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V. R. Singh,
K. Ishigami,
V. K. Verma,
G. Shibata,
Y. Yamazaki,
T. Kataoka,
A. Fujimori,
F. -H. Chang,
D.-J. Huang,
H.-J. Lin,
C. T. Chen,
Y. Yamada, T. Fukumura,
M. Kawasaki
[show abstract]
[hide abstract]
ABSTRACT: We have studied magnetism in anatase Ti1−xCoxO2−δ (x = 0.05) thin films with various electron carrier densities, by soft x-ray magnetic circular dichroism (XMCD) measurements at the Co L2,3 absorption edges. For electrically conducting samples, the magnetic moment estimated by XMCD was <0.3 μB/Co using the surface-sensitive total electron yield mode, while it was 0.3–2.4 μB/Co using the bulk-sensitive total fluorescence yield mode. The latter value is in the same range as the saturation magnetization 0.6–2.1 μB/Co deduced by SQUID measurement. The magnetization and the XMCD intensity increased with carrier density, consistent with the carrier-induced origin of the ferromagnetism.
Applied Physics Letters 06/2012; 100(24):242404. · 3.84 Impact Factor
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V. R. Singh,
K. Ishigami,
V. K. Verma,
G. Shibata,
Y. Yamazaki,
T. Kataoka,
A. Fujimori,
F. -H. Chang,
D.-J. Huang,
H.-J. Lin,
C. T. Chen,
Y. Yamada, T. Fukumura,
M. Kawasaki
[show abstract]
[hide abstract]
ABSTRACT: We have studied magnetism in anatase Ti1−xCoxO2−δ (x = 0.05) thin films with various electron carrier densities, by soft x-ray magnetic circular dichroism (XMCD) measurements at the Co L2,3 absorption edges. For electrically conducting samples, the magnetic moment estimated by XMCD was <0.3 μB/Co using the surface-sensitive total electron yield mode, while it was 0.3–2.4 μB/Co using the bulk-sensitive total fluorescence yield mode. The latter value is in the same range as the saturation magnetization 0.6–2.1 μB/Co deduced by SQUID measurement. The magnetization and the XMCD intensity increased with carrier density, consistent with the carrier-induced origin of the ferromagnetism.
Applied Physics Letters 06/2012; 100(24):242404. · 3.84 Impact Factor
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V. R. Singh,
K. Ishigami,
V. K. Verma,
G. Shibata,
Y. Yamazaki,
T. Kataoka,
A. Fujimori,
F. -H. Chang,
D. -J. Huang,
H. -J. Lin,
C. T. Chen,
Y. Yamada, T. Fukumura,
M. Kawasaki
[show abstract]
[hide abstract]
ABSTRACT: We have studied magnetism in anatase Ti$_{1-x}$Co$_x$O$_{2-\delta}$ ({\it x}
= 0.05) thin films with various electron carrier densities, by soft x-ray
magnetic circular dichroism (XMCD) measurements at the Co $L_{2,3}$ absorption
edges. For electrically conducting samples, the magnetic moment estimated by
XMCD was $<$ 0.3 $\mu_B$/Co using the surface-sensitive total electron yield
(TEY) mode, while it was 0.3-2.4 $\mu_B$/Co using the bulk-sensitive total
fluorescence yield (TFY) mode. The latter value is in the same range as the
saturation magnetization 0.6-2.1 $\mu_B$/Co deduced by SQUID measurement. The
magnetization and the XMCD intensity increased with carrier density, consistent
with the carrier-induced origin of the ferromagnetism.
06/2012;
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V R Singh,
Y Sakamoto,
T Kataoka,
M Kobayashi,
Y Yamazaki,
A Fujimori,
F-H Chang,
D-J Huang,
H-J Lin,
C T Chen,
H Toyosaki, T Fukumura,
M Kawasaki
[show abstract]
[hide abstract]
ABSTRACT: We have studied magnetism in Ti(1-x)Co(x)O(2-δ) thin films with various x and δ by soft x-ray magnetic circular dichroism (XMCD) measurements at the Co L(2, 3) absorption edges. The estimated ferromagnetic moment by XMCD was 0.15-0.24 µ(B)/Co at the surface, while in the bulk it was 0.82-2.25 µ(B)/Co, which is in the same range as the saturation magnetization of 1.0-1.5 µ(B)/Co. These results suggest an intrinsic origin of the ferromagnetism. The smaller moment of the Co atom at the surface is an indication of a magnetically dead layer of a few nanometers thick at the surface of the thin films.
