Stoner Particles Research Articles

Current-induced synchronized magnetization reversal of two-body Stoner particles with dipolar interaction**Project supported by the National Natural Science Foundation of China (Grant No. 11274236) and the Deutsche Forschungsgemeinschaft via SFB 689.

We investigate magnetization reversal of two-body uniaxial Stoner particles, by injecting spin-polarized current through a spin-valve structure. The two-body Stoner particles perform synchronized dynamics and can act as an information bit in computer technology. In the presence of magnetic dipole–dipole interaction (DDI) between the two particles, the critical switching current Ic for reversing the two dipoles is analytically obtained and numerically verified in two typical geometric configurations. The Ic bifurcates at a critical DDI strength, where Ic can decrease to about 70% of the usual value without DDI. Moreover, we also numerically investigate the magnetic hysteresis loop, magnetization self-precession, reversal time and synchronization stability phase diagram for the two-body system in the synchronized dynamics regime.

Zero-field magnetization reversal of two-body Stoner particles with dipolar interaction

We investigate magnetization reversal in a system of two Stoner particles with uniaxial anisotropies both subject to a static and antiparallel magnetic field, and taking into account their mutual dipolar interaction. We identify an interesting regime of stable synchronized magnetic dynamics where the two particles are implementing a single information bit. Here a modified Stoner-Wohlfarth limit occurs which results in a dramatically lower critical switching field Hc (including Hc=0) and also a substantially shorter reversal time. Our analytical results are verified by numerical simulations and offer new technological perspectives regarding devices for information storage and/or fast magnetic response.

Temperature-dependent magnetization dynamics of magnetic nanoparticles

Recent experimental and theoretical studies show that the switching behavior of magneticnanoparticles can be controlled well by external time-dependent magnetic fields. Inthis work, we inspect theoretically the influence of temperature and magneticanisotropy on the spin dynamics and switching properties of single domain magneticnanoparticles (Stoner particles). Our theoretical tools are the Landau–Lifshitz–Gilbertequation extended to deal with finite temperatures within a Langevin framework.Physical quantities of interest are the minimum field amplitudes required forswitching and the corresponding reversal times of the nanoparticle’s magneticmoment. In particular, we contrast the cases of static and time-dependent externalfields and analyze the influence of damping for a uniaxial and a cubic anisotropy.

Optimal spin-current pulse of the Stoner-Wohlfarth problem

Fast magnetization reversal of uniaxial Stoner particles by spin-transfer torque generated by the spin-polarized electric current is investigated. It is found that the critical current required to reverse a magnetization can be dramatically reduced if a current with a properly designed time-dependent polarization direction is used. Under the condition that the magnitude of the current does not vary with time, the optimal time-dependent electron polarization is obtained such that the magnetization reversal is the fastest.

Theoretical Limit in the Magnetization Reversal of Stoner Particles

Magnetization reversal of uniaxial Stoner particles under the Slonczewski spin-transfer torques of polarized electric currents is investigated. Based on the modified Landau-Lifshitz-Gilbert equation of magnetization dynamics, the theoretical limit of critical currents required to reverse a magnetization with an arbitrary polarized current is obtained. Under a constant polarization degree and constant current amplitude, the optimal current pulse for the fastest magnetization reversal is derived. These results can be used as benchmarks to evaluate different reversal strategies besides other possible usages.

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.

Strategy to reduce minimal magnetization switching field for Stoner particles

A 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.

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.