Laser-Induced Magnetization Dynamics of Lanthanide-Doped Permalloy Thin Films

Abstract

We investigate the effect of Ho, Dy, Tb, and Gd impurities on the femtosecond laser-induced magnetization dynamics of thin Permalloy films using the time-resolved magneto-optical Kerr effect. Varying the amount of Ho, Dy, Tb content from 0% to 8%, we observe a gradual change of the characteristic demagnetization time constant from %60 to %150 fs. In contrast, Gd concentrations up to 15% do not influence the time scale of the initial photoinduced magnetization loss. We propose a demagnetization mechanism that relies on strong magnetic inertia of the rare-earth dopant which stabilizes the ferrimagnetic ordering and thereby delays the demagnetization. DOI: 10.1103/PhysRevLett.102.117201 PACS numbers: 75.40.Gb, 76.50.+g Using femtosecond laser pulses in a pump-probe approach, one can directly address the processes which are responsible for the excitation and relaxation of a magnetic system on their characteristic time scales. Various detection methods have been employed ranging from time-resolved linear magneto-optics [1-3] and nonlinear magneto-optics [4-6] to time-resolved photoemission [6][7][8][9]. Very recently time-resolved x-ray magnetic circular dichroism with femtosecond time resolution [10] has been presented. Despite the significant amount of work done on various ferromagnetic materials (e.g., Ni, Fe, Co, CoPt), none of these studies has identified in an unambiguous way the microscopic mechanism responsible for laser-induced demagnetization. On the theoretical side, there have been just a few attempts that address the demagnetization mechanism at a microscopic level [11], still awaiting experimental proof. Recently, Koopmans et al. [12] have proposed a demagnetization model inspired by the Elliot-Yafet (EY) spin relaxation mechanism [13] determined by electronic spin-flip scattering mediated by impurities or defects and phonons. Employing quantum mechanical calculations, an analytical expression was derived that connects the demagnetization time constant M with the Gilbert damping parameter via the Curie temperature T C of the system. The theoretical predictions describe the experimental time-resolved magneto-optic Kerr effect (TR MOKE) data obtained on Ni films [12]. In the EY theory spin relaxation occurs via electron scattering at impurities or defects and phonons that are accompanied by spin-flip events. The spin-flip probability is given by the amount of spin mixing of the electronic wave function which in turn is determined by the spin-orbit coupling strength. In a simpleminded picture one might conclude that in the heavy rare-earth elements the spin-orbit interaction is large and thus doping a Permalloy (Ni 80 Fe 20 ) film with rare-earth impurities may be an adequate experimental approach to study the tunability of the demagnetization time.In this Letter we test the possibility of altering the demagnetization time of the 3d transition metal system Permalloy, by adding magnetic lanthanide (RE) impurities. We investigate the effect of magnetic impurities on the magnetization dynamics of the Ni ...

Classical spin model of the relaxation dynamics of rare-earth doped permalloy

“…A description of the two-temperature model and the use of Langevin dynamics to model the thermal noise follows, leading to a description of the the dynamic simulations that were performed. The results of the simulations are in a good agreement with the experiments of Radu et al 13 We conclude that in complex materials it is important not to expect a single damping parameter but to consider the energy transfer channel relevant to the technique and time scale of the measurement.…”

“…13 in order to obtain insight into the energy transfer processes in ferrimagnets. We require studies of longitudinal and transverse relaxation to calculate the demagnetization time and the effective Gilbert damping respectively.…”

“…The predicted relationship between τ d and α was investigated experimentally by Radu et al 13 The study aimed to use the known dependence of the damping constant of permalloy (Ni 80 Fe 20 ) on the addition of RE impurities 14 to investigate how this changed the demagnetization time. A series of RE doped permalloy samples of different concentrations were prepared and FMR measurements showed the expected increase of damping constant with RE concentration.…”

Perspective: Ultrafast magnetism and THz spintronics

“…Are changes of the electronic structure at the Fermi level resulting in a larger Gilbert damping a connected to a change in the energy dissipation on the ultrafast time scale, e.g., faster demagnetization because of a stronger coupling? Experiments by Walowski et al, 101 similar results have been found by and Radu et al, 102 are summarized in Figure 8(a): rare-earth doping of a ferromagnet is known to increase the magnetic damping. However, surprisingly on ultrafast time scales, the demagnetization does not get faster, it gets even slower.…”

Testing spin-flip scattering as a possible mechanism of ultrafast demagnetization in ordered magnetic alloys

“…Element specific probes [13][14][15] as well as systematic studies using more conventional time resolved techniques [16] on ultrashort time scales have had a tremendous share in the advancement of knowledge in this particular field. For example, for ferrimagnetic 3d-transition metal / 4f-rare earth alloys (3d-4f) such as CoFeGd used for all-optical switching, that a transient ferromagnetic state exists in the process of ultrafast magnetization reversal [17].…”