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Ohkoshi laboratory are trying to open a new field of solid state
chemistry by design and synthesis of novel magnets which have novel
properties and functionalities. The summaries are described by the
phenomena as follows.
1. Synthesis of a metal oxide with a room-temperature photoreversible phase transition
2. Light-induced spin-crossover magnet
3. Synthesis of metal complexes with
novel magnetic functionalities
4. Magnetic property in metal oxides
1. Synthesis of a metal oxide with a room-temperature photoreversible phase transition
We have discovered a new type of metal oxide which shows reversible transition between metal and semiconductor by photo irradiation at room temperature. This new type of metal oxide (lambda type trititanium pentoxide: λ-Ti3O5) (Hereafter, it is called lambda type titanium oxide) was synthesized by the chemical technique using surfactant. This material shows photoinduced phase transition (photoinduced metal-insulator transition) from black colored lamda phase (metallic conductor) to brown colored beta phase (β-Ti3O5) (semiconductor). Moreover, the reverse phase transition was also observed by the photoirradiation. This is the first example of a metal oxide which shows photorewritable phenomenon at room temperature. Lamda titanium oxide is a simple material that consists only of titanium atom and oxygen atom, and hence it is very economical and environmentally benign material. Lamda titanium oxide is promising as a next generation optical storage material because it is obtained in a particle size of 10-20 nm. Furthermore, the lamda titanium oxide can be obtained by a simply calcination of the commercial TiO2 photocatalyst under hydrogen, which is an industrial advantage from the viewpoint of cost and mass production.
(S. Ohkoshi et al., Nature Chemistry, (2010).)
＊This topic appeared on Nature Chemistry "News & Views", Nature Japan, Agence France-Presse (AFP), NHK news, and other many journals and newspapers.
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2. Light-induced spin-crossover magnet
We discovered a new type of photomagnet which exhibits transition from paramagnet (nonmagnetically ordered phase) to ferromagnet (magnetically ordered phase) by the blue light irradiation. The photomagnetic phase shows a ferromagnetic phase transition temperature of 20 K, which is caused by light-induced spin-crossover phenomenon, and returns to the original paramagnetic state by thermal annealing. This material is composed of iron(II) ion, niobium ion, organic ligand (4-pyridinealdoxime), and cyano group. This light-induced spin-crossover magnet contains a lot of organic molecules and this may be the first step toward flexible optical-magnetic material.
(S. Ohkoshi et al., Nature Chemistry, (2011).)
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3. Synthesis of metal complexes with
novel magnetic functionalities
(1)Molecule-based magnet with a high Curie temperature (TC= 210 K)
We prepared a cyano-bridged V-Nb bimetal assembly, K0.59VII1.59VIII0.41[NbIV(CN)8]・(SO4)0.50・6.9H2O, exhibiting ferrimagnetism with a high Curie temperature (TC) of 210 K, which is the highest TC value among octacyano-bridged bimetal assemblies. Such a high TC value derives from the high coordination number of octacyanoniobate and the strong superexchange interaction between VII(S = 3/2) and NbIV(S = 1/2) via the CN groups.
(K. Imoto, S. Ohkoshi et al., Eur. J. Inorg. Chem., 2649-2652 (2012).)
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(2)Spin-ionics
We observed high proton conductivities on Co[Cr(CN)6]2/3・zH2O and V[Cr(CN)6]2/3・zH2O, respectively, and an interference effect between magnetic ordering and ionic conduction below magnetic phase transition temperature. The observation of such an interference effect may open a new field, so-called, "spin-ionics".
(S. Ohkoshi et al., J. Am. Chem. Soc., (2010).)
＊Research Highlights of Nature Asia Materials
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(3)Ferroelectric-ferromagnetic cyano-bridged
metal assembly
We have observed the coexistence of ferroelectricity
and ferromagnetism in Rb0.82Mn[Fe(CN)6]0.94・H2O.
The ferroelectricity is due to the mixing of Fe vacancies and
FeII, FeIII, MnII, and Jahn-Teller-distorted
MnIII centers, and the ferromagnetism is mainly caused
by a parallel ordering of the magnetic spins on the MnIII
centers.
(S. Ohkoshi et al., Angew. Chem. Int. Ed., (2007).)
＊highlighted in the frontispiece
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(4)Ferroelectricity in paramagnetic metal assembly
We have observed the ferroelectricity in a copper octacyanomolybdate-based paramagnet, Cu2[Mo(CN)8]・8H2O (CuII: S= 1/2, MoIV: S= 0). This compound has a freezing point for the fixation of hydrogen bonding at 150 K. Around this temperature, an enhancement in the ferroelectricity and an increase in the dielectric constant are observed. The ferroelectricity of this system is classified into amorphous ferroelectrics, i.e., the electric poling effect induces an electric polarization, maintained by the structural local-disorder of hydrogen bonding and the 3-dimensional CN network.
