Yin-Ming Chang

Extraction of the tunnel magnetocapacitance with two-terminal measurements

The tunnel magnetocapacitance (TMC) of the magnetic tunnel junction has been investigated with a series of complex impedance spectra measured at varying magnetic field. To avoid the circuit complication in the four-terminal measurement with high frequency operation, two-terminal approach was developed by elimination of spin independent contribution apart from the junction area. A subsequent fitting process based on the difference spectra analysis gave the TMC ratio of −0.43% with the opposite dependence on the field as compared to the tunnel magnetoresistance (TMR) of 30.67%. This technique would be applied in the further development and integration of spintronics devices.

Enhanced chemical shift of carbon nanotube from laser assisted gas incorporation

We investigate the position-selected electronic structure of laser modified carbon nanotubes (CNTs) by scanning photoelectron microscopy. After laser pruning removed the top layer of CNTs in air and N2 environment, the modified region shows enhanced chemical shifts of 0.9 and 0.6eV in carbon 1s state, respectively. The modification of electronic structure is shown to be strongly dependent on the gaseous environment. We demonstrate an effective postgrowth process to modify the electronic structure of CNTs for further applications.

Tuning of magnetism in ferromagnetic thin films by reversing the functional groups of molecular underlayer

We demonstrate a molecular approach of tuning the magnetic properties of ferromagnetic (FM) thin films by reversing the functional groups of the organic underlayer. For the CoFe/Langmuir–Blodgett (LB) film system, we find that the coercivity of CoFe thin films (from 4 to 10 nm) made on hydrophobic surfaces is significantly enhanced whereas that on hydrophilic surfaces remains unchanged, as compared with the films directly on glass substrates. These findings suggest an alternative way for tuning the magnetic properties of the FM layer by LB film in which the functional groups play an important role.

Applying Large-Area Molecular Technology to Improve Magnetoresistive Performance of Hybrid Molecular Spin Valves

Applying molecular-level technique to the construction of molecular spin valves is a challenging issue in organic spintronics, in which the magnetoresistive performance is highly sensitive to the devices local contact geometry. Here, we propose a molecular spin-valve design of large-area molecular junction to reduce the geometrical impact. Room-temperature tunneling magnetoresistance and well-defined parallel/antiparallel states are achieved in the spin valve with a molecular spacer. The results hold promise for the development of future molecular-level nonvolatile electronic devices.

Frequency-assisted current-distribution effect in magnetic tunnel junction

Magnetoimpedance effects were studied in a series of magnetic tunnel junctions by means of the 4-terminal measurement. Magnetic tunnel junctions (MTJ) of layered structure NiFe/CoFe/Al/sub 2/O/sub 3//CoFe were prepared in a magnetron sputtering system with base pressure better than 3/spl times/10/sup -8/ torr. The study of magnetoimpedance effects were carried out with Angilent 4284A precision LCR meter at room temperature. The values of the real-part impedance were found to change their signs as the sensing frequency increased. At certain critical sensing frequency, a giant percentage change under sweeping magnetic field was obtained. This abnormal behavior is explained by the picture of frequency-assisted inhomogeneous current distribution effect, in which the effective tunnel resistance is tuned at the variation of sensing frequency.

Field orientation dependence of magnetoresistance in spin-dependent tunnel junctions

The dependence of magnetotransport on field orientation is an important issue in spintronics-related devices where the applied field is not necessarily in the ideal field-in-plane (FIP) geometry. In this study, we perform tunneling magnetoresistance (TMR) measurements on Co-Al/sub 2/O/sub 3/-CoFe-NiFe spin-dependent tunnel (SDT) junctions prepared at different conditions with varying field orientation ranging from FIP to field-perpendicular-to-plane (FPP). The TMR ratio decreases drastically, whereas the switching field of Co increases when the field direction is set close to FPP. Furthermore, in a situation near FPP, a peculiar TMR looping behavior is observed for one set of samples. Interface effect is thought to be related.