Publications by authors named "Yoshinori Kotani"

17 Publications

  • Page 1 of 1

Dehydration of Electrochemically Protonated Oxide: SrCoO with Square Spin Tubes.

J Am Chem Soc 2021 Oct 14;143(42):17517-17525. Epub 2021 Oct 14.

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan.

Controlling oxygen deficiencies is essential for the development of novel chemical and physical properties such as high- superconductivity and low-dimensional magnetic phenomena. Among reduction methods, topochemical reactions using metal hydrides (e.g., CaH) are known as the most powerful method to obtain highly reduced oxides including NdSrNiO superconductor, though there are some limitations such as competition with oxyhydrides. Here we demonstrate that electrochemical protonation combined with thermal dehydration can yield highly reduced oxides: SrCoO thin films are converted to SrCoO by dehydration of HSrCoO at 350 °C. SrCoO forms square (or four-legged) spin tubes composed of tetrahedra, in contrast to the conventional infinite-layer structure. Detailed analyses suggest the importance of the destabilization of the SrCoO precursor by electrochemical protonation that can greatly alter reaction energy landscape and its gradual dehydration (HSrCoO) for the SrCoO formation. Given the applicability of electrochemical protonation to a variety of transition metal oxides, this simple process widens possibilities to explore novel functional oxides.
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http://dx.doi.org/10.1021/jacs.1c07043DOI Listing
October 2021

X-ray study of ferroic octupole order producing anomalous Hall effect.

Nat Commun 2021 Sep 22;12(1):5582. Epub 2021 Sep 22.

Institute for Materials Research, Tohoku University, Sendai, Miyagi, 980-8577, Japan.

Recently found anomalous Hall, Nernst, magnetooptical Kerr, and spin Hall effects in the antiferromagnets MnX (X = Sn, Ge) are attracting much attention for spintronics and energy harvesting. Since these materials are antiferromagnets, the origin of these functionalities is expected to be different from that of conventional ferromagnets. Here, we report the observation of ferroic order of magnetic octupole in MnSn by X-ray magnetic circular dichroism, which is only predicted theoretically so far. The observed signals are clearly decoupled with the behaviors of uniform magnetization, indicating that the present X-ray magnetic circular dichroism is not arising from the conventional magnetization. We have found that the appearance of this anomalous signal coincides with the time reversal symmetry broken cluster magnetic octupole order. Our study demonstrates that the exotic material functionalities are closely related to the multipole order, which can produce unconventional cross correlation functionalities.
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http://dx.doi.org/10.1038/s41467-021-25834-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8458343PMC
September 2021

Strain-induced modulation of temperature characteristics in ferrimagnetic Tb-Fe films.

Sci Rep 2021 Mar 18;11(1):6237. Epub 2021 Mar 18.

Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka, 567-0047, Japan.

This study investigates the effect of strain on the compensation temperature of ferrimagnetic Tb-Fe films formed on a flexible substrate. The compensation temperature is determined by the anomalous Hall measurement, and an application of 1.2% tensile strain reduces the compensation temperature by 12 K. X-ray magnetic circular dichroism reveals that approximately 5% of Fe magnetic moment and approximately 1% of Tb magnetic moment are reduced by an application of 0.9% tensile strain at the room temperature. To understand the greater reduction in Fe magnetization compared with that in Tb and the compensation temperature reduction simultaneously, a model applying molecular field theory is analyzed. Changes in three types of exchange coupling between Fe and Tb atoms are speculated to be caused by the strain.
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http://dx.doi.org/10.1038/s41598-021-85642-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7973561PMC
March 2021

Visualization of elemental distributions and local analysis of element-specific chemical states of an Arachnoidiscus sp. frustule using soft X-ray spectromicroscopy.

PLoS One 2020 16;15(12):e0243874. Epub 2020 Dec 16.

Soft X-ray Spectroscopy Instrumentation Team, Physical and Chemical Research Infrastructure Group, Advanced Photon Technology Division, RIKEN SPring-8 Center, Sayo-gun, Hyogo, Japan.

Using soft X-ray (SX) spectromicroscopy, we show maps of the spatial distribution of constituent elements and local analysis of the density of states (DOS) related to the element-specific chemical states of diatom frustules, which are composed of naturally grown nanostructured hydrogenated amorphous silica. We applied X-ray photoemission electron microscopy (X-PEEM) as well as microprobe X-ray fluorescence (μXRF) analysis to characterize the surfaces of diatom frustules by means of X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES). We successfully demonstrated that SX spectromicroscopy is able to participate in potential observation tools as a new method to spectroscopically investigate diatom frustules.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0243874PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7743981PMC
February 2021

Detection of Spin Transfer from Metal to Molecule by Magnetoresistance Measurement.

