Huy Pham

Charge-carrier localization induced by excess Fe in the superconductor Fe1+yTe1−xSex

We have investigated the effect of Fe nonstoichiometry on properties of the Fe1+y(Te, Se) superconductor system by means of resistivity, Hall coefficient, magnetic susceptibility, and specific heat measurements. We find that the excess Fe at interstitial sites of the (Te, Se) layers not only suppresses superconductivity, but also results in a weakly localized electronic state. We argue that these effects originate from the magnetic coupling between the excess Fe and the adjacent Fe square planar sheets, which favors a short-range magnetic order.

Probing two-dimensional magnetic switching in Co∕SiO2 multilayers using reversible susceptibility experiments

The magnetic switching behavior of Co∕SiO2 multilayers has been studied using reversible susceptibility experiments performed along different orientations in the sample’s plane. A sensitive method for critical curve determination of two-dimensional magnetic systems was proposed. It was shown that this method, based on reversible susceptibility’s singularities detection, is general and can be applied independent of the expression of free energy describing the magnetic system under study. It is found that as the Co∕SiO2 ratio increases in the samples, the switching mechanism is governed by a noncoherent rotation mechanism.

Dynamic critical curve of a synthetic antiferromagnet

In this letter, a dynamic generalization of static critical curves (sCCs) for synthetic antiferromagnet (SAF) structures is presented, analyzing the magnetization switching of SAF elements subjected to pulsed magnetic fields. The dependence of dynamic critical curves (dCCs) on field pulse’s shape and length, on damping, and on magnetostatic coupling is investigated. Comparing sCCs, which are currently used for studying the switching in toggle magnetic random access memories, with dCCs, it is shown that a consistent switching can be achieved only under specific conditions that take into account the dynamics of the systems. The study relies on the Landau–Lifshitz–Gilbert equation.

Dynamic and temperature effects in spin-transfer switching

We have studied the dynamic switching triggered by spin angular momentum transfer in a pulsed current of a spin-valve-type trilayer structure, and its dependence on thermal effects. In order to determine the current pulse parameters, where fast and stable switching can be achieved, we have studied the magnetization’s dynamics properties as a function of applied current pulse amplitude and shape, waiting time, and initial orientation, and also as a function of the Gilbert damping constant. The magnetic layer is assumed to be single domain, ellipsoid shaped. In this paper also we present the thermal fluctuation effects on the switching behavior. The model is based on the Landau–Lifshitz–Gilbert equation and the stochastic Landau–Lifshitz–Gilbert equation with a spin-transfer term included, which are numerically integrated.

Magnetization reversal in interacting magnetic systems

The effect of interparticle interactions on the precessional switching process of coupled ellipsoidally shaped particles subject to pulsed magnetic fields is analyzed by using the Landau–Lifshitz–Gilbert equation. The analysis of switching process is made using two different representations previously proposed in the case of isolated single domain particle. In one switching diagram the final configurational state of the magnetization vector is displayed as a function of the pulse strength and direction whereas in the other representation the final state of magnetization is correlated to the initial position of magnetization vector. It is found that the dynamics of the coupled system, reflected in both switching diagrams is strongly dependent of the interparticle distance and their bond angle.

Switching behavior of a Stoner–Wohlfarth particle subjected to spin-torque effect

Recently, the current-induced spin-transfer torque has been proposed as a convenient writing process in high density magnetic random access memory. A spin-polarized current can switch the magnetization of a ferromagnetic layer more efficiently than a current induced magnetic field. Our paper discusses the switching properties of a Stoner–Wohlfarth magnetic particle for the case when spin torques and external field pulses are simultaneously present. The theoretical investigation of precessional motion is described by using Landau–Lifschitz–Gilbert equation with a spin-transfer torque term included. The main goal is to determine the parameters of field pulse for that the fast and stable switching can be achieved.