Magnetic memory based on magnetic alignment of a paramagnetic ionic liquid near room temperature

Article abstract of DOI:10.1039/c0cc05820c

A paramagnetic ferrocenium-based ionic liquid that exhibits a magnetic memory effect coupled with a liquid-solid phase transformation has been developed. Based on field alignment of the magnetically anisotropic ferrocenium cation, the magnetic susceptibility in the solid state can be tuned by the weak magnetic fields (<1 T) of permanent magnets.

Full text of DOI:10.1039/c0cc05820c

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COMMUNICATION
Magnetic memory based on magnetic alignment of a paramagnetic ionic
liquid near room temperaturew
Yusuke Funasako,a Tomoyuki Mochida,*a Takashi Inagaki,a Takahiro Sakurai,b
Hitoshi Ohta,c Ko Furukawad and Toshikazu Nakamurad
Received 28th December 2010, Accepted 23rd February 2011
DOI: 10.1039/c0cc05820c
A paramagnetic ferrocenium-based ionic liquid that exhibits a
magnetic memory effect coupled with a liquid–solid phase
transformation has been developed. Based on field alignment
of the magnetically anisotropic ferrocenium cation, the magnetic
susceptibility in the solid state can be tuned by the weak
magnetic fields (o1 T) of permanent magnets.
Fig. 1 Chemical formula of [butyloctamethylferrocenium][TFSA] (1).
effect coupled with a liquid–solid phase transformation near
room temperature.
For many years, molecular magnetic materials, whose properties
can be controlled by external stimuli, have attracted considerable
interest for their potential use in electronic devices.1 In practice,
magnetic changes and phase-change mechanisms have been
used for reversible recording of information.2 In this study, we
aimed to design materials that exhibit a magnetic memory
effect coupled with a liquid–solid phase change. It is known
that very strong magnetic fields can control the orientation of
inorganic materials, polymers, and molecular materials during
the solidification process.3 We expected that magnetic fields
would efficiently affect solidification of magnetically anisotropic
molecular liquids. Among such fluids, we have focused on
ionic liquids, which are defined as salts that melt below 100 1C.4
Several intriguing ionic liquids comprising onium cations and
magnetic anions such as MXn (M = transition metal or
rare-earth metal)5 and nitroxides6 have been reported, and
their magnetic properties have attracted special attention.
These magnetic anions, however, are magnetically isotropic.
We recently found that ferrocenium salts with fluorinated
anions form ionic liquids,7 and that these materials contain
paramagnetic cations with magnetic anisotropy. Here we
report that a paramagnetic ferrocenium-based ionic liquid,
[butyloctamethylferrocenium][TFSA] (1, TFSA = bis(trifluoro-
methanesulfonyl)amide, Fig. 1), exhibits a magnetic memory
A dark green ionic liquid of 1 was obtained in almost
quantitative yield by reacting butyloctamethylferrocene with
AgTFSA in CH2Cl2.8 The temperature dependence of the
magnetic susceptibility of 1 was measured with a SQUID
magnetometer, which showed that 1 was paramagnetic in the
measured temperature range of 2–330 K.8 The susceptibility
around room temperature is shown in Fig. 2. The wT value in
the liquid state (0.78 emu K mol–1), which is typical of
ferrocenium cations, is larger than the spin-only value because
of the orbital contribution.9 When the liquid was cooled under
0.5 T (Fig. 2, filled circles), a significant increase in the
susceptibility was observed in association with solidification
at 299 K. Heating the solid led to melting at a higher
temperature, 309 K, where the magnetic susceptibility
decreased and returned to the initial value. Thus, the magnetic
a Department of Chemistry, Graduate School of Science,
Kobe University, Rokkodai, Nada, Hyogo 657-8501, Japan.
E-mail: tmochida@platinum.kobe-u.ac.jp; Fax: +81 78 803 5679;
Tel: +81 78 803 5679
b Center for Supports to Research and Education Activities,
Kobe University, Rokkodai, Nada, Hyogo 657-8501, Japan
c Molecular Photoscience Research Center, Kobe University,
Rokkodai, Nada, Hyogo 657-8501, Japan
d Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585,
Japan
w Electronic supplementary information (ESI) available: Experimental
procedures, preparation, details on thermal and magnetic properties
for compound 1. CCDC 775548. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0cc05820c
Fig. 2 Temperature dependence of magnetic susceptibilities of 1
shown as a wT–T plot measured under 0.5 T (filled circles) and 0.1 T
(open circles).
c
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Fig. 5 Crystal structure of 1 at 100 K. This figure also illustrates the
direction of the field orientation of the crystal with respect to the
external field (H) indicated by the arrow.
