Photoinduced phase transition accompanied with the changes in magnetic properties

Recently, many studies have been started in search for materials which show a photoinduced phase transition (PIPT). In this work, we review two systems as typical examples of PIPT accompanied with changes in magnetic characteristics; (1) organo-metal complex [Fe(2-pic)₃]Cl₂ EtOH (2-pic = 2-amino-methyl-pyridine) and (2) III-V based magnetic semiconductors (In_1-x , Mn _x )As. In the former case, we show several nonlinear characteristics in dynamical process of photoinduced spin state transition from low-spin to high-spin states. In the latter one, photocarrier-induced ferromagnetic order has been observed by both magnetic and transport measurements.     

MBE growth and magnetotransport studies of ferromagnetic Ga1−xMnxSb semiconductor layers on hybrid ZnTe/GaAs substrates

Narrow-gap Ga_1−xMn_xSb layers grown on hybrid ZnTe/GaAs substrates are observed to be ferromagnetic by SQUID magnetization and anomalous Hall effect measurements. The layers display an easy axis of magnetization perpendicular to the layer plane, in contrast to in-plane easy axis orientation observed in Ga_1−xMn_xSb grown on GaSb substrates. Resistivity measured in the Ga_1−xMn_xSb/ZnTe/GaAs system shows a well-defined maximum at temperatures close to the ferromagnetic/paramagnetic transition. We determined the spontaneous resistivity anisotropy in Ga_0.98Mn_0.02Sb grown on hybrid ZnTe/GaAs substrates and compared it to that observed on Ga_0.98Mn_0.02Sb grown on a GaSb buffer. These results should provide a valuable test for future theories of transport in ferromagnetic semiconductors.     

Photo-induced magnetic polarons in semiconductors

Exciton magnetic polarons observed in dilute magnetic semiconductors were investigated by steady-state and pico-second time-resolved photoluminescence measurements and have shown characteristic behavior of exciton localization processes in bulk-Cd_1-x Mn _x Te and also in the quantum structures composed of the dilute magnetic semiconductors. For the quantum structures spin-dependent coherent polarizations associated with excitons and biexcitons were studied by degenerate four-wave mixing experiment. Also investigated for different chalcogenide spinel ferromagnetic semiconductors was photo-induced enhancement of exchange interaction between magnetic ions by direct magnetic flux detection in the vicinity of the Curie temperatures.     

Separated AlxIn1−xN quantum dots grown by plasma-reactive co-sputtering

Separated Al_xIn_1−xN quantum dots (QDs) embedded in amorphous AlN films have been produced by radio-frequency co-sputtering technique on silicon (1 1 1) and quartz glass substrates. The mean size and density of Al_xIn_1−xN QDs can be conveniently monitored by deposition parameters. Transparent electron microscope, and X-ray diffraction were used to detect the structure of the Al_xIn_1−xN QDs system; field-emission scanning-electron microscope was adopted to measure the surface morphology and anticipate the size of the QDs; X-ray photoelectronic spectroscopy was used to measure the stoichiometric ratios of the QDs.     

Growth and properties of ferromagnetic In1−xMnxSb alloys

We discuss a new narrow-gap ferromagnetic (FM) semiconductor alloy, In_1−xMn_xSb, and its growth by low-temperature molecular-beam epitaxy. The magnetic properties were investigated by direct magnetization measurements, electrical transport, magnetic circular dichroism, and the magneto-optical Kerr effect. These data clearly indicate that In_1−xMn_xSb possesses all the attributes of a system with carrier-mediated FM interactions, including well-defined hysteresis loops, a cusp in the temperature dependence of the resistivity, strong negative magnetoresistance, and a large anomalous Hall effect. The Curie temperatures in samples investigated thus far range up to 8.5 K, which are consistent with a mean-field-theory simulation of the carrier-induced ferromagnetism based on the 8-band effective band-orbital method.     

Influence of the built-in electric field on luminescent properties in self-formed single InxGa1−xN/GaN quantum dots

Based on the framework of effective-mass approximation and variational approach, luminescent properties are investigated theoretically in self-formed wurtzite In_xGa_1−xN/GaN quantum dots (QDs), considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects due to the piezoelectricity and spontaneous polarization. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the different structural parameters (the height L and the radius R) are calculated with and without the built-in electric field in detail. The results elucidate that the strong built-in electric field has a significant influence on luminescent properties of In_xGa_1−xN/GaN QDs.     

