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INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER
J. Phys.: Condens. Matter 18 (2006) L619–L624 doi:10.1088/0953-8984/18/50/L01
LETTER TO THE EDITOR
Spin polarized tunnelling investigation of nanometre
Co clusters by means of a Ni bulk tip
M V Rastei and J P Bucher
Institut de Physique et Chimie des Mat ´eriaux de Strasbourg, Universit´e Louis Pasteur, UMR 7504,
23 rue du Loess, F-67037 Strasbourg, France
Received 6 November 2006
Published 27 November 2006
Online at stacks.iop.org/JPhysCM/18/L619
Abstract
A massive Ni tip is used in spin polarized scanning tunnelling microscopy
(SP STM) to explore the magnetization state of nanometre Co clusters, self-
organized on the Au(111) surface. Constant current STM images taken at
4.6 K show a bimodal distribution of the cluster heights, accounting for the spin
polarization of the STM junction. The spin polarization of the tunnel junction as
a function of the bias voltage is found to depend on the local density of states of
the sample examined. Changing the vacuum barrier parameters by bringing the
tip closer to the surface leads to a reduction in the tunnelling magnetoresistance
that may be attributed to spin flip effects.
(Some figures in this article are in colour only in the electronic version)
Spin dependent tunnelling in magnetic tunnel junctions has recently been the object of intense
study due to its technological impact in a phenomenon called tunnelling magnetoresistance
(TMR) [1–3]. The magnetic tunnel junction is also at the origin of spin polarized scanning
tunnelling microscopy (SP STM) capable of visualizing the magnetic structure at surfaces [4]
preferably in an ultrahigh vacuum (UHV) environment. As a matter of fact, SP STM has
proven to be a powerful approach for studying the interplay between structural, electronic and
magnetic properties at the nanometre scale. The technique has been used to detect, with a
nanometre resolution, magnetic domain walls in ferromagnetic materials and magnetization
orientations in small metallic islands, and even to follow the magnetization reversal in small
islands on surfaces [5–8].
In this respect, it should be mentioned that vacuum tunnelling is also an ideal tool for
testing fundamental TMR issues under the best conditions of surface preparation and tunnelling
barrier control; the barrier width can be adjusted within a few 0.001 nm. For example the STM
approach may help to disentangle effects in metal–oxide–metal junctions due to the intrinsic
nature of electrodes and those due to interfaces such as sign changes in the spin polarization
of the Co occurring because of interface bondings [3]. Although the SP STM technique
continues to enrich our understanding, it is foreseen that it has not yet reached its full potential.
For example, most of the approaches to tip material have been limited to tips with as low a
0953-8984/06/500619+06$30.00 ©2006 IOP Publishing Ltd Printed in the UK L619
L620 Letter to the Editor
magnetic stray field as possible; for an exception see the early work of [9] where a bulk CrO2
ferromagnetic tip was used. The work, based on a detailed analysis of SP dI/dVspectroscopy,
uses thin Fe, Gd or Cr film covered tungsten tips [4]. This is legitimate if the purpose is mainly
to measure properties of a soft magnetic material where the probe should be as uninvasive
as possible in order not to perturb the spin state of the sample. Massive ferromagnetic tips
however may open new exciting perspectives in the sense that, due to the important stray field,
they allow magnetic manipulation of the structures or even spin injection experiments.
In this letter, we demonstrate the feasibility of using massive Ni wire probe tips in
SP STM experiments, to measure the magnetic contrasts of single Co nanoclusters, self-
organized on the Au(111) surface. Massive ferromagnetic Ni tips are robust and easy to
characterize. Their advantage compared to other tip materials is that the tip magnetization
is always dictated by the shape anisotropy, since magnetocrystalline anisotropy is weak in Ni.
Therefore, the tip will be magnetized always along its axis, leading to a magnetic configuration
perpendicular to the surface in the STM experiment. In the past, similar ferromagnetic tips
have been characterized by experiments on tunnelling into semiconductor surfaces [10] and by
e-holography where flux quantization gives information about the magnetic stray field [11].
Magnetotransport measurements in wire constrictions, on the other hand, provide unique
information on nucleation and domain wall propagation [12]. An upper value of the tip stray
field can be calculated from the saturation magnetization of 0.494 ×106Am
−1for Ni, which
leads to a stray field of 0.62 T. A simple calculation then shows that the maximum energy shift
due to the tip stray field μBB≈4×10−4eV is negligible.
