Tai Yen Peng

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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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.

-Based Resistance Random Access Memory

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

Impedance behavior of spin-valve transistor

The structural and exchange bias properties of CoFe/IrMn prepared on Si (100) and Si (111) with an Os/Cu buffer layer were investigated. Since the Os (0002) surface mesh has the same atomic arrangement as fcc (111) orientation, and the lattice mismatch between Os (0002) and IrMn (111) is as low as 2.6%, the CoFe/IrMn grown on H-Si(100) showed a strong IrMn (111) diffraction peak, while a very weak IrMn (111) peak appeared on H-Si (111). With increasing Os thickness (dOs), the IrMn (200) peak was weakened, while the IrMn (111) became strong on H-Si (100). For the CoFe/IrMn grown on H-Si(111), no obvious structural change appeared. Os plays an important role on tuning the IrMn to result in the exchange bias. On the other hand, CoFe/IrMn showed an exchange field (Hex) on both H-Si(100) and H-Si(111) with the Os buffer layer; however, the magnetization switching process was different due to different the crystalline degree. A sharp magnetization switching process occurs for IrMn(111) on Os/Cu/H-Si(100) with a square hysteresis loop. A 370 and 310 Oe of Hex was found in textured CoFe/IrMn on Os/Cu/H-Si(100) and Os/Cu/H-Si(111), respectively

Characterization of a Nano-Oxide Layer in a Pseudo Spin Valve by Complex Magneto-Impedance Spectroscopy

The uses of Os as an antidiffusion and buffer layer in IrMn exchange coupled CoFe film were investigated. For the purpose of antidiffusion, the inserted Os layer showed a distinct improvement of S>0.9, with HC slightly increasing by 1.6 times for the CoFe∕Os∕MnOs multilayer after 400 °C annealing, even though the Os thickness was as thin as 0.3 nm. Furthermore, with a 0.3 nm Os barrier, the 350 °C annealed CoFe∕Os∕IrMn∕CoFe showed almost the same magnetic behavior as the as-deposited state, while the Hex of the upper part of the CoFe∕Os∕IrMn changed from 100 to 190 Oe. In addition, as a buffer layer, the Os buffer layer could enhance the diffraction peak intensities of IrMn(111)∕Os(002) and CoFe (111), and the Hex of CoFe∕IrMn was proportional to the Os thickness. A 120 Oe of Hex was achieved by using an 11 nm Os buffer layer in a CoFe10nm∕IrMn15nm bottom type film. These results show that Os has the potential to be an antidiffusion and buffer layer in a magnetic multilayer.

Effects of an Os Buffer Layer on Structure and Exchange Bias Properties of CoFe/IrMn Fabricated on Si(100) and Si(111)

Oscillating voltage (VOs), which depends on the frequency dependence of the magnetoimpedance (MI) effect, was applied to study a magnetic tunneling junction (MTJ) of Ru(5 nm)/Cu(10 nm)/Ru(5 nm)/IrMn(10 nm)/CoFeB(4 nm)/Al2 O3/CoFeB(4 nm)/Ru(5 nm) at frequencies up to 40 MHz. The MI ratio decreased as the VOs was increased. The MI ratio turned from positive to negative at a certain frequency. An equivalent circuit model was employed to analyze the results. The fact that MTJ can be regarded as the composition of a resistance component and two sets of parallel resistance (R) and capacitance (C) components in series has been utilized to describe the individual impedance contribution from the lead of cross pattern, barrier, and interface. The resistance (Rbarrier) and capacitance (Cbarrier) of the barrier effect are functions of VOs. The Rbarrier decreases as the VOs increases, However, C barrier behaves the opposite way. The tendency is for interfacial resistance Rinterface and interfacial capacitance Cinterface to have opposite results with increasing VOs . This work provides a detailed investigation of high-frequency transport behavior subjected to VOs, especially useful for MTJ characterization

Effects of an Os layer on the magnetic properties of CoFe/IrMn

Growing sixfold symmetric osmium (Os) epitaxial films with suitable buffer layers was developed. Using a Cu buffer layer, the lattice mismatch between Os (0002) and Si (100) was significantly reduced from >30% to ∼7% to grow Os films with twin relationships and weak sixfold symmetries. On the other hand, the Cu∕Au buffer layer was selected to form a fcc (111) surface mesh on H–Si (111)-1×1, and therefore sixfold symmetric Os films were grown due to the lower lattice mismatch. Such growth properties may be applied in the high density magnetic random access memory manufacturing processes to connect the magnetic tunnel junction growth and Cu metal line directly.

Oscillating Voltage Dependence of High-Frequency Impedance in Magnetic Tunneling Junctions

Enhancement of exchange field (Hex) and thermal stability of the textured CoFe∕IrMn with Os∕Cu buffer layer and Os diffusion barrier layer were studied. As revealed by x-ray Diffraction (XRD), an Os (0002) surface mesh was observed to form on Cu (100)/Si (100). The growth of CoFe (111)/IrMn (111) on such a template is parallel to the Os (0002). With the Os∕Cu buffer layer, the CoFe∕IrMn presents an enhancement of 70Oe on Hex larger than that without Os∕Cu. The Hex of the textured sample was 230Oe at room temperature and it was increased to 330Oe after 250°C annealing. When the temperature reached 350°C, Hex vanished. The increment of the temperature at which the textured and the nontextured sample obtaining their maximum Hex and the vanishing temperature of Hex were 50 and 75°C, respectively. Furthermore, the CoFe∕Os (d)∕IrMn slowed down the Hex degradation. The sample with d=0.3nm obtained its maximum Hex at 250°C and vanished when it reached 400°C. The combination of CoFe∕IrMn with Os∕Cu buffer layer an...

Heteroepitaxial growth of sixfold symmetric osmium on Si (111) and Si (100)

In this paper, the characteristics of the osmium (Os) in textured CoFe/IrMn magnetic film multilayer structure system is investigated and the characteristics of buffer layer and diffusion barrier is also reported. To examine the magnetic behavior, magneto-optical Kerr effect (MOKE) and vibrating sample magnetometer (VSM) are used and XRD is used to examine the crystal structure. As a result, a textured Ta/CoFe/Os(d)/IrMn is successfully grown on Os(0002)/Cu(002) seed layer by MMES method and at high temperature Os barrier stabilizes the CoFe/IrMn. Also, the exchange field is shown to vary with annealing temperature and Os thickness, and, no exchange bias is observed with dges1 nm.