Enhanced Magnetic Anisotropy of Mn12-acetate
D.M. Seoa, V. Meenakshia, W. Teizera *, H. Zhaob, and K. R. Dunbarb
aDepartment of Physics, Texas A&M University, College Station, TX 77843- 4242,
bDepartment of Chemistry, Texas A&M University, College Station, TX 77842-3012,
*Corresponding author: (Tel) 1-979-845-7730, (Fax) 1-979-845-2590, (e-mail)
Thin films of the Single Molecule Magnet
[Mn12O12(CH3COO)16(H2O)4]·2CH3COOH·4H2O (Mn12-acetate) have been fabricated on
a Si-substrate by the Dip-and-Dry method, a simple and robust technique. Atomic force
microscopy and X-ray photoelectron spectroscopy characterizations reveal that
homogeneous, thin films of a few molecular layers with smoothness at the molecular
level are deposited. Significant changes in magnetic properties of Mn12-acetate exposed
to the same solvent were observed in zero-field-cooled and field-cooled magnetization, as
well as ac-susceptibility measurements. The blocking temperature was found to increase
to TB > 10 K at low magnetic fields, indicating an enhanced magnetic anisotropy.
Keywords: Mn12-acetate; Thin film; Blocking Temperature; Magnetic Anisotropy
Single molecule magnets (SMMs), the most studied of which is
[Mn12O12(CH3COO)16(H2O)4]·2CH3COOH·4H2O (Mn12-acetate) [1-4], provide a model
system for the study of quantum tunneling of the magnetization . Stepwise
magnetization hysteresis loops and out-of-phase ac susceptibility signals are due to a high
spin ground state and a strong uniaxial magnetic anisotropy of Mn12-acetate , in which
eight Mn3+ (S=2) ions and four Mn4+ (S=3/2) ions are magnetically coupled by oxygen
bridges to form the S=10 ground state . Moreover, these compounds have also been
considered for future applications such as quantum computing and information storage
devices [6,7]. The interesting magnetic properties in these materials arise from individual
molecules rather than intermolecular interactions. For this reason, single molecules can,
in principle, be used to store magnetic information. In order to use these molecules in
devices, however, an increase of the blocking temperature (TB) of these materials is
There are few reports on the film production of Mn12 derivatives to date [8-14], and
even less reports on the significant enhancement of magnetic properties from that of the
parent compound [11,12]. Here we report the production of thin, homogeneous Mn12-
acetate films of continuous coverage by the dip-and-dry (DAD) method, and the
enhancement of the magnetic anisotropy of Mn12-acetate, which has undergone similar
solvent exposure. Two studies regarding the magnetic properties of Mn12 derivatives
formed by thermal transformation and gas inclusion as well as incorporation into
mesoporous silica have been reported [15,16]. In the present study, an increase of TB to >
10 K in dc-susceptibility measurements was observed, a significant change from the
parent compound behavior that shows a zero-field blocking temperature of ~ 3.5 K, as
observed in reference  and in our as-produced Mn12-acetate powder. These
observations are of fundamental importance and a promising first step for potential
applications of these materials. Atomic force microscopy (AFM) was used to investigate
the surface morphology and the thickness of the films, which revealed roughness on the
molecular scale and a thickness of ~1 molecular layer per dip. X-ray photoelectron
spectroscopy (XPS) measurements were also carried out to analyze the electronic
structure of the thin films.
A fresh sample of Mn12-acetate was prepared following the customary procedure .
For a typical preparation of films described below by the DAD technique, 2.2 mg ± 0.1
mg of Mn12-acetate was dissolved in 10 ml of acetonitrile (CH3CN) to produce a 1.1 ×
10-4 mol·L-1 solution. Prior to the DAD step, the Si/SiO2 substrate was rinsed with
acetone and isopropanol. The clean wafer was dipped in the prepared Mn12-acetate
solution and immediately removed. A thin film of the solution was subsequently
observed on the substrate, which dried within several seconds to produce a thin film of
Mn12-acetate. All procedures were carried out inside a fume hood under ambient
After preparation of thin films by the DAD technique, the surface morphology of
the films was studied by AFM, with a Digital Instruments Nanoscope IIIa. The AFM
images were acquired in the tapping mode with a silicon cantilever and tip under ambient
conditions. Room temperature core level XPS measurements were performed using a
Kratos AXIS ULTRA spectrometer equipped with a concentric hemispherical analyzer
using the Al Kα radiation (hν=1486.6 eV) and a base pressure of ~ 2 × 10-8 Torr. The
binding energies were calibrated with respect to the C 1s peak (284.8 eV).
