C.Y. Wong

Room-temperature magnetism in Cr-doped AlN semiconductor films

Synthesis and characterization of magnetic semiconductors Al1−xCrxN, in which the atomic fraction of chromium x is up to 0.357, are reported. The films, grown by reactive co-sputtering on silicon, glass, and kapton substrates, have a crystal structure of aluminum nitride. Magnetic and transport properties were studied in the temperature range of 50 to 340 K. The materials are in the dielectric regime and have variable-range-hopping type of conductance. The films are ferromagnetic at temperatures over 340 K.

Room temperature magnetism in sputtered (Zn,Co)O films

Cobalt doped zinc oxide films Zn/sub 1-x/Co/sub x/O with the atomic fraction x in the range 0.035-0.115 were prepared by sputtering from composite ZnO/Co/sub 3/O/sub 4/ targets. X-ray diffraction pattern shows the ZnO phase with [002] preferential orientation. Magnetic measurements show ferromagnetic behavior with Curie temperatures higher than 350 K.

Chemical synthesis of narrowly dispersed SmCo5 nanoparticles

In this article we report a chemical synthetic means for generating a high Ku magnetic material—narrowly dispersed nanoparticles of SmCo5. Using Co2(CO)8 and Sm(acac)3 as the precursors under air-free conditions, we produced SmCo5 nanoparticles according to the procedure reported by Sun et al. [Science 287, 1981 (2000)] but with some modifications. The nanoparticles, with diameters of 6–8 nm, have a SmCo5 composition, as indicated by transmission electron microscopy, electron diffraction, and x-ray photoelectron spectroscopy. The magnetization measurement of the nanoparticles, exhibits superparamagnetism, which is blocked for temperatures below ∼110 K, suggesting Ku to be ∼2.1×106 erg/cm3 for the as-prepared particles.

Magnetization and magnetocaloric effect in magnetic shape memory alloys Ni-Mn-Ga

Magnetization of Ni/sub 52/Mn/sub 24.4/Ga/sub 23.6/ and Ni/sub 52/Mn/sub 24.2/Ga/sub 23.8/ magnetic shape memory alloys was measured as a function of magnetic field up to 50 kOe at temperatures in the range 160-390 K. Anomalies of magnetic properties were observed not only at the temperature of martensitic transition T/sub M/, but also at a second transition temperature T/sub I/<T/sub M/. We assume that T/sub I/ is associated with an intermartensitic transformation.

A novel current-mode sensing scheme for magnetic tunnel junction MRAM

In this paper, we present two integrated circuits for sensing data nondestructively from one-transistor one-magnetic tunnel junction (1T-1MTJ) magnetoresistive random access memory (MRAM). The first one is a low-power sensing circuit for MTJs with a magnetoresistance (MR) ratio larger than 10%, and the second one is a high-sensitivity switched-current sensing circuit for MTJs with an MR ratio as low as 5%. The circuits are designed using 0.60- and 0.18-/spl mu/m CMOS processes, and their performance is verified using HSPICE. Compared with existing sensing circuits at a power supply of 3.0 V, their read access time is 1.46-3.33 times faster and power consumption is 2.67-3.85 times smaller.

Tunneling current and thickness inhomogeneities of ultrathin aluminum oxide films in magnetic tunneling junctions

Tunneling current and thickness inhomogeneities of ultrathin aluminum oxide layers of magnetic tunnel junctions are studied by conducting atomic force microscopy (CAFM). The current inhomogeneities are attributed to thickness inhomogeneities on a nanometer scale. Thickness distributions are derived from the current distributions, using the Simmons’ tunneling model by assuming a constant and uniform barrier height. It is shown that dielectric breakdown at high voltages can cause modifications of the AlOx layer during CAFM measurements, characterized by the irreversibility of both current images and local I–V characteristics. Working at low voltage and low current is a necessary condition for applicability of the CAFM method for testing the insulator layers. The effect of the method of film preparation on the film dielectric strength is studied.

Magnetic and magneto-optical properties of Pd/Cr/Co multilayers

Recent studies found that the (Pt/Co/Pt) trilayers can be used as a unit in combination with nonmagnetic or magnetic layer, X (X=Pd, Ag, Cu, and Ni), to enhance the perpendicular magnetic anisotropy of the films, reduce the Curie temperature, and alter the magneto-optical properties. The effects of intercalating Cr into Pd/Co multilayers on the magnetic and magneto-optical properties are studied in this article. The perpendicular magnetic anisotropy Ku and the coercivity Hc of the system decrease rapidly with increasing the Cr thickness (XCr) up to 0.4 nm, and change slightly when Cr thickness further increases. The dependence of the coercivity Hc on the Cr thickness, which obeys the law: Hc (XCr)=XCr−2.66, indicates that the magnetization reversal is controlled by domain wall moving, mainly due to the interface roughness. Large decrease of the Kerr rotation θk of the Pd/Cr/Co multilayers compared with pure Pd/Co multilayers is also found in the wavelength ranging from 200 to 800 nm. As it is well known, ...

A switched-current sensing architecture for a four-state per cell magnetic tunnel junction MRAM

A current-mode binary-search sensing scheme for a four-state per cell one-transistor one-magnetic tunnel junction magneto-resistive (MR) random access memory is proposed. By using the switched-current technique, it is able to read data nondestructively with a MR ratio as low as 5%. The sensing circuit is designed using a 0.18-/spl mu/m CMOS process and the performance is verified by HSPICE simulation. At a supply voltage of 1.8 V, the data can be accessed in 17.5 ns with a power consumption of 475.9 /spl mu/W. Compared to the parallel sensing approach, the proposed sensing scheme consumes less power and chip area, and requires fewer comparison steps. Compared to the conventional serial sensing approach, it allows a shorter read access time while performing the same number of comparisons.

The reversal mechanism and coercivity of Pt/sub 3/Co alloy film

In this article, Pt/sub 3/Co alloy film is produced using a sputtering system. To understand the reversal mechanism of the Pt/sub 3/Co alloy film, the recoil loops from the major hysteresis loop have been measured. The reversible magnetization M/sub rev/ dependence of the irreversible magnetization M/sub irr/ exhibits a minimum when M/sub irr/ is slightly larger than zero, which is the characteristic of a material that contains domain walls bowing during reversal caused by localized pinning due to dipolar fields for the films. By analyzing the linear dependence of the coercivity on the anisotropy field at different temperatures, the strength of the local dipolar fields N/sub eff/, which influences the creation of the reversed magnetic nucleus, is estimated. We found a positive value of N/sub eff/, which indicates that reversal process is favored by dipolar fields. It is also supposed that the dipolar fields originated from irregular local random anisotropy caused by residual stress, cavities, or inclusions, may be effective sources of pinning in the reversal process, and may strongly affect the coercivity of the film.

VLSI process integration

It has been predicted that microprocessors will use over 13 million transistors by year 2001 and that by year 2010, over 90 million transistors will he deployed to fabricate semiconductor chips with ever increasing efficiency. It has also been generally accepted that the costs of a wafer fab essentially double for each new generation of microprocessor. In the coming 21st Century, VLSI process integration, in product/research/development and in manufacturing, will need to meet the demands of not only the technology innovations, the reliability and scaling challenges, but also the cost effectiveness of yield, factory workflow and capacity management. Issues addressing these factors, sometimes leading in diametric directions, and the novel solutions, are presented and discussed.