C. K. Lo

Enhancement and inverse behaviors of magnetoimpedance in a magnetotunneling junction by driving frequency

The magnetoimpedance effect was employed to study magnetotunneling junction (MTJ) with the structure of Ru(5nm)∕Cu(10nm)∕Ru(5nm)∕IrMn(10nm)∕CoFeB(4nm)∕Al(1.2nm)-oxide∕CoFeB(4nm)∕Ru(5nm). A huge change of more than ±17000% was observed in the imaginary part of the impedance between the magnetically parallel and antiparallel states of the MTJ. The inverse behavior of the magnetoimpedance (MI) loop occurs beyond 21.1MHz; however, the normal MI at low frequency and the inverse MI at high frequency exhibit the same magnetization reversal as checked by the Kerr effect. The reversal in MI was due to the dominance of magnetocapacitance at high frequency.

Effect of Oxygen Concentration on Characteristics of NiO

The resistance bistability and the switching effect has been systematically studied in nickel oxide films with variable oxygen concentrations. Both the switching bias voltage and the resistance ratio of RESET to SET were found to be strongly concentration dependent. Meanwhile, the electrical characteristics can be exactly examined by an Schottky emission. By means of mathematical fitting, the resistance bistability seems to be related to the alternating of dielectric constant and the barrier height. Furthermore, the room temperature cycle endurance with a distinguishable operation window was found to exceed 800. This indicates that the nickel oxide has the potential to be a promising material on resistance random access memory

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Fcc‐Co(110) and hcp‐Co(1100) films of 200 A thickness were grown on MgO(110) and bcc‐Cr(211)/MgO(110), respectively, by the molecular beam epitaxy method. Reflection high energy diffraction was used to in situ characterize the crystal structure. Co films grown directly on MgO(110) were pseudomorphic fcc structure. Twofold symmetrical hcp‐Co(1100) films on top of the bcc‐Cr(211)/MgO(110) were grown and confirmed. The magneto‐optical Kerr effect (MOKE) was used to investigate the magnetic anisotropy of these films. The magnetization of these samples was found to be in‐plane. The magnetization of fcc‐Co films has cubic symmetry with texture induced uniaxial anisotropy. For hcp‐Co(1100)/bcc‐Cr(211) bilayer films, the magnetization was strongly anisotropic, but independent of the thickness of the Cr layer.

-Based Resistance Random Access Memory

A spin transistor which consists of a metallic giant magnetoresistance emitter, a copper base, and a p-n junction was prepared on a Si(100) wafer. The emitter current changes from 1mA at a magnetically parallel state to 0.968mA at a magnetically antiparallel state. At the same states the base currents were 29.3μA and 333nA, respectively, which gave a magnetocurrent ratio of ∼8600% and a transfer ratio of 3×10−2 at room temperature for a common collector configuration. The sensitivity of this spin device is higher than 4000%∕Oe. The memory effect and the high performance make it possible for practical usage. The working principle of this kind of three-terminal spin device can be simply described by circuit theory.

Structure and magnetic anisotropy of epitaxial fcc‐Co(110) and hcp‐Co(11̄00) films

An actuator with single-domain magnetic thin films is designed to investigate the actuation in the application of a magnetic field. The single-domain magnetic thin film used in this study has high shape anisotropy, and the single-domain magnetization is observed using magnetic force microscope. The hysteresis in the planar deflection of the cantilever beam is found, and the magnetic switching behavior of the single-domain magnetic thin films is also observed in the actuation.

Magnetocurrent in a bipolar spin transistor at room temperature

The magnetoimpedance (MZ) effect of the pseudo-spin-valve transistor (PSVT) was investigated at room temperature in the frequency ranged from 100Hzto15MHz. The PSVT can be regarded as a complex combination of resistors, inductors, and capacitors, while the impedance (Z) consists of a real part, the resistance (R), and an imaginary part, the reactance (X). Besides, all these components exhibit magnetic hysteresis. It is due to the frequency dependent behavior that R does not reach a minimum at the resonant frequency (fr). The frequency dependences of MZ and MX ratios cross zero at fx=6.5MHz and at fr=3.65MHz, respectively. The shape of magnetoreactance (MX) loop is reverse to the magnetoresistance (MR) loop; furthermore, MX ratio changes sign from negative at f fr. The MZ loop also reverses shape and sign after crossing fx. For instance, the MZ loop with a ratio of 0.077% at 6MHz switches to −0.086% and −0.125% at 7 and 8MHz, respectively.