Journal of Physics Condensed Matter 05/2011; 23(17):176001. · 2.55 Impact Factor
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V. R. Singh,
Y. Sakamoto,
T. Kataoka,
M. Kobayashi,
Y. Yamazaki,
A. Fujimori,
F. -H. Chang,
D. -J. Huang,
H. -J. Lin,
C. T. Chen,
H. Toyosaki, T. Fukumura,
M. Kawasaki
[show abstract]
[hide abstract]
ABSTRACT: We have studied magnetism in Ti_[1-x]Co_xO_[2-\delta] thin films with various
x and \delta by soft x-ray magnetic circular dichroism (XMCD) measurements at
the Co L_[2,3] absorption edges. The estimated ferromagnetic moment by XMCD was
0.15-0.24 \mu\beta/Co in the surface, while in the bulk it was 0.82-2.25
\mu\beta/Co, which is in the same range as the saturation magnetization of
1.0-1.5 \mu\beta/Co. Theseresults suggest that the intrinsic origin of the
erromagnetism. The smaller moment of Co atom at surface is an indication of a
magnetically dead layer of a few nm thick at the surface of the thin films.
03/2011;
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[show abstract]
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ABSTRACT: We have found that there is a narrow but distinct window in oxygen pressure for growing phase-pure epitaxial EuO films by a pulsed laser deposition. With finely decreasing the oxygen pressure, the electrical property is varied from insulating to metallic with an enhancement in Curie temperature from 70 to 120 K. The anomalous Hall contribution was clearly observed in Hall resistance at 5 K in the highest electron density sample. The saturated anomalous Hall conductivity (0.2 S/cm) is rather high in comparison with those of the other ferromagnetic oxides, probably due to strong spin-orbit coupling in EuO.
Applied Physics Letters 03/2011; · 3.84 Impact Factor
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ABSTRACT: Ultrafast carrier dynamics were investigated in a delafossite CuScO2, a material with a remarkably large binding energy of exciton ( ∼ 0.4 eV), using femtosecond transient transmission spectroscopy. Differential transmission spectra showed dispersive structures in the excitonic resonance energies. We have observed a delayed rise-up on a time scale of 10 ps, suggesting slow carrier cooling. It is followed by a slower decay, time constant of which corresponds to the lifetime of exciton (approximately 0.75 ns). These results were analyzed in terms of the generalized many-body Elliott model, accounting for a screening effect of excitons.
Applied Physics Letters 05/2010; 96(21):211904-211904-3. · 3.84 Impact Factor
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ABSTRACT: A ZnO / Mg <sub>0.2</sub> Zn <sub>0.8</sub> O heterostructure was characterized at T=2 K through capacitance measurements with using a conducting polymer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), as a Schottky contact. The Nyquist diagram, which is the trajectory curve of the complex impedance vector, appeared to be an excellent semicircular shape, implying that the PEDOT : PSS / ZnO / Mg <sub>0.2</sub> Zn <sub>0.8</sub> O junction can be described with an equivalent single RC parallel circuit. Capacitance-voltage characteristics elucidate the existence of a two-dimensional electron gas, where 10<sup>19</sup> cm <sup>-3</sup> electrons are confined within 5 nm at the ZnO / Mg <sub>0.2</sub> Zn <sub>0.8</sub> O heterointerface.
Applied Physics Letters 03/2010; · 3.84 Impact Factor
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ABSTRACT: X-ray photoemission spectroscopy measurements with ultraviolet laser illumination have been performed for anatase Ti <sub>1-x</sub> Co <sub>x</sub> O <sub>2-δ</sub> thin films with x=0.05 and 0.10 in order to investigate the interplay between the Co spins and the photoinduced carriers in the surface region. We have found that the surface band bending is removed by the ultraviolet illumination, indicating that photoinduced carriers are injected into the surface depletion layer. After the carrier injection, the position of the chemical potential is governed by the exchange splitting of the Ti 3d conduction band due to the magnetic interaction between the photoinduced carriers and the Co spins.
