刘晖
个性化签名
- 姓名:刘晖
- 目前身份:
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学术头衔:
博士生导师
- 职称:-
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学科领域:
材料科学基础学科
- 研究兴趣:
学习经历:
1987.9~1991.6:天津大学物理系本科;
1991.9~1994.6:天津大学应用物理学系材料物理专业,工学硕士学位;
2001.2~2004.2:天津大学理学院材料 物理与化学专业,工学博士学位; 教学工作经历
1994.6~1996.11:天津理工学院自动化与能源工程学院,助教;
1996.11~2000.11:天津理工学院自动化与能源工程学院,讲师;
2000.11~2004.11:天津理工大学自动化与能源工程学院,副教授;
2004.11~今: 天津理工大学材料科学与工程学院,研究员;期间:
1998.4~1999.10:香港科技大学物理系,研究助理。
2002.4~2004.3:香港科技大学物理系,访问学者。
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2123
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成果阅读
544
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成果数
10
【期刊论文】Transport and magnetotransport properties of cold-pressed CrO2 powder
刘晖, H. Liu, R. K. Zheng, Y. Wang, H.L. Bai, and X. X. Zhang*
phys. stat. sol. (a) 202, No.1, 144-150 (2005),-0001,():
-1年11月30日
Resistivity of cold-pressed CrO2 powder has been measured over the temperature range of 2 to 350K with a magnetic field up of to 50 kOe. High resolution transmission electron microscopy (HRTEM) images show that the CrO2 particles are needle-shaped with an aspect ratio of 7:1 and have an average length of 200nm, and are covered with an ultra-thin (~2.5nm) native Cr2O3 layer. Temperature dependent conductivity ductivity can be well described by the expression exp [-(Δ/T)1/ 2] below 55K, a feature of electron tunneling, and by the higher-order hopping for temperature above 55 K. The magnetoresistance, MR=[ρ(max)-ρ(H)]/ ρ(max), reaches ~33.5% at 2K and decreases to nearly zero at room temperature. Temperature dependent saturation magnetization follows well the relation M(T)=Ms(1-BT3/2) below 240K. The spin-wave stiffness constant, D, is estimated to be ~91 meV Å2. By fitting the specific heat measured in the temperature range of 1.8-34K by the relation 1yT=βT3+β1T5+β1T5+aT3/2, we obtained the electronic specific heat coefficient γ=1.7mJ/mol K2, this corresponds to a density of states at the Fermi level of 0.73 states/eV, which is in good accordance with the theoretical value of 0.69 states/eV.
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【期刊论文】Giant Hall Effect in Nonmagnetic Granular Metal Films
刘晖, X. X. Zhang, * Chuncheng Wan, H. Liu, Z. Q. Li, and Ping Sheng, J. J. Lin
PHYSICAL REVIEW LETTERS VOLUME 86, NUMBER 24,-0001,():
-1年11月30日
Nearly 3 orders of magnitude enhancement in the Hall coefficient is observed in Cux-(SiO2) 12x granular films. This large enhancement of the Hall coefficient not only is significantly larger than the prediction of the classical percolation theory, but also occurs at a metal concentration identified to be the quantum percolation threshold. Measurements of the electron dephasing length and magnetoresistance, plus the TEM characterization of microstructures, yield a physical picture consistent with the mechanism of the local quantum interference effect.
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【期刊论文】Extraordinary Hall effect in (Ni80Fe20)x (SiO2)1−x thin films
刘晖, Hui Liu, Fuk Kay Lee, Rong Kun Zheng, X. X. Zhang, and Ophelia K. C. Tsui*
PHYSICAL REVIEW B 70, 224431 (2004),-0001,():
-1年11月30日
The extraordinary Hall effect (EHE) in ferromagnetic samples is generally attributed to scatterings of iterant electrons in the presence of spin-orbit interactions. In this work, our study of the thickness dependence of the EHE in the (Ni80Fe20)x(SiO2)1−x system showed the spontaneous Hall resistivity, rxy S to be quite independent of the film thickness while the Hall coefficient, RS (;rxy S /MS, where MS is the saturated magnetization), increased monotonically owing to a depression in MS. We point out that the independence of rxy S with reducing thickness could arise if the morphological structure of the sample becomes two dimensional with decreasing film thickness, which is expected from classical percolation theory. We also find in the (Ni80Fe20)x(SiO2)1−x system (with varying x) that rxy Sρxy∝xg where y=0.53, which disagrees with the value of 2 frequently attributed to the side jump effect, but which can be explained in terms of the more general form rxy Sρxy=ρxy△ye/△SO, where △ye is the side jump displacement and LSO is the spin-orbit mean free path.
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【期刊论文】Exchange bias and the origin of magnetism in Mn-doped ZnO tetrapods
刘晖, R. K. Zheng, H. Liu, and X. X. Zhanga), V. A. L. Roy, A. B. Djurišic
Appl. Phys. Lett., Vol. 85, No.13, 27 September 2004,-0001,():
-1年11月30日
Wurtzite-type ZnO tetrapod nanostructures were prepared by evaporating Zn metal under humid argon flow. After the fabrication, Mn was doped into ZnO nanostructures by diffusion at 600℃. The average concentration of Mn was determined to be 8.4mol% by x-ray fluorescence. X-ray diffraction patterns obtained from the doped and undoped samples are almost the same. High-resolution transmission electron microscopy observations reveal the existence of surface layers. Magnetic measurements show that the sample has a very large coercivity HC=5500 Oe at 5.5K and a Curie temperature TC=43K, which may suggest that ferrimagnetic sZn,MndMn2O4 exists at the surface. Exchange bias is clearly observed below 22 K. Exchange bias is attributed to the exchange interaction between ferrimagnetic sZn,MndMn2O4 and spin-glass-like (or antiferromagnetic) phase in manganese oxides.