(K. Nakagawa et al., Inorg. Chem., 47,
10810 (2008).) |
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(5)Chemical-sensing magnet (Alcohol vapor)
A copper(II) octacyanotungsten(V)-based ferromagnet, CuII3[WV(CN)8]2(pyrimidine)2・8H2O,
was prepared. This magnetic material can reversibly adsorb and
desorb n-propanol vapor, and shows reversible variations in
the crystal structure and magnetic properties. These changes
are due to the coordination geometry switching of CuII
between 6-coordinate and 5-coordinate.
(S. Ohkoshi et al., J. Am. Chem. Soc., 129,
3084 (2007).) |
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(6) Design and Preparation of a Novel
Magnet Exhibiting Two Compensation Temperatures
We have prepared a novel type of magnet exhibiting two compensation
temperatures; i.e., the spontaneous magnetization changes its sign
twice with changing temperature. The key to obtaining this unusual
behavior is the simultaneous incorporation of one antiferromagnetic
and two different ferromagnetic interactions through the use of
four different spin sources, as predicted by a calculation based
on molecular field theory. As a prototype exemplifying this idea,
we have prepared a Prussian blue analog, (NiII0.22MnII0.60FeII0.18)[CrIII(CN)6]2/3・5.1H2O,
which exhibits magnetization reversals at 35 and 53 K.
(S. Ohkoshi et al., Phys. Rev. Lett.,
82, 1285 (1999).) |
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(7) Design and preparation of a bulk magnet exhibiting an inverted hysteresis
loop
We have prepared a bulk magnet exhibiting an inverted magnetic hysteresis
loop, i.e., the magnetization becomes negative in the decreasing
part even when the applied field is still positive, while the magnetization
becomes positive in the increasing part when the applied field is
still negative. The key to obtaining this unusual magnet is to utilize
a competing effect between the spin-flip transition and the uniaxial
magnetic anisotropy. On the basis of the model calculations considering
this effect, we have succeeded in synthesizing a bulk magnet, (SmIII0.52GdIII0.48)[CrIII(CN)6]・4H2O
showing the inverted magnetic hysteresis loop.
(S. Ohkoshi et al., Phys. Rev. B, 64,
132404 (2001).) |
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(8) Humidity-induced magnetization and magnetic pole inversion in a cyano-bridged
metal assembly
We have observed humidity-induced
reversible variations in the magnetic properties of cyano-bridged
metal assemblies, (CoII0.41MnII0.59)[CrIII(CN)6]2/3・zH2O.
The observed magnetic humidity response is due to adsorption
and desorption of a ligand water molecule on the cobalt ion,
which changes CoII between a 6- and 4-fold coordination
geometry and switches the magnetic interaction between ferromagnetic
coupling and antiferromagnetic coupling.
(S. Ohkoshi et al., Nat. Mater., 3,
857 (2004).) |
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(9) A high-spin cluster, a two-dimensional metamagnet compound,
and a pillared layer molecular-based material
We have synthesized molecular-based magnets built by octacyanometalates
[M(CN)8]n- (M = Mo, W) with
controlled its dimensionality. As a zero-dimensional magnet,
{Mn9[W(CN)8]・24C2H5OH}
cluster with the highest ground-state S = 39/2 was synthesized.
Two-dimensional cyanide-bridged copper(II) octacyanotungstates
were prepared and these compounds exhibited metamagnetic behavior
with Neel temperatures. We reported a three-dimensional manganese(II)
octacyanotungsten(V)-based magnet which contains a noncoordinated
aromatic molecule.
(S. Ohkoshi et al., Chem.Commun.2003, JACS
2000, JACS 2003, and JACS 2004) |
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(10) Coexitence of Fe(II) spin crossover and ferromagnetism
Thermal phase transition phenomenon was observed in CsFe[Cr(CN)6]・1.3H2O, due to a spin-crossover on the Fe(II) sites. This compound also showed a spontanepus magnetization with a magnetic ordering temperature of 9 K. This is a first compound which shows spin-crossover and ferromagnetism.
(S. Ohkoshi et al., J. Am. Chem. Soc., 127, 8590 (2005). ) |
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(11) A ferrimagnet with a continuous spin-crossover phenomenon
A spin-crossover from FeII(S=2)-NbIV(S=1/2)-FeII(S=2) to FeII(S=0)-NbIV(S=1/2)-FeII(S=2) occurs in Fe2[Nb(CN)8]・(3-pyCH2OH)8・4.6 H2O (3-py=3-pyridyl) with decreasing temperature. The low-temperature phase shows ferrimagnetism with a Curie temperature of 12 K owing to an antiferromagnetic interaction between the remaining FeII (S=2) and the NbIV (S=1/2) centers.
