Ultrafast probes of nonequilibrium hole spin relaxation in the ferromagnetic semiconductor GaMnAs

Abstract

We report direct measurements of hole spin lifetimes in ferromagnetic GaMnAs carried out by time-and polarization-resolved spectroscopy. Below the Curie temperature, ultrafast photoexcitation of GaMnAs with linearly-polarized light is shown to create a non-equilibrium hole spin population via dynamical polarization of the holes through p-d exchange scattering with ferromagneticallyordered Mn spins. The system is then observed to relax in a distinct three-step recovery process: (i) a femtosecond hole-spin relaxation, on the scale of 160-200 fs, (ii) a picosecond hole-energy relaxation, on the scale of 1-2 ps, and (iii) a coherent, damped Mn spin precession with a period of 250 ps. The transient amplitude of the hole-spin relaxation component diminishes with increasing temperature, directly following the ferromagnetic order of GaMnAs, while the hole-energy amplitude shows negligible temperature change. Our results serve to establish the hole spin lifetimes in the ferromagnetic semiconductor GaMnAs, at the same time demonstrating a novel spectroscopic method for studying non-equilibrium hole spins in the presence of magnetic order and spin exchange interaction.

Manipulating femtosecond spin-orbit torques with laser pulse sequences to control magnetic memory states and ringing

“…The origin of this spin-reversal can be seen by comparing the total populations 1 V k ∆ ĥ † −knĥ −kn for the four different exchange-split HH and LH valence bands n in all {111} k-directions These band-resolved spinpolarized total populations are shown in Figs. 4(c) and (d) as function of time for T 1 =100fs, which is comparable to the measured 62 and calculated 50 hole spin relaxation time. More than one bands are populated simultaneously due to the energy dispersion and laser-pulse-width.…”

“…In the absence of pump, the measured smooth change of static Kerr rotation angle θ K as function of B-field, discussed in Refs. [34,62], reflects this canting. There is no magnetic hysteresis of θ K (B) without the pump.…”

“…Femtosecond demagnetization (decrease in Mn spin amplitude) through dynamical polarization of longitudinal hole spins dominates for high fluences of 100s of µJ/cm 2 . 62 Our proposed pulse-train scheme achieves spin rotational switching by using lower pump intensities that reduce fs demagnetization.…”

“…They relax on a timescale T 1 that is comparable to hole spin relaxation. 50,62 The above characteristic times are expected to be in the 10-200fs range.…”

Critical speeding up of nonequilibrium electronic relaxation near nematic phase transition in unstrained Ba( Fe1−xCox ) 2Patz1, Li2, Luo3 et al. 2017Phys. Rev. B200110View full textAdd to dashboardCite

“…Ultrafast optical spectroscopy represents a powerful tool for studying phase transitions in a complimentary fashion to conventional equilibrium probe methods [10][11][12][13][14][15][16][17] . Ultrafast spectroscopy can be used to potentially reveal temporal fluctuations of the nematic order parameter that would not be seen in static, time-averaged measurements.…”