In order to test the spin polarized contrast provided by the tips, self-organized cobalt
clusters (∼3 nm) on Au(111) [13] are a convenient system since their uniaxial anisotropy
of the magnetization is perpendicular to the Au substrate [14,15] with either up or down
magnetization. The measurements have been performed with a low temperature STM, in an
ultrahigh vacuum chamber with a base pressure of 5 ×10−11 mbar. Due to the working
temperature of 4.6 K the magnetization of the Co clusters is strongly locked in the perpendicular
direction, pointing either upwards or downwards. As a matter of fact, the blocking temperature
of 3 nm Co clusters is measured to be about 70 K in our variable temperature experiments, in
good agreement with a simple estimate1. The probe tips have been prepared by electrochemical
etching of a Ni wire with an appropriate solution [12] and immediately introduced in the
vacuum chamber to be resistively heated in order to get rid of the oxide and to be sputtered
by means of Ar ions. After this treatment, the tips are magnetized in situ along their axis in a
field of 0.3 T by means of a permanent magnet.
The constant current STM image of figure 1(a), taken with a magnetic Ni tip, shows
clusters of different apparent heights as evidenced in the line scans of figures 1(b)and(c)taken
under different tunnelling conditions. The vertical zscale was adjusted to show the topographic
contrast of the topmost part of Co clusters. Under the same conditions, the conventional, non-
magnetic W tip just reveals a unique cluster height of 4.0 ˚
A corresponding to two atomic
Co layers, and the roughness of the top does not exceed 0.1 ˚
A[13]. The presence of two
apparent heights in the case of the Ni tip is exactly what is expected, in the constant current
mode, for clusters which have parallel (P) and antiparallel (AP) magnetization with respect
to the magnetization alignment of the tip. This comes about because it is easier to tunnel SP
electrons in an electrode with the same polarization (P) than in an electrode with opposite
polarization (AP).
1An estimate of the blocking temperature can also be calculated from Tb=K/kBln(t/τ0), by taking the magnetic
anisotropy energy from the literature. An average value of K=0.2meV/atom for bilayer Co clusters can be deduced
for example from [16]. Assuming a typical measuring time t=10 s, a prefactor τ0of the order of 10−10 s, and taking
into account a number of atoms of about 500 in our clusters, we get Tb≈44 K.
Letter to the Editor L621
3.0
3.3
3.6
3.9
4.2
4
3
2
1
Height (Å)
3.0
3.3
3.6
3.9
4.2
4
3
2
1
4 nm
1
24
3
(a) (b)
(c)
Figure 1. (a) STM image (420 ×420 ˚
A2) of self-organized Co clusters recorded with the magnetic
tip at 4.6 K (V=−215 mV, I=250 pA). As an example, (b) and (c) show the cross sections of
the clusters numbered in (a) for (V=−215 mV, I=250 pA) and (V=110 mV, I=600 pA)
respectively.
3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3
0
5
10
15
20
25
Frequency of apparition
Height (Å)
Figure 2. Example of a histogram over many images of the height distribution obtained with the
magnetic tip at I=450 pA and V=60 mV, clearly showing two peaks separated by z≈0.28 ˚
A.
As indicated in the text, the spin polarization, and thus z, depends on the tunnelling parameters,
current and bias voltage.
Furthermore, for as prepared clusters and in the absence of an external field, we expect
as many clusters pointing upwards as downwards. This must be reflected in the statistics of
greyscale levels obtained from a great many SP STM images. Figure 2shows such a statistics
where the frequency of occurrence of a given height has been reported for particular values
of current and bias voltage. A bimodal distribution is clearly visible, corresponding to equal
intensity parallel and antiparallel states separated, in this case, by z=0.28 ˚
A. As will be
shown below, zis directly related to the spin polarization of the tunnelling gap. It must be
stressed however that the apparent height strongly depends on the tunnelling parameters, in
particular the tunnelling current and the bias voltage.
From SP constant current STM data the polarization of the tunnelling junction can easily
be calculated in a simple picture from the distribution of clusters heights. In the topographic
mode of the STM, I0=IP(zP)=IA(zP−z),whereI0is the feedback current of the STM
controller. In order to calculate the polarization of the STM junction, we need to compare
parallel and antiparallel currents for same heights, for example zP=z0.Inotherwords,we
need to calculate IA(zP)from I0=IA(zP−z):
IA(zP)=I0exp[−A1/2z].(1)
L622 Letter to the Editor
-600 -300 0 300 600 -600 -300 0 300 600 -600 -300 0 300 600
24
26
28
30
32
34 (a) I=650pA
Bias Voltage (mV) Bias Voltage (mV) Bias Voltage (mV)
Spin polarization, P (%)
(b) I=450pA (c) I=250pA
Figure 3. Experimental polarization of the STM tunnel junction, as defined in equation (2), as a
function of the bias voltage, for I=650, 450 and 250 pA.
