Spin-density enhancement in a
119
Sn implanted „110…Cr single crystal as evidenced
by Mo
¨
ssbauer spectroscopy
S. M. Dubiel,* J. Cies
´
lak, and J. Z
˙
ukrowski
Faculty of Physics and Nuclear Techniques, The University of Mining and Metallurgy (AGH), aleja Mickiewicza 30,
PL-30-059 Krako ´w, Poland
H. Reuther
Institut fuer Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf, Postfach 510119, D-01314 Dresden, Germany
Received 22 November 2000; published 19 January 2001
Magnetic properties of a presurface zone of the bulk, single-crystal 110Cr, implanted with
119
Sn ions were
studied by means of conversion electron Mo ¨ssbauer spectroscopy. A strong enhancement of the magnetic
hyperfine field, B, was found. The increase is by a factor of 2.7 in the average value of B and by a factor of
2.3 in the most probable value of B the amplitude of the spin-density waves, SDW’s. The observed effects
are explained in terms of the interference of two spin-density waves having the same amplitude phase but
various polarizations. The relative contributions of the interfering SDW’s was estimated as equal to 60% for
those with mutual perpendicular and 40% for those with mutual parallel polarization.
DOI: 10.1103/PhysRevB.63.060406 PACS numbers: 75.30.Fv, 75.50.Ee, 76.80.+y
The antiferromagnetism of metallic chromium has been
attracting great scientific interest since the discovery that it is
constituted by incommensurate spin-density waves
I-SDW’s.
1–3
The importance of SDW’s stems mainly from
the fact that they are related to the density of electrons at the
Fermi surface and its topology. SDW’s set in as linearly
polarized I-SDW structure at the Ne
´
el temperature which for
bulk chromium is equal to 313 K.
4
The structure consists
of a sinusoidal modulation of the magnetic moments, :
r =
0
• sin Q • r , 1
where r is the position vector,
0
is the amplitude of the
SDW equal to 0.6
B
at 4.2 K Ref. 5 and Q is the wave
vector, given by:
Q = 2 / a -2 / • 001 , 2
where a is the lattice constant and is the periodicity of the
SDW’s. The latter is a continuous function of temperature
with a value of 7.8 nm at RT. SDW’s can exist along any
of the 001 crystallographic axes, i.e., three different do-
mains can coexist in an equilibrium state so-called 3 Q
state. The SDW’s are polarized transversely TSDW’s
down to the temperature of 123 K where the polarization
changes to longitudinal LSDW’s. The transition, known as
the spin-flip, is a first-order one with a wide hysteresis.
6
Theoretical calculations predict that the surface properties
of chromium are different. In particular, the magnetic mo-
ment should be enhanced in comparison to its bulk value.
According to the tight binding calculations of Allan, the
001Cr surface has a moment of 2.8
B
.
7
Self-consistent
tight-binding calculations by Victora and Falicov predict the
value of 3
B
Ref. 8 whereas ab initio full-potential linear-
ized augmented-plane-wave FLAPW calculations by Fu
and Freeman give the value of 2.5
B
for the top layer of
bulk chromium.
9
Enhanced magnetic moment was also pre-
dicted for a 001Cr monolayer on substrates other than
Cr,
10,11
as well as for Cr monolayers sandwiched between Fe
on one side and a vacuum on the other.
12
Experimental evidence in favor of such enhancement was
recently obtained from studying epitaxial Cr/Sn multilayers
by
119
Sn Mo
¨
ssbauer spectroscopy.
13
The authors found a
magnetic hyperfine hf field of 1113 T which is twice as
much as in a single-crystal bulk Cr.
14
However, there is still
no evidence that such enhancement exists on the surface or
in the presurface zone of bulk Cr.
To obtain this kind of information 100 m thick foil of
a single-crystal 110Cr was implanted at 65 keV with the
dose of 10
16 119
Sn ions per cm
2
. According to the TRIM
code the mean projected range for the ions is 15.6 nm with a
longitudinal straggling of 5.8 nm. This corresponds to 2
at RT.
As a reference sample, a 100 m thick single-crystal
110Cr foil doped with 0.1 at. %
119
Sn by diffusion was
used. Two Mo
¨
ssbauer spectra were recorded at RT on this
sample: one in the transmission mode using a standard spec-
trometer and a scintillation detector for the -rays supplied
by a Ca
119m
SnO
3
source, and another one in conversion elec-
trons mode CEMS using a gas-flow detector. The spectra,
which were fitted in terms of the hf field distribution method
described in details elsewhere,
15
assuming the value of 1.0
mm/s for the full linewidth at half maximum FWHM. The
line intensities ratio was fixed at the value of 3:2:1 for the
spectrum recorded in the transmission mode and at 3:4:1 for
those recorded in the CEMS mode. The corresponding his-
tograms of the hf field distribution, P ( B ), can be seen in Fig.
1. As can be noticed there is some small difference in the
shape of the two spectra. The one recorded in the transmis-
sion mode is characteristic of the I-SDW structure;
14
it is
symmetric and has a well-defined dip in the central part. On
the other hand, the spectrum obtained in the CEMS mode is
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