arXiv subjects

D. Hinzke

Publications and source records attributed to D. Hinzke.

13 recordsLinked to original sources

arXiv2025

Exploring the impact of the inverse Faraday effect on all-optical helicity-dependent magnetization switching

All-optical helicity-dependent magnetization switching (AO-HDS) is the quickest data recording technique using only ultrashort laser pulses. FePt grains provide an ideal platform for examining the interaction of effects conducting magnetization switching. We identify the magnetic circular dichroism (MCD) and the inverse Faraday effect (IFE) as the primary switching forces. Ultrafast photon absorption rapidly elevates electron temperatures, quenching magnetization. The MCD's helicity-dependent absorption ensures distinct electron temperatures, holding a finite switching probability by generating different spin noise rates in each spin channel. The IFE induces a magnetic moment, enhancing this probability. We present ultrashort laser pulse (<200 fs) AO-HDS experiments in the near-infrared spectral range from 800 nm to 1500 nm, demonstrating a correlation between switching efficiency and absorbed energy density. Elevating electron temperatures to the Curie point enables the IFE to induce a magnetic moment for deterministic switching in the quenched magnetization state. Unlike in films or multilayers, where domain wall motion and domain growth govern the switching process, increasing the MCD in nanometer-sized grains does not enhance switching efficiency. Electrons around the Curie temperature typically reach increased switching rates for higher induced magnetization generated by the IFE. The MCD sets the necessary switching condition, separating electron temperatures. The IFE generates a magnetic moment, directing spins toward the desired orientation and improving switching efficiency. Every laser pulse initiates a new switching probability for each grain, increasing the role of direction indication by the IFE. Stronger absorption assures higher induced magnetization at low switching fluences.

physics.app-ph↗

arXiv2017

Role of temperature-dependent spin model parameters in ultra-fast magnetization dynamics

In the spirit of multi-scale modelling magnetization dynamics at elevated temperature is often simulated in terms of a spin model where the model parameters are derived from first principles. While these parameters are mostly assumed temperature-independent and thermal properties arise from spin fluctuations only, other scenarios are also possible. Choosing bcc Fe as an example, we investigate the influence of different kinds of model assumptions on ultra-fast spin dynamics, where following a femtosecond laser pulse a sample is demagnetized due to a sudden rise of the electron temperature. While different model assumptions do not affect the simulational results qualitatively, their details do depend on the nature of the modelling.

cond-mat.mtrl-sci↗

arXiv2016

Magnetization switching of FePt nanoparticle recording medium by femtosecond laser pulses

Manipulation of magnetization with ultrashort laser pulses is promising for information storage device applications. The dynamic of the magnetization response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage is FePt nanoparticles , on which optical writing with optical angular momentum was demonstrated recently by Lambert et al., although the mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetization switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to write the magnetization of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.

cond-mat.mes-hall↗

arXiv2015

Multiscale modeling of ultrafast element-specific magnetization dynamics of ferromagnetic alloys

A hierarchical multiscale approach to model the magnetization dynamics of ferromagnetic ran- dom alloys is presented. First-principles calculations of the Heisenberg exchange integrals are linked to atomistic spin models based upon the stochastic Landau-Lifshitz-Gilbert (LLG) equation to calculate temperature-dependent parameters (e.g., effective exchange interactions, damping param- eters). These parameters are subsequently used in the Landau-Lifshitz-Bloch (LLB) model for multi-sublattice magnets to calculate numerically and analytically the ultrafast demagnetization times. The developed multiscale method is applied here to FeNi (permalloy) as well as to copper- doped FeNi alloys. We find that after an ultrafast heat pulse the Ni sublattice demagnetizes faster than the Fe sublattice for the here-studied FeNi-based alloys.

cond-mat.mtrl-sci↗

arXiv2010

Constrained Monte Carlo Method and Calculation of the Temperature Dependence of Magnetic Anisotropy

We introduce a constrained Monte Carlo method which allows us to traverse the phase space of a classical spin system while fixing the magnetization direction. Subsequently we show the method's capability to model the temperature dependence of magnetic anisotropy, and for bulk uniaxial and cubic anisotropies we recover the low-temperature Callen-Callen power laws in M. We also calculate the temperature scaling of the 2-ion anisotropy in L10 FePt, and recover the experimentally observed M^2.1 scaling. The method is newly applied to evaluate the temperature dependent effective anisotropy in the presence of the N'eel surface anisotropy in thin films with different easy axis configurations. In systems having different surface and bulk easy axes, we show the capability to model the temperature-induced reorientation transition. The intrinsic surface anisotropy is found to follow a linear temperature behavior in a large range of temperatures.

cond-mat.mtrl-sci↗

arXiv2009

Current-induced domain wall motion including thermal effects based on Landau-Lifshitz-Bloch equation

We employ the Landau-Lifshitz-Bloch (LLB) equation to investigate current-induced domain wall motion at finite temperatures by numerical micromagnetic simulations. We extend the LLB equation with spin torque terms that account for the effect of spin-polarized currents and we find that the velocities depend strongly on the interplay between adiabatic and non-adiabatic spin torque terms. As a function of temperature, we find non-monotonous behavior, which might be useful to determine the relative strengths of the spin torque terms experimentally.

cond-mat.mes-hall↗

arXiv2009

Laser induced magnetization switching in films with perpendicular anisotropy: a comparison between measurements and a multi-macrospin model

