Localized Magnetic Polarons Research Articles

Dual-Color Lasing Lines from EMPs in Diluted Magnetic Semiconductor CdS:NiI Structure.

Have one ever seen a semiconductor that can issue two-color lasing lines? The diluted magnetic semiconductor (DMS) can do this. Here, we have observed dual lasing lines of 530 nm and 789 nm from a DMS structure of CdS:NiI, in which the excitonic magnetic polaron (EMP) and localized excitonic magnetic polaron (LEMP) are excitations out of ferromagnetic (NiS)x nanocluster and NiI2 nanoclusters within CdS lattice; both of them could lead to the collective EMP state at high excitation and therein produce coherent emission lines simultaneously. This occurrence is due to the superposition of EMP near CdS bandedge and the combination of the charge-transfer band of (NiI)n cluster with the LEMP within CdS lattice by overcoming the strong electron correlation of NiI cluster in a DMS structure, evidenced also by ab initio calculation. This finding opens a way to understand the collective behaviour of spin-coupled excitons in DMS and to find novel applications in the spin-related quantum technology.

Optical observation of spin-glass formation in Cd1−XMnXTe compound

The behavior of spin glass (SG) is characteristic of different types of materials (semiconductor, metallic, superconductor, amorphous, etc). However, the nature of this magnetic spin state is still unclear. Usually the study of SG systems near the spin freezing temperature (Tf) is based on magnetic measurements. In this case a spin clustering model was used for the explanation of the obtained results. Here, we have studied the optical properties of the semi-insulating CdMnTe SG compound by using the localized exciton magnetic polarons (LEMPs) as nano-size probes for different microscopic magnetic spin states formed in SGs. This revealed spin cooperative behaviors at temperatures above Tf, which can be considered as presagers of spin freezing at Tf. We also observed an optical signature of the SG transition in diluted magnetic semiconductors (DMSs) and its dependence on the LEMP’s excitation intensity. A schematic of the temperature evolution of the spin’s cooperative behavior and formation of the SG phase based on the obtained results is presented. This opens novel opportunities for future investigations of the dynamical spin correlations in DMS SGs as very promising materials for spintronics using optical measurements.

Magnetic exciton relaxation and spin-spin interaction by the time-delayed photoluminescence spectra of ZnO:Mn nanowires.

ZnO:Mn nanostructures are important diluted magnetic materials, but their electronic structure and magnetic origin are still not well understood. Here we studied the time-delayed and power-dependent photoluminescence spectra of Mn(II) doped ZnO nanowires with very low Mn concentration. From the time-delayed emission spectra, we obtained their electronic levels of single Mn ion replacement of Zn ions in ZnO nanowire. The high d-level emissions show up unusually because of the stronger p-d hybridization than that in ZnS, as well as the spin-spin coupling. After increasing Mn doping concentration, the ferromagentic cluster of the Mn-O-Mn with varied configurations can form and give individual emission peaks, which are in good agreement with the ab initio calculations. The presence of clustered Mn ions originates from their ferromagnetic coupling. The lifetimes of these d levels show strong excitation power-dependent behavior, indication of strong spin-dependent coherent emission. One-dimensional structure is critical for this coherent emission behavior. These results indicate that the d state is not within Mn ion only, but a localized exciton magnetic polaron, Mn-O-Mn coupling should be one source of ferromagnetism in ZnO:Mn lattice, the latter also can combine with free exciton for EMP and produce coherent EMP condensation and emission from a nanowire. This kind of nanowires can be expected to work for both spintronic and spin-photonic devices if we tune the transition metal ion doping concentration in it.

A Model on the Mn2+ Luminescence Band Redshift with Mn(II) Doping and Aggregation within CdS:Mn Microwires

We report the microphoto-luminescence band redshifts with individual and multi-Mn(II) ion emissions within CdS microwires. The localized exciton magnetic polarons (LEMPs) corresponding to the d—d optical transitions of Mn(II) account for this shift. This LEMP emission from the double-, three- and four-Mn(II) ions with ferromagnetic coupling after photoexcitation can happen in diluted magnetic semiconductors, except for the individual Mn(II) doping. In addition, a simple spin-exchange polaronic model is established to account for these emission peaks well. Through this model, we can verify the local geometry of the Mn(II) ions in CdS microwires.