Applied Physics Letters 02/2010; · 3.84 Impact Factor
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ABSTRACT: A distorted delafossite CuMnO <sub>2</sub> , called crednerite, was formed into epitaxial thin films on MgAl <sub>2</sub> O <sub>4</sub> (111) substrates by pulsed laser deposition method. X-ray diffraction revealed that the films had high crystalline quality with epitaxial relationship of CuMnO <sub>2</sub> (001)// MgAl <sub>2</sub> O <sub>4</sub> (111) and CuMnO <sub>2</sub> [100]// MgAl <sub>2</sub> O <sub>4</sub> <11-2> . The ultraviolet-visible optical response revealed a distinct absorption peak at 4.5 eV presumably with excitonic nature and broad peaks at 3.0 and 3.7 eV assignable to O 2p– Mn 3d charge transfer excitation. Small magnetic hysteresis with remanent magnetization of 0.04μ<sub> B </sub>/ Mn was observed below 20 K, representing canted antiferromagnetic spin ordering.
Applied Physics Letters 08/2009; · 3.84 Impact Factor
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ABSTRACT: Room-temperature ferromagnetic oxide semiconductor Co-doped TiO <sub>2</sub> films are grown on glass substrates by sputtering method. Conducting films are ferromagnetic at room temperature that is consistent with the carrier-mediated nature of the ferromagnetism. Nearly full-polarized magnetization, large magneto-optical effect, and anomalous Hall effect are observed at room temperature. The magneto-optical effect shows nearly fourfold enhancement in a one-dimensional magnetophotonic crystal structure with a standard dielectric multilayer ( SiO <sub>2</sub>/ TiO <sub>2</sub>) .
Applied Physics Letters 04/2009; · 3.84 Impact Factor
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M. Nakano,
T. Makino,
A. Tsukazaki,
K. Ueno,
A. Ohtomo, T. Fukumura,
H. Yuji,
S. Akasaka,
K. Tamura,
K. Nakahara,
T. Tanabe,
A. Kamisawa,
M. Kawasaki
[show abstract]
[hide abstract]
ABSTRACT: We report on a high performance visible-blind Schottky ultraviolet photodiode composed of a ZnO (0001) bulk single crystal and a transparent conducting polymer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), fabricated with a simple spin-coating process at room temperature in air. The quantum efficiency as high as unity in ultraviolet region and a visible rejection ratio of about 103 were achieved in the spectral response of the photodiode under zero-bias condition. The normalized detectivity of the photodiode was evaluated to be 3.6×1014 cm Hz1/2/W at 370 nm.
Applied Physics Letters 09/2008; 93(12):123309-123309-3. · 3.84 Impact Factor
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ABSTRACT: We report on a Schottky junction fabricated on O-polar ZnO surfaces with a silane-coupling agent as a protective layer, and a conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a deep work function metal electrode, simply by using wet processes. The silane-coupling agent prevented the O-polar ZnO surface from an etching reaction in contact with the acidic PEDOT:PSS solution, resulting in a good rectification with a current rectification ratio of 10<sup>7</sup> at ±1 V . The junction characteristics were systematically controlled in accord with the electron density in ZnO ranging from 8×10<sup>14</sup> to 4×10<sup>16</sup> cm <sup>-3</sup> .
Applied Physics Letters 08/2008; · 3.84 Impact Factor
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ABSTRACT: Electric and magnetic properties of a high temperature ferromagnetic oxide semiconductor, cobalt-doped rutile TiO2, are summarized. The cobalt-doped rutile TiO2 epitaxial thin films with different electron densities and cobalt contents were grown on r-sapphire substrates with laser molecular beam epitaxy. Results of magnetization, magnetic circular dichroism, and anomalous Hall effect measurements were examined for samples with systematically varied electron densities and cobalt contents. The samples with high electron densities and cobalt contents show the high temperature ferromagnetism, suggesting that charge carriers induce the ferromagnetism. Comment: 14 pages, 12 figures
05/2008;
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ABSTRACT: Presence of spin polarized electrons was confirmed by observing anomalous Hall effect in a ferromagnetic semiconductor anatase Ti1−xCoxO2−δ up to 600 K. The anomalous Hall resistivity exhibited apparent ferromagnetic hysteresis loop from 300 to 600 K with insignificant change in the magnitude, indicating that the Curie temperature is higher than 600 K. The measurements above 500 K induced an annealing effect represented by the decrease in resistivity, whereas kept the anomalous Hall resistivity nearly constant.
