arXiv subjects

U. Nowak

Publications and source records attributed to U. Nowak.

At least 19 recordsLinked to original sources

arXiv2024

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↗

arXiv2021

Polarized phonons carry the missing angular momentum in femtosecond demagnetization

Magnetic phenomena are ubiquitous in our surroundings and indispensable for modern science and technology, but it is notoriously difficult to change the magnetic order of a material in a rapid way. However, if a thin nickel film is subjected to ultrashort laser pulses, it can lose its magnetic order almost completely within merely femtosecond times. This phenomenon, in the meantime also observed in many other materials, has connected magnetism with femtosecond optics in an efficient, ultrafast and complex way, offering opportunities for rapid information processing or ultrafast spintronics at frequencies approaching those of light. Consequently, the physics of ultrafast demagnetization is central to modern material research, but a crucial question has remained elusive: If a material loses its magnetization within only femtoseconds, where is the missing angular momentum in such short time? Here we use ultrafast electron diffraction to reveal in nickel an almost instantaneous, long-lasting, non-equilibrium population of anisotropic high-frequency phonons that appear as quickly as the magnetic order is lost. The anisotropy plane is perpendicular to the direction of the initial magnetization and the atomic oscillation amplitude is 2 pm. We explain these observations by means of circularly polarized phonons that quickly absorb the missing angular momentum of the spin system before the slower onset of a macroscopic sample rotation. The time that is needed for demagnetization is related to the time it takes to accelerate the atoms. These results provide an atomistic picture of ultrafast demagnetization under adherence to all conservation laws but also demonstrate the general importance of polarized phonons for non-equilibrium dynamics and provide innovative ways for controlling materials on atomic dimensions.

cond-mat.mtrl-sci↗

arXiv2021

Exceptional sign changes of the non-local spin Seebeck effect in antiferromagnetic hematite

Low power spintronic devices based on the propagation of pure magnonic spin currents in antiferromagnetic insulator materials offer several distinct advantages over ferromagnetic components including higher frequency magnons and a stability against disturbing external magnetic fields. In this work, we make use of the insulating antiferromagnetic phase of iron oxide, the mineral hematite $\alpha$-Fe$_2$O$_3$ to investigate the long distance transport of thermally generated magnonic spin currents. We report on the excitation of magnons generated by the spin Seebeck effect, transported both parallel and perpendicular to the antiferromagnetic easy-axis under an applied magnetic field. Making use of an atomistic hematite toy model, we calculate the transport characteristics from the deviation of the antiferromagnetic ordering from equilibrium under an applied field. We resolve the role of the magnetic order parameters in the transport, and experimentally we find significant thermal spin transport without the need for a net magnetization.

cond-mat.mtrl-sci↗

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↗

arXiv2013

Exchange Bias driven by Dzyaloshinskii-Moriya interactions

The exchange bias effect in compensated IrMn3/Co(111) system is studied using multiscale modeling from "ab initio" to atomistic calculations. We evaluate numerically the out-of-plane hysteresis loops of the bi-layer for different thickness of the ferromagnetic layer. The results show the existence of a perpendicular exchange bias field and an enhancement of the coercivity of the system. In order to elucidate the possible origin of the exchange bias, we analyze the hysteresis loops of a selected bi-layer by tuning the different contributions to the exchange interactions across the interface. Our results indicate that the exchange bias is primarily induced by the Dzyaloshinskii-Moriya interactions, while the coercivity is increased mainly due to a spin-flop mechanism.

cond-mat.mtrl-sci↗

arXiv2012

Thermally induced error: density limit for magnetic data storage

Magnetic data storage is pervasive in the preservation of digital information and the rapid pace of computer development requires ever more capacity. Increasing the storage density for magnetic hard disk drives requires a reduced bit size, previously thought to be limited by the thermal stability of the constituent magnetic grains. The limiting storage density in magnetic recording is investigated treating the writing of bits as a thermodynamic process. A 'thermal writability' factor is introduced and it is shown that storage densities will be limited to 15 to 20 TBit/in^2 unless technology can move beyond the currently available write field magnitudes.

cond-mat.mtrl-sci↗

arXiv2010

Atomistic spin-model based on a new spin-cluster expansion technique: Application to the IrMn3/Co interface

In order to derive tensorial exchange interactions and local magnetic anisotropies in itinerant magnetic systems, an approach combining the Spin-Cluster Expansion with the Relativistic Disordered Local Moment scheme is introduced. The theoretical background and computational aspects of the method are described in detail. The exchange interactions and site resolved anisotropy contributions for the IrMn3/Co(111) interface, a prototype for an exchange bias system, are calculated including a large number of magnetic sites from both the antiferromagnet and ferromagnet. Our calculations reveal that the coupling between the two subsystems is fairly limited to the vicinity of the interface. The magnetic anisotropy of the interface system is discussed, including effects of the Dzyaloshinskii-Moriya interactions that appear due to symmetry breaking at the interface.

cond-mat.str-el↗

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↗

arXiv2010

Non-adiabatic spin torque investigated using thermally activated magnetic domain wall dynamics