Time-resolved photoluminescence spectroscopy of localized exciton magnetic polarons in Cd0.70Mn0.30Te spin glass compound

We have investigated dynamics of different localized exciton magnetic polarons (LEMPs) in Cd0.70Mn0.30Te spin glass (SG) compound below the freezing temperature Tf in the crystal regions, where various microscopic magnetic spin states (MMSSs), namely, “loose” spins, finite, and infinite clusters, are formed. It was shown that there is a broad distribution of the LEMPs lifetimes. The presence of the long-lived LEMPs is caused by the admixture of the optically active bright exciton states to the dark exciton states, i.e., the “brightening” of the dark LEMPs which exist along with the bright LEMPs. The lifetimes of the dark LEMPs correspond to hundreds of nanoseconds. It was found that the time decay of photoluminescence band intensity is approximated by the sum of two functions: a single exponential function and the Kohlrausch–Williams–Watts stretched exponential function. The stretched exponential function describes the recombination processes of the LEMPs formed in the crystal regions of the finite clusters as well as the infinite cluster. This reflects the appearance of spatially heterogeneous dynamics in Cd0.70Mn0.30Te SG compound below Tf which is due to the disorder in the spin distribution caused by the formation of different MMSSs.

Photoluminescence study of magnetic spin clusters and their temperature evolution in Cd0.70Mn0.30Te spin-glass compound

We have investigated microscopic magnetic spin states (MMSSs) (“loose spins, finite superparamagnetic, ‘locked’ and infinite clusters”) both above and below the freezing temperature in Cd0.70Mn0.30Te spin glass (SG). We used the localized exciton magnetic polarons, which we observed in the photoluminescence spectra, as a probe. This makes it possible to estimate the MMSS's relative concentrations and to study their temperature evolution and thus to elucidate one of the most important issues in this field of research. Furthermore, the findings described here open new prospects for further studies of spin freezing in the different SGs, especially, in dilute magnetic semiconductors.

The photoexcited spin-aligned state of high-density exciton magnetic polarons and the effect of magnetic field in semimagnetic semiconductor Cd0.8Mn0.2Te

In Cd0.8Mn0.2Te, nonlinear photoluminescence (PL) appears only when localized excitons are selectively excited to high-density states. Here, the effect of a magnetic field is compared between nonlinear PL and PL due to localized magnetic polarons. Nonlinear PL shows a shift towards lower energy under an applied magnetic field, whereas PL of a localized magnetic polaron band shows a slight shift towards higher energy. The experimental results support the hypothesis that the origin of the nonlinear PL is a spin-aligned state of high-density exciton magnetic polarons. In the spin-aligned state, most spins of electrons (holes) in many magnetic polarons point in the same direction. In this new high-density photoexcited state, the s, p–d exchange interaction between photoexcited electrons (holes) and magnetic ions plays an important role.

Polaron-induced anti-Stokes photoluminescence under selective excitation in diluted magnetic semiconductors

Anti-Stokes photoluminescence is observed under selective excitation in a bulk sample of the diluted magnetic semiconductor ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ with $x$ $=$ 0.2. The anti-Stokes photoluminescence is induced by a two-step optical-absorption process that creates excitons via an alloy potential state and an impurity center. Both localized exciton magnetic polarons bound to the fluctuating alloy potential and exciton magnetic polarons bound to neutral acceptors participate in forming intermediate states with long lifetimes that cooperatively contribute to the anti-Stokes photoluminescence. The reabsorption of the anti-Stokes photoluminescence is also examined by considering the role of bound magnetic polarons. The localized Mn${}^{2+}$ spin ordering in magnetic polarons bound to the impurity center largely contributes in reducing reabsorption.

Magnetooptical properties of a single CdMnSe/CdMgSe quantum well

The spectra of photoluminescence and reflectance in magnetic fields up to 7 T are studied for a 3.8 nm semimagnetic CdMnSe quantum well confined by two CdMgSe barriers. A noticeable magnetic shift in the σ+-polarized emission line of the heavy exciton to low energies and a decrease in the halfwidth of the line by more than one-half are detected with increasing magnetic field. It is established that a localized magnetic polaron is formed, with the polaron energy of 19.8 meV determined from the change in the degree of circular polarization in magnetic field. A σ−-polarized emission line is observed in magnetic fields ranging from 0.4 to 2 T. This line can be interpreted as being produced by the complex of two electrons, with oppositely directed spins, and a heavy hole, i.e., by the trion X− or the exciton localized at a donor, D0X. The binding energy of such complex is 10 meV.

