Bertschat, H H

Forkel-Wirth, Doris

Granzer, H

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;>1(.%%!procedures are described. OCR Outputcomplete these experiments, several runs at ISOLDE are requested and the experimentalsurfaces and on magnetic multilayer systems are obtained and briefly reported. In order tointeractions were proposed in our ISOLDE experiment IS 318. First results on siliconMonolayer—resolved measurements at interfaces by means of magnetic and electric hyperfineSUMMARY1995BerlinContact person: D. Forkel-WirthSpokesperson: H.H. BertschatG. Schatzd, G. Weyeifl, and W.—D. ZeitzlH.H. Bertschatl, D. Forkel—Wirth2, H. Granzerl, H. Haasl, J. Lohmiillerd,1Berlin—CERN 2-Konstanz3—Arhus4 Collaboration95-r,·g;_;igy._)» 1%,; RADIOACTIVE IONS‘SURFACE AND INTERFACE STUDIES WITH, _ . C/Illx lRequest ..,....... .... ne for IS 318 `mmapmL>IONE lllllllllllllllllllllll|IIll||||l||l||l|||| 19.01.1995CERN 1.1BRAR1Es, GENEVAISC/P29 Add- 1JU? "CERN/ISC 95-4with beam time requests. OCR Outputbrief status report on the results achieved so far, and finally we propose further experimentsWe describe the specific motivation for the particular experiments in progress, then we give adegree and were thus the cause for severe problems.contaminations by accompanying elements could often not be avoided to a comparablegenerally be detected. In laboratory experiments (University Konstanz or HM1 Berlin)the samples are loaded with radioactive probe atoms, no additional contamination canessential for all investigations on surfaces and interfaces, especially for semiconductors. AfterISOLDE mass separator can be deposited with extreme cleanliness on surfaces. This isIn our UHV chamber ASPIC, ca1rier—free radioisotopes provided from the BOOSTERsurfaces, ad 2) on ferromagnetic multilayer systems.During the first year (1994), exploratory experiments were performed ad 1) on semiconductor2) metallic multilayer systems.1) semiconductor—metal contacts,materials:model systems in order to develop a better understanding of the properties of two classes ofobtain infomation on structural and magnetic properties at microscopic scales for severalThe experiment IS 318 "Surface and interface studies with radioactive ions" was started t0INTRODUCTIONFig. 1: PAC time spectmfor HBr Oil Si(100)2.r1 smfacefor two rlgjferent rzmteczling temperatures. OCR Outputtime (ns)O 10 20 30 40 50'O.4i-1-0.20.0 l . lll . .*.1 YJMJ1: Lid. llll ll ~ll.ll¤lin illlll ll lllllll {xTA:420K0.2-0.4-0.2llll’· l l 0.0 ln my { id JL Jlil l. ll= K TA 2400.2assignment). Thermal desorption occurs at 650 K (see Fig. 2b).diffusion along these rows to missing dimer sites occurs ending up at step sites (tentativeoccupy sites at the dimer rows, characteristic for this reconstructed surface. Above RTdiffusive displacements could be explained. At lower temperatures, the bromine atomsinterest for technical applications [l]. With the help of theoretical models [2], the differentmeasurements yield detailed information about the silicon-bromine system, which is also ofof another one with *00:482 (7) MHz was observed around 320 K (see Fig. 2a). Theseposition at the surface. With increasing temperature, a depopulation of this position in favor120:526 (7) MHz was measured (see Fig. l), indicating the occupancy of a well defineddecaying to the PAC nucleus "Se, a single electric quadrupole interaction frequency ofA Isolated "Br atoms were adsorbed on Si(l0O)2xl surfaces at a temperature of 90 K. With "BrThe halogen probe "Br (half—life 57 h) was obtained either from a "Kr beam or a "Rb beam.1.1 Status of Experiments with "Br on Si(100)2x1 surfacessurfaces as a function of temperature.we have started to study the adsorption sites of various probe atoms on semiconductorabout the position of the probe atom on a clean surface is a necessary prerequisite. Therefore,semiconductor surface covered with a metal can be successfully investigated, knowledgesince this method is most suitable to provide infomation exactly from the interface. Before aOur final aim in this project is to study semiconductor-metal contacts with radioactive ions,1 Semiconductor