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Register a new userDynamics of magnetization and carriers at the onset of the photo-excited precession of magnetization in (Ga,Mn)As
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Photo-excited precession of magnetization in (Ga,Mn)As is investigated by measuring time-resolved magneto-optical response and transient differential reflectivity with pump-and-probe technique. In the time region less than 1 ps, rapidly oscillating and spike-like signals are observed, respectively, with excitation of below and above the GaAs band gap. Analysis with gyromagnetic model and autocorrelation function concludes that those signals are not attributed to ultrafast demagnetization but due to interference between pump and probe pulses incorporating sub-ps carrier dynamics characteristic of low-temperature grown semiconductors. Photo-ionization of Mn ions (Mn2+ -> Mn3+) is proposed as a mechanism which dynamically induces orbital angular momentum and affects hole-mediated magnetic anisotropy in (Ga,Mn)As.
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We present a study of photo-excited magnetization dynamics in ferromagnetic (Ga,Mn)As films observed by time-resolved magneto-optical measurements. The magnetization precession triggered by linearly polarized optical pulses in the absence of an external field shows a strong dependence on photon frequency when the photo-excitation energy approaches the band-edge of (Ga,Mn)As. This can be understood in terms of magnetic anisotropy modulation by both laser heating of the sample and by hole-induced non-thermal paths. Our findings provide a means for identifying the transition of laser-triggered magnetization dynamics from thermal to non-thermal mechanisms, a result that is of importance for ultrafast optical spin manipulation in ferromagnetic materials via non-thermal paths.
Aiming at studying the mechanism of photo-excited precession of magnetization in ferromagnetic (Ga,Mn)As, magneto-optical (MO) and differential reflectivity (DR) temporal profiles are studied at relatively long and ultra-short time scales for samples with different Mn contents (x=0.01-0.11). As to the oscillatory MO profiles observed in the long time scale, simulation based on the LLG equation combined with two different MO effects confirms photo-inducement of the perpendicular anisotropy component p-Heff. As for the profiles observed in the ultra-short time scale, they are consistently explained in terms of the dynamics of photo-generated carriers, but not by the sudden reduction in magnetization. With those experimental results and analyses, new mechanism which accounts for the photo-induced p-Heff is addressed; photo-ionization like excitation of Mn(II). Namely, Mn(II) is excited into Mn(III) and e . It is discussed that such excitation tips magnetic anisotropy toward the out-of-plane direction through the inducement of orbital angular momentum and the gradient dMn(II*)/dz. Validity of the proposed mechanism is examined by estimating the efficiency of excitation on the basis of the Lambert-Beer law and the experimental p-Heff values, through which the efficiency of 1-10 ppm with the nominal optical cross section of around 5 x 10^12 m^2 are obtained.
We report single-color, time resolved magneto-optical measurements in ferromagnetic semiconductor (Ga,Mn)As. We demonstrate coherent optical control of the magnetization precession by applying two successive ultrashort laser pulses. The magnetic field and temperature dependent experiments reveal the collective Mn-moment nature of the oscillatory part of the time-dependent Kerr rotation, as well as contributions to the magneto-optical signal that are not connected with the magnetization dynamics.
Non-thermal laser induced spin excitations, recently discovered in conventional oxide and metal ferromagnets, open unprecedented opportunities for research and applications of ultrafast optical manipulation of magnetic systems. Ferromagnetic semiconductors, and (Ga,Mn)As in particular, should represent ideal systems for exploring this new field. Remarkably, the presence of non-thermal effects has remained one of the outstanding unresolved problems in the research of ferromagnetic semiconductors to date. Here we demonstrate that coherent magnetization dynamics can be excited in (Ga,Mn)As non-thermally by a transfer of angular momentum from circularly polarized femtosecond laser pulses and by a combination of non-thermal and thermal effects due to a transfer of energy from laser pulses. The thermal effects can be completely suppressed in piezo-electrically controlled samples. Our work is based on pump-and-probe measurements in a large set of (Ga,Mn)As epilayers and on systematic analysis of circular and linear magneto-optical coefficients. We provide microscopic theoretical interpretation of the experimental results.
Kerr rotation and Superconducting QUantum Interference Device (SQUID) magnetometry measurements were performed on ultrathin (Ga$_{0.95}$Mn$_{0.05}$)As layers. The thinner layers (below 250 AA) exhibit magnetic properties different than those of thicker ones, associated with different microstructure, and some degree of inhomogeneity. The temperature dependence of the field-cooled-magnetization of the layers is recorded after successive low temperature annealings. While the Curie temperature of the thicker layer (250 AA) is nearly unchanged, the critical temperature of the thinner layers is enhanced by more than 23 K after two annealings. Secondary Ion Mass Spectrometry (SIMS) experiments on similar layers show that Mn is displaced upon annealing. The results are discussed considering a possible segregation of substitutional and interstitial Mn atoms at the surface of the (Ga,Mn)As layers.
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