3. Results and discussion
3.1. AFM and XPS characterizations
Fig. 1 shows AFM images of the Mn12-acetate thin film. A topographical top-view,
the corresponding 3D image, and height profile are shown for 1 × 1 µm2 scan size,
respectively from (a) to (c). This figure shows a Mn12-acetate thin film, which,
considering the simplicity of the DAD method, is surprisingly homogeneous and smooth.
In a control experiment, the DAD method was used with pure acetonitrile instead of the
Mn12-acetate solution. As expected, the resulting images (not shown) did not show any
substantial surface corrugations. Detailed examination of the AFM images (Fig. 1) reveal
that the typical horizontal size of pictured Mn12-acetate clusters is about 25 nm and the
average vertical height is about 2 nm, close to a molecular diameter. As a result of the
radius of curvature of an AFM tip (~ 10 nm) , the typical horizontal size of a cluster
appears larger than a single molecule. The height information, however, indicates that
nearly all of the particles on the film form a monolayer instead of clusters. The root mean
square (RMS)-roughness of the surface is 0.73 nm, considerably smaller than the size of
a single molecule (1.7 nm) .
The thickness of the films after a single DAD step was measured and found to be ~ 2
nm in AFM measurements of artificial Mn12-acetate patterns (not shown). The Mn12-
acetate films were surprisingly homogeneous, both microscopically and macroscopically,
as indicated by the fact that all the AFM images we have acquired in different regions of
the film show similar images .
In order to analyze the electronic structure of the thin films after the DAD process, X-
ray photoelectron spectroscopy (XPS) measurements have been carried out. Multiple
peaks of C 1s, and O 1s, which show the existence of different C and O sites in Mn12-
acetate , were observed in narrow scan spectra with pass energy of 40 eV (not
shown). Core-level spectra of Mn 2p for the crystalline Mn12-acetate (pellet) and the
Mn12-acetate thin film, which was used for AFM, are shown in Fig. 2. The two peaks for
the Mn 2p core-level of Mn12-acetate thin films, at 642 eV and 653.6 eV, correspond to
2p3/2 and 2p1/2, respectively. A small shift, observed in the binding energy for different
samples, was within the XPS resolution (0.5 eV). The films are thin enough to observe
the Si 2p peak from the substrate, at 99.5 eV and 103.1 eV, that correspond to a pure
silicon phase and a silicon oxide phase, respectively (not shown) .
3.2. Magnetic characterizations
We were not successful in acquiring magnetization data using these DAD samples
due to the small amount of material and the background signal from the substrate.
Therefore, a sample for the magnetization measurements was made by a similar method
to the DAD samples but on a larger scale to accumulate sufficient sample material. A
solution of ~ 1 × 10-3 mol·L-1 (21 mg of Mn12-acetate in 10 mg acetonitrile) concentration
was prepared in a beaker. The top ~ 90 % portion of this solution was used to assure that
no sediment was present. Subsequently, this solution was evaporated over the course of 1
hour in a glass Petri dish. The magnetization sample was obtained by scraping the
powder from the dish. Magnetic measurements were acquired in a Quantum Design
MPMS-XL SQUID magnetometer.