Fig. 3 Magnetic susceptibility changes of 1 (DwT = wT270 K – wT310 K
)
plotted as a function of magnetic field strength (open circles). Data are
were well reproduced by these values. When quartz wool
(14 wt%) was added to the liquid sample to inhibit magnetic
orientation during solidification, the susceptibility change DwT
was effectively suppressed (Fig. 3, filled circles).
also shown for a sample filled with quartz wool (filled circles).
response accompanies the thermal hysteresis based on phase
changes.10 Under 0.1 T, however, only a small change was
observed at the phase change (Fig. 2, open circles). This result
shows that the magnetic susceptibility in the solid state can be
controlled by the intensity of the magnetic field that is applied
upon solidification.
For magnetic orientation to occur, it is important that the
crystal structure is magnetically anisotropic. Therefore, the
molecular arrangement of 1 in the solid state was determined
at 100 K (Fig. 5).11 In the unit cell, the C5 axes of ferrocenium
cations are aligned parallel to the c-axis. Given that the largest
principal value of the g-tensor of a ferrocenium cation is along
its C5 axis,12 this arrangement produces the largest magnetic
susceptibility along the c-axis. Out-of-plane XRD patterns for
samples crystallized under magnetic fields exhibited a significant
increase in the intensities of peaks corresponding to [0 0 2],
indicating that the c-axis is oriented perpendicular to the
surface. These results demonstrate that the field orientation
is consistent with the magnetic anisotropy of the crystal.
In this study, control of the magnetic response near room
temperature was achieved using a paramagnetic ionic liquid
with magnetic anisotropy. Without using magnetic ordering,
the memory mechanism cannot be realized by typical molecular
magnets, inorganic materials, or conventional magnetic fluids.
In particular, it is highly advantageous that the phenomenon is
(i) observable near room temperature, (ii) accompanied by
hysteresis, and (iii) controllable by weak magnetic fields
(o1 T). Although the magnetic-field alignment of materials
upon solidification has been widely investigated,3a–c very
strong magnetic fields or field gradients are required. To our
knowledge, the remarkable magnetic orientation we observed
under weak magnetic fields is hitherto unknown. Magnetic
susceptibility changes coupled with a liquid–solid transformation
are also observed in several molecular materials. For
example, slight susceptibility changes are observed for para-
magnetic ionic liquids such as [ethylmethylimidazolium][FeCl4]
(DwTsolidÀliquid E 0.05 emu K mol–1).5a Another example is the
transformation of diamagnetic organic solids composed of
radical dimers to a paramagnetic liquid upon melting.13 In
these cases, however, the magnetic changes are smaller and
cannot be controlled by external fields.
Next, the detailed field dependence of the magnetic suscept-
ibility changes was investigated, the results of which are
plotted in Fig. 3 (open circles). The value increases as a
function of the magnetic field, exhibiting saturation above
2 T. Therefore, the material can record the applied magnetic
field up to about 1 T. The dependencies of the magnetic
susceptibility on the temperature and magnetic field were
independent of the scan rate in the range of 0.5–10 K min–1.
The magnetic response is attributed to the magnetic-field
orientation of the material upon solidification, which can be
observed directly by polarized optical microscopy (POM).
Fig. 4a shows the POM image of a liquid of 1 on a glass plate
at room temperature. Solidification of the liquid in the absence
of magnetic fields resulted in the formation of microscopic
domains (Fig. 4b). However, under a magnetic field of
permanent magnets (0.6 T), the liquid crystallized into needles
arranged perpendicular to the field (Fig. 4c). The magnetic
orientation was quantitatively confirmed by observing
angle-dependent ESR spectra at 3.8 K on a sample crystallized
under a magnetic field of 0.8 T. The anisotropic g values
obtained from the angular dependence were g> = 4.3,
g8 = 1.7, and gav = 2.8. The SQUID magnetic susceptibilities
Fig. 4 Polarized optical microscopy images of 1 under plain-polarized
light on a glass plate at RT. (a) Liquid state, (b) after solidification
without magnetic field, and (c) after solidification under a magnetic
field of 0.6 T applied parallel to the surface. The optical axis is
indicated by the arrow in (a).
In summary, a paramagnetic ferrocenium-based ionic liquid
[butyloctamethylferrocenium][TFSA] that exhibits a magnetic
response due to field-oriented solidification near room
temperature has been developed. This phenomenon was achieved
c
4476 Chem. Commun., 2011, 47, 4475–4477
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by a room-temperature ionic liquid and a ferrocenium ion with
magnetic anisotropy. The physical chemistry of liquids, in
addition to the characteristic magnetic features as well as the
flexibility of the material, may lead to novel molecular electronic
applications using molecular liquids.
Industrial Applications to Green Chemistry, ed. R. D. Rogers and
K. R. Seddon, ACS Symposium Series, American Chemical
Society, Washington, DC, 2002; (c) M. Armand, F. Endres,
D. R. MacFarlane, H. Ohno and B. Scrosati, Nat. Mater., 2009,
8, 621; (d) P. Hapiot and C. Lagrost, Chem. Rev., 2008, 108, 2238;
(e) N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37,
123; (f) H. Weingartner, Angew. Chem., Int. Ed., 2008, 47, 654;
¨
This work was supported by a Grant-in-Aid for Scientific
Research (No. 21350077) from the Japan Society for the
Promotion of Science; Research for Promoting Technological
Seeds A (No. 11-145, 2009) from the Japan Science and
Technology Corporation, and the IMS (Institute for Molecular
Science) Joint Studies Program. We thank Prof. Y. Shinoda
(Waseda University) for his English support and Y. Furuie
(Kobe University) for the elemental analysis.