For the calculation we took =4.0 eV as obtained from the I–zmeasurement and A=
1.025 (eV)−1/2˚
A−1. The polarization of the STM junction is then given by2
P=IP−IA
IP+IA
.(2)
In figure 3the polarization of the STM junction as a function of the bias voltage has
been reported from the zmeasurements. As can be seen in figure 3, the polarization as
a function of bias voltage is not a monotonic function. Depending on tunnelling current I
and bias voltage V(note that on varying Vthe barrier width varies as well), the polarization
of the tunnelling junction is found to vary between 25% and 34%. The many features that
appear in the polarization Pas a function of Vcannot be attributed to an effect of voltage
dependent barrier width which normally induces an even variation with respect to the bias
voltage. They must rather be assigned to structures in the sample’s local density of states
(LDOS). A good estimate of the LDOS can be obtained from scanning tunnelling spectroscopy
(STS) measurements performed on the same system at 4.6 K [19]. Two peaks dominated
by minority d electrons are found respectively above (+0.3eV)andbelow(−0.15 eV) the
Fermi energy. These spin polarized features lead to a steep increase of the spin polarization in
the corresponding energy ranges. Furthermore, when the SP current is changed from 250 to
650 pA, a dip appears in the curves of figure 3close to zero bias, indicating a strong influence
of barrier narrowing on the polarization of the Ni tip–vacuum–Co cluster junction. A similar
behaviour has also been observed in spin polarized tunnelling experiments performed on a Co
single crystal as a function of tip distance when the tip comes closer to the surface [20].
The polarization Pof the junction can be related to the polarization Piof the electrodes
where i=1,2. Using the phenomenological Julli`ere model [21], under the assumption of
low bias, and utilizing P=P1P2, the TMR expressed as TMR =2P/(1−P)can then be
calculated. Due to the fact that the spin polarization drastically depends on the LDOS, TMR
values can only be compared for well defined bias voltages, for example in the vicinity of
the Fermi level. The low bias spin polarization Pand the TMR are reported in table 1for
2In STM experiments this formula was used for the first time in [9].
Letter to the Editor L623
Tab le 1. Spin polarization Pand tunnelling magnetoresistance (TMR), calculated from images
taken at three different tunnelling currents Iand a bias of 30 mV. See the text for details.
I(pA) PTMR
250 0.30 ±0.01 0.86 ±0.01
450 0.29 ±0.02 0.82 ±0.03
650 0.26 ±0.01 0.71 ±0.02
three values of the tunnelling current I. As can be observed, Pas well as the TMR decreases
for increasing current. It should be recalled here that in the normal STM mode, a current
increase brings the tip closer to the surface and therefore reduces the tunnel barrier width. As
a matter of fact, spin flip effects, originating from electrode interactions, have been suggested
as an explanation for the decrease in TMR at small barrier widths [17,18]. In a free electron
treatment of this problem, that takes into account the barrier parameters, the TMR is given by
TMR =2(1−γ)P/(1−P+γ(1+P)),where0γ1 represents the ratio between
the square matrix elements of the spin flips and spin-conserved tunnelling. In the case of spin
conservation, γis zero and the above formula reduces to Julli`ere’s one. In this model, the γ
factors calculated from the TMR values of table 1increase as a function of current (decreasing
barrier width) but remain small. An upper value of γ=0.06 is obtained for a tunnelling
current of 650 pA. From I(z)measurements a rough value of the vacuum barrier width can be
evaluated; for 250 pA and bias voltages of a few tens of mV a barrier width of 6.5 ˚
A is found.
Therefore, by changing Ifrom 250 to 450 and to 650 pA, we calculate a decrease of the barrier
width of z=0.38 and 0.55 ˚
A, respectively. These small zvariations clearly reveal the
high sensitivity of the spin polarization and TMR to small changes in the width of thin vacuum
barriers.
In conclusion, we analysed the magnetization state, at low temperature, of self-organized
nanometre Co clusters by SP STM using massive Ni wire probe tips. A bimodal cluster height
distribution was found, due to the spin polarized tunnelling currents in parallel and antiparallel
configurations, leading to tunnelling magnetoresistance of the junction. Although the SP STM
junctions are dominated by features of the LDOS it is found that the geometry of the junction
plays a significant role. In particular, by decreasing the vacuum barrier width it was found
that the polarization and the tunnelling magnetoresistance at the Fermi level decrease as well.
The findings are important for understanding the magnetotransport in tunnel junctions and may
have an impact in the development of new spin electronic devices.
The authors acknowledge technical support from J G Faullumel and G Biechel. This work was
supported by the Growth Project No G5RD-CT-2001-00478 from the EC.
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