Nano Lett 2020 Jan 10;20(1):75-80. Epub 2019 Dec 10.

Graduate School of Engineering Science , Osaka University , Toyonaka , Osaka 560-8531 , Japan.

Localized electronic spin state in molecules has a relatively long spin lifetime and has thus attracted much attention. In this study, we characterize the magnetoresistance of a system comprising Pt and Fe(II)-phthalocyanine (FePc) molecules. The magnetoresistance measurement with the weak antilocalization analysis reveals that a magnetic moment in FePc acts as magnetic impurities for conduction electrons in Pt. Moreover, we find that the magnetoresistance involves a component that possesses the same symmetry as spin-Hall magnetoresistance. These results reveal the spin-angular momentum transfer from metallic Pt to a magnetic moment in FePc molecules, which can be used as a spin torque in a molecular system.
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http://dx.doi.org/10.1021/acs.nanolett.9b03110DOI Listing
January 2020

Electron-Transfer Activity in a Cyanide-Bridged Fe Nanomagnet.

Inorg Chem 2019 Aug 11;58(15):10160-10166. Epub 2019 Jul 11.

Institute for Materials Research , Tohoku University , Katahira , Sendai 980-8577 , Japan.

The ability to switch a molecule between different magnetic states is of considerable importance for the development of new molecular electronic devices. Desirable properties for such applications include a large-spin ground state with an electronic structure that can be controlled via external stimuli. Fe is a cyanide-bridged stellated cuboctahedron of mixed-valence Fe ions that exhibits an extraordinarily large = 45 spin ground state. We have found that the spin ground state of Fe can be altered by controlling the humidity and temperature. Dehydration results in a 15 μ reduction of the saturation magnetization that can be partially recovered upon rehydration. The complementary use of UV-vis, IR, L-edge X-ray absorption spectroscopy and X-ray magnetic circular dichroism is applied to uncover the mechanism for the observed dynamic behavior. It is identified that dehydration is concurrent with metal-to-metal electron transfer between Fe pairs via a cyanide π hybridization. Upon dehydration, electron transfer occurs from low-spin {Fe(Tp)(CN)} sites to high-spin Fe centers. The observed reduction in magnetization upon dehydration of Fe is inconsistent with a ferrimagnetic ground state and is proposed to originate from a change in zero-field splitting at electron-reduced high-spin sites.
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http://dx.doi.org/10.1021/acs.inorgchem.9b01216DOI Listing
August 2019

Effective protocol for realizing contamination-free X-ray reflective optics.

Rev Sci Instrum 2019 Feb;90(2):021704

Japan Synchrotron Radiation Research Institute, Koto 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan.

The surface contamination of reflective X-ray optics has long been a serious problem that degrades beam quality. We evaluated the total organic content at the surface by gas chromatography to clarify the source of contamination. We found that various materials that can become contamination sources are used around the optical elements. After covering the optics with cleaned materials and applying synchrotron radiation cleaning during commissioning, the observed reflected intensity at the beamline has not reduced for 2.5 years.
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http://dx.doi.org/10.1063/1.5063262DOI Listing
February 2019

Realization of a scanning soft X-ray microscope for magnetic imaging under high magnetic fields.

J Synchrotron Radiat 2018 Sep 2;25(Pt 5):1444-1449. Epub 2018 Aug 2.

Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Sayo 679-5198, Japan.

For the purpose of imaging element- and shell-specific magnetic distributions under high magnetic fields, a scanning soft X-ray microscope has been developed at beamline BL25SU, SPring-8, Japan. The scanning X-ray microscope utilizes total electron yield detection of absorbed circularly polarized soft X-rays in order to observe magnetic domains through the X-ray magnetic circular dichroism effect. Crucially, this system is equipped with an 8 T superconducting magnet. The performance and features of the present system are demonstrated by magnetic domain observations of the fractured surface of a NdFeCuB sintered magnet.
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http://dx.doi.org/10.1107/S1600577518009177DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6140398PMC
September 2018

Correlation of the Dzyaloshinskii-Moriya interaction with Heisenberg exchange and orbital asphericity.

Nat Commun 2018 04 25;9(1):1648. Epub 2018 Apr 25.

Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611-0011, Japan.