(g) S. Lee, Chem. Commun., 2006, 1049; (h) Y. Yoshida and
G. Saito, Phys. Chem. Chem. Phys., 2010, 12, 1675;
(i) T. Torimoto, T. Tsuda, K. Okazaki and S. Kuwabata, Adv.
Mater., 2010, 22, 1196.
5 (a) Y. Yoshida, A. Otsuka, G. Saito, S. Natsume, E. Nishibori,
M. Takata, M. Sakata, M. Takahashi and T. Yoko, Bull. Chem.
Soc. Jpn., 2005, 78, 1921; (b) M. Okuno, H. Hamaguchi and
S. Hayashi, Appl. Phys. Lett., 2006, 89, 132506; (c) B. Mallick,
B. Balke, C. Felser and A.-V. Mudring, Angew. Chem., Int. Ed.,
¨
2008, 47, 7635; (d) T. Peppel, M. Kockerling, M. Geppert-
Rybczynska, R. V. Ralys, J. K. Lehmann, S. P. Verevkin and
´
Notes and references
A. Heintz, Angew. Chem., Int. Ed., 2010, 49, 7116;
(e) B. M. Krieger, H. Y. Lee, T. J. Emge, J. F. Wishart and
E. W. Castner, Jr., Phys. Chem. Chem. Phys., 2010, 12, 8919.
6 Y. Yoshida, H. Tanaka and G. Saito, Chem. Lett., 2007, 36, 1096.
7 T. Inagaki and T. Mochida, Chem. Lett., 2010, 39, 572.
8 See ESIw for details.
9 J. S. Miller, D. T. Glatzhofer, D. M. O’Hare, W. M. Reiff,
A. Chakraborty and A. J. Epstein, Inorg. Chem., 1989, 28,
2930.
10 Y. Funasako, K. Abe and T. Mochida, Thermochim. Acta, 2011,
DOI: 10.1016/j.tca.2011.01.013.
11 Crystallographic parameters for 1: Pna21, a = 26.602(3) A,
b = 8.773(5) A, c = 11.901(6) A, V = 2777.8(5) A3, Z = 4 and
R1 = 0.0254. See ESIw for details.
1 (a) J. S. Miller and A. J. Epstein, Angew. Chem., Int. Ed. Engl.,
1994, 33, 385; (b) O. Kahn, Molecular MagnetismWiley-VCH, New
York, 1993; (c) W. Fujita and K. Awaga, Science, 1999, 286, 261;
(d) M. E. Itkis, X. Chi, A. W. Cordes and R. C. Haddon, Science,
2002, 296, 1443; (e) O. Sato, J. Tao and Y.-Z. Zhang, Angew.
Chem., Int. Ed., 2007, 46, 2152.
2 (a) J. Feinleib, J. P. deNeufville, S. C. Moss and S. R. Ovshinsky,
Appl. Phys. Lett., 1971, 18, 254; (b) A. Hamada, T. Kurosu,
M. Saito and M. Kikuchi, Appl. Phys. Lett., 1972, 20, 9.
3 (a) P. de Rango, M. Lees, P. Lejay, A. Sulpice, R. Tournier,
M. Ingold, P. Germi and M. Pernet, Nature, 1991, 349, 770;
(b) A. E. Mikelson and Y. K. Karklin, J. Cryst. Growth, 1981,
52, 524; (c) T. Sugiyama, M. Tahashi, K. Sassa and S. Asai, ISIJ
Int., 2003, 43, 855; (d) H. Ezure, T. Kimura, S. Ogawa and E. Ito,
Macromolecules, 1997, 30, 3600; (e) S. Takami, Y. Shirai,
Y. Wakayama and T. Chikyow, Thin Solid Films, 2008, 516, 2438.
4 (a) A. Stark and K. R. Seddon, in Kirk-Othmer Encyclopedia of
Chemical Technology, ed. A. Seidel, Wiley-Interscience, Hoboken,
New Jersey, 5th edn, 2007, vol. 26, p. 836; (b) Ionic Liquids:
12 J. S. Miller, J. C. Calabrese, H. Rommelmann, S. R. Chittipeddi,
J. H. Zhang, W. M. Reiff and A. J. Epstein, J. Am. Chem. Soc.,
1987, 109, 769.
13 (a) K. Nishimura and G. Saito, Synth. Met., 2005, 153, 385;
(b) W. Fujita, K. Awaga, Y. Nakazawa, K. Saito and M. Sorai,
Chem. Phys. Lett., 2002, 352, 348.
c
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