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刘晖, Hui Liu, , E Y Jiang, H L Bai, R K Zheng, and X X Zhang
J. Phys. D: Appl. Phys. 36(2003)2950-2953,-0001,():
-1年11月30日
At room temperature, polycrystalline Fe3O4 films with thicknesses in the range 5-1120 nm have been prepared by reactive sputtering. Transmission electron microscopy imaging shows that uniform Fe3O4 grains are well separated by grain boundaries and their size decreases with film thickness. The change of resistivity as a function of temperature reveals a grain boundary dominated electron tunnelling mechanism. The magnetoresistance MR=(ρH−ρ0)/ρ0 measured at room temperature for the films thicker than 200 nm is ~-7.4% under a magnetic field of 46 kOe, which is among the largest values ever reported for Fe3O4 films under the same measuring conditions. As the thickness reduces from 80 to 5nm, MR decreases from −6.5% to −1.1% due to the enhanced spin-flip scattering at film and grain surfaces.
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【期刊论文】Structures and transport properties of polycrystalline Fe3O4 films
刘晖, Hui Liu, E Y Jiang, RK Zheng, and H L Bai,
J. Phys.: Condens. Matter 15(2003)8003-8009,-0001,():
-1年11月30日
Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature. A transmission electron microscope image shows that the films consist of Fe3O4 grains well separated by grain boundaries with long-range and atomic scale disorders. The width of the long-range disordered grain boundaries observed by high resolution transmission electron microscopy is about ~4nm, which is consistent with the boundary dimension derived from the resistivity and magnetization measurements. The temperature dependence of resistivity indicates that the transport properties of the films are dominated by the mechanism of fluctuation-induced tunnelling of electrons across the grain boundaries. Ordinary and extraordinary Hall constants were measured to be −8.22×10−11 and ~−1.45×10−8Ωcm G-1, which are two and three orders of magnitude larger than those of iron, respectively.
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刘晖, Hui Liu, E. Y. Jiang, and H. L. Baia), R. K. Zheng
J. Appl. Phys., Vol. 95, No.10, 15 May 2004,-0001,():
-1年11月30日
(Fe3O4)12x-(SiO2)x composite films have been prepared by reactive sputtering iron and SiO2 targets in Ar1O2 mixture at room temperature. Transmission electron microscopy bright field images show that with the increase of SiO2 addition, uniform Fe3O4 grains are well separated by the amorphous SiO2 matrix, forming a well-defined granular structure. Temperature dependence of resistivity ρ(T) indicates that the electron tunneling mechanism featured by logρ∝T-1/2 dominates the transport properties of the films, which smears out the Verwey transition intrinsic to Fe3O4. This tunneling transport of electrons causes a spin-dependent magnetoresistance {=(ρH-ρ0)/ρ0% of about -4.7% for Fe3O4 films and -1.8% for (Fe3O4)0.6(SiO2)0.4 composite films under a 46 kOe magnetic field at room temperature. Magnetic and magnetoresistance measurements reveal that the antiferromagnetically coupled Fe3O4 grains are decoupled and show the behavior of superparamagnetism at x≥0.4.
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【期刊论文】Magnetic properties of Mn doped ZnO tetrapod structures
刘晖, V. A. L. Roy, A. B. Djurišica), H. Liu and X. X. Zhang, Y. H. Leung, M. H. Xie, and J. Gao, H. F. Lui and C. Surya
Appl. Phys. Lett., Vol. 84, No.5, 2 February 2004,-0001,():
-1年11月30日
ZnO tetrapod nanostructures were prepared by evaporating Zn metal under humid argon flow. After the fabrication, Mn diffusion doping was performed at two different temperatures (600 and 800℃) The samples were characterized by scanning electron microscopy, transmission electronmicroscopy, x-ray fluorescence, x-ray diffraction (XRD), superconducting quantum interferencedevice magnetometer, and photoluminescence. Diffusion doping resulted in the increase of the sizeof tetrapods, but no new peaks were found in XRD spectrum. Mn doped ZnO tetrapod structureswere found to be ferromagnetic with Curie temperature ~50K, and showed large coercive field (~3500 Oe for 800℃ sample, 5500 Oe for 600℃ sample).
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【期刊论文】Cr2O3 surface layer and exchange bias in an acicular CrO2 particle
刘晖, R. K. Zheng, Hui Liu, Y. Wang, and X. X. Zhanga)
Appl. Phys. Lett., Vol. 84, No.5, 2 February 2004,-0001,():
-1年11月30日
It has been found by electron diffraction and high-resolution transmission electron microscopy that each commercial CrO2 particle is enclosed by an epitaxial Cr2O3 layer ~2.5nm thick. The epitaxial relationships of Cr2O3 layers on CrO2 particles have been identified. Magnetic hysteresis loops measured at different temperatures (5-300K), after the sample was cooled from 350K with a 50kOe field, were found to be perfectly symmetrical, i.e., no exchange bias was observed, which is unexpected for ferromagnetic/antiferromagnetic core/shell structured particles. To gain a deeper understanding of the phenomenon, the "as-received" commercial CrO2 particles were annealed at 600℃ in air for 1h to thicken the Cr2O3 shells. Exchange bias was clearly observed in the annealed particles with thicker Cr2O3 shells. At 5K, the exchange bias field reached 220 Oe. These experimental observations can be explained well within the framework of the random field model.
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刘晖
,-0001,():
-1年11月30日
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