We note, that the as-produced Mn12-acetate displayed magnetic data as seen before
by others. Specifically, differences between the zero-field cooled (ZFC) and field cooled
(FC) magnetization data display the customary signatures of a blocking temperature at ~
3K. Furthermore, the ac-susceptibility shows a typical frequency dependence with peaks
in the imaginary part at 4.2K (1Hz), 6K (100Hz) and 7.8K (1000Hz). In contrast, novel
effects were observed in the magnetization data of the solvent exposed Mn12-acetate in
both ac-susceptibility, ZFC and FC magnetization measurements. The temperature
dependence of the in-phase (χ') and out-of-phase (χ") ac-susceptibility for the sample
were investigated in an ac field of 3 Oe at different frequencies ranging from 1 Hz to
1000 Hz (Fig. 3). A magnetic transition was observed at ~ 11 K in the χ' data. As the
frequency was increased from 1 Hz to 1000 Hz, an increase in the transition temperature
was observed. The inset shows a blow up around the transition temperature. Maxima, at
10.5 K (1 Hz) and 10.8 K (10 Hz), were observed in the χ" data. Moreover, a frequency
dependence of the peaks for 100 Hz and 1000 Hz is also evident at ~ 11.0 K and ~ 11.6
K, respectively, although the data are noisier. A shift of the maxima in the χ" data from
those of crystalline Mn12-acetate (e.g. 5.0 K at 10 Hz)  is consistent with an increase
of the blocking temperature (TB) in our film material.
Fig. 4 shows both ZFC and FC magnetization curves, which are split below TB ~ 9.6
K in Happ = 0.05 T . TB is shown in the inset of Fig. 4 as a function of applied
magnetic fields, Happ = 0.001, 0.005, 0.05, 0.3, and 0.5 T, respectively. An increased TB
around 10 K is consistent with AC data. The magnetization of the sample was measured
as a function of the applied magnetic field at 1.8 K. A hysteresis loop with a coercive
field of ~ 760 Oe was obtained. The low coercive field may be due to a random
orientation of the sample.
Considering the fact that the core of Mn12-acetate molecules is not affected by
acetonitrile, as was shown in a previous report , we suggest the following possible
reasons for the shifted TB compared to that of the parent compound: (1) The change in
magnetic properties may be the result of structural changes to the Mn12-acetate complex,
e.g. missing ligands from the Mn12-acetate molecules or missing water of crystallization
(2) The total spin of the Mn12-acetate molecules may have changed in this process. (3)
Intermolecular interaction between the Mn12-acetate molecules may be enhanced due to
missing ligands. At this point we cannot distinguish which of the effects or combination
thereof is present in our sample.
In summary, the deposition of Mn12-acetate directly onto a Si/SiO2-substrate using
the DAD method yields very smooth, homogeneous thin films. More interestingly,
altered magnetic properties were observed from the ac-susceptibility, as well as ZFC and
FC magnetization measurements. The blocking temperature of the film material increased
to ~ 10 K, indicating that the magnetic anisotropy of the Mn12-acetate film may have
significantly changed during exposure to acetonitrile. This study is an important step
towards the use of molecular magnets in temperature ranges that lend themselves to
This research was supported by the Texas Advanced Research Program (010366-
0038-2001), the Robert A. Welch Foundation(A-1585), the National Science Foundation
(DMR-0103455 and NSF-9974899 ), DOE (DOE--DE-FG03-02ER45999), and the
Telecommunications and Informatics Task Force (TITF 2001-3) at Texas A&M
University. Use of the TAMU/CIMS Materials Characterization Facility is
acknowledged. We thank D. Naugle for helpful discussions
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Fig. 1. AFM images of a Mn12-acetate thin film formed by the Dip-and-Dry method. (a)
Topographical top-view with 1 × 1 µm2 scan size. (b) The corresponding 3D image of the
same area. (c) Height profile, which is taken at the location indicated by the white line in
(a). The height scale bar is shown in the topographical top-view.
640 644648 652656
Intensity [arb. unit]
Binding Energy [eV]
Fig. 2. XPS Spectra of the Mn 2p core level for crystalline Mn12-acetate (pellet) and the
Mn12-acetate thin film.
' [arb. unit]
χ" [arb. unit]
Fig. 3. Temperature dependence of real part (χ') and imaginary part (χ") of ac-
susceptibility at different frequencies. The inset shows the frequency dependence of the χ'
data around the transition temperature.
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at 500 [Oe]
Magnetization [arb. unit]
Fig. 4. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization vs temperature
curves at Happ= 500 Oe. The inset shows the blocking temperature (TB) as a function of
different applied magnetic fields (Happ = 10, 50, 500, 3000, 5000 Oe), respectively. The
connected line is a guide for the eye.