Thermally-assisted ultra-fast magnetization reversal in a DC magnetic field for magnetic multilayer thin films with perpendicular anisotropy has been investigated in the time domain using femtosecond laser heating. The experiment is set-up as an optically pumped stroboscopic Time Resolved Magneto-Optical Kerr Effect magnetometer. It is observed that a modest laser fluence of about 0.3 mJ/square-cm induces switching of the magnetization in an applied field much less than the DC coercivity (0.8 T) on the sub-nanosecond time-scale. This switching was thermally-assisted by the energy from the femtosecond pump-pulse. The experimental results are compared with a model based on the Landau Lifschitz Bloch equation. The comparison supports a description of the reversal process as an ultra-fast demagnetization and partial recovery followed by slower thermally activated switching due to the spin system remaining at an elevated temperature after the heating pulse.

cond-mat.mtrl-sci↗

arXiv2002

Simulation of Magnetization Switching in Nanoparticle Systems

Magnetization reversal in magnetic nanostructures is investigated numerically over time-scales ranging from fast switching processes on a picosecond scale to thermally activated reversal on a microsecond time-scale. A simulation of the stochastic Landau-Lifshitz equation of motion is used as well as a time quantified Monte Carlo method for the simulation of classical spin systems modeling magnetic Co nanoparticles. For field pulses larger than the Stoner-Wohlfarth limit spin precession effects govern the reversal behavior of the particle while for lower fields a magnetization reversal is only possible when it is assisted by thermal fluctuations.

cond-mat.stat-mech↗

arXiv2000

Magnetization Switching in Nanowires: Monte Carlo Study with Fast Fourier Transformation for Dipolar Fields

For the investigations of thermally activated magnetization reversal in systems of classical magnetic moments numerical methods are desirable. We present numerical studies which base on time quantified Monte Carlo methods where the long-range dipole-dipole interaction is calculated with the aid of fast Fourier transformation. As an example, we study models for ferromagnetic nanowires comparing our numerical results for the characteristic time of the reversal process also with numerical data from Langevin dynamics simulations where the fast Fourier transformation method is well established. Depending on the system geometry different reversal mechanism occur like coherent rotation, nucleation, and curling.

cond-mat.stat-mech↗

arXiv1999

Magnetic relaxation in a classical spin chain as model for nanowires

With decreasing particle size, different mechanisms dominate the thermally activated magnetization reversal in ferromagnetic particles. We investigate some of these mechanisms for the case of elongated, single-domain nanoparticles which we describe by a classical Heisenberg spin chain driven by an external magnetic field. For sufficiently small system size the magnetic moments rotate coherently. With increasing size a crossover to a reversal due to soliton-antisoliton nucleation sets in. For even larger systems many of these soliton-antisoliton pairs nucleate at the same time. These effects give rise to a complex size dependence of the energy barriers and characteristic time scales of the relaxation. We study these quantities using Monte Carlo simulations as well as a direct integration of the Landau-Lifshitz-Gilbert equation of motion with Langevin dynamics and we compare our results with asymptotic solutions for the escape rate following from the Fokker-Planck equation. Also, we investigate the crossover from coherent rotation to soliton-antisoliton nucleation and multi-droplet nucleation, especially its dependence on the system size, the external field and the anisotropy of the system.

cond-mat.stat-mech↗

arXiv1999

Uniform susceptibility of classical antiferromagnets in one and two dimensions in a magnetic field

We simulated the field-dependent magnetization m(H,T) and the uniform susceptibility χ(H,T) of classical Heisenberg antiferromagnets in the chain and square-lattice geometry using Monte Carlo methods. The results confirm the singular behavior of χ(H,T) at small T,H: \lim_{T \to 0}\lim_{H \to 0} χ(H,T)=1/(2J_0)(1-1/D) and \lim_{H \to 0}\lim_{T \to 0} χ(H,T)=1/(2J_0), where D=3 is the number of spin components, J_0=zJ, and z is the number of nearest neighbors. A good agreement is achieved in a wide range of temperatures T and magnetic fields H with the first-order 1/D expansion results [D. A. Garanin, J. Stat. Phys. 83, 907 (1996)]

cond-mat.stat-mech↗

arXiv1998

Magnetization Switching in Small Ferromagnetic Particles: Nucleation and Coherent Rotation

The mechanisms of thermally activated magnetization switching in small ferromagnetic particles driven by an external magnetic field are investigated. For low uniaxial anisotropy the spins rotate coherently while for sufficiently large uniaxial anisotropy they behave Ising-like, i.e. the switching then is due to nucleation. The crossover from coherent rotation to nucleation is studied for the classical three-dimensional Heisenberg model with uniaxial anisotropy by Monte Carlo simulations. From the temperature dependence of the metastable lifetime the energy barrier of a switching process can be determined. For the case of infinite anisotropy we compare numerical results from simulations of the Ising model with theoretical results for energy barriers for both, single-droplet and multi-droplet nucleation. The simulated barriers are in agreement with the theoretical predictions.

cond-mat.stat-mech↗

arXiv1998

Magnetization switching in a Heisenberg model for small ferromagnetic particles

We investigate the thermally activated magnetization switching of small ferromagnetic particles driven by an external magnetic field. For low uniaxial anisotropy the spins can be expected to rotate coherently, while for sufficient large anisotropy they should behave Ising-like, i.e., the switching should then be due to nucleation. We study this crossover from coherent rotation to nucleation for the classical three-dimensional Heisenberg model with a finite anisotropy. The crossover is influenced by the size of the particle, the strength of the driving magnetic field, and the anisotropy. We discuss the relevant energy barriers which have to be overcome during the switching, and find theoretical arguments which yield the energetically favorable reversal mechanisms for given values of the quantities above. The results are confirmed by Monte Carlo simulations of Heisenberg and Ising models.

cond-mat.stat-mech↗