Monitoring statistical magnetic fluctuations on the nanometer scale

Insight into the Spin Properties in Undoped and Mn-Doped CdSe/CdS-Seeded Nanorods by Optically Detected Magnetic Resonance

Controlling the spin degrees of freedom of photogenerated species in semiconductor nanostructures via magnetic doping is an emerging scientific field that may play an important role in the development of new spin-based technologies. The current work explores spin properties in colloidal CdSe/CdS:Mn seeded-nanorod structures doped with a dilute concentration of Mn²⁺ ions across the rods. The spin properties were determined using continuous-wave optically detected magnetic resonance (ODMR) spectroscopy recorded under variable microwave chopping frequencies. These experiments enabled the deconvolution of a few different radiative recombination processes: band-to-band, trap-to-band, and trap-to-trap emission. The results uncovered the major role of carrier trapping on the spin properties of elongated structures. The magnetic parameters, determined through spin-Hamiltonian simulation of the steady-state ODMR spectra, reflect anisotropy associated with carrier trapping at the seed/rod interface. These observations unveiled changes in the carriers' g-factors and spin-exchange coupling constants as well as extension of radiative and spin-lattice relaxation times due to magnetic coupling between interface carriers and neighboring Mn²⁺ ions. Overall, this work highlights that the spin degrees of freedom in seeded nanorods are governed by interfacial trapping and can be further manipulated by magnetic doping. These results provide insights into anisotropic nanostructure spin properties relevant to future spin-based technologies.

Directed Exciton Magnetic Polaron Formation in a Single Colloidal Mn2+:CdSe/CdS Quantum Dot

One of the most prominent signatures of transition-metal doping in colloidal nanocrystals is the formation of charge carrier-induced magnetization of the dopant spin sublattice, called exciton magnetic polaron (EMP). Understanding the direction of EMP formation, however, is still a major obstacle. Here, we present a series of temperature-dependent photoluminescence studies on single colloidal Mn²⁺:CdSe/CdS core/shell quantum dots (QDs) performed in a vector magnetic field providing a unique insight into the interaction between individual excitons and numerous magnetic impurities. The energy of the QD emission and its full width at half-maximum are controlled by the interplay of EMP formation and statistical magnetic fluctuations, in excellent agreement with theory. Most important, we give the first direct demonstration that anisotropy effects-hypothesized for more than a decade-dominate the direction of EMP formation. Our findings reveal a pathway for directing the orientation of optically induced magnetization in colloidal nanocrystals.

Persuasive Evidence for Electron-Nuclear Coupling in Diluted Magnetic Colloidal Nanoplatelets Using Optically Detected Magnetic Resonance Spectroscopy

The incorporation of magnetic impurities into semiconductor nanocrystals with size confinement promotes enhanced spin exchange interaction between photogenerated carriers and the guest spins. This interaction stimulates new magneto-optical properties with significant advantages for emerging spin-based technologies. Here we observe and elaborate on carrier-guest interactions in magnetically doped colloidal nanoplatelets with the chemical formula CdSe/Cd_1-xMn_xS, explored by optically detected magnetic resonance and magneto-photoluminescence spectroscopy. The host matrix, with a quasi-type II electronic configuration, introduces a dominant interaction between a photogenerated electron and a magnetic dopant. Furthermore, the data convincingly presents the interaction between an electron and nuclear spins of the doped ions located at neighboring surroundings, with consequent influence on the carrier's spin relaxation time. The nuclear spin contribution by the magnetic dopants in colloidal nanoplatelets is considered here for the first time.

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

In semiconductors, quantum confinement can greatly enhance the interaction between band carriers (electrons and holes) and dopant atoms. One manifestation of this enhancement is the increased stability of exciton magnetic polarons in magnetically doped nanostructures. In the limit of very strong 0D confinement that is realized in colloidal semiconductor nanocrystals, a single exciton can exert an effective exchange field B_ex on the embedded magnetic dopants that exceeds several tesla. Here we use the very sensitive method of resonant photoluminescence (PL) to directly measure the presence and properties of exciton magnetic polarons in colloidal Cd_1-xMn_xSe nanocrystals. Despite small Mn²⁺ concentrations (x = 0.4-1.6%), large polaron binding energies up to ∼26 meV are observed at low temperatures via the substantial Stokes shift between the pump laser and the resonant PL maximum, indicating nearly complete alignment of all Mn²⁺ spins by B_ex. Temperature and magnetic field-dependent studies reveal that B_ex ≈ 10 T in these nanocrystals, in good agreement with theoretical estimates. Further, the emission line widths provide direct insight into the statistical fluctuations of the Mn²⁺ spins. These resonant PL studies provide detailed insight into collective magnetic phenomena, especially in lightly doped nanocrystals where conventional techniques such as nonresonant PL or time-resolved PL provide ambiguous results.

