Magnetic polaron formation and exciton spin relaxation in single Cd1−xMnxTe quantum dots

Abstract

We study the formation dynamics of a spontaneous ferromagnetic order in single self-assembled Cd1−xMnxTe quantum dots. By measuring time-resolved photoluminescence, we determine the formation times for QDs with Mn ion contents x varying from 0.01 to 0.2. At low x these times are orders of magnitude longer than exciton spin relaxation times evaluated from the decay of photoluminescence circular polarization. This allows us to conclude that the direction of the spontaneous magnetization is determined by a momentary Mn spin fluctuation rather than resulting from an optical orientation. At higher x, the formation times are of the same order of magnitude as found in previous studies on higher dimensional systems. We also find that the exciton spin relaxation accelerates with increasing Mn concentration. Doping semiconductor quantum dots (QDs) with magnetic ions offers a possibility of controlling magnetic properties of matter at nanoscale. Notably, several theoretical reports have proposed tailoring of QD magnetization by tuning the number of carriers in a dot.1-3 However, in order to achieve the control over magnetization a detailed knowledge of its dynamics is required. In compound II -VI QDs the Mn doping is performed routinely enabling studies of very dilute systems including QDs with single Mn ions 4 and of highly doped ones with molar contents up to 7%.5 Magnetic properties are comfortably monitored through optical experiments, since exchange interaction between the localized magnetic ions and the band carriers leads to pronounced magnetooptical effects.6 In particular, energy minimization of a complex consisting of a photocreated electron-hole pair (an exciton) interacting with Mn ions, results in a spontaneous formation of a local ferromagnetic order -a magnetic polaron (MP).Static and dynamic properties of MPs have been subject to intensive experimental and theoretical studies 5,[7][8][9][10][11][12][13][14][15][16][17][18][19] Experimental fingerprint of the MP formation is a redshift of the exciton photoluminescence (PL) by polaron energy E P -the energy gained by formation of the ferromagnetic order. The development of the magnetization can therefore be monitored in a time-resolved (TR) PL experiment, in which a transient shift of the exciton energy is observed allowing to evaluate the MP formation time, τ f .10,11 However, in bulk and 2D systems a prerequisite for the MP formation is an initial localization of the exciton.12 A precise experimental identification of E P and τ f is then hindered by processes related to trapping of the exciton. On the other hand, excitons in QDs are inherently localized by the QD potential, and thus the studies of MP formation dynamics in these nanostructures are free of the obscuring localization effects.14,18 The studies reported so far were performed on QD ensembles, in which the obtained τ f may be inaccurate due to inhomogeneities in dot morphology leading to variations in exciton lifetimes, 20 τ X , affecting in turn the TRPL transients. Previous reports have also lef...

Optical control of the spin state of two Mn atoms in a quantum dot

“…In opposition to the observation in QDs containing a large number of magnetic atoms, 28 the spin polarization of the exciton coupled to the 2 Mn is well conserved during its lifetime. Nevertheless, a bright exciton spin relaxation time of about 10 ns can be extracted from the time decay of the circular polarization rate of the exciton observed under pulsed (2 ps pulses) quasiresonant excitation [exponential fit in Fig.…”

Time-resolved magnetophotoluminescence studies of magnetic polaron dynamics in type-II quantum dots

“…[1][2][3][4] Compared to their bulk counterparts, [5][6][7][8][9] magnetically doped semiconductor QDs could provide control of the magnetic ordering, [10][11][12][13][14][15][16] with the onset of magnetization at substantially higher temperatures. [17][18][19][20][21] Experiments typically focus on Mn-doped II-VI and III-V QDs, in which it is possible to include both single [22][23][24][25] and several magnetic impurities, [17][18][19][20][21][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] having similarities with nuclear spins. 41,42 In the first case (single magnetic ion), such systems could be considered as potential quantum bits, quantum memories, or probes to detect an unconventional orbital ordering.…”

Direct Measurements of Magnetic Polarons in Cd1–xMnxSe Nanocrystals from Resonant Photoluminescence

“…As such, local spin-spin interactions can be greatly enhanced by strong quantum confinement, which compresses carrier wavefunctions to nanometer-scale volumes and therefore increases |ψ e,h (r)| 2 . The extent to which these exchange interactions can be enhanced and controlled via quantum confinement is an area of significant current interest and has recently been studied in a variety of magnetically-doped semiconductor nanostructures, including nanoribbons [12, 13], nanoplatelets [14], epitaxial quantum dots [14][15][16][18][19][20][21][22][23][24][25][26], and colloidal nanocrystals [1-8, 27, 28, 30].…”

“…As such, local spin-spin interactions can be greatly enhanced by strong quantum confinement, which compresses carrier wavefunctions to nanometer-scale volumes and therefore increases |ψ e,h (r)| 2 . The extent to which these exchange interactions can be enhanced and controlled via quantum confinement is an area of significant current interest and has recently been studied in a variety of magnetically-doped semiconductor nanostructures, including nanoribbons [12, 13], nanoplatelets [14], epitaxial quantum dots [14][15][16][18][19][20][21][22][23][24][25][26], and colloidal nanocrystals [1-8, 27, 28, 30].A particularly striking consequence of sp-d interactions in II-VI semiconductors is the formation of exciton magnetic polarons (EMPs), wherein the effective magnetic exchange field from a single photogenerated exciton -B ex -induces the collective and spontaneous ferromagnetic alignment of the magnetic dopants within its wavefunction envelope, generating a net local magnetization even in the absence of any applied field [31][32][33]. In turn, these aligned local moments act back on the exciton's spin, which lowers the exciton's energy, further localizes the exciton, and further stabilizes the polaron.…”

“…In turn, these aligned local moments act back on the exciton's spin, which lowers the exciton's energy, further localizes the exciton, and further stabilizes the polaron. The stability and binding energy of EMPs therefore depends on the detailed interplay between many factors including the exciton lifetime, the polaron formation time, the exchange field B ex , sample dimensionality, and temperature.EMPs and collective magnetic phenomena have been experimentally studied in a variety of Mn 2+ -doped semiconductor nanostructures, including CdMnSe and CdMnTe-based epilayers and quantum wells [34][35][36][37][38][39][40][41][42], selfassembled CdMnSe and CdMnTe quantum dots grown by molecular-beam epitaxy [14][15][16][18][19][20][21][22][23][24][25], and most recently in CdMnSe nanocrystals synthesized via colloidal techniques [8,28]. Common measurement techniques include the analysis of conventional (i.e., non-resonant) PL [8, 14, 15,18,21,28,39] and time-resolved PL [8, 16,28,34,35,40].…”