Original
Paper
phys. stat. sol. (b) 242, No. 9, 1775 – 1778 (2005) / DOI 10.1002/pssb.200461714
Spin magnetic moments from single atoms to small Cr clusters
C. Boeglin*, 1, P. Ohresser2, R. Decker1, H. Bulou1, F. Scheurer1, I. Chado1, S. S. Dhesi**, 3,
E. Gaudry4, and B. Lazarovits5
1
2
3
4
5
IPCMS-GSI – UMR 7504, 67037 Strasbourg Cedex, France
LURE, 91405 Orsay, France
ESRF, BP 220, 38043 Grenoble Cedex, France
LMCP, 4, place Jussieu, 75252 Paris, France
CCMS, T.U. Vienna, Gumpendorfstr. 1a, 1060 Wien, Austria
Received 11 October 2004, revised 24 March 2005, accepted 24 March 2005
Published online 17 June 2005
PACS 75.70.–i, 81.16.Dn, 81.16.Ta
Morphology studies at the first stages of the growth of Cr/Au(111) are reported and compared to the magnetic properties of the nanostructures. We analyze by Scanning Tunneling Microscopy and Low Energy
Electron Diffraction the Cr clusters growth between 200 K and 300 K. In the early stages of the growth
the morphology of the clusters shows monoatomic high islands located at the kinks of the herringbone reconstructed Au(111) surface. By X-ray Magnetic Circular Dichroism performed on the Cr L2, 3 edges it is
shown that the temperature dependent morphology strongly influences the magnetic properties of the Cr
clusters. We show that in the sub-monolayer regime Cr clusters are antiferromagnetic and paramagnetic
when the size reaches the atomic limit.
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction
The constant demand for higher storage density in the magnetic device industry has resulted in an increasing interest in systems with reduced dimensionality, such as ultra-thin magnetic films, and more
recently, wires and dots. For application purposes, strong magnetization, and long-range magnetic order
at room temperature are needed. From a more fundamental point of view, interesting metastable magnetic states with controllable anisotropy are sought by adjusting the structure and/or the morphology
(dimensionality) of the objects [1]. Until now experimental work has been essentially dedicated on
nanostructures of ferromagnetic materials [2 –4]. However, antiferromagnetic materials have great potential as they may show new magnetic properties once they form structures of nanometer size. Among
them, Cr is a particularly interesting material. Not only are Cr clusters predicted to show a non-zero net
magnetic moment but also the magnetic properties of Cr show highly atomic structure dependent properties [5, 6]. For instance, a stretched Cr in a hexagonal symmetry is predicted to become ferromagnetic
[7, 8]. A (111) gold surface is a priori a good candidate to seek new magnetic Cr phases. Its reconstruction allows the growth of self-organized clusters of easily adjustable size for a number of metals (e.g. Fe
[9], Co [10], Ni [11, 12]). In the case of Cr, a similar growth is expected on the basis of simple surface
energy considerations [12], but was not yet verified experimentally. This would allow a fine tuning of
Corresponding author: e-mail: christine.boeglin@ipcms.u-strasbg.fr, Phone: +00 33 (0)3 88 10 70 28,
Fax: +00 33 (0)3 88 10 72 48
**
Present permanent address: Diamond Light Source, Chilton, Didcot OX11 0QX, U.K.
*
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1776
C. Boeglin et al.: Spin magnetic moments from single atoms to small Cr clusters
the dimensionality, from clusters (0D) to chains (1D) and films (2D) by simply increasing the coverage.
The other advantage of the Au(111) surface is that it constitutes a hexagonal template with an expanded
lattice parameter (2.88 Å) compared to the bulk Cr bcc nearest neighbor distance (2.48 Å). Consequently, a pseudomorphic growth (as observed for Fe/Au(111) [9]) could lead to a ferromagnetic Cr
phase.
In this context we have studied by means of scanning tunneling microscopy (STM) and X-ray magnetic circular dichroism (XMCD) the morphology and the magnetic properties of Cr dots on the Au(111)
herringbone surface reconstruction. We use the temperature dependent nucleation and growth to promote
either self-organized clusters nucleated at the kinks of the herringbone reconstruction, or a collection of
single atoms. These later are obtained by evaporation of minute amount of Cr at low temperature. By
STM we established that below 0.3 ML the room temperature growth of Cr leads to monoatomic high
islands aligned along the periodic reconstruction lines without connections between nearest neighbor
islands. These clusters contain up to several hundreds of Cr atoms per cluster. The 1D coalescence of the
Cr into monolayer high stripes is obtained for 0.75 ML. Using XMCD [13, 14] the magnetic moment is
determined from isolated Cr adatoms to clusters up to several hundred atoms.
2
Experiment
The experiments were carried out in two independent ultra-high vacuum (UHV) systems. The growth
characterization was done in a chamber equipped with STM, Low Energy Electron Diffraction (LEED)
and Auger Electron Spectroscopy (AES) allowing evaporation from 200 to 300 K. The XMCD experiment was carried out at beamline ID08 of the European Synchrotron Radiation Facility in Grenoble. The
magnetic characterization and the thin film growth were performed in situ by XMCD in an ultrahighvacuum chamber (5 × 10–11 mbar) containing a superconducting magnet and the Cr evaporation cell. Cr
was evaporated onto the clean Au(111) reconstructed surface between 10 K and 300 K. The X-ray absorption spectra (XAS) at the Cr L2, 3 edges were recorded in the total electron yield mode using circularly polarized light (P) with 99% polarization in a magnetic field of ±7 T. The applied magnetic field is
aligned with the photon propagation vector. The XMCD signal was recorded by switching both P and the
sample magnetization. The Au(111) single crystal was prepared by cycles of Ar+ at 300 K and annealing
at 1000 K. The pressure measured during evaporation was 1.0 × 10–10 mbar. Residual gas contamination
was always lower than that detectable by AES or O K-edge spectra recorded before and after evaporation.