Surfacespromising alternative to investigate isolated adatoms. OCR Outputon very clean surfaces with the probe nucleus l`”“Cd on the Si(lOO)2xl surface offer aand chlorine contaminations caused during the deposition. At ISOLDE, investigationsthe Si(lO0)2xl surface were problematic until now. This is probably due to carbonusing the isotope In(Cd) [3, 4], but diffusion experiments on isolated probe atoms onmThe system In on Si—surface has been subject of successful experimental PAC studiesexperiments. Here we plan a complete temperature dependent measurement.Br atoms at two distinct sites have already been observed in our preliminaryAt a Si(1l l) surface with the well known 7x7 reconstruction signals characteristic foroffered to the probe atoms.crystals where stepped surfaces are obtained. Thus preferential surface positions areIn order t0 verify the above mentioned findings, we plan experiments on vicinally cutFuture experiments on silicon surfaces1.2I2) Fraction of desorbed "Br probe atoms as a function of annealing temperature.jimction of annealing temperature. The solid lines are guides to the eye.Fig. 2: a) Fractions ja and ]Q of probe atoms exposed t0 two dyyerent field gradients as aTemperature 00Temperature (K)100 200 300 400 500 600 200 400 500 000T T2010*6 4020e 60SOL fr4080soia)100ferromagnetic crystals combined interaction OCR Outputshowing several fractions. This is expected, because close to the surface ofmeasurements with `°”’Pd/W’Rh [7], we did observe spin precession frequencies, usuallysurfaces were covered with one or several monolayers of Pd. In contrast to our earlierConsiderable progress has been achieved in the present experiment series when the Niearlier data [9, 10].quadrupole interaction measurements in the Pd surface essentially confirmed thementioned above. Our magnetic measurements in the Ni surface and electricof these results, we have used `"'"Cd from ISOLDE, without any contaminations asmeasured earlier in laboratory experiments using ln [9]. In order to take advantagema) Magnetic hyperfine fields at impurities incorporated in the Ni surface have been2.1 Status of Ni-Pd experimentseasily available at ISOLDE.these experiments with nonmagnetic probe atoms such as Cd or Se, and Cd and Se arem77larger lattice constant, the frequencies are not resolved. It therefore seems advisable to repeatsystems. If the magnetic field exceeds considerably this value in polarized Pd due to theprecession frequencies just at the limit of time resolution of conventional PAC detectormagnetic hyperfine field of Rh in Ni has been measured to be 20.2T at RT [8] leading to"”assumption of very strong magnetic hyperfine fields at the Rh nuclei in polarized Pd. The"`"precession of the 4d element probe Rh could be observed. One possible explanation is the'°°However, no detailed study of the depth dependence was possible since no resolved spinthe PAC probe Pd/Rh and found a magnetic interaction of a surprisingly long range [7].'m’°°In a laboratory experiment at HMI Berlin we had started to measure the Pd polarization withthe interface in dependence with the number of Pd monolayers grown on it.from the Ni crystal as well as to study the magnetic properties within the Ni crystal close tocase the magnetic hyperfine field at the probe nuclei) in Pd in dependence of the distanceregion close to the interface. It is of great importance to study the magnetic properties (in ourmeasurements are necessary for the quantitative determination of the magnetic profile in thea ferromagnetic metal. Ni—Pd is a most interesting model system, where monolayer-resolvedpredicted [5] and experimentally confirmed [6], that Pd polarizes magnetically in contact withPd single crystals covered with a few monolayers of Pd or Ni, respectively. It is theoreticallyNi-Pd-systems as mentioned in our original proposal. In simple cases, they consist of Ni orOut of the huge amount of magnetic multilayer systems of interest, we have concentrated on2) Eerromazuetic multilaver svstemsFig. 3.