Spintronics_and

Yihong Wu, PhD Kyoto

Professor

Department of Electrical and Computer Engineering

National University of Singapore

Tel: (65) 6516-2139; Fax: (65) 6779-1103; Room: E4-05-19

E-mail : elewuyh@nus.edu.sg

Lab: Information Storage Materials Laboratory

Bio Sketch of Yihong Wu

Teaching (existing)

EE5202 Nanometer Scale Information Storage (Semester II)

EE5209 Nanoelectronics (Semester I)

EE6504 Nanoscale Engineering (Semester I)

TE2002 Engineering Mathematics II (Semester I)

Teaching (past)

EE5209 Physics and Modeling of Spin Electronic Devices (changed to Nanoelectronics in 2005)

EE5202 Information Storage Materials and Devices (changed to Nanometer Scale Information Storage in 2005)

EE4409 Principle and Practice of Optical Data Storage

EE2461 Engineering Mathematics II

Current Group Members (Gallery)

Research fellow:

Ang Ran

Huang Leihua

Yang Dezheng

Postgraduate Students:

Saidur Rahman Bakaul, Wang Jiayi, Wu Baolei, Zhang Chi

Awards received by past and current group members

Dr. Yuankai Zheng (former member of Nano Spintronics Group) received the TR100 Award from MIT Technology Review magazine in 2004 (one of the top 100 innovators in the world).

Liu Tie received the Best Poster Award in ICMAT 2005 and a Poster Award (3rd prize) in MRS-S Conference on Advanced Materials 2006..

Maureen Tay received a Poster Award (3rd prize) in MRS-S Conference on Advanced Materials 2006.

Han Gang received a Poster Award (2nd prize) in MRS-S Conference on Advanced Materials 2006.

Awards received by collaborators

G. C. Han, E. L. Tan, B. Y. Zong, K. B. Li, B. Liu and Y. H. Wu, “Temperature dependence of thermally activated ferromagnetic resonance in tunneling magnetoresistive heads”, Asia-Pacific Data Storage Conference, 28-30 August 2006, Hsin Chu, Taiwan (APDSC’06 Outstanding Poster Award).

Selected Publications

Z. H. Ni, H. M. Wang, Y. Ma, J. Kasim, Y. H. Wu, Z. X. Shen, "Tunable stress and controlled thickness modification in graphene by annealing", to appear in ACS Nano.

G. Q. Teo, H. M. Wang, Y. H. Wu, Z. B. Guo, J. Zhang, Z. H. Ni and Z. X. Shen, "Visibility study of graphene multilayer structures", to appear in J. Appl. Phys.

H. M. Wang, Y. H. Wu, Z. H. Ni, and Z. X. Shen, “Electronic transport and layer engineering in multilayer graphene structures”, Appl. Phys. Lett. 92, 053504 (2008).

Y. Y. Wang, Z. H. Ni, Z. X. Shen, H. M. Wang, and Y. H. Wu, “Interference enhancement of Raman signal of graphene”, Appl. Phys. Lett. 92, 043121 (2008).

Haomin Wang, Catherine Choonga, Jun Zhang, Kie Leong Teo and Yihong Wu, "Differential conductance fluctuation of curved nanographite sheets in the mesoscopic regime", Solid State Comm. 145, 341-345 (2008).

T. Dietl, T. Andrearczyk, A. Lipińska, M. Kiecana, Maureen Tay, and Yihong Wu, "Origin of ferromagnetism in Zn1−xCoxO from magnetization and spin-dependent magnetoresistance measurements ", Phys. Rev. B 76, 155312 (2007).

Z. H. Ni, H. M. Wang, J. Kasim, H. M. Fan, T. Yu, Y. H. Wu, Y. P. Feng and Z. X. Shen, "Graphene Thickness Determination Using Reflection and Contrast Spectroscopy", Nano Lett. 7, 2758-2763 (2007).

Kebin Li, Yihong Wu, Zaibing Guo, Yuankai Zheng, Guchang Han, Jinjun Qiu, Ping Luo, Lihua An, and Tiejun Zhou, "Exchange Coupling and Its Applications in Magnetic Data Storage", J. Nanosci. Nanotechnol. 7, 13–45 (2007) (Review).