Effects of the Mn/Co ratio on the magnetic transition and magnetocaloric properties of Mn1+xCo1-xGe alloys

We have investigated the magnetic transition and magnetocaloric effects of Mn 1+x Co 1 x Ge alloys by tuning the ratio of Mn/Co.With increasing Mn content,a series of first-order magnetostructural transitions from ferromagnetic to paramagnetic states with large changes of magnetization are observed at room temperature.Further increasing the content of Mn (x=0.11) gives rise to a single second-order magnetic transition.Interestingly,large low-field magnetic entropy changes with almost zero magnetic hysteresis are observed in these alloys.The effects of Mn/Co ratio on magnetic transition and magnetocaloric effects are discussed in this paper.     

Lattice, magnetic and transport properties in antiperovskite compounds

The temperature-dependent magnetization, lattice, and transport properties of Mn₃Sn_1−xGe_xC (0≤x≤0.5) compounds are systematically investigated. The Mn–Mn atomic distance decreases as Ge content is increased, and the transition temperature from ferromagnetic (or ferrimagnetic) to paramagnetic state decreases too. Mn₃SnC has a large magnetovolume effect (MVE). However, Ge-doping in Mn₃SnC gradually reduces the MVE, till the MVE disappears. Whether there is an abnormal lattice change or not, there always exists an anomalous increase in resistivity near the magnetic phase transition point with decreasing temperature.     

Temperature-dependent magnetization in (Mn, N)-codoped ZnO-based diluted magnetic semiconductors

The influences of Mn doping on the structural quality of the Zn_xMn_1−xO:N alloy films have been investigated by XRD. Chemical compositions of the samples (Zn and Mn content) and their valence states were determined by X-ray photoelectron spectrometry (XPS). Hall effect measurements versus temperature for Zn_xMn_1−xO:N samples have been designed and studied in detail. The ferromagnetic transitions happened at different T_C should explain that the magnetic transition in field-cooled magnetization of Zn_1−xMn_xO:N films at low temperature is caused by the strong p-d exchange interactions besides magnetic transition at 46 K resulting from Mn oxide, and that the room temperature ferromagnetic signatures are attributed to the uncompensated spins at the surface of anti-ferromagnetic nano-crystal of Mn-related Zn(Mn)O.     

Landau-level lifetimes in PbTe nipi superlattices, PbTe/PbEuTe and InAs/AlSb quantum wells

Landau-level lifetimes are determined from saturation cyclotron resonance (CR) in wide parabolic wells, quantum wells and bulk PbTe–Pb_1−xEu_xTe systems. These narrow gap structures exhibit strong band non-parabolicity necessary to terminate the normally equi-spaced Landau-level ladder. It was not possible to saturate the bulk sample, but short lifetimes, of between 1.5 and 8 ps, were obtained for the wide parabolic well and the quantum well, respectively, utilising a multi-level rate equation model. We also report the first pump–probe cyclotron resonance result in an InAs–AlSb quantum structure. The pump–probe experiment provides a direct determination of the lifetime, giving τ=40 ps in this InAs–AlSb sample. This shows good agreement with an 8×8k·p calculation.     

Microfabrication of FeMnPt films involving magnetic phase change due to structural transformation caused by ion irradiation

Correlations between crystal structures and magnetic properties of Fe_1–xMn_x Pt films were studied. The disordered films with x ≥ 0.44 had paramagnetic properties and the ordered films with x ≥ 0.46 had antiferromagnetic properties, viz. a difference of 0.02 in the x ‐value (i.e., 1.0 at%) was found. At x = 0.44 with the ordered structure, the uniaxial magnetocrystalline anisotropy was about 2.1 × 10⁷ erg cm^–3. A microfabrication process involving the slight composition difference of 0.02, which results in ferromagnetic–paramagnetic phase change due to the structural transformation caused by ion irradiation, was investigated. Only the area irradiated by Mn ions changed from ferromagnetic to paramagnetic phase. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

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.     

