 2024. [3] R. Wiesendanger, D. Burgler, G. Tarrach, A. Wadas, D. Brodbeck, H.-J. Giintherodt, G. Giintherodt, R.J. Gambino and R. Ruf, J. Vac. Sci. Technol. B 9 (1991) 519. [4] A.A. Minakov and I.V. Shvets, Surf. Sci. Lett. 236 (1990) L377. [5] K.P. K~imper, W. Schmin, G. Giintherodt, R.J. Gambino and R. Ruf, Phys. Rev. Lett. 59 (1987) 2788. [6] R. Wiesendanger, G. Tarrach, D. Biirgler, T. Jung, L. Eng and H.-J. Giintherodt, Vacuum 41 (1990) 386

344

R. Wiesendanger et al. / Recent adt:ances in spin-polarized scanning tunneling microscopy

[7] R. Wiesendanger, D. Biirgler, G. Tarrach, D. Anselmetti, H.R. Hidber and H.-J. Gfintherodt, J. Vac. Sci. Technol. A 8 (1990) 339. [8] R. Wiesendanger, D. Biirgler, G. Tarrach, T. Schaub, U. Hartmann, N.J. Giintherodt, I.V. Shvets and J.M.D. Coey, Appl. Phys. A 53 (1991) 349. [9] R. Wiesendanger, D. Bfirgler, G. Tarrach, I.V. Shvets and H.-J. Gfintherodt, in: Mater. Res. Soc. Syrup. Proc. of the MRS'91 Spring Meeting, Anaheim, CA, USA, in press. [10] A. Yanase and K. Siratori, J. Phys. Soc. Jpn. 53 (1984) 312. [11] R. Aragon, D.J. Buttrey, J.P. Shepherd and J.M. Honig, Phys. Rev. B 31 (1985) 430. [12] R. Aragon, R.J. Rasmussen, J.P. Shepherd, J.W. Koenitzer and J.M. Honig, J. Magn. Magn. Mater. 54-57 (1986) 1335.

[13] R. Wiesendanger, I.V. Shvets, D. Biirgler, G. Tarrach, H,-J. Gfintherodt and J,M.D. Coey, submitted. [14] R.M. Tromp, R.J. Hamers and J.E. Demuth, Phys. Rev. Lett. 55 (1985) 1303. [15] R.J. Hamers, R.M. Tromp and J.E. Demuth, Phys. Rev. B 34 (1986) 5343. [16] R.J. Hamers, Ph. Avouris and F. Bozso, Phys. Rev. Lett. 59 (1987) 2071. [17] R.J. Hamers and U.K. Koehler, J. Vac. Sci. Technol. A 7 (1989) 2854. [1~8] E. Artacho and F. Yndurain, Phys. Rev. B 42 (1990) 11310. [19] I.V. Shvets, R. Wiesendanger, D. Biirgler, G. Tarrach, H.-J. Giintherodt and J.M.D. Coey, submitted.