Revealing giant internal magnetic fields due to spin fluctuations in magnetically doped colloidal nanocrystals

Strong quantum confinement in semiconductors can compress the wavefunctions of band electrons and holes to nanometre-scale volumes, significantly enhancing interactions between themselves and individual dopants. In magnetically doped semiconductors, where paramagnetic dopants (such as Mn(2+), Co(2+) and so on) couple to band carriers via strong sp-d spin exchange, giant magneto-optical effects can therefore be realized in confined geometries using few or even single impurity spins. Importantly, however, thermodynamic spin fluctuations become increasingly relevant in this few-spin limit. In nanoscale volumes, the statistical fluctuations of N spins are expected to generate giant effective magnetic fields Beff, which should dramatically impact carrier spin dynamics, even in the absence of any applied field. Here we directly and unambiguously reveal the large Beff that exist in Mn(2+)-doped CdSe colloidal nanocrystals using ultrafast optical spectroscopy. At zero applied magnetic field, extremely rapid (300-600 GHz) spin precession of photoinjected electrons is observed, indicating Beff ∼ 15 -30 T for electrons. Precession frequencies exceed 2 THz in applied magnetic fields. These signals arise from electron precession about the random fields due to statistically incomplete cancellation of the embedded Mn(2+) moments, thereby revealing the initial coherent dynamics of magnetic polaron formation, and highlighting the importance of magnetization fluctuations on carrier spin dynamics in nanomaterials.

Tuning Equilibrium Compositions in Colloidal Cd1-xMnxSe Nanocrystals Using Diffusion Doping and Cation Exchange

The physical properties of semiconductor nanocrystals can be tuned dramatically via composition control. Here, we report a detailed investigation of the synthesis of high-quality colloidal Cd1-xMnxSe nanocrystals by diffusion doping of preformed CdSe nanocrystals. Until recently, Cd1-xMnxSe nanocrystals proved elusive because of kinetic incompatibilities between Mn(2+) and Cd(2+) chemistries. Diffusion doping allows Cd1-xMnxSe nanocrystals to be prepared under thermodynamic rather than kinetic control, allowing access to broader composition ranges. We now investigate this chemistry as a model system for understanding the characteristics of nanocrystal diffusion doping more deeply. From the present work, a Se(2-)-limited reaction regime is identified, in which Mn(2+) diffusion into CdSe nanocrystals is gated by added Se(2-), and equilibrium compositions are proportional to the amount of added Se(2-). At large added Se(2-) concentrations, a solubility-limited regime is also identified, in which x = xmax = ∼0.31, independent of the amount of added Se(2-). We further demonstrate that Mn(2+) in-diffusion can be reversed by cation exchange with Cd(2+) under exactly the same reaction conditions, purifying Cd1-xMnxSe nanocrystals back to CdSe nanocrystals with fine tunability. These chemistries offer exceptional composition control in Cd1-xMnxSe NCs, providing opportunities for fundamental studies of impurity diffusion in nanocrystals and for development of compositionally tuned nanocrystals with diverse applications ranging from solar energy conversion to spin-based photonics.

