Magnetization reversal in europium sulfide nanocrystals

Article abstract of DOI:10.1063/1.2396915

The authors report the observation of the reversal in the magnetization hysteresis curve of europium sulfide nanocrystals. This phenomenon was investigated through the temperature-dependent magnetization of two classes of nanomaterials, nanocrystalline (2

Full text of DOI:10.1063/1.2396915

Magnetization reversal in europium sulfide nanocrystals
Marcela L. Redígolo, Dmitry S. Koktysh, Sandra J. Rosenthal, James H. Dickerson, Zheng Gai, Lan Gao, and
Jian Shen
Citation: Applied Physics Letters 89, 222501 (2006); doi: 10.1063/1.2396915
View online: http://dx.doi.org/10.1063/1.2396915
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APPLIED PHYSICS LETTERS 89, 222501 ͑2006͒
Magnetization reversal in europium sulfide nanocrystals
Marcela L. Redígolo
Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235
and Vanderbilt Institute for Nanoscale Science and Engineering, Vanderbilt University,
Nashville, Tennessee 37235
Dmitry S. Koktysh
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235
and Vanderbilt Institute for Nanoscale Science and Engineering, Vanderbilt University,
Nashville, Tennessee 37235
Sandra J. Rosenthal
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235; Department of Physics
and Astronomy, Vanderbilt University, Nashville, Tennessee 37235 and Vanderbilt Institute for Nanoscale
Science and Engineering, Vanderbilt University, Nashville, Tennessee 37235
James H. Dickersona͒
Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235
and Vanderbilt Institute for Nanoscale Science and Engineering, Vanderbilt University,
Nashville, Tennessee 37235
Zheng Gai
Center for Nanophase Materials Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee
37831 and Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37831
Lan Gao
Center for Nanophase Materials Science Division, Oak Ridge National Laboratory,
Oak Ridge, Tennessee 37831
Jian Shen
Center for Nanophase Materials Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee
37831 and Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37831
͑Received 21 August 2006; accepted 9 October 2006; published online 27 November 2006͒
The authors report the observation of the reversal in the magnetization hysteresis curve of europium
sulfide nanocrystals. This phenomenon was investigated through the temperature-dependent
magnetization of two classes of nanomaterials, nanocrystalline ͑2.0 nmഛdNCsഛ100 nm͒ and
quantum confined ͑dNCsഛ2.0 nm͒, where dNCs is the diameter of the nanomaterial. The effect of the
size of the nanomaterial on the magnetization is attributed to the competition between the magnetic
properties of strained surface atoms and unstrained core atoms. Superconducting quantum
interference device probed the magnetic response. Electron microscopy and X-ray diffraction
spectroscopy revealed the crystallinity and monodispersivity of the nanomaterials. © 2006
American Institute of Physics. ͓DOI: 10.1063/1.2396915͔
Europium sulfide ͑EuS͒ nanocrystals have been the sub-
ject of considerable research interest recently due to their
intriguing ferromagnetic and optical properties.1–5 Studies
have suggested that europium monochalcogenide nanocrys-
tals will exhibit enhancements in their magnetic properties,
compared to bulk, when the nanocrystal diameter reaches a
critical size. Such traits may lead to a variety of nanomaterial
applications, which could supplant those based on bulk EuS
or on other magnetic materials.4,6–12 Recent research has in-
vestigated how the nanocrystal size and surface oxidation
affect the magnetic characteristics of EuS nanocrystals.4,12
However, the origin of the magnetization in these nanomate-
rials remains largely unexplored.
is removed, and have discovered an unexpected reversal in
their magnetization hysteresis curve. We observed this phe-
nomenon in nanocrystalline ͑2.0 nmഛdNCsഛ100 nm͒ eu-
ropium sulfide. However, the reversal vanished as the nano-
crystal diameter decreased below 2.0 nm, the critical size
that defines the so-called quantum-confined ͑dNCsഛ2.0 nm͒
regime for EuS. We assert that strain on the surface and core
atoms of the nanocrystal creates competing magnetic re-
sponses. Thus, the magnetic properties of the strained surface
atoms and the unstrained core atoms engender the observed
magnetization reversal.