Current-induced effect on resistivity and magnetoresistance of La 0.67Ba 0.33MnO 3 manganite

The influence of dc biasing current on temperature dependence of resistivity and low-field magnetoresistance (MR) of La 0.67Ba 0.33MnO 3 bulk sample is reported. A prominent finding is the change in resistivity around the insulator-to-metal transition temperature ( T IM) and the change in MR around the ferromagnetic transition temperature ( T C). The decrease in MR around T C at higher biasing current indicates a strong interaction between carrier spin and spin of Mn ions resulting in a higher alignment of Mn ion spins. Change in resistivity around T IM is interpreted in the framework of percolative conduction model based on the mixed phase of itinerant electrons and localized magnetic polarons.

Long-lived localized magnetic polarons in ZnMnSe/ZnSSe type-II superlattices

The kinetics and polarized spectra of low-temperature photoluminescence in semiconductor type-II superlattices based on ZnMnSe/ZnSSe structures have been studied in detail. Processes responsible for the formation of short-lived (about 1 ns) and long-lived (above 10 ns) localized exciton-type magnetic polarons (EMPs) in these systems are determined, and the relative contributions due to magnetic and nonmagnetic localization of heavy holes to the formation of such polarons are evaluated. A phenomenological model is constructed that takes into account the energy distribution of charge-carrier traps with respect to their level depths and employs the EMP parameters determined for ZnMnSe quantum wells. Within the proposed model, all spectral, temporal, and temperature-dependent features in the behavior of magnetophotoluminescence observed for the system under consideration can be consistently and quantitatively described.

Excitons and magnetic polarons in semimagnetic double quantum wells CdSe/CdMgSe/CdMnSe and CdSe/CdMnSe/CdSe

We have studied at high magnetic fields, exciton photoluminescence and reflection spectra of two groups of semimagnetic double quantum wells, CdSe/CdMgSe/CdMnSe and CdSe/CdMnSe/CdSe, with either one of the wells or a coupling barrier being semimagnetic. Owing to strong exchange interaction between band carriers and manganese ions, magnetic tuning of energy levels or the height of the coupling barrier could control the interaction of excitons of the coupled wells and their tunneling. In the first group of structures, we have observed direct and spatially indirect excitons. Photoluminescence intensity of the indirect exciton strongly increased and its half-width decreased with the magnetic field. Dependence of excitons and spin tunneling on the barrier thickness was studied. We also addressed the problem of localized magnetic polarons formation and its peculiarities for indirect and direct excitons. In the second group with a semimagnetic coupling barrier, the energy shifts of the excitons in a magnetic field exceeded significantly the calculated values obtained without account of interface effects. A proper account of both interdiffusion of Mn and Cd ions in the surface layers of the coupling barrier and decrease of antiferromagnetic coupling in these layers, allowed to fit experimental data.

Direct and indirect excitons and magnetic polarons inCdSe∕CdMgSe∕CdMnSesemimagnetic double quantum wells

We have studied exciton photoluminescence and reflection spectra as well as formation of localized magnetic polarons and spin tunneling in $\mathrm{Cd}\mathrm{Se}∕\mathrm{Cd}\mathrm{Mg}\mathrm{Se}∕\mathrm{Cd}\mathrm{Mn}\mathrm{Se}$ asymmetric double quantum well structures with different barrier thicknesses. Measurements were performed in strong magnetic fields both in Faraday and Voigt configurations. Direct excitons from nonmagnetic CdSe and semimagnetic CdMnSe quantum wells were observed, as well as a spatially indirect exciton formed by the electron in the CdSe quantum well and the heavy hole in the CdMnSe one. The problem of the direct and indirect exciton forming localized magnetic polarons is addressed. Polarization properties of direct and indirect excitons are studied. In the case of a thin barrier, spin tunneling is observed under magnetic field. Calculations of the exciton energies are performed. The calculated values are found to be in reasonable agreement with the measured ones.

Origin of peculiar chalcogen and temperature dependence of the spectrum width for the luminescence inEuXc(Xc=S,Se, and Te)

The origin of the peculiar chalcogen and temperature dependence of the spectrum width of the broad luminescence band in EuX c (X c =S, Se, and Te) at about 1.1-1.3 eV for EuS, 1.4-1.6 eV for EuSe, and 1.4-1.5 eV for EuTe [J. Phys. Soc. Jpn. 63, 4616 (1994)], is studied theoretically. The study confirms the validity of the photo-induced localized magnetic polaron model accompanied with large lattice relaxations around the 4f-hole produced in the absorption process [Phys. Rev. B 52, 8140 (1995)]. It is shown that the origin for the temperature variation of the spectrum width is attributed to the temperature variation of the lattice relaxation which is closely related with the degree in the screening of the localized 4f hole by the photoexcited electron. In other words, the temperature variation of the spectrum width is directly related to how the extent of the photoexcited electron varies with temperature by the magnetic polaron effect. The chalcogen dependence is also understood on this basis by considering the difference due to the magnetic ordering from EuS (ferromagnet) to EuTe (antiferromagnet). A brief comment concerning a narrow photoluminescence band recently observed at about 1.9 eV is also given.

