Resolving the role of femtosecond heated electrons in ultrafast spin dynamics

Abstract

Magnetization manipulation is essential for basic research and applications. A fundamental question is, how fast can the magnetization be reversed in nanoscale magnetic storage media. When subject to an ultrafast laser pulse, the speed of the magnetization dynamics depends on the nature of the energy transfer pathway. The order of the spin system can be effectively influenced through spin-flip processes mediated by hot electrons. It has been predicted that as electrons drive spins into the regime close to almost total demagnetization, characterized by a loss of ferromagnetic correlations near criticality, a second slower demagnetization process takes place after the initial fast drop of magnetization. By studying FePt, we unravel the fundamental role of the electronic structure. As the ferromagnet Fe becomes more noble in the FePt compound, the electronic structure is changed and the density of states around the Fermi level is reduced, thereby driving the spin correlations into the limit of critical fluctuations. We demonstrate the impact of the electrons and the ferromagnetic interactions, which allows a general insight into the mechanisms of spin dynamics when the ferromagnetic state is highly excited, and identifies possible recording speed limits in heat-assisted magnetization reversal.

Controlling the polarity of the transient ferromagneticlike state in ferrimagnets

“…Namely, an initial sub-ps demagnetization followed by a much slower demagnetization process of the order of several ps. Similar results where also obtained in Ni 18 and FePt thin films 19 keeping the initial temperature fixed but increasing the laser fluency, arXiv:1308.0993v2 [cond-mat.mtrl-sci] 3 Feb 2014 and thus reaching higher temperatures. The observed slowing down of the magnetization response was associated to the increasing role played by the spin fluctuations for temperatures approaching T c .…”

Conditions for thermally induced all-optical switching in ferrimagnetic alloys: Modeling of TbCo

“…I 0 can be estimated from theory or experimental reflectivity, and has the units of m −1 s −1 . In this paper and because the experimental data are not available, its value has been taken from literature for FePt (I 0 = 3 × 10 19 m −1 s −1 ) [ 40]g i v i n g laser pulse fluences in agreement with the experimental data on TbCo. We note that the results, while dependent on I 0 , will be simply rescaled should a different value be used.…”

Temperature-dependent ferromagnetic resonance via the Landau-Lifshitz-Bloch equation: Application to FePt

“…2) uses the functions for FePt that were presented in Ref. [17], however, similar functions could be calculated using mean-field theory [27].…”

“…In this work, these are calculated separately from a Langevin dynamics simulation of an atomistic spin model, however, it is possible to calculate these properties from mean-field calculations [27]. We use a model for FePt which was introduced earlier and which is meanwhile well established in the literature [26,[28][29][30].…”