Yuankai Zheng, Yihong Wu, Kebin Li, Jinjun Qiu, Guchang Han, Zaibing Guo, Ping Luo, Lihua An, Zhiyong Liu, Li Wang, Seng Ghee Tan, Baoyu Zong, and Bo Liu, "Magnetic Random Access Memory (MRAM)", J. Nanosci. Nanotechnol. 7, 117–137 (2007) (Review)

Hongliang Li, Yihong Wu, Zaibing Guo, Ping Luo, and Shijie Wang, "Magnetic and electrical transport properties of Ge_1−xMn_x thin films",J. Appl. Phys. 100, 103908 (2006).

M. Tay, Y. H. Wu, G. C. Han, T. C. Chong, Y. K. Zheng, S. J. Wang, Y. B. Chen and X. Q. Pan, "Ferromagnetism in inhomogeneous Zn_1–xCo_xO thin films", J. Appl. Phys. 100, 063910 (2006).

T. Liu, Y. H. Wu, Z. L. Zhao, Y. K. Zheng and A. O. Adeyeye, "Transport properties and micromagnetic modeling of magnetic nanowires with multiple constrictions", Thin Solid Films, 505 (1-2): 35-40 MAY 18 2006.

KB Li, Wu YH, Han GC, Qiu JJ, Zheng YK, Guo ZB, An LH, Luo P, "Electrical and magnetic properties of nano-oxide added spin valves", Thin Solid Films, 505 (1-2): 22-28 MAY 18 2006.

G.C. Han, Li KB, Zheng YK, Qiu JJ, Luo P, An LH, Guo ZB, Liu ZY, Wu YH, "Fabrication of sub-50 nm current-perpendicular-to-plane spin valve sensors", Thin Solid Films, 505 (1-2): 41-44 MAY 18 2006.

P. Esquinazi, D. Spemann, K. Schindler, R. Höhne, M. Ziese, A. Setzer, K.-H. Han, S. Petriconi, M. Diaconu, H. Schmidt, T. Butz and Y.H. Wu, “Proton irradiation effects and magnetic order in carbon structures”, Solid Films 505, 85-89 (2006).

Li HL, Wu YH, Guo ZB, Wang SJ, Teo KL, Veres T, “Effect of antiphase boundaries on electrical transport properties of Fe3O4 nanostructures”, Appl. Phys. Lett. 86, 252507 (2005).

Li KB, Qiu JJ, Han GC, Guo ZB, Zheng YK, Wu YH, Li JS, “Effect of capping layer on interlayer coupling in synthetic spin valves”, Phys. Rev. 71, 014436 (2005).

Wang L, Qiu JJ, McMahon WJ, Li KB and Wu YH, “Nano-oxide-layer insertion and specular effects in spin valves: Experiment and theory”, Phys. Rev. B 69, 214402 (2004).

Wang L, McMahon WJ, Liu B, Wu YH, Chong CT, “Interface or bulk scattering in the semiclassical theory for spin valves”, Phys. Rev. 69 (21): Art. No. 214403 JUN 2004.

Wu YH, Yang BJ, Zong BY, Sun H and Feng YP, “Carbon nanowalls and related materials”, Journal of Materials Chemistry 14, 469-477 (2004) (Feature article).

Han GC, Wu YH, Tay M, Li KB, and Chong TC, “Epitaxial growth of ferromagnetic Co:TiO2 thin films by co-sputtering”, J. Magn. Magn. Mat. 268, 159-164 (2004).

Wu YH, “Nano Spintronics for Data Storage”, Encyclopedia for Nanoscience and Nanotechnology, vol.7, 493-544, American Scientific Publishers, 2003.

Wu YH, Shen YT, Liu ZY, Li KB, and Qiu JJ, “Point dipole response from a magnetic force microscopy tip with a synthetic antiferromagnetic coating”, Appl. Phys. Lett. 82, 1748-1750 (2003).

Han GC, Zong BY, Luo P, and Wu YH, “Angular dependence of the coercivity and remanence of electrodeposited nano-wire”, J. Appl. Phys. 93, 9202-9207 (2003).