Bound magnetic exciton polaron accompanied by large lattice displacement and luminescence in EuSe

We have studied the luminescence in EuSe using the photoinduced localized magnetic polaron (PILMP) model previously proposed for EuTe (Phys. Rev. B 52 (1995) 8140). This paper shows the characteristics of the luminescence around 1.47 eV recently reported (J. Phys. Soc. Japan 63 (1994) 4616) for EuSe is consistent with this model. The PILMP is bound to the localized 4f hole that is created in the photoabsorption process, and accompanies a large lattice displacement induced by the 4f hole. By taking into account this large lattice displacement, we can understand the characteristics of the luminescence, such as the emission energy, the Stokes shift and the spectrum width, quantitatively. The existence of the large lattice displacement is a common feature for EuTe and EuSe. On the other hand, the magnetic polaron effect in EuSe is quite large in comparison to that in EuTe, which causes the difference in the temperature dependence of the emission energy and the spectrum width between EuTe and EuSe. Through the analysis of the luminescence in EuSe, we can see a typical magnetic polaron bound to the 4f hole, that is a bound magnetic exciton polaron accompanied by the large lattice displacement.

Magnetic polarons in semimagnetic-semiconductor-based heterostructures

Specific features in the behavior of localized magnetic polarons formed under optical excitation in heterostructures based on semimagnetic semiconductors are considered. These features are due to the strong anisotropy of the hole g factor in low-dimensional systems based on zincblende crystals. The anisotropy is due to the strong spin-orbit coupling in the valence band which, in quantum confinement conditions, results in quadrupole splitting of the hole spin levels. The g factor anisotropy manifests itself in a strong anisotropy of the magnetic and magneto-optical characteristics of localized magnetic polarons.

Collective exciton magnetic polarons in quantum wells with semimagnetic barriers

A mechanism for exciton Bose condensation is suggested. We show that the interaction of localized exciton magnetic polarons (EMP's) can lead to the formation of a free collective exciton magnetic polaron (CEMP) able to move as a whole particle. Critical conditions for this transition and characteristics of the CEMP are found. We interpret the recent data on giant magnetic polaron mobility in ${\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{M}\mathrm{n}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ quantum wells [G. A. Balchin, C. D. Poweleit, and L. M. Smith, in Proceedings of the 23rd International Conference on the Physics of Semiconductors, edited by M. Scheffler and R. Zimmermann (World Scientific, Singapore 1996), p. 2055] as a manifestation of the free CEMP formation due to clusterization of long-living indirect EMP's.

Dynamics of exciton magnetic polaron in the [formula omitted] quantum wells

To study dynamics of exciton magnetic polaron in a quantum-well (QW) structure, we have grown the Cd 0.95Mn 0.05Te ZnTe QW in which the Mn ions are incorporated into well layers. Magnetic field dependence of the steady-state and time-resolved photoluminescence (PL) spectra under the selective excitations demonstrates the formation of localized magnetic polaron. The evaluated formation time of localized magnetic polaron is found to be considerably small as compared with that in the bulk.

Localized magnetic polarons in diluted magnetic semiconductors

A theory of localized magnetic polarons in diluted magnetic semiconductors is developed. The theory involves below band gap states and alloy fluctuations utilizing the optimal fluctuation method. A comparison of the theoretical calculations with published experimental results is performed. The implications of the comparison are discussed and the need for extending the theory to include an additional impurity/native lattice defect potential described. The magnetic field and temperature dependence of the polaron energies are also evaluated.

Spin-state transition and high-spin polarons in LaCoO3.

The spin gap energy (\ensuremath{\approx} 30 meV) associated with the low-spin ($S=0$) to high-spin ($S=2$) transition in LaCo${\mathrm{O}}_{3}$ was proved to be considerably smaller than the charge gap energy (\ensuremath{\approx} 0.1 eV) which was estimated by optical spectroscopy. Hole doping in the low-spin ground state of LaCo${\mathrm{O}}_{3}$ leads to formation of localized magnetic polarons with unusually high spin number ($S=10\ensuremath{-}16$), which can be viewed as a precursor of the doping-induced ferromagnetic metallic state.