Ultrafast carrier and spin dynamics in GaMnAs across the Curie temperature

Yaohua Jiang,Yanda Ji,Ye Yuan,Jia Guo,Hao Yang,Weizheng Liang,Ning Li

doi:10.1063/5.0032728

Abstract

The carrier and spin dynamics of the diluted ferromagnetic semiconductor GaMnAs are crucial to understanding its ferromagnetic nature, especially near the Curie temperature (TC). Herein, transient differential reflectivity measurements and the temporal evolution of Kerr rotation are shown using ultrafast optical spectroscopy at various temperatures. The energy and angular momentum transfer routes after femtosecond laser pumping are revealed. The hole dynamics consist of two processes: a fast process that arises from carrier recombination and a slow process caused by carrier–lattice interactions. The spin dynamics are determined from the spin–orbital coupling and spin–lattice interactions in the ferromagnetic and paramagnetic phases, respectively. Critical phenomena are observed from the competition between the hole-mediated ferromagnetic orders and thermal perturbations near TC. These experimental results provide deep insights into the ultrafast decay behavior in GaMnAs, which is regarded as one of the most promising spintronic materials.

Highlights

Based on the p-d Zener model proposed by Dietl et al, Tc is proportional to the effective Mn concentration or the cube root of the hole concentration
We studied the carrier and spin dynamics of GaMnAs across TC, which have not been well investigated in previous studies
The TRS gives a reflectivity change ΔR(t) from the pumping divided by the static reflectivity R0(ΔR(t)/R0), which is proportional to the photo-induced change in refractive index as ascribed to heat transfer of the carriers

Summary

INTRODUCTION

Dilute ferromagnetic semiconductors (DFSs) have received attention due to their carrier-mediated ferromagnetism and electrical compatibility with current semiconductor devices. Among the various types of DFSs, GaMnAs is the most studied as a promising candidate for spintronics due to its high Curie temperature (Tc) above liquid nitrogen temperatures. The substitutional Mn ions induce holes and local spins into the classic direct bandgap III–V semiconductor GaAs, and the ferromagnetism is mediated by these holes over long range exchanges between the Mn 3d orbitals. Recently, super-paramagnetism beyond ferromagnetism has been evidenced in the insulating phases of GaMnAs at low Mn concentrations, while the metallic GaMnAs presents global ferromagnetism. the Mn-induced impurity band merges with the valence band (VB) when gradually increasing the Mn concentration. Based on the p-d Zener model proposed by Dietl et al, Tc is proportional to the effective Mn concentration or the cube root of the hole concentration. Across Tc, a critical fluctuation in the intrinsic degrees of freedom in the magnetic system is exhibited, which provides emerging insights into this prototype DFS. The theoretical results from Chen et al showed that the itinerant hole spins and spins at the Mn 3d orbital co-contribute to spin–orbit coupling (SOC) with different decay lifetimes of 300 fs and 700 fs, respectively.27 These results are based on different theoretical models, the energy and angular momentum transfer channels after ultrafast laser pumping are feasible routes to fundamentally understand the physics of DFSs. Here, we studied the carrier and spin dynamics of GaMnAs across TC, which have not been well investigated in previous studies. We captured femtosecond transient reflectivity spectra (fs-TRS) and time-resolved magneto-optical Kerr effect (TR-MOKE) measurements where the net spin polarization is excited by circularly polarized light These measurements are based on ultrafast pumpprobing (PP) techniques without an external magnetic field. The physical processes of the fitting parameters are discussed around TC

EXPERIMENTAL

RESULTS AND DISCUSSION

CONCLUSION