(M. Arai et al., Angew.Chem. Int. Ed., 47, 6885 (2008) ) |
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(12) Photomagnetism
We have synthesized five types of photomagnetic materials, i.e. photo-induced
magnetic pole-inversion material of (Fe0.40Mn0.60)1.5[Cr(CN)6],
time-dependent photomagnetic material of RbMn[Fe(CN)6],
visible light reversible photomagnetic material of Cu2[Mo(CN)8],
and photomagnetic material of [{Co(3-CNpy)2}{W(CN)8}]
and Fe[Cr(CN)6]. The examples are as follows;
(A) Photoinduced magnetic pole inversion
We designed the magnet exhibiting magnetic pole (N and S)
inversion by photostimuli. A ferro-ferrimagnet (FeII0.40MnII0.60)1.5CrIII(CN)6・7.5H2O
mixed by ferromagnetic (Fe-Cr system showing the change of magnetization
by optical stimuli) site and ferrimagnetic (Mn-Cr system showing
no optical response) site showed negative magnetization at the
temperature lower than compensation temperature (Tcomp
= 19 K). In this mixed metal cyanide magnet we have succeeded
in demonstrating a novel magnetic behavior ‘‘photoinduced magnetic
pole inversion’’. Moreover, the magnetic pole inversion can
be induced repeatedly by alternate optical and thermal stimulations.
(S. Ohkoshi et al., Appl. Phys. Lett., 70,
1040 (1997); J. Am. Chem. Soc., 121, 10591
(1999).) |
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(B) Time dependent photo-induced magnetic phase
We studied on the photomagnetic effect on RbMn[Fe(CN)6].
By irradiation with only one-shot of laser pulse, magnetization
of the LT phase was disappeared at 3 K. In contrast, when the
LT phase was irradiated by the weak CW laser light at 3 K, the
spontaneous magnetization suddenly disappeared. This demagnetization
state was maintained for several minuets, and then, the magnetization
abruptly recovered to the initial state.
(S. Ohkoshi et al., J. Phys. Chem B, 106,
2423 (2002); H. Tokoro et al., Appl. Phys. Lett.,
82, 1245 (2003).) |
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(C) Photo-induced reversible magnetization
We have syntesized a visible light-induced reversible photomagnetism
in CuII2[MoIV(CN)8]・8H2O.
By the irradiation with a 473 nm blue light, a spontaneous magnetization
with a magnetic ordering temperature (Tc)
of 30 was observed. Conversely, the magnetization and Tc
values were reduced by the irradiation with light above 520
nm. This photomagnetism is due to the reversible photo-induced
electron transfer between paramagnetic CuII-NC-MoIV
and its valence isomer CuI-NC-MoV exhibiting
ferromagnetism.
(S. Ohkoshi et al., Chem. Lett., 4,
312 (2001); J. Am. Chem. Soc., 128, 270
(2006); T. Hozumi et al., J. Am. Chem. Soc., 127,
3864 (2005).) |
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(D) A high-performance photo-reversible magnet
A high-performance photo-reversible magnet has been developed. The prepared material, Co3[W(CN)8]2(pyrimidine)4・6H2O, has the three-dimensional crystal structure composed of Co and W ions. When this material was irradiated by 840 nm light, the non-magnetic (paramagnetic) phase was changed to the magnetic (ferromagnetic) phase. This photo-generated magnetic phase showed a Curie temperature of 40 K (Kelvin) and magnetic coercive field of 12 kOe (kilo Oernsted). These values are the highest values in photo-magnets reported so far. In addition, when 532 nm light was irradiated to the photo-generated magnetic phase, the reverse change, i.e., from the magnetic phase to the non-magnetic phase, was observed. This material is a high-performance photo-reversible magnet for an optical magnetic memory device on next generation (for example, optical magnetic memory media of needless external magnetic filed).
(S. Ohkoshi et al., Chem. Mater., 20,
3048 (2008).)
＊highlighted at the Cover picture
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(13) Colored magnetic films composed of cyano-bridged metal
assemblies and magneto-optical functionalities
Magnetic thin films of (FeIIxCrII1-x)3[CrIII(CN)6]2・15H2O
was electrochemically prepared by reducing aqueous solutions
containing three compounds of K3[Cr(CN)6],
CrCl3, and FeCl3.
Their colors could be controlled by controlling the compositional
factor x, e.g., colorless (x = 0), violet
(x = 0.20), red (x = 0.42), and orange (x
= 1). We also synthesized film-type vanadium hexacyanochromate-based
magnets with high critical temperatures (Tc
= 345 K) by electrochemically method. With these films, we have
succeeded in observing the Faraday spectra in a ferromagnetic
region for the first time among molecular-based magnet.