3
Results
STM shows that Cr grown at room temperature nucleates preferentially on the elbows of the Au(111)
herringbone reconstruction as predicted [12]. In Fig. 1 we represent an STM overview of the Cr cluster
morphology during the growth of the first atomic monolayer. One notices that for a coverage of 0.07 ML
(monolayer) the clusters are regularly distributed along the still visible Au(111) reconstructed surface
(Fig. 1a). The Cr clusters begin to coalesce into monolayer-high stripes at 0.5 ML (Fig. 1b) whereas the
stripes are completely connected at 0.75 ML (Fig. 1c). Following the morphology with the coverage
several observations can be made. Below ~1 ML the growth is very close to a layer-by-layer growth,
only few second layer nuclei on top of the islands can be distinguished above 1 ML (Fig. 1d). The island
height is approximately 2 Å. This distance corresponds to the interplanar distance between [110] planes
for the bulk bcc Cr. The LEED study did not allow further structure determination since the diffraction
patterns shows large background and blurring of the LEED spots upon Cr deposition (the diffraction
spots disappear above 10 ML). This is characteristic of a poor long-range order. It seems difficult to
stretch the Cr lattice: Rather than pseudomorphic, the structure of the clusters is probably close to a distorted [110] lattice.
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Original
Paper
phys. stat. sol. (b) 242, No. 9 (2005) / www.pss-b.com
a
b
c
d
1777
Fig. 1 STM topographic images 110 × 110 nm2
obtained for (a) 0.07 ML, (b) 0.5 ML, (c) 0.75 ML
and (d) 1.5 ML Cr/Au(111) grown at room temperature. All images are obtained with a tunnel current of
0.15 nA and a bias voltage of 0.2 V.
If we take a closer look to the STM images one can see that below the 1D coalescence, around
0.3 ML, the islands are well separated but their shape is quite round and irregular (Fig. 1a, 1b), unlike the
Fe/Au(111) where the geometrical island shape is governed by the fcc structure [9]. After the 1D coalescence the clusters broaden keeping their irregular shape until the 2D percolation takes place around
0.9 ML. One can still observe that the connections between the lines are really perpendicular to the rows
accrediting the bcc(110) symmetry. For higher coverage the layer-by-layer growth mode turns into a
rougher growth mode where the typical structure size is ~10 Å. This morphology coincides with the
complete disappearance of the LEED pattern. The AES signal of the Au(111) substrate vanishes completely after 10 ML deposition. Cr deposition in the 200 –300 K range shows basically the same kind of
the morphology evolution. This allows us to conclude that interdiffusion, if any, remains weak during the
growth of Cr on Au(111).
The XMCD measurements are done at the Cr L2, 3 absorption edges. Strong spin – orbit coupling in the
core shell leads to an XAS signal which depends on the relative alignment of photon spin and sample
magnetization. The XMCD spectrum is the difference between the two XAS spectra recorded with opposite orientation of the magnetic field with respect to the helicity of the light. The magnetic properties
observed for films grown at 300 K are characteristic of antiferromagnetic behaviors. Conversely, a col-
Intensity (arb. units)
1.00
0.004 ML Cr @ 10 K
0.98
0.96
0.94
Fig. 2 (online colour at: www.pss-b.com) XAS
and XMCD spectra for 0.004 ML of Cr grown and
measured at 10 K. The applied field was 7 T. The
dashed curve corresponds to the theoretical XMCD
spectrum obtained by atomic multiplet calculation
for Cr.
0.00
XMCD
atomic Cr
570
580
590
600
Energy (eV)
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1778
C. Boeglin et al.: Spin magnetic moments from single atoms to small Cr clusters
lection of single adatoms deposited randomly at 10 K, leads to paramagnetic Cr with a magnetic moment
close to the one expected for a free Cr ion as shown by the absorption spectrum obtained for 0.004 ML
with an applied magnetic field of 7 T (Fig. 2 top lines), the corresponding XMCD is shown along with a
theoretical spectrum obtained by multiplet calculation for a d5 electronic configuration. The fine structures observed in the spectra are characteristics of atom-like electronic structure. XMCD magnetization
curves (not shown) together with the application of XMCD sum rules [13, 14] allow to determine the
magnetic moments via a Brillouin function fit. For these single adatoms on Au(111) one finds magnetic
eff
moment of mspin
= 4.5 ± 0.4µB/at and an orbital moment close to zero.
The number of 3d holes used to determine the spin and orbital moments are taken from fully relativistic band structure calculations: Nh = 5.4 holes per atom. The average spin moment is rapidly decreasing
as a function of cluster size, indicating antiferromagnetic coupling even in clusters containing few atoms.
4
Conclusion
The Cr is found to have a preferential nucleation at the kinks of the Au(111) reconstruction. Below the
1D coalescence, it forms ML high islands with a narrow size distribution. We demonstrate that individual Cr adatoms obtained by deposition at low temperature have the expected magnetic behaviour. They
are paramagnetic and show large magnetic spin moment (4.5 ± 0.4µB/at). Due to surface diffusion these
atoms form clusters of few atoms for higher temperature of growth. Such clusters are found to be antiferromagnetic, no hint of ferromagnetism or net moment was found.
The authors would like to express many thanks to the ID08 staff for their technical support.
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© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim