Magnetic moments of CoNMnM and CoNVM clusters (N ≤ 60; M ≤ N/3) are measured in molecular beams using the Stern-Gerlach deflection method. Surprisingly, the per atom average moments of CoNMnM clusters are found to increase with Mn concentration, in contrast to bulk CoMn. The enhancement with Mn doping is found to be independent of cluster size and composition in the size range studied. Meanwhile, CoNVM clusters show reduction of average moments with increasing V doping, consistent with what is expected in bulk CoV. The results are discussed within the virtual bound states model

de Heer, Walter A.

Moro, Ramiro

Xu, Xiaoshan

Yin, Shuangye

English

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University of Nebraska - Lincoln
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Xiaoshan Xu Papers Research Papers in Physics and Astronomy
2007
Magnetic Enhancement in Cobalt-Manganese
Alloy Clusters
Shuangye Yin
Georgia Institute of Technology
Ramiro Moro
Georgia Institute of Technology
Xiaoshan Xu
University of Nebraska-Lincoln, xiaoshan.xu@unl.edu
Walter A. de Heer
Georgia Institute of Technology
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Yin, Shuangye; Moro, Ramiro; Xu, Xiaoshan; and de Heer, Walter A., "Magnetic Enhancement in Cobalt-Manganese Alloy Clusters"
(2007). Xiaoshan Xu Papers. 1.
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Magnetic Enhancement in Cobalt-Manganese Alloy Clusters
Shuangye Yin, Ramiro Moro, Xiaoshan Xu, and Walter A. de Heer
School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
(Received 10 October 2006; published 15 March 2007)
Magnetic moments of CoNMnM and CoNVM clusters (N  60; M  N=3) are measured in molecular
beams using the Stern-Gerlach deflection method. Surprisingly, the per atom average moments of
CoNMnM clusters are found to increase with Mn concentration, in contrast to bulk CoMn. The enhance-
ment with Mn doping is found to be independent of cluster size and composition in the size range studied.
Meanwhile, CoNVM clusters show reduction of average moments with increasing V doping, consistent
with what is expected in bulk CoV. The results are discussed within the virtual bound states model.
DOI: 10.1103/PhysRevLett.98.113401 PACS numbers: 36.40.Cg, 75.20.Hr, 75.50.Ee, 82.80.Rt
Magnetism of clusters has been intensively investigated
in the past decade. Significant enhancements of magnetism
are often observed due to the low coordination numbers
and slight lattice expansion. Experiments on Fe, Co, and Ni
clusters show that they are ferromagnetic and have en-
hanced magnetic moments when they contain less than
about 1000 atoms [1]. Rh is found to be ferromagnetic in
clusters with magnetic moments of as large as 0.8 Bohr
magnetons (B) per atom, even though bulk Rh is para-
magnetic [2]. Mn clusters were determined to be ferro-
magnetic or ferrimagnetic in the size range of 5–99, while
bulk Mn is antiferromagnetic [3,4]. These discoveries to-
gether with theoretical work on cluster magnetism have
enriched the understanding of nanomagnetism. While early
work mainly concentrated on elemental clusters, there are
some studies of alloy species in the search for novel nano-
magnetic materials. Stern-Gerlach (SG) experiments have
been performed on BiCo [5], DyTe, and DyBi clusters [6].
Recently, our own work on BiMn [7] has shown highly
magnetic cluster species as well as ferromagnetic and
antiferromagnetic coupling behavior in these clusters.
Magnetic enhancement is also found in chemically synthe-
sized CoRh nanoparticles [8]. For magnetic alloys, the
most important property is the saturation magnetization,
which is usually presented as a function of average electron
number in the Slater-Pauling plot. The major trends were
first explained by Slater and Pauling, based on a rigid band
model [9,10]. But the generally accepted explanation is
from Friedel [11], who considered the formation of virtual
bond states (VBS) near impurity atoms in the host. To our
knowledge, there has been no attempt to study the Slater-
Pauling behavior in magnetic alloy clusters. It is interesting
to examine if the physical picture developed from bulk
alloys is still valid in alloy clusters.
The study of deposited alloy magnetism in nanoparticles
is both an experimental and theoretical challenge. It is
difficult to control the stoichiometry in nanoparticles using
chemical methods. The nanoparticles inevitably interact
with each other and with the substrate, which makes the
measurements difficult and explanations ambiguous. These
difficulties are resolved in molecular cluster beams [12]
since the clusters, once formed, are isolated from contam-
inations and other interactions during the measurement.
Moreover, their chemical composition is accurately deter-
mined using high-resolution mass spectrometry methods.
Here, we present magnetic measurements on CoNMnM and
CoNVM clusters, for N  60 and M  N=3. The photo-
ionization thresholds as well as reactivities were studied in
molecular beams [13,14], but their magnetic properties
have not been reported. The experimental method used
here is described elsewhere [15,16]. Briefly, alloy clusters
are produced in a cryogenically cooled pulsed laser vapor-
ization source, which is maintained at low temperature
using a temperature controlled closed cycle cryogenic
refrigerator. A high power laser pulse (Continuum
Surelite I20, 355 nm) is focused on a 2 mm diameter alloy
target rod. Several alloy rods are used in these experiments:
Co0:85Mn0:15 and Co0:5Mn0:5 rods were purchased from
Alfa Aesar; Co0:88V0:12 rods were produced using a home-
made induction-heating furnace. A pulse of helium is
injected in the source to form the cold cluster beam, which
is collimated to a width of 0.3 mm before entering the
inhomogeneous field of a Stern-Gerlach magnet that
causes the magnetic clusters to deflect. The deflection (d)
of a cluster is proportional to the average magnetization
(Meff) in the magnetic field, determined by
 d  KMeffdB=dz=mv2 (1)
where K is the geometry factor of the apparatus, m and v
are the mass and speed of the cluster, dB=dz is the mag-
netic field gradient.
Meff is related to the intrinsic magnetic moment of the
cluster () [17,18]. Both for clusters in molecular beams
and for supported clusters, the magnetization is in the same
direction of the magnetic field. When B=kBT  1,
Meff  2B=3kBT and when B=kBT  1, Meff  ;
but there are differences for intermediate fields [18]. At
the experimental conditions for the results presented in this
Letter, Meff can be approximately expressed using the
Langevin equation,
PRL 98, 113401 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending16 MARCH 2007
0031-9007=07=98(11)=113401(4) 113401-1 © 2007 The American Physical Society
 Meff  cothB=kBT 	 kBT=B
: (2)
Hence, the intrinsic value of the magnetic moment is
determined from the deflections. The deflected clusters
are photoionized using an ArF excimer laser (Lambda
Physik Optex, 193 nm), and detected in a position-sensitive
time-of-flight mass spectrometer (PSTOFMS) [19], which
simultaneously measures the cluster deflections and their
masses (Fig. 1). The method requires that mass resolution
be traded for position sensitivity so that it is restricted to
small clusters (i.e., less than 100 atoms), to preserve ade-
quate mass resolution. The composition of the alloy clus-
ters species is determined from their masses, which in most
cases is unambiguous. Statistics are used for the few cases
where ambiguity exists.
The magnetic moments () of CoNMnM clusters are
determined using Eqs. (1) and (2) (Fig. 2). The total mo-
ments generally increase with the addition of Co or Mn
atoms. The size effect of Mn doped Co clusters is remark-
able. In Fig. 3(a), we fix the total of atoms (Nt  N M)
and plot at the average moment per atom (hi  =Nt) as
a function of impurity concentration (x  M=Nt), for sev-
eral representative cluster sizes.
The slopes of these curves are equivalent to the change
of total moment after substituting one Co atom by one
Mn atom. For CoNMnM clusters, the average moments
increase with increasing Mn concentration. The average
magnetic moment enhancement after substitution of a Co
with a Mn atom is 1:7B, for all the alloy clusters in our
experiments. The average moment also depends on the
cluster size, as discussed below. In separate experiments
using Mn rich samples, it is seen that the increasing trend
only persists for Mn concentrations up to 30% to 40%,
after which the average moment decreases with increasing
Mn concentration.
This magnetic enhancement is contrasted with that of
bulk CoMn, where the substitution of Co with Mn atoms
tends to decrease the average moment [20] by about 6:0B
per substitution. The observed trends in bulk are explained
by the formation of virtual bond states (VBS) near the
impurity state. For ferromagnetic transition metals, it is
well known that the magnetic moment is due to the d
electrons, which occupy spin-up and spin-down bands
that are split by the exchange interaction:
 nd  n"d  n#d;
where nd, n"d, and n
#
d are the total, majority, and minority d
band occupations, respectively. The average magnetic mo-
ments (in units of B) are determined from the imbalance
between spin-up and spin-down occupations,  
n"d 	 n#d, which can be written as
   2n"d 	 nd: (3)
If the spin-up bands are fully occupied, the material is
called a strong magnet; otherwise, it is a weak magnet
[21]. For a strong magnet, n"d  5 so that   10	 nd.
While this explains the right-hand-side slope of the Slater-
Pauling curve, it is not applicable for early transition metal
impurities in Co or Ni, which manifest themselves as the
fast dropping branches that deviate from the Slater-Pauling
curve. In this case (as for the bulk [11,21,22]), one has to
consider the formation of VBS near the impurity sites. If
the difference between the atomic number of the impurity
 
FIG. 1 (color online). Part of the position-sensitive mass spec-
trum of CoNMnM clusters. Dashed line is the spectrum without
magnetic field. Solid line is with magnetic field (0.98 T).
Magnetic deflections cause the cluster mass peaks to shift to
the right when the magnet is activated. The magnetization is
determined from the deflections of the mass peaks. The compo-
sitions of the CoNMnM clusters as determined from the masses
are denoted as (N, M).
 
FIG. 2 (color online). Total magnetic moment of CoNMnM
clusters as a function of N. Each series represents clusters
with the same number of Mn atoms N. The experimental
uncertainty is about 1:5B.
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113401-2
and the host Z  ZImpurity 	 Zhost is sufficiently large
(	Z > 2), the VBS will rise above EF because electrons
are less attracted to the impurity ions. As a result, the VBS
may be completely unoccupied. Note that the degeneracy
of the VBS is exactly 5 for each impurity atom as first
pointed out by Friedel [11] (See also Refs. [21,22]). In this
case, we have n"d  5	 5x and nd  nHostd  Zx, where
x is the impurity concentration per atom. From Eq. (3), we
find the average moment of the alloy to be
   10	 nHostd 	 Z 10x  Host 	 Z 10x:
(4)
Since Z 10 is always positive, this explains the de-
creasing trend of the magnetic moment of transition metal
ferromagnets with increased doping with early transition
metals like V and Mn.
For Mn impurities in a Co host, Z  	2. The slope of
average moment as a function of x from Eq. (4) is expected
to be 	8B. The trend of CoMn bulk ( 	 6B slope) is
close to this expectation. The difference can be explained
by the fact that Z  	2 is not large enough to cause the
VBS to rise entirely above EF. From band structure calcu-
lations of dilute Mn impurities in a Co host [23], the Mn
local moments are found to have both antiferromagnetic
and ferromagnetic couplings with the Co, where the anti-
ferromagnetic state has the lower energy. In both cases, the
calculated average moment decreases with increasing Mn
concentration.
However, in CoMn clusters, we observe a positive slope,
suggesting that the VBS are below EF. In this case, n"d  5
and nd  nHostd Zx. From Eq. (3), we find the average
moment of the alloy to be
   10	 nHostd 	 Zx  Host 	Zx: (5)
With Z  	2 for Mn impurity in Co host, one finds the
slope to be 2B, which is close to the 1:7B found in
Fig. 3(a). Since the spin-up bands in CoNMnM clusters are
filled, they should follow the Slater-Pauling curve. This
suggests an avenue for synthesizing high moment materi-
als based on clusters.
Experiments on CoNVM clusters are more consistent
with the case that the VBS are above EF. The average
magnetic moments as a function of V concentration are
plotted in Fig. 3(b). For CoNVM clusters, the average mo-
ments generally decrease with increasing V concentration.
The decreasing trends are similar for all clusters with only
a few exceptions (Co41V2, Co17V2, Co23V3, Co25V4, and
Co19V6, which are not shown in the figure). Although CoV
alloy magnetism data are not available for comparison,
calculations show [23] that V in Co is the perfect case to
form VBS well above the Fermi level because of the large
nuclear charge difference Z  	4. Experimental data for
Fe-V, Ni-V, and CoNi-V alloys [21,22] all agree with this
picture. According to Eq. (4), the predicted slope is 	6B,
consistent with our experimental measurements [Fig. 3(b)].
Note that this effect cannot be explained by considering the
Co moment only. If V atoms were considered as inert
additions to Co clusters, the slope would have been 	2B.
Another observation is that the magnetic enhancement
by doping with Mn atoms is uniform and does not depen-
 
FIG. 4 (color online). Normalized average moment for
CoNMnM clusters. The contribution due to the electron defi-
ciency of Mn compared to Co (i.e., 1.7 electrons per substituted
atom) is subtracted. The normalized average moments only de-
pends on the total number of atoms, which indicates the presence
of the VBS below EF. If the spin-up band of the VBS is
completely below EF , then the strong magnetism picture applies
and the enhancement is 2B. We observe that the enhancement
is 1:7B, which is reasonably close to this prediction.
 
FIG. 3 (color online). Magnetic moments per atom of
CoNMnM (a) and CoNMnM (b) clusters. Representative cluster
sizes (N M) are shown,; other clusters have similar behavior.
The data for bulk CoMn are from [20]. Dotted line is the
expected magnetic moments assuming the VBS are above the
Fermi level.
PRL 98, 113401 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending16 MARCH 2007
113401-3
dent on the size or composition of the CoNMnM clusters.
This is illustrated in Fig. 4 where we subtracted the en-
hancement by the Mn atoms (i.e., we subtracted 1:7B for
each Mn atom), which results in a universal curve. Hence,
we treat the Mn atoms as if they were Co atoms, however
with an electron deficiency of 1.7 per atom. The resulting
curve is consistent with pure Co clusters. This implies that,
except the valence difference, Mn atoms play the same role
as Co atoms. Note that the ionic radii of Mn and Co (in 12
coordinated metals) are very close (1.26 and 1.25 A˚ , re-
spectively) [24]. Further note that in bulk, up to 20% Mn
can be dissolved in solid Co without altering the host
crystal structure [25]. These facts suggest that substitution
of Co atoms with Mn atoms in clusters does not signifi-
cantly alter the geometric structure of the clusters. Besides
forming the additional VBS above EF, the band structure
of the cluster appears to be essentially unchanged.
In summary, a magnetic moment enhancement is found
for cobalt clusters doped with manganese atoms, which is
opposite to that of the bulk. In contrast, doping vanadium
atoms in cobalt clusters reduces the average magnetic
moment, which is consistent with bulk behavior. The mag-
netic enhancement does not depend on the cluster size and
composition, indicating the robustness of the geometric
and electronic structure upon doping with Mn. Hence,
the magnetic moments of these alloy clusters can still be
approximately understood in an electronic band picture
similar to bulk alloys. This emphasizes the intrinsic con-
nections between the electronic properties of clusters and
the bulk. We interpret the overall trends of experimental
results (linear dependence on impurity concentration, and
size-dependence on total number of atoms) using a virtual
bound states model that was initially developed for bulk
alloys. The prediction from the model qualitatively agrees
with experiments for CoV clusters. However, in order to
explain the magnetic enhancement trends for CoMn clus-
ters, the assumption that VBS are below Fermi level has to
be made, the mechanism of which is not clear in this
framework. Theoretical investigations using more ad-
vanced tools, such as density functional theory, will defi-
nitely shed light on this point. Such studies will also help to
elucidate more details about the local moment distribution,
size-dependence, and detailed electronic structures in these
alloy clusters, as has been demonstrated in earlier studies
[26].
This work is supported by NSF and DARPA. The
authors are grateful to Uzi Landman for stimulating
discussions.
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Magnetic Enhancement in Cobalt-Manganese Alloy Clusters

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Magnetic Enhancement in Cobalt-Manganese Alloy Clusters

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