Zheng YK, Wu YH, Qiu JJ, Li KB, Guo ZB, Han, GC, An LH, Luo P, You D, Liu ZY, and Shen YT, “A low-switching-current flux-closed magneto-resistive random access memory”, J. Appl. Phys. 93, 7307-7309 (2003).

Guo ZB, Zheng YK, Li KB, Liu ZY, Shen YT, and Wu YH, “Observation of a flux closure state in NiFe/IrMn exchange biased rings”, J. Appl. Phys. 93, 7435-7437 (2003).

Wu YH, Qiao PW, Chong TC, and Shen ZX, “Carbon nanowalls grown by microwave plasma enhanced chemical vapor deposition”, Adv. Mater. 14, 64-67 (2002).

Wu YH, Li KB, Qiu JJ, Guo ZB, and Han GC, “Antiferromagnetically coupled hard/Ru/soft layers and their applications in spin valves”, Appl. Phys. Lett. 80, 4413-4415 (2002).

Liu ZY, Dan Y, Qiu JJ, and Wu YH, “Magnetic force microscopy using focused ion beam sharpened tip with deposited antiferro-ferromagnetic multiple layers”, J. Appl. Phys. 91, 8843-8845 (2002).

Shen YT, Wu YH, Xie H, Li KB, Qiu JJ, and Guo ZB, “Exchange bias of patterned NiFe/IrMn film”, J. Appl. Phys. 91, 8001-8003 (2002).

Wu YH, Yang BJ, Han GC, Zong BY, Ni HQ, Luo P, Chong TC, Low TS, and Shen ZX, “Fabrication of a class of nanostructured materials using carbon nanowalls as the templates”, Adv. Funct. Mater.12, 489-494 (2002).

Wu YH, Qiao PW, Qiu JJ, Chong TC, and Low TS, “Magnetic nanostructures grown on vertically aligned carbon nanotube templates”, Nano Letters 2, 161-164 (2002).

Shen YT, Wu YH, Chong TC, Xie H, Guo ZB, Li KB, and Qiu JJ, “Asymmetry diffraction magneto-optical phenomenon of NiFe grating”, Appl. Phys. Lett. 79, 2034-2036 (2001).

Li KB, Wu YH, Qiu JJ, Han GC, Guo ZB, Xie H, and Chong TC , “Suppression of interlayer coupling and enhancement of magnetoresistance in spin valves with oxide layer”, Appl. Phys. Lett. 79, 3663-3665 (2001).

Zheng YK, Wang XY, You D, and Wu YH, “Switch-free read operation design and measurement of magnetic tunnel junction magnetic random access memory arrays”, Appl. Phys. Lett. 79, 2788-2790 (2001).

Wu YH, Arai K, and Yao T, “Temperature dependence of the photoluminescence of ZnSe/ZnS quantum-dot structures”, Phys. Rev. B 53, 10485-10488 (1996).

Wu YH, “Structure-dependent threshold current-density for CdZnSe-Based II-VI semiconductor-lasers”, IEEE J. Quan. Elect. 30, 1562-1573 (1994).

Wu YH, Khoo H, and Kogure T, “Read-only optical disk with superresolution”, Appl. Phys. Lett. 64, 3225-3227 (1994).

Kawakami Y, Yamaguchi S, Wu YH, Ichino K, Fujita S, Fujita S, “Optically pumped blue-green laser operation above room-temperature in Zn_0.80Cd_0.20Se-ZnS_0.08Se_0.92 multiple quantum-well structures grown by metalorganic molecular-beam epitaxy”, Jpn. J. Appl. Part 2 - Lett.30 (4A): L605-L607 APR 1 1991.

Wu YH, Kawakami Y, Fujita S, Fujita S, “Growth and optical-properties of novel wide-band-gap strained-layer single quantum-wells - Zn_1-yCd_ySe/ZnS_xSe_1-x”, Jpn. J. Appl. Part 2 - Lett.30 (4A): L555-L557 APR 1 1991.

