Thermally assisted spin-transfer torque magnetization reversal in uniaxial nanomagnets Appl. Phys. Lett. 101, 262401 (2012) Thick CoFeB with perpendicular magnetic anisotropy in CoFeB-MgO based magnetic tunnel junction AIP Advances 2, 042182 (2012) Erase/restorable asymmetric magnetization reversal in polycrystalline ferromagnetic films

Optical pump-probe measurements have been performed on three materials with pump pulses of 120 fs duration and variable helicity. The samples were probed in a reflection geometry. We compare the transient intensity and rotation signals induced in intrinsic GaAs wafers and sputtered polycrystalline Ni and Al thin films. An initial peak is observed in the transient rotation signal. The dependence of the peak height on the polarization of the pump is found to be qualitatively different for the three materials, indicating different amounts of linear and circular birefringence in each case. For ferromagnetic Ni the peak is superimposed on a longer-lived ultra-fast demagnetization signal. For GaAs and Al we also observe a tail in the rotation signal that decays on time-scales of several picoseconds. In the case of GaAs, we attribute this to the relaxation of a spin polarization that is induced by the pump beam within the thermal electron population. In the case of Al, the dependence of the amplitude of the tail on the pump beam's polarization is characteristic of linear birefringence. We suggest that this signal is associated with an ultra-fast excitation of the lattice rather than with optical orientation of spin.

Femtosecond optical pump-probe measurements have been performed upon a 500 Å polycrystalline Al film at an interband transition at a wavelength of 800 nm. The dependence of the intensity and polarization of the reflected probe pulse upon the polarization of the pump pulse has been determined. A sharp peak was observed in the polarization signal at zero time delay that contains contributions from both the specular inverse Faraday effect (SIFE) and the specular optical Kerr effect (SOKE). The SIFE and SOKE are associated with the optical orientation of angular and linear momentum, respectively. The dependence of the peak height upon the pump polarization has allowed us to deduce values of χ xxyy = (1 + 0.6i) × 10 −8 rad cm 3 erg −1 and χ xyyx = (0.5 − 0.1i) × 10 −8 rad cm 3 erg −1 for the non-vanishing components of the local cubic susceptibility tensor. A distortion of the peak was observed when the pump was elliptically polarized. Its detailed shape shows that the characteristic relaxation time for the SOKE is about 30 fs longer than that for the SIFE in measurements of ellipticity. This suggests a similar difference in the relaxation times for the angular and linear momentum of the optically excited hot electrons. The initial peak was generally followed by a tail that decayed with a time constant of several picoseconds, which we attribute to an ultrafast anisotropic excitation of the lattice.

No abstract

No abstract