Hysteresis in a Microactuator With Single-Domain Magnetic Thin Films

Co(1100)/Cr(211) and Co(1120)/Cr(100) multilayers have been simultaneously prepared on MgO(110) and MgO(100) substrates, respectively, by molecular beam epitaxy. They show however distinct magnetic anisotropic behavior which coincides with their magneto–crystalline anisotropy. Magneto–optical Kerr effect shows the existence of a unique easy axis and strong in‐plane uniaxial magnetic anisotropy in Co(1100)/Cr(211) multilayers, which is induced by the well‐defined hexagonal crystalline of the Co(1100) layers. For Co(1120)/Cr(100) multilayers, on the other hand, an in‐plane biaxial magnetic anisotropy is found due to the bicrystalline structure of the Co(1120) layers.

Impedance behavior of spin-valve transistor

This study demonstrated the preparation of a Cr-doped ZnO wurtzite structure without any impurity phases (metallic Zn, Cr, Zn(OH) 2 , ZnCrO 4 , etc.) via electrodeposition. The surface morphology, lattice structure, Cr content, chemical binding characteristics, and optical properties of the deposits were examined by field-emission-scanning electron microscopy, X-ray diffraction, inductive coupled plasma mass spectroscopy, X-ray photoelectron spectroscopy, UV-visible spectroscopy, and photoluminescence, respectively. Cr-doped ZnO in the shape of hexangular columns appears when the applied potential is equal to or more positive than -1.2 V SSCE . The thickness of the deposits was within the range of 1.07-2.25 μm. Cr was in its trivalent state in the ZnO lattice. Both the high concentration of Cr ions in baths and the more negative applied potential impede the formation of the ZnO(002) plane. The redshift of the bandgap of the deposits from 3.31 to 3.18 eV occurs after the introduction of Cr impurity into the ZnO lattice. The photoluminescence results show both UV and visible light emissions from the electrodeposited specimens.

Studies of the magnetic anisotropies of Co(1100)/Cr(211) and Co(1120)/Cr(100) multilayers

Epitaxial Co/Cr bilayered films have been successfully grown on the MgO(100) and MgO(110) substrates by molecular‐beam epitaxy. According to the reflection high‐energy electron‐diffraction and x‐ray‐diffraction measurements the crystal structure of the film depends on orientation of the buffer and substrate. Epitaxial growth of biaxial Co(1120)/Cr(100) on MgO(100) substrate and of uniaxial Co(1100)/Cr(211) on MgO(110) substrate has been confirmed. The anisotropy magnetoresistance (AMR) is strongly influenced by the orientation of the Cr buffer. In Co(1120)/Cr(100) on MgO(100) AMR is isotropic for all in‐plane fields. However, for Co(1100)/Cr(211) on MgO(110) we observed enhancement of AMR along the easy axis for temperatures below 150 K, while along the hard axis AMR has a local maximum at about 150 K. The easy axis data suggest that the longitudinal spin density wave of Cr and the crystal anisotropy of Co on Cr(211) plane dominate the enhancement of the AMR.

Cr-Doped ZnO Prepared by Electrochemical Deposition

The ac properties of a pseudo spin valve (PSV) consisting of NOL top/Co/Cu/Co/Ni80Fe20/NOLbottom with a top and bottom nano-oxide layer (NOL) were studied as functions of NOL and thickness, d, by magneto impedance spectroscopy. The NOLs were formed by natural oxidizing the top and bottom ferromagnetic or Ta layers in situ. The increase in the thickness of NOL resulting in the increment of effective capacitance caused the shift of roll-off frequency from low to high. The frequency dependence behavior of this NOLtop/PSV/NOLbottom system can be modeled by equivalent circuit theory. PSV with a different NOL structure can be checked by such a nondestructive analysis method