XAFS of Mn multilayers on Ru(0 0 1)

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Physica B 208&209 (1995) 469-470

XAFS of Mn multilayers on Ru(001) T.K. Sham a, M.L. Shek b, M. Kuhn c, J.

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aDepartment of Chemistry, University of Western Ontario, London, Ont., Canada N6A 5B7 bNSLS, Brookhaven National Laboratory, Upton, NY 11973-5000, USA c Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000, USA

Abstract

Mn K-edge XAFS together with LEED and thermal desorption has been used to investigate a series of Mn multilayers deposited on a Ru(00 1) substrate under UHV conditions. It is found that a meta-stable phase with a (1 x 1) LEED pattern can be prepared at thin as well as thick layers. The XAFS of such a system appears to exhibit an intense HCP-like EXAFS pattern indicating that the as-deposited Mn overlayer is pseudomorphic with the substrate. Upon annealing, the XAFS of the meta-stable phase collapses to a pattern similar to that of the bulk Mn.

The structure and electronic properties of magnetic overlayers on single crystal metal surfaces has been an interesting subject for many recent studies [1]. Of particular concern to us has been the system of Mn on Ru(00 1) in connection with its coverage and morphology dependent electronic structure [2, 3] as well as its chemical and magnetic properties revealed by LEED, thermal desorption and photoemission studies I-4, 5-1. It has been found that Mn overlayers on Ru(00 1) exhibit a (1 x 1) LEED pattern from thin (several monolayer, ML) to thick multilayer ( > 100 ML) coverages under certain conditions [2, 3, 5-1, indicating that the structure of the overlayers is much simpler than the structure of bulk Mn. Despite these results, local structural information for Mn on Ru is still very limited [2, 3, 6]. In this paper, we report our preliminary XAFS results for the structure of Mn multilayers on Ru(00 1). Mn overlayer on Ru(001) was prepared in situ in a UHV chamber at the X-23B beamline of the NSLS X-ray ring (2.5 GeV, 150mA), which is equipped with Si(1 1 1) double-crystal monochromator and a pre-mirror with a cutoff at 10 keV. This chamber was outfitted with * Corresponding author.

a beryllium window at the photon entrance port, a quadrupole mass spectrometer, LEED optics and a channeltron electron multiplier, for total electron yield XAFS measurements. No significant Bragg peak was detected in this arrangement. The XAFS data displayed herein are the sum of multiple scans and have been normalized to the beam intensity. Fig. 1 shows the Mn K-edge XAFS of the as-deposited overlayers as a function of coverage. It can be seen that the XAFS are essentially identical; that is that they all exhibit a simple sinusoidal oscillation with an amplitude of > 10% relative to the edge jump. The only difference is the first peak at ~ 6540 eV which is well developed at 100 ML (curve c) but not at smaller overlayer coverages (curves a and b). This observation probably indicates the lack of long range order normal to the surface in the latter. In the following, we will focus on the result of the 100 ML sample, which has better statistics. The XAFS of the as-deposited thick overlayer (Fig. l(c)) changes after annealing at 700 K (Fig. l(d)); a Mn metal spectrum (Fig. l(e)) recorded in transmission is also shown for comparison. The most noticeable change is seen in the region of 6600-6680 eV where the single sinusoidal curve of the as-deposited sample has

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T.K. Sham et al. /Physica B 208&209 (1995) 469-470

Mn K-edge

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split into two peaks after annealing indicating a phase transition has occurred. Similar behavior is also seen for the two smaller coverages. This observation immediately suggests the presence of a Mn environment with welldefined multiple shells (long range order) in the overlayers after annealing. A comparison of the Mn metal and the annealed Mn multilayer reveals gross similarities although the latter exhibits a larger amplitude of oscillations. The sinusoidal EXAFS for the as-deposited multilayer clearly indicates a simpler Mn local structure than that of the bulk. This may be due to either a Mn structure of which all the nearest neighbors have very similar bond length with little long range order or HCP-like Mn domains grown on the H C P Ru substrate. The as-deposited Mn overlayer X A F S are arguably very similar to that of H C P Co of which the first four wiggles appear sinusoidal without any noticeable beating. The k 2 weighted E X A F S of curves c, d and e in Fig. 1 are shown in Fig. 2. We see a single sinusoidal oscillation for the as-deposited sample (c), while strong beating of waves with different frequency is observed in the region of k = 4.0-5.8/~- 1 for the annealed sample (d) and Mn

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Fig. 1. Mn K-edge XAFS of Mn overlayers supported on a Ru(001) crystal. Curve (a) 5 monolayers (ML); (b) 15 ML: (c) 100 ML; (d) 100 ML annealed to 700 K and (e) Mn metal in transmission.

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Fig. 2. k 2 weighted EXAFS of curves (c), (d) and (e) from Fig. 1. metal (e). The FT shows a single peak for the as-deposited sample; signs for long range order (multiple shells) are apparent in the annealed sample as well as Mn metal. Quantitative E X A F S analysis which is necessary for understanding the structure of these films is presently in progress. Part of this research was carried out at BNL under contract DE-AC02-76CH00016 with the US Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. Work at the U W O is supported by O C M R and N S E R C of Canada.

References

[1] B. Heinrich and J.F. Cochran, Adv. Phys. 42 (1993) 523 and references therein. [2] B. Heinrich, C. Liu and A.S. Arrott, J. Vac. Sci. Technol. B 3 (1985) 766. [3] A.S. Arrott, B. Heinfich, C. Liu and S.T. Purcell, J. Magn. Magn. Mater. 54 57 (1986) 1025. [4] J. Hrbek, T.K. Sham and M.L. Shek, Surf. Sci. 191 (1987) L772. [5] M.L. Shek, T.K. Sham and J. Hrbek, unpublished. [6] V. Dupuis, M. Maurer, M. Piecuch, M.F. Ravet, J. Dekoster, S. Andrieu, J.F. Bobo, F. Baudelet, P. Bauer and A. Fontaine, Phys. Rev. B 48 (1993) 5585.