Chiral spin textures of a ferromagnetic layer in contact to a heavy non-magnetic metal, such as Néel-type domain walls and skyrmions, have been studied intensively because of their potential for future nanomagnetic devices. The Dyzaloshinskii-Moriya interaction (DMI) is an essential phenomenon for the formation of such chiral spin textures. In spite of recent theoretical progress aiming at understanding the microscopic origin of the DMI, an experimental investigation unravelling the physics at stake is still required. Here we experimentally demonstrate the close correlation of the DMI with the anisotropy of the orbital magnetic moment and with the magnetic dipole moment of the ferromagnetic metal in addition to Heisenberg exchange. The density functional theory and the tight-binding model calculations reveal that inversion symmetry breaking with spin-orbit coupling gives rise to the orbital-related correlation. Our study provides the experimental connection between the orbital physics and the spin-orbit-related phenomena, such as DMI.
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http://dx.doi.org/10.1038/s41467-018-04017-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5916936PMC
April 2018

Voltage controlled interfacial magnetism through platinum orbits.

Nat Commun 2017 06 23;8:15848. Epub 2017 Jun 23.

Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka 560-8531, Japan.

Electric fields at interfaces exhibit useful phenomena, such as switching functions in transistors, through electron accumulations and/or electric dipole inductions. We find one potentially unique situation in a metal-dielectric interface in which the electric field is atomically inhomogeneous because of the strong electrostatic screening effect in metals. Such electric fields enable us to access electric quadrupoles of the electron shell. Here we show, by synchrotron X-ray absorption spectroscopy, electric field induction of magnetic dipole moments in a platinum monatomic layer placed on ferromagnetic iron. Our theoretical analysis indicates that electric quadrupole induction produces magnetic dipole moments and provides a large magnetic anisotropy change. In contrast with the inability of current designs to offer ultrahigh-density memory devices using electric-field-induced spin control, our findings enable a material design showing more than ten times larger anisotropy energy change for such a use and highlight a path in electric-field control of condensed matter.
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http://dx.doi.org/10.1038/ncomms15848DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5490004PMC
June 2017

Ultrahigh-spatial-resolution chemical and magnetic imaging by laser-based photoemission electron microscopy.

Rev Sci Instrum 2015 Feb;86(2):023701

The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan.

We report the first experiments carried out on a new chemical and magnetic imaging system, which combines the high spatial resolution of a photoemission electron microscope (PEEM) with a continuous-wave deep-ultraviolet laser. Threshold photoemission is sensitive to the chemical and magnetic structures of the surface of materials. The spatial resolution of PEEM is limited by space charging when using pulsed photon sources as well as aberrations in the electron optics. We show that the use of a continuous-wave laser enabled us to overcome such a limit by suppressing the space-charge effect, allowing us to obtain a resolution of approximately 2.6 nm. With this system, we demonstrated the imaging of surface reconstruction domains on Si(001) by linear dichroism with normal incidence of the laser beam. We also succeeded in magnetic imaging of thin films with the use of magnetic circular dichroism near the Fermi level. The unique features of the ultraviolet laser will give us fast switching of the incident angles and polarizations of the photon source, which will be useful for the characterization of antiferromagnetic materials as well as ferromagnetic materials.
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http://dx.doi.org/10.1063/1.4906755DOI Listing
February 2015

Controlled doping of semiconducting titania nanosheets for tailored spinelectronic materials.

Nanoscale 2014 Nov;6(23):14227-36

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan.

Ti₁-x-yFexCoyO₂ nanosheets are synthesized in which the (Fe/Co) content is systematically controlled in the range of 0 ≤ x ≤ 0.4 and 0 ≤ y ≤ 0.2. A key feature of this new preparation is the use of (Li/Fe)-, (Fe/Co)- and (Li/Co)-co-substituted layered titanates as starting materials. In exfoliated nanosheets, the composition can be intentionally modified by controlled Fe/Co substitution into Ti sites during the solid-state synthesis of the starting layered compounds. The composition of the host layers is maintained in the subsequent exfoliation process, which is very helpful in the rational design of nanosheets through the use of controlled doping. Through this controlled doping, we achieve exquisite control of the electronic properties of Ti₁-δO₂ nanosheets, including the position of impurity bands, the Fermi energy and ferromagnetic properties. From photoelectron spectroscopy and first-principles studies, we have observed that the use of Fe/Co co-doping with higher Fe and Co oxidation states is necessary to bring the highest occupied Fe/Co impurity states to the Fermi level. This band engineering transforms the Ti₁-x-yFexCoyO₂ nanosheet into a room-temperature half-metallic ferromagnet, thus accomplishing the main requirements of future spinelectronics.
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http://dx.doi.org/10.1039/c4nr04465gDOI Listing
November 2014

X-ray magnetic circular dichroism investigation of the electron transfer phenomena responsible for magnetic switching in a cyanide-bridged [CoFe] chain.