Journal of Applied Physics 02/2008; 103(7):07D114-07D114-3. · 2.17 Impact Factor
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ABSTRACT: Anomalous Hall effect (AHE) is a generally observed phenomenon in ferromagnetic metals representing spin polarized nature
of itinerant carriers. However, the microscopic mechanism has not been clarified for long debates. Recent advances in the
theory are to unveil the mechanism. Here, we present an AHE in a room temperature ferromagnetic semiconductor cobalt doped
TiO2. This compound shows a scaling behavior of the AHE: the anomalous Hall conductivity σAH approximately follows the relation σAH ∞ σxx
1.6 (σxx, conductivity) over five decades of σxx, irrespective of the electronic state, i.e., metallic or insulating conduction.
12/2007: pages 87-92;
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ABSTRACT: High quality Schottky junctions were fabricated on a ZnO (0001) bulk single crystal by spin coating a commercial conducting polymer, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), as the metal electrodes. The junctions exhibited excellent rectifying behavior with a typical ideality factor of 1.2. Such parameters as Schottky barrier height (ϕb) and built-in potential (Vbi) show negligible variation among junctions. The electron affinity of ZnO derived from ϕb and qVbi values show a slight deviation ( ∼ 0.2 eV), suggesting the existence of spontaneously formed interfacial dipole layer between ZnO (0001) polar surface and anionic PSS molecules.
Applied Physics Letters 10/2007; 91(14):142113-142113-3. · 3.84 Impact Factor
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ABSTRACT: Anomalous Hall effect (AHE) of a ferromagnetic semiconductor anatase Ti <sub>1-x</sub> Co <sub>x</sub> O <sub>2-δ</sub> thin film is studied from 10 to 300 K . Magnetic field dependence of anomalous Hall resistance is coincident with that of magnetization, while the anomalous Hall resistance decreases at low temperature in spite of nearly temperature-independent magnetization. Anomalous Hall conductivity σ<sub> AHE </sub> is found to be proportional to the square of Hall mobility, suggesting that charge scattering strongly affects the AHE in this system. The anatase Ti <sub>1-x</sub> Co <sub>x</sub> O <sub>2-δ</sub> also follows a scaling relationship to conductivity σ<sub>xx</sub> as σ<sub> AHE </sub>∝σ<sub>xx</sub><sup>1.6</sup> , which was observed for another polymorph rutile Ti <sub>1-x</sub> Co <sub>x</sub> O <sub>2-δ</sub> , suggesting an identical mechanism of their AHE.
Applied Physics Letters 03/2007; · 3.84 Impact Factor
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ABSTRACT: In this work, geometric structures for TM-doped (TM = Mn and Fe) ZnO film grown by combinatorial laser molecular beam epitaxy (CLMBE) were investigated using fluorescence EXAFS measurement. The EXAFS analysis has revealed that the majority of TM ions heavily doped in ZnO ([Mn] ~ 20% and [Fe] ~ 1%) substituted the Zn atoms in ZnO lattice.
Physica Scripta 11/2006; 2005(T115):614. · 1.20 Impact Factor
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ABSTRACT: Magnetic tunnel junctions are fabricated by laser molecular-beam epitaxy employing a room temperature ferromagnetic semiconductor Ti <sub>1-x</sub> Co <sub>x</sub> O <sub>2-δ</sub> and a ferromagnetic metal Fe <sub>0.1</sub> Co <sub>0.9</sub> as electrodes and an AlO <sub>x</sub> tunnel barrier. The thickness of the AlO <sub>x</sub> barrier is systematically varied on a substrate during the growth by stencil mask. The junction resistance increases with the barrier thickness exponentially. The differential conductance and the tunneling magnetoresistance are significantly asymmetric with respect to bias voltage at low temperature, possibly due to the asymmetric junction structure and/or the degraded interface of AlO <sub>x</sub>/ Fe <sub>0.1</sub> Co <sub>0.9</sub> .
Journal of Applied Physics 05/2006; · 2.17 Impact Factor