Links: Information Storage Materials Lab @ Department of Electrical and Computer Engineering, NUS; Data Storage Institute

Scholarships:

Singapore Millennium Foundation

NUS Graduate Study Program

NUS Graduate School for Integrated Science and Engineering

Research

My past research activities center on nanostructured materials / devices and their applications in data storage (see Special Issue on Nanotechnology for Information Storage: J. Nanosci. Nanotechnol. 7 (1), (2007)). From 1986 to1999, I worked on semiconductor nanostructures, optoelectronics and optical data storage. Since 1998, I have been mainly working on nanomagnetism, spintronics, magnetic data storage and 2D nanomaterials. My current research activities are focused on nanomagnetism / spintronics and 2D carbon. Below is a brief introduction of some of the research topics.

Dilute Magnetic semiconductors and related spintronic devices

Electrons possess both charges and spins. The motion of charges forms the current. The ability to control or modulate the charge transport has made it possible to form functional devices such as diodes and transistors. This is so far only possible in semiconductors instead of metals because the latter have too many electrons per unit volume; the variation of charge distribution, if any, is limited to a few atomic layers at the surface that can hardly cause any measurable change in the conductance of the metal. Therefore, functional electronic devices have not been realized in pure metals with dimensions larger than the mean-free path of electrons. Although they belong to the same family of metals, metallic magnetic materials have another additional degree of freedom which can be used to vary their electronic transport properties - the spin of electrons. As the spin of electrons in magnetic materials can be easily manipulated using an external magnetic field without suffering the electrostatic screening effect as the charges do when they are subjected to an electric field, it is possible to alter the conductivity of magnetic materials without changing the carrier distribution inside the material. This forms the basis of GMR-based electronics or sometimes is also called magnetoelectronics. The spintronics in its broader sense contains all types of electronics that make use of both charges and spins. In semiconductor-based electronics, we only control the charge motion and ignore the spins. In fact, it is difficult to control the spins in semiconductors because they don’t have specific directions under normal conditions. On the other hand, in metallic magnetic materials it is easy to control the spins due to the strong ferromagnetic coupling among the spins, but it is difficult to control the charges. Therefore, a question naturally arises here: can we have a kind of material in which we can control both charges and spins? The answer is ‘yes’ and this type of material is called a magnetic semiconductor. The magnetic semiconductor is usually made through adding magnetic impurities to host semiconductors. It is not necessary, however, that every semiconductor can be made magnetic using this approach because some of them still don’t exhibit any magnetic properties even after they are doped with substantial amount of magnetic impurities. Some of them, although being magnetic, show a very low Curie temperature. However, the situation changed drastically in recent years due to the intensive efforts made by researchers in this field in many research organizations. Several different types of magnetic semiconductors having a Curie temperature higher than room temperature have been found (though the results are still controversial). Our work are focused on oxide-based as well as group IV based magnetic semiconductors. We choose these materials because the oxides are suitable for integration with current metal-based spintronic devices and group IV materials are suitable for integration with existing Si technology. The focus of our work is on the understanding of origin of ferromagnetic ordering in these materials.

Development of a combined nanofabrication / characterization tool for research on nanometer scale spintronics

Electron as a negatively charged elementary particle was discovered in 1897. About two decades later, photon (1922) and spin (1925) were discovered. Each of them has played a central role in the development of microelectronics, optoelectronics and magnetics industry. Although charge, photon and spin are deeply connected with one another, they have been used more or less “independently” by human being to create the infocom industry of the 20^th century. Looking forward, a deep understanding on how these three interact with one another on the nanoscale is of great importance for creating devices with new functionalities. Although both single charge and single spin detection techniques have become available recently, they are still being probed “independently” in both the time and spatial domains, except for the case of vary small systems such as quantum dots. A true simultaneous detection technique for probing the spin-charge interactions in “real time” is a necessity for increasing our understanding of spin-charge interactions. To this end, we are currently developing a combined nano-fabrication and characterization tool consisting of (i) a scanning electron microscope with spin-polarization analysis (SEMPA) (ii) a scanning tunnel microscope (STM) or spin-dependent STM (SPSTM) (iii) four nano-probes (including the STM probe) (iv) a focused ion beam (FIB) (v) a sample preparation and fabrication chamber with variable temperature and magnetic field features.