Inorg Chem 2013 Dec 4;52(24):13956-62. Epub 2013 Dec 4.

Institute for Materials Research, Tohoku University , Sendai 980-8577, Japan.

The cyanide-bridged [CoFe] one-dimensional chain, [Co(II)((R)-pabn)][Fe(III)(Tp)(CN)3](BF4)·MeOH·2H2O, where (R)-pabn = (R)-N2,N(2')-bis(pyridin-2-ylmethyl)-1,1'-binaphthyl-2,2'-diamine and Tp = hydrotris(pyrazolyl)borate, exhibits magnetic and electric bistabilities originating from an electron transfer coupled spin transition between Fe-CN-Co pairs. The use of L-edge X-ray absorption spectroscopy (XAS) in combination with L-edge X-ray magnetic circular dichroism (XMCD) is explored for the investigation of the electronic structure and magnetization of Co and Fe ions separately, in both diamagnetic and paramagnetic states. It has been established from susceptibility results that the switching between diamagnetic and paramagnetic phases emanates from electron transfer between low spin Fe(II) and Co(III), resulting in low spin Fe(III) (S = 1/2) and high spin Co(II) (S = 3/2). The XAS and XMCD results are consistent with the bulk susceptibility measurements, where greater detail regarding the charge transfer process is determined. The Fe-CN-Co electron transfer pathway is highlighted by a strongly XMCD dependent transition to a cyanide back bonding orbital, giving evidence for strong hybridization with Fe(III) t2g orbitals. In addition to thermally induced and photoinduced switching, [CoFe] is found to exhibit a switching by grinding induced dehydration. Analysis of XAS shows that on grinding diamagnetic [CoFe], 75% of metal ions lock into the magnetic Co(II)Fe(III) phase. Density functional theory calculations based on the [CoFe] crystal structure in the magnetic and nonmagnetic phases aid the spectroscopic results and provide a complementary insight into the electronic configuration of the [CoFe] 3d shells, quantifying the change in ligand field around Co and Fe centers on charge transfer.
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http://dx.doi.org/10.1021/ic402580nDOI Listing
December 2013

Orbital reconstruction and interface ferromagnetism in self-assembled nanosheet superlattices.

ACS Nano 2011 Sep 15;5(9):6871-9. Epub 2011 Aug 15.

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan.

We have investigated the interface electronic states in self-assembled (Ti(0.8)Co(0.2)O(2)/Ti(0.6)Fe(0.4)O(2))(n) superlattices by X-ray photoelectron spectroscopy. A charge of about -0.3 electron is transferred from Fe to Co ions across the interface and induces a major reconstruction of the orbital occupation at the interfacial (Ti(0.8)Co(0.2)O(2)/Ti(0.6)Fe(0.4)O(2)) layers. Supported by first-principles calculations, the Co(3+) state is partially occupied at the interface by superlattice formation, and this new magnetic state directly influences the coupling between Ti(0.8)Co(0.2)O(2) and Ti(0.6)Fe(0.4)O(2) nanosheets. These data indicate that the orbital reconstruction is indeed realized by the interface charge transfer between Co and Fe ions in the adjoined nanosheets, and the generic feature of engineered interfaces can be extended to self-assembled superlattices of oxide nanosheets.
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http://dx.doi.org/10.1021/nn200835vDOI Listing
September 2011

The experimental and clinical pharmacology of propofol, an anesthetic agent with neuroprotective properties.

CNS Neurosci Ther 2008 ;14(2):95-106

Department of Biofunctional Evaluation, Molecular Pharmacology, Gifu Pharmaceutical University, Gifu 502-8585, Japan.

