An analysis of the mechanism of Kerr effect enhancement in Mn/Dy/Bi

A study is reported of the structural, magnetic, and magneto-optic properties of Mn/Dy/Bi films obtained by multilayer technology. The maximum Kerr rotation angle in such films is shown to be θ _k =2.25°. Possible reasons for such a large Kerr effect enhancement are considered, namely, an increase in the 6p–3d transition probability caused by symmetry distortion, polarization of the Bi6p band, and a change in the density of states near the Fermi level. The latter reason has been analyzed by simulating the electronic structure of Mn/Dy/Bi through superposition of Dy levels on the MnBi band structure. This approach has revealed possible additional transitions which may be induced by the presence of a Dy buffer and could contribute to the Kerr magneto-optic effect. Fiz. Tverd. Tela (St. Petersburg) 41, 91–97 (January 1999)     

Laser-plasma deposited nanosized layers of ferromagnetic semiconductors and silicon- and germanium-based Heusler alloys

The electric, magnetic resonance, and magneto-optical properties of thin laser-plasma deposited 50–100-nm layers of diluted magnetic semiconductors Ge:(Mn, Al)/GaAs, Ge:(Mn, Al)/Si, and Heusler alloys Co₂MnSi/Si, Co₂MnSi/GaAs, and Fe₂CrSi/GaAs with T _c > 293 K were studied. Anomalous ferromagnetic resonance in Ge:(Mn, Al) layers, ferromagnetism in CoSi/Si characterized by strong hysteresis in the magneto-optic Kerr effect, and the anomalous Hall effect at 293 K were observed.     

Excitonic effects in diluted magnetic semiconductor nanostructures

Excitonic properties and the dynamics are reported in quantum dots (QDs) and quantum wells (QW) of diluted magnetic semiconductors. Transient spectroscopies of photoluminescence and nonlinear-optical absorption and emission have been made on these quantum nanostructures. The Cd_1−x Mn_xSe QDs show the excitonic magnetic polaron effect with an increased binding energy. The quantum wells of the Cd_1−x Mn_xTe/ZnTe system display fast energy and dephasing relaxations of the free and localized excitons as well as the tunneling process of carriers and excitons in the QWs depending on the barrier widths. The observed dynamics and the enhanced excitonic effects are the inherent properties of the diluted magnetic nanostructures. Fiz. Tverd. Tela (St. Petersburg) 40, 846–848 (May 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Magnetoexciton light absorption in inhomogeneous quasi-two-dimensional systems

The problem of exciton light absorption in quasi-two-dimensional inhomogeneous systems in a strong transverse magnetic field H is analyzed. We assume that a random Gaussian field (“white noise”) acting separately on an electron and a hole is due to (1) fluctuations in the quantum well thickness or (2) fluctuations in the concentrations of the solid solution components. The problem of a magnetoexciton in a random Gaussian white noise field has been reduced to the problem of the motion in an H-dependent effective field of a single particle with the effective magnetic mass of the exciton, which is a function of the magnetic field and parameters of the quantum wells, in a field characterized by “colored noise,” whose correlation function is different from that of the white noise field. In this approximation, the problem of a magnetoexciton in isolated and coupled quantum dots is considered. In the coherent-potential approximation, the exciton absorption in random fields of the first and second type in single and coupled quantum wells has been calculated. The absorption decreases as H increases in the range of strong magnetic fields, which is in agreement with experimental data. Zh. éksp. Teor. Fiz. 114, 1451–1465 (October 1998)     

Intraband population inversion and amplification of IR radiation through charge-carrier injection into quantum wells and quantum dots

