67 research outputs found
Universal Criteria for Single Femtosecond Pulse Ultrafast Magnetization Switching in Ferrimagnets
Single-pulse switching has been experimentally demonstrated in ferrimagnetic GdFeCo and Mn2RuxGa alloys. Complete understanding of single-pulse switching is missing due to the lack of an established theory accurately describing the transition to the nonequilibrium reversal path induced by femtosecond laser photoexcitation. In this work, we present general macroscopic theory for the magnetization dynamics of ferrimagnetic materials upon femtosecond laser excitation. Our theory reproduces quantitatively all stages of the switching process observed in experiments. We directly compare our theory to computer simulations using atomistic spin dynamics methods for both GdFeCo and Mn2RuxGa alloys. We provide explicit expressions for the magnetization relaxation rates in terms of microscopic parameters, which allows us to propose universal criteria for switching in ferrimagnets
Atomistic spin model of single pulse toggle switching in MnRuGa Heusler alloys
Single femtosecond pulse switching of ferrimagnetic alloys is an essential
building block for ultrafast spintronics. However the most promising class of
material for ultrafast spintronics showing switching capabilities,
MnRuGa Heusler alloys, has barely been investigated theoretically from
an atomistic perspective. Here, we propose an atomistic spin model for the
simulation of single pulse toggle switching of the Heusler alloy MnRuGa
for a range of Ru concentrations. The magnetic parameters entering the spin
Hamiltonian are obtained by mapping our computer results to previous
experimental results. We investigate the Ru-concentration dependence of single
pulse toggle switching and find that it is restricted to a range of
concentrations, similar to experimental findings. We expect that our atomistic
model serve to further investigate toggle switching in MnRuGa Heusler
alloys for ultrafast spintronics applications.Comment: Submitted to Appl. Phys. Lett. Special Topic: 'Ultrafast and THz
spintronics
Multiscale modeling of magnetic materials: Temperature dependence of the exchange stiffness
For finite-temperature micromagnetic simulations the knowledge of the temperature dependence of the exchange stiffness plays a central role. We use two approaches for the calculation of the thermodynamic exchange parameter from spin models: (i) based on the domain-wall energy and (ii) based on the spin-wave dispersion. The corresponding analytical and numerical approaches are introduced and compared. A general theory for the temperature dependence and scaling of the exchange stiffness is developed using the classical spectral density method. The low-temperature exchange stiffness A is found to scale with magnetization as m(1.66) for systems on a simple cubic lattice and as m(1.76) for an FePt Hamiltonian parametrized through ab initio calculations. The additional reduction in the scaling exponent, as compared to the mean-field theory (A similar to m(2)), comes from the nonlinear spin-wave effects
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
Ultrafast double magnetization switching in GdFeCo with two picosecond-delayed femtosecond pump pulses
The recently discovered thermally induced magnetization switching (TIMS) induced by single femtosecond laser pulses in ferrimagnetic GdFeCo alloys proceeds on the picosecond time-scale. The rate at which data can be changed for use of TIMS in technological devices is limited by the processes leading to thermal equilibrium. In the present work, we address the question of whether it is possible to further excite switching via TIMS well before thermal equilibrium between subsystems is reached. In particular, we investigate the conditions for double thermally induced magnetic switching by the application of two shortly delayed laser pulses. These conditions become relevant for potential applications as it sets both a limit to rewrite data and demonstrates the importance of spatial confinement of a heat pulse to bit size, as neighboring bits may be accidentally re-switched for spatially extended pulse spots. To demonstrate this effect, we theoretically study the switching behavior in a prototypical ferrimagnetic GdFeCo alloy as a function of composition. We use computer simulations based on thermal atomistic spin dynamics and demonstrate the possibility of inducing a second switching event well before thermal equilibrium is reached and define the conditions under which it can occur. Our theoretical findings could serve as a guidance for further understanding of TIMS as well as to act as a guide for future applications
Ultra-fast magnetisation rates within the Landau-Lifshitz-Bloch model
The ultra-fast magnetisation relaxation rates during the laser-induced
magnetisation process are analyzed in terms of the Landau-Lifshitz-Bloch (LLB)
equation for different values of spin . The LLB equation is equivalent in
the limit to the atomistic Landau-Lifshitz-Gilbert (LLG)
Langevin dynamics and for to the M3TM model [B. Koopmans, {\em et al.}
Nature Mat. \textbf{9} (2010) 259]. Within the LLB model the ultra-fast
demagnetisation time () and the transverse damping ()
are parameterized by the intrinsic coupling-to-the-bath parameter ,
defined by microscopic spin-flip rate. We show that for the phonon-mediated
Elliott-Yafet mechanism, is proportional to the ratio between the
non-equilibrium phonon and electron temperatures.
We investigate the influence of the finite spin number and the scattering
rate parameter on the magnetisation relaxation rates. The relation
between the fs demagnetisation rate and the LLG damping, provided by the LLB
theory, is checked basing on the available experimental data. A good agreement
is obtained for Ni, Co and Gd favoring the idea that the same intrinsic
scattering process is acting on the femtosecond and nanosecond timescale.Comment: 9 pages, 7 figure
The Landau-Lifshitz-Bloch equation for ferrimagnetic materials
We derive the Landau-Lifshitz-Bloch (LLB) equation for a two-component
magnetic system valid up to the Curie temperature. As an example, we consider
disordered GdFeCo ferrimagnet where the ultrafast optically induced
magnetization switching under the action of heat alone has been recently
reported. The two-component LLB equation contains the longitudinal relaxation
terms responding to the exchange fields from the proper and the neighboring
sublattices. We show that the sign of the longitudinal relaxation rate at high
temperatures can change depending on the dynamical magnetization value and a
dynamical polarisation of one material by another can occur. We discuss the
differences between the LLB and the Baryakhtar equation, recently used to
explain the ultrafast switching in ferrimagnets. The two-component LLB equation
forms basis for the largescale micromagnetic modeling of nanostructures at high
temperatures and ultrashort timescales
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