- The SEMPA in this system will be used to study surface magnetism of various types of magnetic, half-metallic, and magnetic semiconductor thin films and nanostructures at nanometer scale.

- The STM and SPSTM will be used for atomic scale electronic and spin state detection applications.

- The nano-probes will be used to study the transport properties of various types of nanostructures including nanowires, nanotubes, nano-rings of magnetic, semiconductor and biological materials and the associated devices.

- The combination of SEM with STM ensures that the above-mentioned experiments can be performed at controlled positions with a nanometer scale spatial accuracy.

- The combination of SEMPA with the four nano-probes makes some of the measurements which so far are impossible an attainable task, such as in-situ study of spin-injection and spin transfer in magnetic nanostructures.

- The inclusion of an FIB makes it possible to perform in-situ modification and fabrication of nanostructures while performing magnetic and electrical measurements.

Development of new MFM tips

Magnetic force microscopy (MFM) has beenand continues to be one of the primary imaging toolsfor studying magnetic nanostructures. There are two major issues with the MFM which havebeen addressed frequently during the last decade: (i) tip-sample interaction and (ii) moderate resolution. Although many techniques have been proposed and developed to resolve these two issues, the success still remains moderate. Most of these techniques are based on the modification of the MFM tips one at a time which suffer from a very low yield and poor reproducibility. To address this issue, in the last 2-3 years, we have proposed and verified experimentally a novel type of synthetic magnetic force microscopy tip which does not only allow for batch fabrication but also exhibits a resolution which approaches the theoretical limit. The key to achieving a superior performance over commercial tips is the introduction of a new tip coating structure which allows the tip to function as a point dipole in the as-fabricated form. The usefulness of the new tip has been tested based on a small number of prototypes. Currently, we are exploring the possibility of commercializing the tips and at the mean time using the new tips to study magnetic nanostructures. (info about different types of MFM tips).

Two-dimensional (2D) carbon and related nanostructures

Although the recent work on nanomaterials has been focused on the 0D and 1D systems, it was in the 2D system where the top-down approach of nanotechnology has been developed. This has led to the discovery of quantum Hall effect and the creation of new devices such as high electron mobility transistors, intersubband infrared detectors and quantum cascade lasers in semiconductor systems and the discovery of giant magnetoresistance and invention of spin-valves in metallic systems. The work on the 2D systems had also become the foundation for the subsequent work on 1D and 0D systems. The above-mentioned 2D systems are obtained by the so-called top-down approach. Most of these 2D systems were realized in laminar structures of semiconductors, insulators and metals. In addition to these artificially structured 2D systems, there are also many naturally formed 2D systems such as graphite, MgB2, transition metal dichalcogenides, intercalation compounds of graphite, high-Tc superconductors, and many others. The common feature of these materials is that the electrical conduction is highly anisotropic, with a low resistance in the layer plane but a high resistance or being insulating perpendicular to the layers. It is worth noting that most of the layered compounds are also good superconductors. Regardless of whether it is an artificial laminar structure or a naturally formed layered compound, the 2D system formed in such a fashion is not a perfect 2D system in the sense that each 2D layer still has to interact with the adjacent layers either chemically or electronically. An ideal 2D system would be such that it consists of only a single nano-sheet without any electronic or chemical interactions with other types of materials or layers.

We questioned why there is no report on graphene synthesis and reported on the growth of 2D carbon by MPECDV in Adv. Mater. 14, 64-67 (2002). We also pointed out in Journal of Materials Chemistry 14, 469-477 (2004) that graphene can be obtained by "peeling off" carbon layer-by-layer from graphite.

The 2D carbon nano-sheets are very useful not only for fundamental physics studies but also for practical applications due to their large surface-to-volume ratio. In contrast to the closed boundary structure of 0D and 1D materials, the 2D nanosheets are characterized by open boundaries. Theoretical studies have shown that this may bring about nano-sheets unique transport and magnetic properties. We are currently focusing on the ferromagnetism, superconducting instability and spintronic properties of graphene and 2D carbon.