A mechanism is proposed for obtaining intraband population inversion of electrons in size-quantization levels through the injection of electron-hole pairs into the i region of a heterostructure with quantum wells or quantum dots. Key elements of the mechanism are the simultaneous generation of interband (hv≈E _g ) near-IR radiation and the presence of a “metastable” level. In quantum wells such a level can be produced by making use of the weak overlap of the wave functions of electrons in the levels of a quantum well of complicated configuration and exploiting the characteristic features of the interaction of electrons with optical phonons in polar semiconductors. In quantum dots such a level forms as a result of the phonon bottleneck effect. Estimates are made of the gain for mid-IR radiation in intraband optical transitions of electrons. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 392–399 (10 September 1998)     

Polar magneto-optic Kerr effect in the near-light field of a small nonmagnetic particle

A theory of elastic light scattering by a small resonantly polarizing particle located near a flat surface of a magnetic medium has been developed. The effective polarizability of the particle was calculated self-consistently taking account of the dynamic “image forces” in all orders of perturbation theory in the interaction of the particle with the demagnetized ferromagnet, and the magneto-optic perturbation was determined to first order in the magnetization. In the case of a ferromagnet magnetized perpendicular to the surface, the light conversion factors and the magneto-optic corrections to the transverse cross sections of all processes in which the scattering of light by a particle and the polar magneto-optic Kerr effect are elementary events, have been calculated. The results, including an analysis of the near-field contribution to light scattering, comprise the physical foundation for constructing a theory of near-field magneto-optic spectroscopy of ferromagnets and magnetic structures. Fiz. Tverd. Tela (St. Petersburg) 39, 560–567 (March 1997)     

Magnetoexcitons in type-II quantum dots

The ground state of a spatially indirect exciton in type-II quantum dots with a short-range potential acquires nonzero angular momentum in the presence of a magnetic field oriented perpendicular to the plane of the system. The critical magnetic field of the transition to a ground state with nonzero angular momentum depends on the radius of the quantum dot. Such a transition can be observed as quenching of luminescence by a magnetic field in quantum dots of the GaSb/GaAs system, for example. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 8, 634–637 (25 October 1998)     

Spin–spin interaction in magnetic semiconductor quantum dots

Self-assembled Cd(Mn)Se/Zn(Mn)Se quantum dots have been investigated by means of spatially and time-resolved magneto-optical spectroscopy. In such quasi zero-dimensional diluted magnetic semiconductors, the exchange interaction couples the spins of optically generated charge carriers with localized magnetic ion spins. We demonstrate that this can be used on the one hand to monitor nanoscale magnetization with a resolution of <100 μ_B by a purely optical technique and on the other hand to optically manipulate the magnetization in a semiconductor quantum dot.     

Room-temperature ferromagnetism of diamagnetically-doped ZnO aligned nanorods fabricated by vapor reaction

Large-scale monocrystalline oxide-diluted magnetic semiconductor (ODMS) Zn_1−x Bi _x O nanorods arrays (NAs) were prepared within a large doping concentration range from 5% to 20% by a simple chemical vapor deposition. X-ray diffraction and high-resolution transmission electron microscopy reveal the monotonous expansion of the lattice constants with increasing Bi content, due to the effective Bi doping. In particular, room-temperature ferromagnetic (RTFM) behavior with Curie temperature over 363.7 K has been observed based on Bi-doped ZnO nanoarrays, whereas undoped ZnO NAs disappear. The RTFM origin is suggested, in which vacancies can be controlled to tune the FM. The as-formed RTFM NAs would have potential applications in many areas of advanced nanotechnology, such as new spintronic devices and magneto-optic components.     

Orbital magnetism in small quantum dots with closed shells

It is found that various kinds of shell structure which occur at specific values of the magnetic field lead to the disappearance of the orbital magnetization for particular magic numbers in small quantum dots with electron number A<30. Pis'ma Zh. éksp. Teor. Fiz. 68, No. 12, 870–875 (25 December 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Raman scattering of light by optical phonons in Si-Ge-Si structures with quantum dots