Electron–hole symmetry and spin–orbit splitting in IV–VI asymmetric quantum wells

It is shown that, due to the electron–hole symmetry of the fundamental gap of the lead–salts (PbTe, PbSe and PbS), the Rashba spin splitting in their flat band asymmetric quantum wells is much reduced with the usual equal conduction and valence band-offsets. Different from the III–V case, we find that the important structure inversion asymmetry for the Rashba splitting in IV–VI quantum wells with different left and right barriers is not a material property (i.e., barrier height, effective mass or band gap) but results from the band alignment. This is shown by specific envelope function calculations of the spin-dependent subband structure of Pb_1−xEu_xTe/PbTe/Pb_1−yEu_yTe asymmetric quantum wells (x≠y), based on a simple but accurate four-band kp model for the bulk band structure near the gap, which takes into account band anisotropy, nonparabolicity and multi-valley effects.     

Band-edge exciton transitions temperature in multiple stacked self-assembled (In1−xMnx)As quantum dot arrays

Multiple stacked self-assembled (In_1−xMn_x)As quantum-dot (QD) arrays were grown on GaAs (100) substrates by using molecular-beam epitaxy with a goal of producing (In_1−xMn_x)As QDs with a semiconductor phase and a high ferromagnetic transition temperature (T_c). Atomic force microscopy, magnetic force microscopy, high-resolution transmission electron microscopy, and energy dispersive X-ray fluorescence measurements showed that crystalline multiple stacked (In_0.84Mn_0.16)As with symmetric single-domain particle were formed on GaAs substrates. Near-field scanning optical spectroscopy spectra at 10 K for the (In_0.84Mn_0.16)As multiple stacked QDs showed that the band-edge exciton transitions were observed. The magnetization curve as a function of the magnetic field at 5 and 300 K indicated that the multiple stacked (In_0.84Mn_0.16)As QDs were ferromagnetic, and the magnetization curve as a function of the temperature showed that the T_c was as high as 400 K. These results provide important information on the optical and magnetic properties for enhancing the T_c of (In_1−xMn_x)As-based nanostructures.     

Diluted magnetic semiconductor superlattices and heterostructures

Diluted magnetic semiconductors (DMS) are mixed semiconducting crystals whose lattice is made up in part of substitutional magnetic ions. Cd_1−xMn_xTe and Hg_1−xMn_xTe are examples of such materials. Their structural and band parameters can be “tuned” by composition over a wide range. They can thus be exploited in situations completely similar to those involving Ga_1−xAl_xAs. Using molecular beam epitaxy, we have grown Cd_1−xMn_xTe superlattices with alternating Mn content, having up to 150 layers, with layer thickness ranging from 50 to 100 Å. The superlattice structure is clearly revealed by transmission electron microscopy and by zone-folding of the phonon spectrum observed in Raman scattering. Photoluminescence observed on Cd_1−xMn_xTe superlattices is several orders of magnitude greater than that from a Cd_1−xMn_xTe film with uniform Mn content, or from bulk Cd_1−xMn_xTe specimens. The presence of localized magnetic moments in DMS results in a strong exchange interaction between these moments and band electrons. This in turn leads to gigantic Zeeman splittings of impurity states, exciton levels, Landau levels, and the bands themselves. Zeeman splittings as large as 20 meV (which in non-magnetic semiconductors would require unrealistic megagauss fields) are easily achieved in DMS in fields of several kilogauss. Since the magnitude of this exchange-induced splitting in DMS can be comparable to the binding energies and to the minigaps encountered in multiple quantum wells, DMS superlattices hold promise of a host of novel effects of both fundamental and applied interest.     

Low temperature photoluminescence of GaxIn1−xAs/GaAs strained layer superlattices

The low temperature photoluminescence spectra of several Ga_xIn_1−xAs/GaAs strained layer superlattices have been measured. Excitomic recombination between electrons and holes confined in the ternary layers and conduction band-acceptor transitions have been observed. The excitonic transition energies calculated with a simple model which takes into account both strain and quantization are in good agreement with the measured values provided the additional strain due to the mismatch between the SLS and the buffer layer is taken into account. The hydrogenic acceptor binding energy is smaller in the SLS than in the bulk ternary because the reduction due to the decrease of the hole mass under strain appears to be more important than the two dimensional quantization enhancement in the present samples.     