Using transmission electron microscopy, we investigate the thermally activated motion of domain walls (DWs) between two positions in permalloy (Ni80Fe20) nanowires at room temperature. We show that this purely thermal motion is well described by an Arrhenius law, allowing for a description of the DW as a quasi-particle in a 1D potential landscape. By injecting small currents, the potential is modified, allowing for the determination of the non-adiabatic spin torque: the non-adiabatic coefficient is 0.010 +/- 0.004 for a transverse DW and 0.073 +/- 0.026 for a vortex DW. The larger value is attributed to the higher magnetization gradients present.

cond-mat.mes-hall↗

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↗

arXiv2008

Ultra-fast spin dynamics: the effect of colored noise

Recent experimental results have pushed the limits of magnetization dynamics to pico- and femtosecond timescales. This ultra-fast spin dynamics occurs in extreme conditions of strong and rapidly varying fields and high temperatures. This situation requires new description of magnetization dynamics, even on a phenomenological level of the atomistic Landau-Lifshitz-Gilbert equation, taking into account that the correlation time for electron system could be of the order of the inverse characteristic spin frequency. For this case we introduce the thermodynamically correct phenomenological approach for spin dynamics based on the Landau-Lifshitz-Miyasaki-Seki equation. The influence of the noise correlation time on longitudinal and transverse magnetization relaxation is investigated. We also demonstrate the effect of the noise correlation time on demagnetisation rate of different materials during laser-induced dynamics.

cond-mat.mtrl-sci↗

arXiv2008

Giant magnetic anisotropy of the bulk antiferromagnets IrMn and IrMn3

Theoretical predictions of the magnetic anisotropy of antiferromagnetic materials are demanding due to a lack of experimental techniques which are capable of a direct measurement of this quantity. At the same time it is highly significant due to the use of antiferromagnetic components in magneto-resistive sensor devices where the stability of the antiferromagnet is of upmost relevance. We perform an ab-initio study of the ordered phases of IrMn and IrMn3, the most widely used industrial antiferromagnets. Calculating the form and the strength of the magnetic anisotropy allows the construction of an effective spin model, which is tested against experimental measurements regarding the magnetic ground state and the Neel temperature. Our most important result is the extremely strong second order anisotropy for IrMn3 appearing in its frustrated triangular magnetic ground state, a surprising fact since the ordered L12 phase has a cubic symmetry. We explain this large anisotropy by the fact that cubic symmetry is locally broken for each of the three Mn sub-lattices.

cond-mat.other↗

arXiv2006

Dynamic approach for micromagnetics close to the Curie temperature

In conventional micromagnetism magnetic domain configurations are calculated based on a continuum theory for the magnetization which is assumed to be of constant length in time and space. Dynamics is usually described with the Landau-Lifshitz-Gilbert (LLG) equation the stochastic variant of which includes finite temperatures. Using simulation techniques with atomistic resolution we show that this conventional micromagnetic approach fails for higher temperatures since we find two effects which cannot be described in terms of the LLG equation: i) an enhanced damping when approaching the Curie temperature and, ii) a magnetization magnitude that is not constant in time. We show, however, that both of these effects are naturally described by the Landau-Lifshitz-Bloch equation which links the LLG equation with the theory of critical phenomena and turns out to be a more realistic equation for magnetization dynamics at elevated temperatures.

cond-mat.mtrl-sci↗

arXiv2005

Transition to linear domain walls in nano-constrictions

Domain walls in nano-constrictions are investigated with a focus on thermal properties. In general, the magnetization component perpendicular to the easy axis which in a domain wall usually occurs has a value different from the easy-axis bulk magnetization value with a separate phase transition at a critical temperature below the Curie temperature. Since this effect is the more pronounced the smaller the domain wall width is we investigate it especially in domain walls with a confined geometry, using analytical arguments, mean-field theory and Monte Carlo simulations. Our findings may contribute to the understanding of magneto-resistive effects in domain walls with sizes of only a few atomic layers, as e. g. in nano-contacts or nano-constrictions.

cond-mat.mtrl-sci↗

arXiv2004

Temperature dependent magnetic properties of FePt: effective spin Hamiltonian model

A model of magnetic interactions in the ordered ferromagnetic FePt is proposed on the basis of first-principles calculations of non-collinear magnetic configurations and shown to be capable of explaining recent measurements of magnetic anisotropy energy (MAE). The site (Fe,Pt) resolved contributions to the MAE have been distinguished with small Fe easy-plane and large Pt easy-axis terms. This model has been tested against available experimental data on the temperature dependence of MAE showing scaling of uniaxial MAE (K$_{1}$(T)) with magnetization (M(T)) $K_{1}(T) \sim M(T)^γ$ characterized by the unusual exponent of $γ= 2.1$. It is shown that this unusual behavior of the FePt can be quantitatively explained within the proposed model and originates from an effective anisotropic exchange mediated by the induced Pt moment. The latter is expected to be a common feature of 3d-5d(4d) alloys having 5d/4d elements with large spin-orbit coupling and exchange enhanced Stoner susceptibility.

physics.flu-dyn↗