Raman scattering of light by optical phonons in Si-Ge-Si structures with pseudomorphic germanium quantum dots has been investigated. Resonance amplification of the scattering intensity on E ₀ (Γ₇−Γ₈) transitions has been observed. It is shown that as a result of the formation of the layer of germanium quantum dots, the resonance energy is ∼0.3 eV higher than in the two-dimensional case. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 203–207 (10 August 1996)     

Quantum melting of mesoscopic clusters

The phase diagram of a two-dimensional mesoscopic system of charges or dipoles, whose realizations could be electrons in a semiconductor quantum dot or indirect excitons in a system of two vertically coupled quantum dots, is investigated. Quantum calculations using ab initio Monte Carlo integration along trajectories determine the properties of such objects in the temperature-quantum de-Boer-parameter plane. At zero (sufficiently low) temperature, as the quantum fluctuations of the particles increase, two types of quantum disordering phenomena occur with increasing quantum de Boer parameter q: first, for q∼10⁻⁵ the systems transform into a radially ordered but orientationally disordered state wherein various shells of the “atom” rotate relative to one another. For much larger q∼0.1, a transition occurs to a disordered state (a superfluid in the case of a system of bosons). Fiz. Tverd. Tela (St. Petersburg) 41, 1856–1862 (October 1999)     

Optical orientation of particles with a random g-factor in semiconductor nanostructures

In real quasi-two-dimensional semiconductor nanostructures (quantum wells, quantum dots), the transverse g-factor of holes is a stochastic quantity. This fact should be taken into account in analyzing the optical orientation and Hanle effect of holes. The Hall effect for an ensemble of particles with a “random” g-factor has been treated theoretically. In the case where the spin relaxation time of a hole with a characteristic g-factor is shorter than the hole lifetime, there can occur a narrowing of the depolarization contour and an increase in its amplitude. In the opposite case of long spin relaxation times (trions in quantum dots), a formula has been derived, which generalizes the previously obtained result to the case of an arbitrary tilt angle of the magnetic field with respect to the plane of the layer (Hanle effect in the tilted form).     

Emission and percolation of excitons in dense sensembles of quantum dots on the spherical surface

We study core/shell heterostructures in which a core (SiO₂) is overcoated with a shell of ZnO quantum dots, randomly distributed on the sphere surface with the surface filling factor P∼0.45. Due to the high surface energy of SiO₂ spheres, ZnO quantum dots have the shape of disks in which, in spite of the large radii, quantum size effects of excitons are retained. The height of ZnO disks estimated by the effective mass approximation is comparable with the exciton diameter in bulk ZnO. Analysis of optical spectra has shown that, at the given density of ZnO quantum dots, the exciton system is above the percolation threshold. The quantitative parameters characterizing such phenomenon are obtained using the elements of percolation theory and the topology of the samples.     

“Spherical” quantum dots

Quantum dots with a three-dimensional confining potential, i.e. “spherical” quantum dots, are considered with inclusion of electron-electron interaction (a quantum analog of the Thomson atom). The energy spectrum of two-electron parabolic quantum dots has been determined by numerical diagonalization of the full Hamiltonian in a one-particle basis. Fiz. Tverd. Tela (St. Petersburg) 40, 2134–2135 (November 1998)     

Quantum orientational melting and the phase diagram of a mesoscopic system

The “phase diagram” of a two-dimensional mesoscopic system of bosons is investigated. An example of such a system is a system of indirect magnetoexcitons in semiconductor double quantum dots. Quantum Monte Carlo calculations show the existence of quantum orientational melting. At zero (quite low) temperature, as quantum fluctuations of the particles intensify, two quantum disordering phenomena occur with increasing de Boer parameter q. First, at q≈10⁻³ the system passes to a radially ordered but orientationally disordered state, where different shells of a cluster rotate relative to one another. Then at q≈0.16 a transition to a superfluid state occurs. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 11, 817–822 (10 December 1998)     

Magneto-optic study of spatial magnetic-field distribution relaxation in an HTSC film strip after transport current turn-on

