Ming-wen Xiao

Bias dependent tunneling in ferromagnetic junctions and inversion of the tunneling magnetoresistance from a quantum mechanical point of view

In the framework of the free-electron approximation, we have developed a quantum mechanical treatment for describing the bias dependent tunneling in FM/I/FM ferromagnetic junctions. In our theory, the Slonczewski model is extended to include the bias effect. In the barrier region, the Wentzel–Kramers–Brillouin wave function is used following Harrison. The main point of our treatment is to match the wave functions at both sides of the electrode/barrier interfaces quantum mechanically. We find that apart from the usual density of states effect, there exists a quantum coherent factor D(Ex,V)=κR2(Ex,V)−kR↑(Ex,V)kR↓(Ex,V), which decreases monotonously with the increasing applied bias and could change its sign at a sufficiently high bias. The characteristic of this coherent factor can explain the observed rapid decrease of tunneling magnetoresistance (TMR) with increasing bias and the sign change of TMR in some ferromagnetic junctions. Furthermore, numerical results for asymmetry barrier junctions provide a goo...

Oscillation effect and sign-change behaviour of the bias-dependent tunnelling magnetoresistance in ferromagnetic junctions

In this paper, we present a spintronic tunnelling theory for ferromagnet/insulator/ferromagnet (FM/I/FM) junctions. With the use of Airy functions, it can analytically account for both the low-bias and the high-bias tunnelling magnetoresistances (TMRs). We find that the sign-change behaviour of TMR can only occur in the low-bias region, due to the quantum coherence in FM/I/FM junctions. In the high-bias region, the TMR will oscillate between positive and negative with increasing bias voltage. Physically, this oscillation arises from the interference between the incident and reflected electron waves in the barrier region. The effects of the barrier height, the barrier width and the electron effective mass in the barrier are studied systematically. The theoretical results obtained from the exact Airy functions agree well with TMR experiments on Ta2O5- and Al2O3-barrier junctions, within the whole measurable range of bias voltage.

Barrier-height and bias-voltage-controlled spin-filter effect and tunneling magnetoresistance in full ferromagnetic junctions

Within the framework of the single electron spintronic model, we systematically studied the barrier-height and bias-voltage-controlled spin-filter effect and tunneling magnetoresistance (TMR) in ferromagnetic metal/ferromagnetic insulator/ferromagnetic metal (FM/FI/FM) tunnel junctions. We find that it is the extended quantum-coherence factor of Slonczewski, κL↑2−kL↑kL↓, that physically controls the sign of the zero-bias TMR. This factor is a linear function of the mean barrier height. The zero-bias TMR is positive when the mean barrier is high, and negative when the mean barrier gets low, which agrees well with the experimental results observed in GdOx-barrier junctions. As a cooperative result of the mean barrier and spin-filter effect, a positively or negatively large TMR can be maintained in a rather wide range near the zero bias if the mean barrier of the FI spacer is much higher or much lower. This property is believed to be of practical use in designing spintronic devices. Besides, the TMR can osci...