L. C. Hsieh

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.

Magnetocurrent in a bipolar spin transistor at room temperature

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.

Impedance behavior of spin-valve transistor

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.

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

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

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

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

Spin-valve transistor

A spin transistor consisting of a metallic pseudo-spin-valve emitter, a copper base, and a p‐n junction collector was prepared on a Si(100) wafer. This spin transistor can provide high magnetocurrent output, high magnetocurrent variation, and high transfer ratio. The common collector configuration with an emitter bias of 5.12V at 77K gives a large magnetocurrent variation of more than 95.5μA in the collector, and the change is more than 3400% with a transfer ratio of 2.59×10−3. At room temperature, these changes go down to 98.3μA and 55.3%, respectively, and the transfer ratio rises to 5.98×10−3. The high performance of this spin-valve transistor is due to the use of a p‐n junction as an energy barrier to select spin electrons.

Room temperature stability study in silicon base magnetic tunneling transistor

A spin tunneling transistor (STT) was designed by growing a magnetic tunneling junction (MTJ) on a p-n junction. The magnetocurrent (MC) ratio of the collector can be stabilized roughly above 40% at V/sub E/=1.25/spl plusmn/0.25 V with the transfer ratio (I/sub C//I/sub E/) of 2.88%, while the transistor is operated in the common collector circuitry with an emitter bias and a base resistor at room temperature. The output current can be more than 4 /spl mu/A when the magnetic moment of the base layer is oriented parallel to that of the emitter layer. The high performance is achieved mainly due to the base resistor, which can push our STT to the right working region and enlarge the MC ratio of the collector.

The optimum magneto-current of collector in a silicon base spin valve transistor

New samples of a new type of spin transistor consisting of a metallic pseudo spin-valve emitter, a copper base, and a Si (100) p-n junction collector were made. For better operating conditions, the magneto-resistance between the emitter and base (MR/sub EB/) had different ratios. The value of MR/sub EB/ is an important factor for better magneto-current (MC) ratios; however, the working range for the emitter bias, V/sub E/, is largely improved and the maximum of MC could be enhanced by one order of the magnitude of MR/sub EB/.