(S. Ohkoshi et al., J. Am. Chem. Soc., 120,
5349 (1998); J. Phys. Chem. B, 104,
9365 (2000).) |
(14) Magnetization-induced second harmonic generation (MSHG)
Second harmonic generation (SHG) and magnetization-induced
second harmonic generation (MSHG) were observed in pyroelectric
ferromagnets of (FexCr1-x)1.5[Cr(CN)6]・7.5H2O
thin films and piezoelectric ferromagnets of RbMn[Fe(CN)6]
and CsCo[Cr(CN)6]･0.5H2O.
(S. Ohkoshi et al., Electrochem. Soc. Interface,
34 (2002); K. Ikeda et al, Chem. Phys.
Lett., 349, 371 (2001); T. Nuida et al,
J. Am. Chem. Soc., 127, 11604 (2005).) |
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4. Magnetic property in metal oxides
(1) Giant coercive field of nanometer-sized iron oxide
We have recently presented a nanocrystal of iron oxide in silica
matrix that exhibited the largest Hc value
reported to date, 20 kOe at room temperature. This nanocrystal consisted
of a particular phase of iron oxide ε-Fe2O3
and was obtained in silica matrix by combining reverse-micelle and
sol-gel techniques.
(S. Ohkoshi et al., J. Appl. Phys., 97,
10K312 (2005); J. Jin et al., Adv. Mater., 16,
48 (2004)) |
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(2) Magnetization-induced third harmonic generation (MTHG)
MTHG in a Bi,Al:YIG thin film was observed. In this phenomenon,
a longitudinal external magnetic field to a Bi,Al:YIG magnetic
film rotated the polarization plane of the TH wave. This TH
rotation is understood by the contribution of the magnetic term
in a third-order nonlinear optical susceptibility.
(S. Ohkoshi et al., J. Opt. Soc. Am. B, 22,
196 (2005); J. Shimura et al., Appl. Phys. Lett.,
82, 3290 (2003).) |
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(3) Millimeter Wave Absorption
Finding a material that effectively
restrains electromagnetic interference in the region of millimeter
wave has received much attention. Herein, we report a new EM
absorber composed of ε-GaxFe2-xO3
(0.10 ≦ x ≦ 0.67) nanomagnets, which shows a ferromagnetic
resonance in the region of 35-147 GHz. The possibility that
the ferromagnetic resonance can achieve 190 GHz at x
→ 0 is also suggested. The magnetic material absorbing millimeter
wave of such a high frequency has not been reported to date.
(S. Ohkoshi et al., Angew. Chem. Int. Ed.,
46, 8392 (2007).)
＊highlighted at the Inside Cover |
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(4) High performance and cheap electromagnetic wave absorber
for next generation wireless communications
A new and cheap electromagnetic (EM) wave absorber for high-speed wireless communication, which absorbs millimeter wave in the region above 180 GHz was developed. Wireless communication using millimeter wave receives one's attention as a next generation wireless communication system which realizes a fast exchange of huge data such as movie. On the other hand, electromagnetic interference (EMI) is a fatal problem in a wireless communications. To avoid potential health effect from high exposure to EM waves, unnecessary EM waves should be eliminated to protect human bodies, especially expectant mothers and children. Ohkoshi et al. developed a high-performance millimeter wave absorber composed of a series of aluminum-substituted ε-iron oxide, ε-AlxFe2-xO3, nanomagnets (0 ≦ x ≦ 0.66). This materials shows frequency selective EM absorption in the region of 94-182 GHz, depending on chemical composition. ε-AlxFe2-xO3 are metal oxide, therefore they are stable over long periods. Furthermore, because aluminum is the third abundant atom, ε-AlxFe2-xO3 are very economical, and thus, these materials are advantageous for industrial applications. Considerable application of these materials are (1) building materials for a medical room or office, paint on the bodies of a car, train, or airplane as for the prevention of EMI, (2) electronic devices such as isolator or circulator for stabilizing wireless communications.
( A. Namai, et al., J. Am. Chem. Soc., 131, 1170 (2009).)
Please see also "newspaper, No. 80,81"
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(5) Ferroelectric ferromagnet
(PLZT)x(BiFeO3)1-x
solid solutions ( PLZT : (Pb0.9La0.1)(Zr0.65Ti0.35)O3
) have been prepared by the solid-state reaction. The materials
for x = 0.10-0.45 showed both magnetic hysteresis loops
and ferroelectric hysteresis loops at room temperature. Moreover,
a film-type of the material has been prepared by a sol-gel method
and MSHG was observed with this film.
(T.Kanai and S. Ohkoshi et al., Adv. Mater.,
13, 487 (2001). )
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