arXiv subjects

Martin Stier

Publications and source records attributed to Martin Stier.

10 recordsLinked to original sources

arXiv2021

Creating arbitrary sequences of mobile magnetic skyrmions and antiskyrmions

Magnetic skyrmions and their anti-particles, the antiskyrmions, are stable magnetic solitons existing down to the nanometer scale. Their stability and size as well as the possibility to propel them by, e.g., electric currents make them promising candidates for the use in memory devices, such as racetrack memories. Skyrmions and antiskyrmions share those same advantages individually, but may annihilate each other when they coexist in the same device. Yet, combining them to represent logical bits of 0 (skyrmion) and 1 (antiskyrmion) in one device opens new possibilities to create new types of densely packed racetrack memory devices. For this, a controlled creation and annihilation procedure, i.e., a writing or deletion operation, is necessary. Here, we propose a method to create arbitrary sequences of coexisting skyrmions and antiskyrmions by rotations of the magnetic moments at the edge of a rectangular slab. The skyrmions and antiskyrmions remain stable and do not annihilate each other.

cond-mat.mes-hall↗

arXiv2021

Controlled Creation of Quantum Skyrmions

We study the creation of quantum skyrmions in quadratic nanoscopic lattices of quantum spins coupled by Dzyaloshinkii-Moriya and exchange interactions. We numerically show that different kinds of quantum skyrmions, characterized by the magnitude of their spin expectation values and strong differences in their stability, can appear as ground state and as metastable excitations. In dependence on the coupling strengths and the lattice size, an adiabatic rotation scheme of the magnetization at the lattice boundaries allows for a controlled creation of quantum skyrmions.

cond-mat.mes-hall↗

arXiv2020

Rotating edge-field driven processing of chiral spin textures in racetrack devices

Topologically distinct magnetic structures like skyrmions, domain walls, and the uniformly magnetized state have multiple applications in logic devices, sensors, and as bits of information. One of the most promising concepts for applying these bits is the racetrack architecture controlled by electric currents or magnetic driving fields. In state-of-the-art racetracks, these fields or currents are applied to the whole circuit. Here, we employ micromagnetic and atomistic simulations to establish a concept for racetrack memories free of global driving forces. Surprisingly, we realize that mixed sequences of topologically distinct objects can be created and propagated over far distances exclusively by local rotation of magnetization at the sample boundaries. We reveal the dependence between the chirality of the rotation and the direction of propagation and define the phase space where the proposed procedure can be realized. The advantages of this approach are the exclusion of high current and field densities as well as its compatibility with an energy-efficient three-dimensional design.

cond-mat.mes-hall↗

arXiv2018

Dragging of magnetic domain walls by interlayer exchange

Magnetic domain walls can be moved by spin-polarized currents due to spin-transfer torques. This opens the possibility to use them in spintronic memory devices as, e.g., in racetrack storage. Naturally, in miniaturized devices domain walls can get very close to each other and affect each others dynamics. In this work we consider two separated domain walls in different layers which interact via an interlayer exchange coupling. One of these walls is moved by a spin-polarized current. Depending on several parameters as the current density, the interlayer coupling or the pinning potential, the combined dynamics of the two domain walls can change very strongly allowing, e.g., for a correlated motion of the walls. In addition, more subtle effect appear as a suppression of the Walker breakdown accompanied by an increase of the domain wall velocity.

cond-mat.mes-hall↗

arXiv2017

Skyrmion-Antiskyrmion pair creation by in-plane currents

Magnetic skyrmions can be considered as topologically protected localized vortex-like spin textures. Due to their stability, their small size, and the possibility to move them by low electric currents they are promising candidates for spintronic devices. Without violating topological protection, it is possible to create skyrmion-antiskyrmion pairs, as long as the total charge remains unchanged. We derive a skyrmion equation of motion which reveals how spin-polarized charge currents create skyrmion-antiskyrmion pairs. It allows to identify general prerequisites for the pair creation process. We corroborate these general principles by numerical simulations. On a lattice, where topological protection becomes imperfect, the antiskyrmion partner of the pairs is annihilated and only the skyrmion survives. This eventually changes the total skyrmion number and yields a new way of creating and controlling skyrmions.