Ferromagnetic transition in GaAs/Mn/GaAs/In x Ga1 − x As/GaAs structures with a two-dimensional hole gas

The transport properties of GaAs/Mn/GaAs/In _x Ga_{1 ? x} As/GaAs structures with a layer that is separated from the quantum well and contains Mn impurities in the concentration range 4–10 at % corresponding to the reentrant metal-insulator transition observed in the bulk GaMnAs material [17] have been investigated. The hole mobility in the objects under investigation is more than two orders of magnitude higher than the known values for the GaMnAs semiconductor and GaMnAs-based magnetic heterostructures. This makes it possible to observe Shubnikov-de Haas oscillations, which confirm a two-dimensional character of the hole energy spectrum. The calculated Curie temperature for heterostructures with indirect exchange interaction through a two-dimensional hole channel is in good agreement with the position of the maximum (at 25–40 K) in the temperature dependences of the electrical resistance of the channel. This suggests that two-dimensional holes play an important role in ferromagnetic ordering of the Mn layer under these conditions. The observations of a negative spin-dependent magnetoresistance and an anomalous Hall effect, whose magnitude correlates well with the results of theoretical calculations for two-dimensional ferromagnetic systems based on III-Mn-V, also indicate a significant role of the two-dimensional channel in ferromagnetic ordering.     

Influence of strain on built-in dipole moment in asymmetric In x Ga1−x As quantum dot molecules

Strain effects on a built-in electron-hole dipole moment are investigated in asymmetric In _x Ga_1?x As coupled quantum dots. We compute electron-hole separation as a function of alloy compositions for both electron and hole resonance cases. It is noted that the inclusion of strain enhances the built-in dipole moments and that the inverted electron-hole alignment is found for electron and hole resonances. Furthermore, the reversal of dipole moments gives rise to different asymmetric Stark shifts in each transition spectrum.     

Suppression of charge ordering and thermal hysteresis of electronic transport and magnetisation in La0.5Ca0.5Mn1−xNixO3

A series of polycrystalline La_0.5Ca_0.5Mn_1?xNi_xO₃ (x = 0.00, 0.025, 0.050, 0.075, 0.100 and 0.125) was synthesised using solid state reaction. Measurements in a cooling and warming cycle between 300 and 80 K were carried out to study the Ni-doping effects on the electrical resistivity, thermopower and magnetisation of single-phase La_0.5Ca_0.5Mn_1?xNi_xO₃. Partial substitution of Ni for Mn leads to the suppression of charge ordering state, the evidence of which is shown by the dramatic decrease in electrical resistivity and thermal hysteresis width in electrical resistivity, thermopower and magnetisation. However, the magnitude of both electrical resistivity and thermopower increases with increasing Ni content. This can be attributed to an increase in the Mn⁴⁺ concentration, which favours the antiferromagnetic state and leads to a gradual disappearance of ferromagnetic double exchange interaction. Besides, the metal–nonmetal transition temperature decreases with increasing Ni content until x = 0.075, which might arise from increased electron–phonon coupling due to less ordered spins at temperatures above ferromagnetic transition. For samples with x greater than 0.075, no metal–nonmetal transition is observed due to the suppression of double exchange mechanism.     

Enhancement of ferromagnetism in polycrystalline Si0.965Mn0.035:B films by boron plasma treatment

This paper reports that the polycrystalline Si0.965Mn0.035:B films have been prepared by cosputtering deposition followed by rapid thermal annealing for crystallization. The polycrystalline thin films consist of two ferromagnetic phases. The low temperature ferromagnetic phase with Curie temperature (Tc) of about 50 K is due to the Mn4Si7 phase in the films, while the high temperature one (Tc～250 K) is resulted from the incorporation of Mn into silicon. The films are treated by boron plasma excited with the approach of microwave plasma enhanced chemical vapor deposition for 40 minutes. After plasma treatment, it is observed that no extra magnetic phases or magnetic complexes exist in the films, while both the high temperature saturation magnetization and the hole concentration in the films increase. The obvious correlation between the magnetic properties and the electrical properties of the polycrystalline Si0.965Mn0.035:B films suggests that the hole carriers play an important role in Si:Mn diluted magnetic semiconductors.