FEMTOSPIN - Multiscale Modelling of Femtosecond Spin Dynamics
- AG Nowak (Theoretische Physik mit SP Moderne Materialeigenschaften)
|(2015): Ab initio investigation of light-induced relativistic spin-flip effects in magneto-optics Physical Review B. 2015, 91(17), 174415. ISSN 1098-0121. eISSN 1550-235X. Available under: doi: 10.1103/PhysRevB.91.174415
Excitation of a metallic ferromagnet such as Ni with an intensive femtosecond laser pulse causes an ultrafast demagnetization within approximately 300 fs. It was proposed that the ultrafast demagnetization measured in femtosecond magneto-optical experiments could be due to relativistic light-induced processes. We perform an ab initio investigation of the influence of relativistic effects on the magneto-optical response of Ni. To this end, first, we develop a response theory formulation of the additional appearing ultrarelativistic terms in the Foldy-Wouthuysen transformed Dirac Hamiltonian due to the electromagnetic field, and second, we compute the influence of relativistic light-induced spin-flip transitions on the magneto-optics. Our ab initio calculations of relativistic spin-flip optical excitations predict that these can give only a very small contribution (≤0.1%) to the laser-induced magnetization change in Ni.
|(2015): Multiscale modeling of ultrafast element-specific magnetization dynamics of ferromagnetic alloys Physical Review B. 2015, 92(5), 054412. ISSN 1098-0121. eISSN 1095-3795. Available under: doi: 10.1103/PhysRevB.92.054412
A hierarchical multiscale approach to model the magnetization dynamics of ferromagnetic random 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 parameters). These parameters are subsequently used in the Landau-Lifshitz-Bloch (LLB) model for multisublattice 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.
|(2014): Temperature-dependent ferromagnetic resonance via the Landau-Lifshitz-Bloch equation : Application to FePt Physical Review B. 2014, 90(9), 094402. ISSN 0163-1829. eISSN 1095-3795. Available under: doi: 10.1103/PhysRevB.90.094402
Temperature-dependent ferromagnetic resonance via the Landau-Lifshitz-Bloch equation : Application to FePt
Using the Landau-Lifshitz-Bloch (LLB) equation for ferromagnetic materials, we derive analytic expressions for temperature-dependent absorption spectra as probed by ferromagnetic resonance. By analyzing the resulting expressions, we can predict the variation of the resonance frequency and damping with temperature and coupling to the thermal bath. We base our calculations on the technologically relevant L10FePt, parametrized from atomistic spin dynamics simulations, with the Hamiltonian mapped from ab initio parameters. By constructing a multimacrospin model based on the LLB equation and exploiting GPU acceleration, we extend the study to investigate the effects on the damping and resonance frequency in μm-sized structures.
|(2014): Controlling the polarity of the transient ferromagneticlike state in ferrimagnets Physical Review B. 2014, 89(22), 224421. ISSN 1098-0121. eISSN 1095-3795. Available under: doi: 10.1103/PhysRevB.89.224421
It was recently observed that the two antiferromagnetically coupled sublattices of a rare earth-transition metal ferrimagnet can temporarily align ferromagnetically during femtosecond laser heating, but always with the transition metal aligning in the rare earth direction. This behavior has been attributed to the slower magnetization dynamics of the rare earth sublattice. The aim of this work was to assess how the difference in the speed of the transition metal and rare earth dynamics affects the formation of the transient ferromagneticlike state and consequently controls its formation. Our investigation was performed using extensive atomistic spin simulations and analytic micromagnetic theory of ferrimagnets, with analysis of a large area of parameter space such as initial temperature, Gd concentration, and laser fluence. Surprisingly, we found that at high temperatures, close to the Curie point, the rare earth dynamics become faster than those of the transition metal. Subsequently we show that the transient state can be formed with the opposite polarity, where the rare earth aligns in the transition metal direction. Our findings shed light on the complex behavior of this class of ferrimagnetic materials and highlight an important feature which must be considered, or even exploited, if these materials are to be used in ultrafast magnetic devices.
|(2013): Exchange Bias Driven by Dzyaloshinskii-Moriya Interactions Physical Review Letters. 2013, 111(21). ISSN 0031-9007. eISSN 1079-7114. Available under: doi: 10.1103/PhysRevLett.111.217202
The exchange bias effect in a compensated IrMn3/Co(111) system is studied using multiscale modeling from ab initio to atomistic spin model calculations. We evaluate numerically the out-of-plane hysteresis loops of the bilayer for different thicknesses of the ferromagnetic layer. The results show the existence of a perpendicular exchange bias and an enhancement of the coercivity of the system. To identify the origin of the exchange bias, we analyze the hysteresis loops of a selected bilayer by tuning the different contributions to the exchange interaction across the interface. Our results indicate that the exchange bias is primarily induced by Dzyaloshinskii-Moriya interactions, while the coercivity is increased mainly due to a spin-flop mechanism.
|(2013): Orbital-resolved spin model for thermal magnetization switching in rare-earth-based ferrimagnets Physical Review B. 2013, 88(2). ISSN 1098-0121. eISSN 1095-3795. Available under: doi: 10.1103/PhysRevB.88.020406
The switching of rare-earth-based ferrimagnets triggered by thermal excitation is investigated on the basis of an atomistic spin model beyond the distinguishing magnetic moments due to electrons in d and f orbitals of the rare earth. It is shown that after excitation of the conduction electrons a transient ferromagneticlike state follows from a dissipationless spin dynamics where energy and angular momentum are distributed between the two sublattices. The final relaxation can then lead to a new state with the magnetization switched with respect to the initial state. The time scale of the switching event is to a large extent determined by the exchange interaction between the two sublattices.
|(2012): Temperature dependence of the frequencies and effective damping parameters of ferrimagnetic resonance Physical Review B. 2012, 86(21). ISSN 1098-0121. eISSN 1550-235X. Available under: doi: 10.1103/PhysRevB.86.214416
Temperature dependence of the frequencies and effective damping parameters of ferrimagnetic resonance
Recent experiments on all-optical switching in GdFeCo and CoGd have raised the question about the importance of the angular momentum or the magnetization compensation point for ultrafast magnetization dynamics. We investigate the dynamics of ferrimagnets by means of computer simulations as well as analytically. The results from atomistic modeling are explained by a theory based on the two-sublattice Landau-Lifshitz-Bloch equation. Similarly to the experimental results and unlike predictions based on the macroscopic Landau-Lifshitz equation, we find an increase in the effective damping at temperatures approaching the Curie temperature. Further results for the temperature dependence of the frequencies and effective damping parameters of the normal modes represent an improvement of former approximated solutions, building a better basis for comparison to recent experiments.
|01.06.2012 – 31.05.2015