To synthesize the EuS nanomaterials, we employed a
thermolysis procedure of a single source EuS precursor.4,11,12
This synthesis provided marked control over the EuS nano-
crystals’ diameter and the size distribution, compared to pre-
viously reported techniques. The preparation of the EuS
single source precursor was conducted under a nitrogen at-
mosphere to minimize oxidation. A 1 mM methanolic solu-
To this end, we have investigated the remanent magne-
tization ͑MR͒ of EuS nanocrystals, the magnitude of magne-
tization a material maintains once the external magnetic field
a͒
Electronic mail: james.h.dickerson@vanderbilt.edu
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0003-6951/2006/89͑22͒/222501/3/$23.00 89, 222501-1 © 2006 American Institute of Physics
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222501-2
Redígolo et al.
Appl. Phys. Lett. 89, 222501 ͑2006͒
FIG. 1. ͑a͒ TEM micrograph of quantum-confined, sub-2.0-nm EuS nano-
crystals; ͑b͒ SAED pattern of the nanocrystals. Inset: Characteristic x-ray
diffraction spectrum for 5.0 nm EuS nanocrystals.
tion of 1,10-phenanthroline was added under vigorous stir-
ring to a 1 mM methanolic solution of europium ͑II͒
chloride. Thereafter, 2 mM of diethylammonium dieth-
yldithiocarbamate were added into the reaction mixture, pro-
ducing an orange precipitate.13,14 The resulting solid, dried
under vacuum, was transferred to a horizontal tube furnace
for a time-sensitive thermolysis. Adjustments to the ther-
molysis time and temperature yielded narrowly dispersed
EuS nanocrystals, devoid of ligand molecules. The nanoma-
terials were dispersed in 2,2 -bipyridine to inhibit the spon-
Ј
taneous formation of a nonmagnetic surface layer of
Eu2O2S.12
Transmission electron microscopy ͑TEM͒ images were
obtained using a Hitachi HF-2000 microscope operating at
200 kV. Figure 1͑a͒ shows a group of quantum-confined EuS
nanocrystals with an average diameter of 2.0 nm. The inter-
planar distance, approximately 2.1 Å, matches the ͑220͒ lat-
tice plane spacing of face-centered-cubic ͑fcc͒ europium sul-
fide. Selected area electron diffraction ͑SAED͒ patterns of
the nanocrystals are presented in Fig. 1͑b͒. The measured
ring diameters match the values for the ͑200͒, ͑222͒, and
͑311͒ planes of fcc EuS. X-ray diffraction ͑XRD͒ measure-
ments of our nanocrystals were obtained using a Scintag
X1␪/␪ automated powder diffractometer, with a Cu target
͑Cu K␣ radiation of ␭a=1.54 Å͒. The XRD spectrum of the
EuS nanocrystals is shown in the inset of Fig. 1. The diffrac-
tion peak positions and their full width at half maxima were
determined from profiles, calculated for each observed peak
using a diffraction management system for NT ͑DMSNT͒ soft-
ware package provided by Scintag.15 The EuS compound
was identified by major diffraction peaks at the 2␪ angles of
30.0°, 42.8°, 50.7°, and 53.4°, which match the Powder Dif-
fraction File ͑PDF͒ for this material16 and correspond, re-
spectively, to the ͑200͒, ͑220͒, ͑311͒, and ͑222͒ planes of fcc
EuS.
FIG. 2. ͑a͒ Magnetization hysteresis curves of 5.0 nm EuS nanocrystals at
different temperatures; ͑b͒ details of the hysteresis curves about the origin.
exchange among next-nearest neighbor Eu2+ ions.17 For
monochalcogenide nanocrystals with small lattice constants
͑a0=0.5968 nm for EuS at 300 K͒, the ferromagnetic ex-
change dominates. When the size of a EuS nanocrystal ap-
proaches the quantum-confinement regime at 2.0 nm, it con-
tains less than 250 atoms. As the number of lattice sites with
a full complement of nearest neighbors and next-nearest
neighbors decreases, the antiferromagnetic superexchange is
suppressed.18–22 The reduced size increases the strain on the
surface atoms due to the 1/R dependence of the surface-to-
volume ratio, which influences the ferromagnetic exchange
parameters.23–25 To perturb the 4f-5d dipole transitions of the
Eu2+ ions, we must change the ions’ local environment, using
effects such as surface strain.26,27
Figure 2 illustrates the magnetization hysteresis curves
of the nanocrystalline samples ͑dNCsϳ5 nm͒ as measured
via SQUID magnetometry. We surveyed the magnetization
response across a temperature range from 2 to 25 K, which
includes the Curie temperature ͑TC=17 K͒ for the nanoma-
terials. At 2 K, we observed a typical, counterclockwise hys-
teresis curve. This meant that the moments aligned parallel
with the magnetic field while the magnitude of the field de-
creased. However, when the temperature increased above
11 K, the hysteresis curve reversed direction, becoming
After identifying the quantum-confined and the nano-
crystalline materials, we investigated their magnetization
properties via superconducting quantum interference device
͑SQUID͒ magnetometry. The magnetic properties of eu-
ropium monochalcogenides are dominated by the competi-
tion between the ferromagnetic indirect exchange among
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nearest neighbor Eu2+ ions and the antiferromagnetic super-
clockwise. This reversal persisted from 11 to 25 K. When
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Redígolo et al.
