Research

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Spintronics

The world has witnessed a rapid growth of an emerging research field of spintronics in passed two decades. Spintronics is driving the development of a revolutionary new class of electronics based on utilization of the spin degree of freedom of the electron besides the charge.

The main goal of spintronics is to gain knowledge on spin dependent transport phenomena and to exploit the device applications with new functionalities. The field of spintronics started with the discovery of giant magnetoresistance (GMR) in layered systems consisting of FM layers separated by nonmagnetic (NM) layers These structures, in particular spin valves (SV), have been used in magnetic recording as read heads, which significantly increased the areal density in magnetic recording and led to an unprecedented economic development in computer industries since 1990. In late 1990’s, another structure, magnetic tunnel junction (MTJ) in which two FM layers are separated by an insulating barrier (Al2O3), has attracted more and more attention. The electron tunneling phenomenon has been known since the advent of quantum mechanics. However it continues to enrich our understanding in many fields. It is crucial to understand the mechanism of MTJ in order to understand spin polarized transport. MTJ also possesses great potential in applications and they are recognized as a promising candidate in non-volatile random access memories and the next generation magnetic field sensor.

Our research is conducted to elucidate the underlying mechanisms that affect the transfer of spin across interfaces between materials and the loss of spin polarization of the spin current as it propagates through a material. Particularly, we focus on the characterization of interface and barrier structure, bias dependence of tunneling magnetoresistance (TMR), and the interplay among different length scales arising from charge and spin transports. We also developed several theoretical models to explain the various observed experimental results.

Affilicated People: Xin Fan, Xiaoming Kou

Soft Magnetic material

The objective is to develop high temperature magnetic materials characterized by low coercivity, high permeability, and low core loss (magnetically soft). These materials will be used in electric airplanes, ships and automobiles where no hydraulic cooling system is needed. The major obstacle is the mechanical strength of the materials. We are developing composite materials using various reinforcement agents such as fibers to overcome this.

Affilicated People: Xiaokai Zhang, Xing Chen, Xiaoming Kou

High Frequency Magnetic Composite Materials

We are interested in High Frequency Magnetic Composite Materials recently because new properties for high-frequency application can be achieved by different types of magnetic composite materials.

  • I. One new type of so-called "left-handed" materials (LHM) with negative permittivity, ε, and negative permeability, μ, at microwave frequencies can be obtained in a periodic array of "conducting non-magnetic split ring resonators and continuous wires"[1,2]. Except using this resonate method, we are trying to develop“left-handed” material in magnetic composites.
  • II. Develop low loss magneto-dielectric composites in high frequency
  • III. Explore the applications:patch antenna, DC-DC converters, and novel microwave devices based on LHM

Facilities for ε/μ measurements:

Hysteresis Loop Tracer: Dynamic magnetic measurement up to 1MHz; LCR meter: Both magnetic and dielectric measurement up to 100MHz; Network Analyzer: UHF properties up to GHz range with different fixtures.

Affiliated People: Xin Fan, Xing Chen, Xiaoming Kou

Nano Structure Fabrication and Applications

One objective of this project is to fabricate nano-particles, nanowires, and nanotubes, which are composed of metals, alloys, oxides, or composite materials, then they are self-assembled in different ways for different applications. The high frequency, magnetic, optical, transport and catalytic properties of these materials can be manipulated by controlling the size, fabrication conditions, and self assemble ways. They have potential applications in high frequency (low loss, controllable permittivity and permeability), magnetic recording, photonic, spintronics, drugs transferring, catalyst, hydrogen storage etc.

The other objective of this project is to fabricate film or multilayer film with thickness in the nanometer scale by magnetron sputtering, e-beam evaporation, thermal evaporation, electrochemical deposition, electroless plating and spin coating. It has wide applications in magnetic recording, photonics, spintronics, corrosive-resistant coating, and surface modification. Sample quality and properties are studied by XRD, EDS, XPS, AFM, SEM, TEM, TOF-SIMS, VSM, SQUID and photospectrometer.

Affiliated People: Xing Chen, Xiaoming Kou, Qi Lu

Super Capacitors

Fast, high current pulses from a portable, rechargeable energy source is in great demand in many applications By utilizing supercapacitors in combination with a battery source they are therefore ideally suited to meeting peak power requirements while the battery supplies the average load.

Typically there are two main categories in supercapacitors: electrochemical double layer capacitor (EDLC) and pseudocapacitor. The former, which has already been commercialized, uses double layer at the surface of electrochemical electrode to store charges (ions). Due to ultrathin double layer (0.5~1nm), large dielectric constant of the electrolyte, and large electrode surface area (~103m2/g), EDLC can provide capacitance five to seven orders of magnitudes larger than conventional capacitors. Pseudocapacitors utilize redox reaction to store more charges during the charging process by reducing the valance states of the electrodes, and return those charges to the electrolyte in the discharging process.

The most common electrode in EDLC is activated carbon because of its low cost, environmental compatibility, abundance, and larger surface area (~1500m2/g). However, it suffers from its inherit low specific capacitance (10-50μF/cm2) and thus low energy density. In addition, the charging/discharging time is largely influenced by the random pore size distributions and labyrinth microstructure.

Metal oxides such as RuO2 and IrO2 are used as electrode materials in pseudocapacitors. They possess large specific capacitance which is usually more than 10 times larger than that of porous carbon materials. Especially for RuO2?xH2O, one can achieve a capacitance of 720F/g, leading to an energy density of about 27 Wh/kg at less than 1 voltage. However, those rare elements like Ru and Ir are not only toxic but also too expensive to be commercialized.

The objective of our team is to fabricate pseudocapacitor with both better electrode and electrolyte performances, to address the energy requirements in Navy application. Our team consists of a material physicist, a composite scientist, and a chemical engineer with expertise in fabrication and characterization of oxide and electrolyte properties. The team also collaborates with Navy laboratory personnel.


Affiliated People: Qi Lu, Xing Chen