Bo-Zang Li

CONDUCTANCE, MAGNETORESISTANCE, AND INTERLAYER COUPLING IN TUNNEL JUNCTIONS MODULATED BY NONMAGNETIC METALLIC INTERLAYERS

Based on the two-band model and free-electron approximation, we study the magnetism and transport properties of tunnel junctions with nonmagnetic interlayers (NM) between the ferromagnetic electrodes and tunneling barrier. We find that properties of the junctions are intermediate between tunnel junctions and metallic magnetic multilayers. The mean conductance, tunnel magnetoresistance, and interlayer coupling are all the oscillatory functions of the thickness of NM. It suggests that weak antiferromagnetic coupling can be attained by controlling the thickness of NM. Our results have potential in designing spin-polarized tunneling devices with large field sensitivity.

Spin-polarized resonant tunneling and quantum-size effect in ferromagnetic tunnel junctions with double barriers subjected to an electric field

Abstract Based on the two-band model, we present a transfer-matrix treatment of tunnel magnetoresistance (TMR) for tunneling through double ferromagnetic tunnel junctions with any magnetization angle of the middle ferromagnetic layer subjected to an electric field. Our results show that the TMR changes non-monotonically with the magnetization angle of the middle ferromagnetic layer in double junctions, which is different from that in a single junction. Owing to the comprehensive role of the quantum-size effect and the spin-polarized resonant tunneling, the TMR oscillates with the thickness of the middle ferromagnetic layer and can reach very large values under suitable conditions. Furthermore, the quantum-size effect can also give rise to a positive TMR (inverse spin-valve effect).

THEORY OF SPIN-POLARIZED TUNNELING BETWEEN FERROMAGNETIC FILMS

Abstract We consider the spin-flip effect and present a theoretical analysis of tunnel conductance and magnetoresistance for a ferromagnetic tunnel junction subjected to a dc bias by using the nonequilibrium Green function method. Our results are in qualitative agreement with the experimental measurements. The spin-flip tunneling is found to diminish the magnitude of tunnel magnetoresistance while improving the value of tunnel conductance. In certain cases, by considering the spin-flip effect, quantitative agreement is obtained between our theoretical results and recent experimental data.

Bias dependence of the tunneling magnetoresistance in double spin-filter junctions

With the intention of providing reference materials for research, manufacture and application of magnetoresistance devices, we calculate the dependences of tunneling magnetoresistance (TMR) in a NM/FI/FI/NM double spin-filter junction (DSFJ) on the bias (voltage) and, secondarily, on the thickness, barrier height and molecular field of FIs [here the NM and FI represent the nonmagnetic electrode and the ferromagnetic insulator (semiconductor) spacer, respectively]. Our results show that for the TMR of the DSFJ besides its very high value it does not decrease monotonously and rapidly with a rise of bias, but increases slowly at first and then decreases after a maximum value is reached. This feature is in distinct contrast to the ordinary magnetic tunnel junction FM/NI/FM and thus facilitates the application of the DSFJ as a magnetoresistance device [here FM and NI denote the ferromagnetic electrode and the nonmagnetic insulator (semiconductor) spacer, respectively]. The influence of the thickness, barrier h...

The interlayer exchange coupling in magnetic multilayers: the effect of the thickness of the ferromagnetic layer

In this work the effect of the ferromagnetic layer thickness on the interlayer exchange coupling in magnetic multilayers, which has rarely been considered so far, is investigated by employing the one-band tight-binding hole-confinement model. The numerical calculations for a simple cubic lattice show that although the oscillatory variation of coupling parameter J with the variation of non-magnetic layer thickness N has periods insensitive to the ferromagnetic layer thickness M, when M becomes smaller the amplitude and phase of such oscillations depend strongly on M, and J varies with M in an oscillatory-like fashion. Our theoretical results may be helpful in understanding the recent experimental data.

Exact evolving states for a class of generalized time-dependent quantum harmonic oscillators with a moving boundary

Abstract In this Letter, a class of the generalized time-dependent quantum harmonic oscillators with a moving boundary is studied. The Hamiltonian is of the non-homogeneous quadratic form of space coordinate and momentum with time-dependent coefficients. An orthonormalized and complete set of evolving states has been constructed, and the necessary and sufficient conditions for existence of such a type of states have been given. Our results are of considerable generality, which encompass as particular cases almost all the literature results.

Berry phases for many-spin systems with anisotropy exchange interaction in a strong magnetic field

An approximate instantaneous eigenstate is presented for the many-spin system possessing the anisotropy exchange interaction in a strong magnetic field by the perturbation method. Based on the instantaneous eigenstate, the Berry phase of this state is worked out. Furthermore, the effect of the anisotropy exchange interaction on the Berry phase of the state for the many-spin system is studied.

Numerical solutions of the generalized Moore's equations for a one-dimensional cavity with two moving mirrors

Extending the approach proposed by Cole and Schieve for a one-dimensional (I D) cavity with one moving mirror, we develop a numerical method to solve exactly the generalized Moores (GM) equations for a I D cavity with two moving mirrors. As an example of applying our method, the GM equations are solved in detail in the case where the two mirrors oscillate resonantly, and the dependencies of the solutions on frequency, amplitude and dephasing are investigated systematically

Giant tunneling magnetoresistance in ferromagnet/insulator (semiconductor) coupling double-tunnel-junction subjected to electric field

Based on the two-band model, a transfer-matrix treatment of the tunnel conductance and magnetoresistance is presented for tunneling through ferromagnet/insulator (semiconductor) double-junction subjected to dc bias. There exist the spin polarized resonant tunneling and the giant tunnel magnetoresistance. The highest value of the magnetoresistance in double-junction can reach 90%. It is expected that these results can cause the interest in experimental efforts in designing spin polarized resonant tunneling devices. Our theories can also be extended to the single-junction and the superlattice easily. For the single-junction, our results are qualitatively in agreement with the experimental measurements.

Geometric Phase in Path Integral and Quantisation Rule

A description of geometric phase in terms of path integral formalism is presented. It is proved that this adiabatic phase can appear in the propagator or Green function of an adiabatic system. In the semiclassical approximation, following the Green function expression of the electronic density of states, the corresponding generalized Bohr-Sommerfeld quantisation rule can thus be obtained. It is shown that this rule has been corrected by the geometric phase.