cond-mat.mes-hall↗

arXiv2015

Nonlocal spin torques in Rashba quantum wires with steep magnetic textures

We provide a general procedure to calculate the current-induced spin-transfer torque which acts on a general steep magnetic texture due to the exchange interaction with an applied spin-polarized current. As an example, we consider a one-dimensional ferromagnetic quantum wire and also include a Rashba spin-orbit interaction. The spin-transfer torque becomes generally spatially non-local. Likewise, the Rashba spin-orbit interaction induces a spatially nonlocal field-like nonequilibrium spin-transfer torque. We also find a spatially varying nonadiabaticity parameter and markedly different domain wall dynamics for very steep textures as compared to wide domain walls.

cond-mat.mes-hall↗

arXiv2015

Implications of a temperature-dependent magnetic anisotropy for superparamagnetic switching

The macroscopic magnetic moment of a superparamagnetic system has to overcome an energy barrier in order to switch its direction. This barrier is formed by magnetic anisotropies in the material and may be surmounted typically after 10^9 to 10^12 attempts per second by thermal fluctuations. In a first step, the associated switching rate may be described by a Neel-Brown-Arrhenius law, in which the energy barrier is assumed as constant or a given temperature. Yet, magnetic anisotropies in general depend on temperature themselves which is known to modify the Neel-Brown-Arrhenius law. We illustrate quantitatively the implications of a temperature-dependent anisotropy on the switching rate and in particular for the interpretation of the prefactor as an attempt frequency. In particular, we show that realistic numbers for the attempt frequency are obtained when the temperature dependence of the anisotropy is taken into account.

cond-mat.mes-hall↗

arXiv2014

Rashba induced chirality switching of domain walls and suppression of the Walker breakdown

In conventional domain wall systems the aim of a high domain wall velocity may be hindered by the occurrence of a Walker breakdown at comparably low current density. We show how a Rashba interaction can stabilize the domain wall dynamics and thereby shift the Walker breakdown to higher current densities. The Rashba interaction creates a field like spin torque, which breaks the symmetry of the system and modifies the internal structure of the domain wall. Besides a shift of the Walker breakdown it can additionally induce a chirality switch of the domain wall at sufficient Rashba fields. The preferred chirality may then be chosen by the direction of the current flow. Both, the suppression of the Walker breakdown and the chirality switching, affect the domain wall velocity. This is even more pronounced for short current pulses, where an additional domain wall movement after the pulse in either positive or negative direction can determine the final position of the domain wall.

cond-mat.mes-hall↗

arXiv2013

Nonequilibrium Rashba field driven domain wall motion in ferromagnetic nanowires

We study the effects of spin-orbit interaction (SOI) on the current-induced motion of a magnetic (Bloch) domain wall in ultrathin ferromagnetic nanowires. The conspiracy of spin relaxation and SOI is shown to generate a novel strong nonequilibrium Rashba field, which is dominant even for moderate SOI. This field causes intricate spin precession and a transition from translatory to oscillatory wall dynamics with increasing SOI. We show that current pulses of different lengths can efficiently be used to control the domain wall motion.

cond-mat.mes-hall↗

arXiv2007

Extensions to the Kondo lattice model to achieve realistic Curie temperatures and appropriate behavior of the resistivity for manganites

We investigate the influence of the Jahn-Teller distortion and a direct antiferromagnetic moment coupling as extensions to a two-band Kondo lattice model for the magnetic and electronic properties of manganites. Those are calculated self-consistently via an interpolating self-energy model and a modified RKKY technique using finite Hund coupling and quantum spins. We found that both effects are essential to achieve realistic Curie temperatures if we regard intraband Coulomb repulsion. Using reliable model parameters we got results which are in very good agreement with experimental data in the whole ferromagnetic doping range. In the calculated phase diagram there are ferromagnetic metal to paramagnetic insulator transitions, accompanied by a Colossal Magnetoresistance (CMR) behavior. To improve the comparability of the measured behavior of the resistivity with the calculated one, we have to switch on interband Coulomb correlations.

cond-mat.str-el↗