Picosecond Dynamics of Excitonic Magnetic Polarons in Colloidal Diffusion-Doped Cd(1-x)Mn(x)Se Quantum Dots

Spontaneous magnetization is observed at zero magnetic field in photoexcited colloidal Cd(1-x)Mn(x)Se (x = 0.13) quantum dots (QDs) prepared by diffusion doping, reflecting strong Mn(2+)-exciton exchange coupling. The picosecond dynamics of this phenomenon, known as an excitonic magnetic polaron (EMP), are examined using a combination of time-resolved photoluminescence, magneto-photoluminescence, and Faraday rotation (TRFR) spectroscopies, in conjunction with continuous-wave absorption, magnetic circular dichroism (MCD), and magnetic circularly polarized photoluminescence (MCPL) spectroscopies. The data indicate that EMPs form with random magnetization orientations at zero external field, but their formation can be directed by an external magnetic field. After formation, however, external magnetic fields are unable to reorient the EMPs within the luminescence lifetime, implicating anisotropy in the EMP potential-energy surfaces. TRFR measurements in a transverse magnetic field reveal rapid (<5 ps) spin transfer from excitons to Mn(2+) followed by coherent EMP precession at the Mn(2+) Larmor frequency for over a nanosecond. A dynamical TRFR phase inversion is observed during EMP formation attributed to the large shifts in excitonic absorption energies during spontaneous magnetization. Partial optical orientation of the EMPs by resonant circularly polarized photoexcitation is also demonstrated. Collectively, these results highlight the extraordinary physical properties of colloidal diffusion-doped Cd(1-x)Mn(x)Se QDs that result from their unique combination of strong quantum confinement, large Mn(2+) concentrations, and relatively narrow size distributions. The insights gained from these measurements advance our understanding of spin dynamics and magnetic exchange in colloidal doped semiconductor nanostructures, with potential ramifications for future spin-based information technologies.

Size-dependent magneto-optical effects in CdMnTe diluted magnetic quantum dots

We report on the growth and high spatial resolution magneto-photoluminescence of CdMnTe quantum dots embedded in a ZnCdTe matrix with Mn content of about 3%. The giant Zeeman spitting of excitonic emission coming from individual quantum dots and magnetization fluctuations inside them are studied in magnetic fields up to 7 T. We have found that the description of these effects requires the inclusion of antiferromagnetic Mn-Mn interaction in the considerations. Moreover, the values of the Zeeman shift are strongly related to the size of the dot. In particular, the higher spectral position of the line is, i.e., the smaller the dot, the smaller the Zeeman splitting that is observed. This effect is quantitatively explained with model calculations made under the assumption of magnetic polaron formation in diluted magnetic quantum dots and the presence of ion-ion exchange interaction.

Spin-polarizable excitonic luminescence in colloidal Mn2+-doped CdSe quantum dots

The photoluminescence of colloidal Mn2+-doped CdSe nanocrystals has been studied as a function of nanocrystal diameter. These nanocrystals are shown to be unique among colloidal doped semiconductor nanocrystals reported to date in that quantum confinement allows tuning of the CdSe bandgap energy across the Mn2+ excited-state energies. At small diameters, the nanocrystal photoluminescence is dominated by Mn 2+ emission. At large diameters, CdSe excitonic photoluminescence dominates. The latter scenario has allowed spin-polarized excitonic photoluminescence to be observed in colloidal doped semiconductor nanocrystals for the first time.

Probing the spin state of a single magnetic ion in an individual quantum dot

The magnetic state of a single magnetic ion (Mn2+) embedded in an individual quantum dot is optically probed using microspectroscopy. The fine structure of a confined exciton in the exchange field of a single Mn2+ ion (S=5/2) is analyzed in detail. The exciton-Mn2+ exchange interaction shifts the energy of the exciton depending on the Mn2+ spin component and six emission lines are observed at zero magnetic field. Magneto-optic measurements reveal that the emission intensities in both circular polarizations are controlled by the Mn2+ spin distribution imposed by the exchange interaction with the exciton, the magnetic field, and an effective manganese temperature which depends on both the lattice temperature and the density of photocreated carriers. Under magnetic field, the electron-Mn interaction induces a mixing of the bright and dark exciton states.

Ferromagnetism mediated by few electrons in a semimagnetic quantum dot

A (II,Mn)VI diluted magnetic semiconductor quantum dot with an integer number of electrons controlled with a gate voltage is considered. We show that a single electron is able to induce a collective spontaneous magnetization of the Mn spins, overcoming the short range antiferromagnetic interactions, at a temperature order of 1 K, 2 orders of magnitude above the ordering temperature in bulk. The magnetic behavior of the dot depends dramatically on the parity of the number of electrons in the dot.