Z. Z. Sun

Magnetization reversal through synchronization with a microwave

Based on the Landau-Lifshitz-Gilbert equation, it can be shown that a circularly polarized microwave can reverse the magnetization of a Stoner particle through synchronization. In comparison with magnetization reversal induced by a static magnetic field, it can be shown that when a proper microwave frequency is used the minimal switching field is much smaller than that of precessional magnetization reversal. A microwave needs only to overcome the energy dissipation of a Stoner particle in order to reverse magnetization, unlike the conventional method with a static magnetic field where the switching field must be of the order of magnetic anisotropy.

Fast magnetization switching of Stoner particles: A nonlinear dynamics picture

The magnetization reversal of Stoner particles is investigated from the point of view of nonlinear dynamics within the Landau-Lifshitz-Gilbert formulation. The following results are obtained. 1) We clarify that the so-called Stoner-Wohlfarth (SW) limit becomes exact when damping constant is infinitely large. Under the limit, the magnetization moves along the steepest energy descent path. The minimal switching field is the one at which there is only one stable fixed point in the system. 2) For a given magnetic anisotropy, there is a critical value for the damping constant, above which the minimal switching field is the same as that of the SW-limit. 3) We illustrate how fixed points and their basins change under a field along different directions. This change explains well why a non-parallel field gives a smaller minimal switching field and a short switching time. 4) The field of a ballistic magnetization reversal should be along certain direction window in the presence of energy dissipation. The width of the window depends on both of the damping constant and the magnetic anisotropy. The upper and lower bounds of the direction window increase with the damping constant. The window width oscillates with the damping constant for a given magnetic anisotropy. It is zero for both zero and infinite damping. Thus, the perpendicular field configuration widely employed in the current experiments is not the best one since the damping constant in a real system is far from zero.

Theoretical limit of the minimal magnetization switching field and the optimal field pulse for Stoner particles.

The theoretical limit of the minimal magnetization switching field and the optimal field pulse design for uniaxial Stoner particles are investigated. Two results are obtained. One is the existence of a theoretical limit of the smallest magnetic field out of all possible designs. It is shown that the limit is proportional to the damping constant in the weak damping regime and approaches the Stoner-Wohlfarth (SW) limit at large damping. For a realistic damping constant, this limit is more than 10 times smaller than that of so-called precessional magnetization reversal under a noncollinear static field. The other is on the optimal field pulse design: if the magnitude of a magnetic field does not change, but its direction can vary during a reversal process, there is an optimal design that gives the shortest switching time. The switching time depends on the field magnitude, damping constant, and magnetic anisotropy.

Proximity and anomalous field-effect characteristics in double-wall carbon nanotubes

Proximity effect on field-effect characteristic in double-wall carbon nanotubes is investigated. In a semiconductor-metal double-wall carbon nanotube, the penetration of electron wave functions in the metallic shell to the semiconducting shell turns the original semiconducting shell into a metal where the local density of states is not zero at the Fermi level. By using a two-band tight-binding model on a ladder of two legs, it is demonstrated that anomalous field-effect characteristic in semiconductor-metal-type double-wall carbon nanotubes can be fully understood by the proximity effect of metallic phases.

Theoretical limit in the magnetization reversal of stoner particles.

Fast magnetization reversal of uniaxial Stoner particles by spin-transfer torque due to the spin-polarized electric current is investigated. It is found that a current with a properly designed time-dependent polarization direction can dramatically reduce the critical current density required to reverse a magnetization. Under the condition that the magnitude and the polarization degree of the current do not vary with time, the shape of the optimal time-dependent polarization direction is obtained such that the magnetization reversal is the fastest.

Strategy to reduce minimal magnetization switching field for Stoner particles

A new strategy is proposed aimed at substantially reducing the minimal magnetization switching field for a Stoner particle. Unlike the normal method of applying a static magnetic field which must be larger than the magnetic anisotropy, a much weaker field, proportional to the damping constant in the weak damping regime, can be used to switch the magnetization from one state to another if the field is along the motion of the magnetization. The concept is to constantly supply energy to the particle from the time-dependent magnetic field to allow the particle to climb over the potential barrier between the initial and the target states.

Resonance and antiresonance effects in electronic transport through several-quantum-dot combinations

We investigated the resonance and antiresonance effects in electronic transport through several-quantum-dot combinations by using the nonequilibrium Green’s function method. All distinctive quantum-dot (QD) arrangements with one to three QDs and with different architectures were studied systematically. The theoretical and numerical results show that a peak in the current-voltage spectrum can be attributed to the resonance effect, whereas a dip is due to the antiresonance effect. The results will help experimenters to better understand their electronic measurements.

Explanation to the resistance anomaly observed in nanowires

A possible model for a resistance anomaly of nanowires is proposed. As the radius of a quantum wire shrinks below either the impurity (scatterer) potential ranges or the carrier sizes, the usual inverse (of wire cross section) Ohm’s law fails. Instead, each scatterer contributes almost equally to the wire resistance, and the resistance is proportional to the cross section (number of scatterers). The model explains well the recent resistance measurement on InN nanowires [C.-Y. Chang et al., J. Electron. Mater. 35, 738 (2006)].

Negative differential resistance and tunable peak-to-valley ratios in a silicon nanochain

The current-voltage characteristics of a silicon nanochain is investigated. The nanochain is viewed as a superlattice structure of quantum dots (QDs), where silicon cores in a chain act as QDs while silicon dioxides covering the cores act as potential barriers. It is found that the whole nanochain structure can display the negative differential conductance (NDC) feature as the tunneling current through each barrier has the NDC property individually. Importantly, large peak-to-valley ratios of the current are observed and tunable by the number of QDs involved. This feature will be useful in device design.

Spin Dynamics: Fast Switching of Macro-spins

Recent progress on the theoretical studies of fast magnetization reversal of Stoner particles is reviewed. The following results are discussed: (1) The Stoner–Wohlfarth (SW) limit becomes exact when the damping constant is infinitely large. Under the limit, magnetization moves along the steepest energy descent path. (2) For a given magnetic anisotropy, there is a critical damping constant, above which the minimal switching field is the same as that of the SW-limit. (3) The field of a ballistic magnetization reversal should be along a certain direction window in the presence of energy dissipation. (4) Since a time-dependent magnetic field can be an energy source, two new reversal strategies are possible. One is to use a field following magnetization motion, and the other is to use a circularly polarized microwave near the ferromagnetic resonance frequency. The critical switching fields of both strategies are substantially lower than that of precessional reversal for realistic materials. (5) The theoretical limits for both field-induced and current-induced magnetization reversal are presented for uniaxial Stoner particles.