The paper provides the first demonstration of the efficiency of applying the magneto-optic method to studies of the spatial and temporal magnetic-field relaxation in an YBa₂Cu₃O₇ film strip after the transport current is switched on. It is shown that the evolution of magnetic flux distribution is adequately described in terms of a modified Bean model with time-dependent critical current. At a time 50 ms after the current is switched on, the critical current of the samples studied decreases by ≈15%. This proves the significance of thermally activated magnetic flux motion (creep) in the regime investigated. The magnetic vortex pinning energy has been estimated as U ₀≈20 kT. Fiz. Tverd. Tela (St. Petersburg) 41, 965–968 (June 1999)     

Electron states in quantum-dot and antidot arrays placed in a strong magnetic field

Quantum electronic states in a dot (antidot) array in the presence of a dc magnetic field are studied. A new method of numerical calculation of the electron spectrum and wave functions in a two-dimensional periodic potential and perpendicular magnetic field is proposed. The magnetic-subband energies, density of electron states, and electron density |ψ(x,y)|², as well as the amplitude of the potential, and lattice period and degree of anisotropy for different magnetic fields have been found. The calculations were performed for quantum dots in the In_0.2Ga_0.8As-GaAs and GaAs-Al_0.3Ga_0,7As systems. The rearrangement of the spectrum with variation of magnetic field and with transition from the tight-binding to weak-binding approximation is studied (ω _c is the cyclotron frequency, and V ₀ is the periodic-potential amplitude). The calculations show that the two-dimensional lattices epitaxially grown presently on semiconductor surfaces permit observation of quantum effects associated with rearrangement of the spectrum (electron transport and optical absorption) in magnetic fields H⩽1 MG. Fiz. Tverd. Tela (St. Petersburg) 40, 1134–1139 (June 1998)     

Grain boundary layers in nanocrystalline ferromagnetic zinc oxide

The complete solubility of an impurity in a polycrystal increases with decreasing grain size, because the impurity dissolves not only in the crystallite bulk but also on the grain boundaries. This effect is especially strong when the adsorption layers (or the grain boundary phases) are multilayer. For example, the Mn solubility in the nanocrystalline films (where the size of grains is ∼20 nm) is more than three times greater than that in the ZnO single crystals. The thin nanocrystalline Mn-doped ZnO films in the Mn concentration range 0.1–47 at % have been obtained from organic precursors (butanoates) by the “liquid ceramic” method. They have ferromagnetic properties, because the specific area of the grain boundaries in them is greater than the critical value [B.B. Straumal et al., Phys. Rev. B 79, 205206 (2009)]. The high-resolution electron transmission microscopy studies show that the ZnO nanocrystalline grains with the wurtzite lattice are separated by amorphous layers whose thickness increases with the Mn concentration. The morphology of these layers differs greatly from the structure of the amorphous prewetting films on the grain boundaries in the ZnO:Bi₂O₃ system.     

Ferromagnetism in Cu-doped ZnSe semiconducting quantum dots

The projection of integrating optical, magnetic and electronic functionalities into a single material have aggravated passionate attention in mounting wide band gap diluted magnetic semiconductor (DMS) in the midst of room temperature ferromagnetism. We report the evidence of ferromagnetism in Cu-doped ZnSe quantum dots (QDs) below room temperature, grown from a single source precursor by lyothermal method with the sizes of approximately 3.2–5.14 nm. QDs mainly exhibit paramagnetic behavior between 80 and 300 K, with a weak ferromagnetic/anti-ferromagnetic exchange at lower temperature as observed by superconducting quantum interference device (SQUID) magnetometer. From the Curie–Weiss behavior of the susceptibility, Curie temperature (T _c) of Cu-doped ZnSe sample has been evaluated. From EPR, we obtain the Lande-g factor in the Zeeman interaction term as 2.060. Photoluminescence and EPR measurements support and confirm the view that Cu²⁺ substitutes for Zn²⁺ in Cu-doped ZnSe quantum dots.