Appl. Phys. Lett. 89, 222501 ͑2006͒
for nanocrystalline materials, the different temperature de-
pendences of the two states will dictate the response. Accord-
ingly, Fig. 3 evinces the evolving positive and negative val-
ues of MR as the magnetization of the surface and core atoms
responds to the changing temperature.
In summary, we have observed the reversal in the mag-
netization hysteresis curves of nanocrystalline europium sul-
fide. Competition between the magnetic response of strained
surface atoms and unstrained core atoms engenders a size-
dependent and temperature-dependent magnetization re-
sponse in quantum-confined and nanocrystalline EuS materi-
als. Further investigation of the magnetic properties of these
nanomaterials may lead to applications, such as bioimaging
reagents.
The authors thank Leonard Feldman for fruitful discus-
sions. A portion of this research was conducted at the Center
for Nanophase Materials Sciences, which is sponsored at
Oak Ridge National Laboratory by the Division of Scientific
User Facilities, U.S. Department of Energy. One of the au-
thors ͑J.H.D.͒ recognizes support from the Ralph E. Powe
Junior Faculty Enhancement Award from Oak Ridge Associ-
ated Universities. Another author ͑D.S.K.͒ recognizes sup-
port from the Vanderbilt Institute for Nanoscale Science and
Engineering, Vanderbilt University.
FIG. 3. Remanent magnetization vs temperature for nanocrystalline and
quantum-confined EuS materials. Negative values indicate a reversed hys-
teresis curve.
similar temperature-dependent studies were performed on
quantum-confined, sub- 2.0-nm nanocrystals, no reversal was
observed. The values for MR for quantum-confined and
nanocrystalline materials are summarized in Fig. 3. Tracking
the evolution of the magnitude of MR, as a function of tem-
perature, confirmed the magnetization reversal phenomena.
MR remained positive at all temperatures up to 90 K for the
quantum-confined EuS materials. Although MR was negative
from 11 to 25 K for the nanocrystalline EuS, the trend sug-
gested that the value would have become positive at tem-
peratures higher than 25 K. Further investigation of this tem-
perature dependence is being pursued.
The observed results can be described as a competition
between the magnetic response of unstrained, core atoms in
the nanocrystal and that due to surface anisotropies.26–29 The
magnetization of bulk EuS is uniform in zero applied field
since surface anisotropies are negligible. Once the material
becomes nanocrystalline, for which surface strain effects are
pronounced, the magnetic moments of the surface atoms will
align along their easy directions.26,27 The innermost core at-
oms will not experience the surface effect. This places the
nanocrystal in a bimodal magnetization state, with compet-
ing magnetizations from unstrained core atoms and strained
surface atoms.
When the material enters the quantum-confined regime,
for which the vast majority of the atoms reside at the surface,
the net magnetization response of the core atoms will be
substantially weaker than that of the surface atoms. This is
because most of the atoms will be surface atoms. If the sur-
face anisotropy field is large enough, the magnetic moments
of the core atoms of the nanocrystal will be forced to align
with those moments at the surface. Thus, if the nanocrystal is
so small that the majority of its atoms are at the surface, then
the alignment of the moments will be dictated by the surface.
The nanocrystal then will respond as if it consisted of just
one state, the surface state. When only one dominant state
exists ͑surface atom state, core atom state, etc.͒, magnetiza-
tion reversal is not observed. When the magnetization of the
core atoms and surface atoms are comparable, as is the case
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