Propofol (2,6-diisopropylphenol) is a versatile, short-acting, intravenous (i.v.) sedative-hypnotic agent initially marketed as an anesthetic, and now also widely used for the sedation of patients in the intensive care unit (ICU). At the room temperature propofol is an oil and is insoluble in water. It has a remarkable safety profile. Its most common side effects are dose-dependent hypotension and cardiorespiratory depression. Propofol is a global central nervous system (CNS) depressant. It activates gamma-aminobutyric acid (GABA A) receptors directly, inhibits the N-methyl-d-aspartate (NMDA) receptor and modulates calcium influx through slow calcium-ion channels. Furthermore, at doses that do not produce sedation, propofol has an anxiolytic effect. It has also immunomodulatory activity, and may, therefore, diminish the systemic inflammatory response believed to be responsible for organ dysfunction. Propofol has been reported to have neuroprotective effects. It reduces cerebral blood flow and intracranial pressure (ICP), is a potent antioxidant, and has anti-inflammatory properties. Laboratory investigations revealed that it might also protect brain from ischemic injury. Propofol formulations contain either disodium edetate (EDTA) or sodium metabisulfite, which have antibacterial and antifungal properties. EDTA is also a chelator of divalent ions such as calcium, magnesium, and zinc. Recently, EDTA has been reported to exert a neuroprotective effect itself by chelating surplus intracerebral zinc in an ischemia model. This article reviews the neuroprotective effects of propofol and its mechanism of action.
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http://dx.doi.org/10.1111/j.1527-3458.2008.00043.xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6494023PMC
August 2008

Propofol exerts greater neuroprotection with disodium edetate than without it.

J Cereb Blood Flow Metab 2008 Feb 25;28(2):354-66. Epub 2007 Jul 25.

Department of Biofunctional Molecules, Gifu Pharmaceutical University, Gifu, Japan.

The main objective of this study, on mice, was to compare the neuroprotective effects of propofol with those of propofol plus disodium edetate (propofol EDTA). We also administered propofol EDTA (0.005% (w/v) EDTA) to mice intravenously, and measured the changes in zinc concentrations occurring after permanent middle cerebral artery occlusion. In the in vivo study, propofol EDTA displayed stronger neuroprotective effects than propofol alone. Furthermore, we examined the neuroprotective effects of EDTA administered alone, and found that EDTA Na significantly reduced the infarct volume. The number of terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling-positive cells in the ischemic penumbra was reduced more by propofol EDTA than by propofol alone. We performed in the in vitro study in five groups (aerobic, vehicle (control), propofol, EDTA, and propofol plus EDTA). Propofol and EDTA each protected PC12 cells against oxygen-glucose deprivation-induced cell damage, and the effect of propofol was increased by adding EDTA. Because the chelating action of EDTA was a potential causal mechanism, we examined the effect of propofol EDTA on intracerebral zinc homeostasis. When propofol EDTA was given intravenously 10 mins before cerebral ischemia, the zinc concentration decreased significantly in the cortical area, but not in the subcortex. In conclusion, (a) propofol provides neuroprotection against both in vivo and in vitro ischemic damage, and its effects are enhanced when EDTA is added; and (b) EDTA itself protects against ischemic neuronal damage, possibly, owing to its zinc-chelating action.
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http://dx.doi.org/10.1038/sj.jcbfm.9600532DOI Listing
February 2008

Fasudil, a Rho kinase (ROCK) inhibitor, protects against ischemic neuronal damage in vitro and in vivo by acting directly on neurons.

Brain Res 2007 Jun 11;1154:215-24. Epub 2007 Apr 11.

Department of Biofunctional Molecules, Gifu Pharmaceutical University, 5-6-1 Mitahora-higashi, Gifu 502-8585, Japan.

Background And Purpose: Recently, fasudil, a Rho kinase (ROCK) inhibitor, was reported to prevent cerebral ischemia in vivo by increasing cerebral blood flow and inhibiting inflammatory responses. However, it is uncertain whether a ROCK inhibitor can directly protect neurons against ischemic damage. Our purpose was to evaluate both the involvement of ROCK activity in ischemic neuronal damage and any direct neuroprotective effect of fasudil against cerebral infarction.

Methods: In vivo, focal cerebral ischemia was induced by permanent middle cerebral artery occlusion in mice, and the resulting infarction was evaluated 24 h later. ROCK expression and activity were assessed using Western blotting and immunohistochemistry. In vitro, the effects of fasudil and hydroxyfasudil (a main metabolite of fasudil) were examined on oxygen-glucose deprivation (OGD)-induced PC12 cell death and on glutamate-induced neurotoxicity in primary cerebral neuronal culture.

Results: ROCK expression and activity increased in the striatum, especially in axons, in the early phase of ischemia. Fasudil reduced this ROCK activity and protected against cerebral infarction in vivo. Hydroxyfasudil inhibited OGD-induced PC12 cell death, and fasudil and hydroxyfasudil each attenuated glutamate-induced neurotoxicity in vitro.

Conclusions: These findings indicate that ROCK plays a pivotal role in the mechanism underlying ischemic neuronal damage and that a direct effect of fasudil on neurons may be partly responsible for its protective effects against such damage.
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http://dx.doi.org/10.1016/j.brainres.2007.04.013DOI Listing
June 2007
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