The magnetic properties of Ni(II) and Cu(II) complexes were measured. In the case of Ni(II) samples strong enhancement of the magnetic susceptibility below 23 K was observed. The model of structural transition was proposed to explain this behavior

A Glebowska

A Krówczyński

A Twardowski

B Witek

D Pociecha

E Górecka

H Turski

J Szczytko

J Szydlowska

P Budzowski

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Vol. 114 (2008) ACTA PHYSICA POLONICA A No. 5
Proc. XXXVII International School of Semiconducting Compounds, Jaszowiec 2008
Magnetic Liquid Crystals
for Molecular Spintronics
J. Szczytkoa, P. Budzowskia, H. Turskia, B. Witeka,
A. Glebowskab, A. Krówczyńskib, D. Pociechab,
J. Szydlowskab, E. Góreckab and A. Twardowskia
aInstitute of Experimental Physics, Faculty of Physics, University of Warsaw
Hoża 69, 00-681 Warsaw, Poland
bDepartment of Chemistry, University of Warsaw
al. Żwirki i Wigury 101, 02-089 Warsaw, Poland
The magnetic properties of Ni(II) and Cu(II) complexes were measured.
In the case of Ni(II) samples strong enhancement of the magnetic suscep-
tibility below 23 K was observed. The model of structural transition was
proposed to explain this behavior.
PACS numbers: 75.20.Ck, 75.50.Xx
1. Introduction
Growing interest in spintronics and spin control on the atomic level arises
from predicted superiority of the spin of the electron over the electron charge
in terms of energy and speed efficiency. Advances in this field require a new
approach to the concept of spin manipulation [1]. The bottom-up strategy can
only be realized by extensive studies of spin interactions in well defined clusters of
atoms or molecules. In this communication we present the study of the coupling
between paramagnetic cations of copper Cu2+ and nickel Ni2+ incorporated to
metal–complex molecules with mesogenic properties [2].
These compounds possess the property of adjustable lattice structure, de-
pending on the type of the compound and the temperature of the system. Still,
within the accuracy of measurements, modifications of the external structure of
the compounds do not seem to affect in a significant manner the magnetic proper-
ties of the complex, which seem to depend solely on the internal structure of the
compounds.
2. Samples and magnetization measurement
The new nickel(II) complexes of aminomethylene derivatives of barbituric
acid [3] and enaminoketone copper(II) [4] has been obtained (Fig. 1). The phase
(1383)
1384 J. Szczytko et al.
Fig. 1. The example of investigated liquid-crystal complexing compound (Cu2+, Ni2+).
TABLE
General structure of metal complexes studied
at liquid crystalline phases, phase transition
temperatures (in ◦C).
Cr Colho Colhd
Cu 138.0
Ni 119.9 141.7 184.3
Cr — crystal,
Colho — columnal hexagonal ordered,
Colhd — columnal hexagonal disordered.
sequence and phase transition temperatures for studied complexes are summarized
in Table [3–5]. Due to the discoidal molecular shape this substances form columnar
mesophases.
Magnetization measurements were performed using superconducting quan-
tum interference device (SQUID) in the range of magnetic fields 0–6.0 T and
temperatures 2.0–300.0 K.
3. Results and discussion
It is expected that the ground state of Cu(II) ion in tetragonal configuration
of the chelating plane is a spin doublet (d9, S = 1/2), while the Ni(II) complex
should be non-magnetic singlet (d8, S = 0) [6]. Indeed, for the copper(II) complex
magnetization demonstrates paramagnetic behavior (Fig. 2a), which can be fitted
with the Curie law and S = 1/2. However, magnetic characterization of nickel(II)
complex revealed its paramagnetic behavior in a broad temperature range. Below
23 K the low field magnetization starts to increase rapidly (Fig. 2b).
Magnetic Liquid Crystals for Molecular Spintronics 1385
Fig. 2. The magnetization of investigated compound as a function of temperature
(a) for the sample with Cu(II) measured with 1.0 T; (b) for the sample with Ni(II)
measured in 0.10 T. The black curve is the fit to (a) the Curie law, (b) mean field model
with TC = 23 K.
Fig. 3. Chains of magnetic moments in columnar structure of Ni(II) imposed by chelat-
ing of metal center by atoms of neighboring complexes.
Observed paramagnetism of this Ni(II) complex is most probably of Van
Vleck type [6]. The low-temperature ferromagnetism may originate from Ni–Ni in-
teractions, resulting in chains of magnetic moments in columnar structure (Fig. 3).
Using mean field approximation model for the Ni(II) ion chains and assuming
only nearest neighbors coupling one can estimate Ni–Ni exchange interaction to
be J/kB = +17 K (for S = 1). However, it is not possible at present to show
1386 J. Szczytko et al.
clearly what kind of molecular interactions facilitates paramagnetic-ferromagnetic
phase transition. It is possible that the transition is accompanied by the change in
the complex geometry from flat to tetrahedral or octahedral, formed in additional
chelating by neighboring axial molecules in the column, which affects the Ni(II)
ion ground electronic state (Fig. 3).
4. Summary
Summarizing, some of the transition metals complexes seem to be promis-
ing materials for pseudo one-dimensional ferromagnets. The ferromagnetic state
is observed for Ni(II) complexes at low temperature. The spin interactions, lead-
ing to ferromagnetism most probably are enhanced by bridging the complexes
along the columns by chelating centers of neighboring molecules. Low-temperature
X-ray diffraction should be performed in order to decide whether the structure
change can account for the ferromagnetic transition.
Acknowledgments
The partial support from European Network of Excellence MTKD-CT-2005-
029671 is acknowledged.
References
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819 (1994).
[2] J.L. Serrano, in: Metallomesogens: Synthesis, Properties and Applications, VCH,
Weinheim 1996, p. 43; B. Donnio, D. Guillon, R. Deschenaux, D.W. Bruce, in:
Comprehensive Coordination Chemistry II, Eds. J.A. McCleverty, J.J. Meyer, M.
Fujita, A. Powell, Vol. 7, Elsevier, Oxford 2003, p. 353.
[3] A. GÃlȩbowska, K. Kamieńska-Trela, Adam Krówczyński, D. Pociecha, J.
SzydÃlowska, J. Szczytko, A. Twardowski, J. Wójcik, E. Górecka, J. Mater. Chem.
18, 3419 (2008), DOI: 10.1039/b807656a.
[4] A. Krowczynski, J. Szydlowska, D. Pociecha, J. Przedmojski, E. Gorecka, Liq.
Cryst. 25, 117 (1998).
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J. Am. Chem. Soc. 124, 8884 (2002).
[6] C. Kittel, in: Introduction to Solid State Physics, Wiley, 2005; L. Piela, in: Ideas
of Quantum Chemistry, Elsevier, 2006.


Magnetic Liquid Crystals for Molecular Spintronics

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Magnetic Liquid Crystals for Molecular Spintronics

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