· PAC spectrum of"’"'Cd/I/[Cd on Ni(1]1) covered with 3 monolayers ofPd at 90 K. OCR Outputti me (ns)100 200I Grad/sec0,,-0.15 I- "FilData-0.10 kl °°“53 -0.05¤»¤¤> +3 tt 1 1Ig. $ 1..,. ,•£°•§°% rs';. ‘ 1 -% - *, · Fnh t l - V W J; " “ ` M `il(ml0.05l-l il0.10from these exploratory experimentssuggests a new result for magnetic interactions, a definite conclusion cannot be drawnnor with the known magnetic interaction frequencies. Although the PAC patternThe frequencies obtained cannot be identified with any of the known surface positionsdeposited on it) was covered with three monolayers of Pd, at a temperature of 90 K.Fig. 3 shows an example, where a Ni(l 1 1) surface (after Cd probe atoms had beenc0L = gN uN Bhf/H magnetic dipole interaction.electric quadrupole interaction andwQ = e Q VZZ / 41 (21-Uhwill be observed, consisting ofwi: wi(coQ, mL)Fig. 4: PAC spectrum 0fHBr/ HSE 011 N[(I]]) surface ar 300 K. OCR Outputst R(t) Hill]. Br?`7 T(ness):30°C after ann. 400°C35.0 i ns 70.0iiiuir il_. qtillinteraction is of magnetic or of electric origin.However, from this experiment alone it cannot be concluded to which extent thedifferent temperatures a single interaction frequency was observed (see Fig. 4).change in the hyperfine field value. In our experiment for Br/Se on Ni(ll1) at"77surface. There the most drastic change in coordination number should lead to a drasticIn no case any hyperfine field has yet been measured for impurities on top of aFe, Co, Ni. These values are quite well reproduced within theoretical models [e. g. 1 1].most of the elements of the periodic table as impurities in the ferromagnetic elementsadatoms on top of a ferromagnetic surface. Magnetic hyperfine fields are known forsurfaces with the aim to measure for the first time the magnetic hyperfine field foras a new possibility for surface and interface studies. "Br was positioned on Ni(1ll)b) When Rb 0r Kr beams were produced at ISOLDE, we have used Br/Se probe nuclei777718 shiftsx 3 shifts of . half of 1996 Ni-Pd multilayers OCR Output77Brx 3 shifts of . half of 1996 Si(1 1 1)lllcdx 3 shifts of . half of 1995 Ni—Pd multilayerslllCdx 3 shifts of . half of 1995 Ni surface77Brx 3 shifts of . half of 1995 Si(100)7'/Brx 3 shifts of . half of 1995 Ni—Pd multilayersl1lCd3. Beam-time requestexcluded.the Curie temperature, magnetic contributions to the hyperfine interactions areb) The Br/Se—on—nickel experiment shall be continued to higher temperatures. Above7777magnetically.into a Pd single crystal and Ni layers will be grown on top of it in order to polarize Pdrespective copper one. For a cross—check of these experiments, Cd will be embeddedmmagnetic and electric interactions, the nickel crystal will also be replaced by ain palladium grown on a nickel single crystal is planned. In order to disentanglea) A systematic investigation 0f the depth dependence of the magnetic interaction of Cd2.2 Future experiments on Ni-Pd[1 1] J. Kanamori, H. K. Yoshida, and K. Terakura, Hyp. Int. 9 (1981) 363 OCR Output[10] E. Hunger, H. Haas, Surf. Science 234 (1990) 273and G. Schatz, Phys. Rev. Lett. 64 (1990) 2202[9] J. Voigt, R. Fink, G. Krausch, B. Luckscheiter, R. Platzer, U. Wohrmann, X. L. Ding,J ahresbericht, HMI-B 507 (1992) 112.S. Seeger, R. Kowallik, H.H. Bertschat, W.-D. Zeitz, A. Mayer, and G. Schatz,[8] G. N. Rao, Hyp. Int. 24-26 (1985) 1119.Jahresbericht, HMI-B 520 (1993) 114[7] S. Seeger, H. Granzer, R. Kowallik, H. H. Bertschat, H. Haas, and W.—D. Zeitz,Gudat, Surf. Science 287, 288 (1993) 736[6] O. Rader, C. Carbone, W. Clemens, E. Vescovo, S. Bltigel, W. Eberhardt, and W.Hyp. Int. 78 (1993) 341[5] P. H. Dederichs, P. Lang, K. Willenberg, R. Zeller, N. Papanikolaou, and N. Stefanou,Wohrmann, and G. Schatz, Surface Science 285 (1993) 81[4] G. Krausch, R. Fink, K. Jacobs, B. Luckscheiter, J. Lohmtiller, B.—U Runge, U.Wohrmann and G. Schatz, Europhys. Letters 19 (1992) 611[3] G. Krausch, K. Jacobs, R. Fink, J. Lohmiiller, B. Luckscheiter, B.-U. Runge, U.S. M. Mohapatra, Rev. Sol. State Sci. 4 (1990) 873.[2] Pe-Liu Cao and Ru Hong Zhou, J. Phy. Cond. Matter 5 (1993) 2897;[1] H. F. Winters and J. W. Cobum, Surf. Sci. Rep. 14 (1992) 161References

Surface and interface studies with radioactive ions

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