Doping Ni80Fe20 by heavy rare earth atoms alters the magnetic relaxation properties of this material drastically. We show that this effect can be well explained by the slow relaxing impurity mechanism. This process is a consequence of the anisotropy of the on site exchange interaction between the 4f magnetic moments and the conduction band. As expected from this model the magnitude of the damping effect scales with the anisotropy of the exchange interaction and increases by an order of magnitude at low temperatures. In addition, our measurements allow us to determine the relaxation time of the 4f electrons as a function of temperature.

GaAs/GaMnAs core-shell nanowires were grown by molecular beam epitaxy. The core GaAs nanowires were synthesized under typical nanowire growth conditions using gold as catalyst. For the GaMnAs shell the temperature was drastically reduced to achieve low-temperature growth conditions known to be crucial for high-quality GaMnAs. The GaMnAs shell grows epitaxially on the side facets of the core GaAs nanowires. A ferromagnetic transition temperature of 20 K is obtained. Magnetic anisotropy studies indicate a magnetic easy axis parallel to the nanowire axis.

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 ...