Magnetic Order in GaMnAs Layers

MBE grown Ga_1–x Mn _x As layers were investigated by means of magnetic resonance techniques. Two phases can be distinguished: an almost isotropic ferromagnetic phase in insulating layers and an anisotropic ferromagnetic phase in the metallic Ga_1–x Mn _x As. Under a strong magnetic field the field-induced insulator-to-metal transition is accompanied by the change from the ferromagnetic to the ferrimagnetic phase.     

Magnetic-Field-Induced Level Crossing and the Spin Dynamics of Excitons in Zn1?xMnxTe/ZnTe Quantum Wells

Magnetic-field-induced level crossing and the spin dynamics of excitons in a Zn_1–x Mn _x Te/ZnTe single quantum well are studied. The circularly-polarized photoluminescence (PL) shows that the down spin branch of the Zn_1–x Mn _x Te exciton overlaps with both the up and down spin branches of the ZnTe exciton at a crossing field (H _c) of 4 T, due to the giant Zeeman shift of Zn_1–x Mn _x Te. The PL intensities and lifetimes in each layer become gradually equal toward H _c, which shows the mixing of wavefunctions of the excitons generated in each layer. Above H _c, each branch of the spin-polarized exciton separates again. The lifetimes of the spin-polarized exciton PL reflect the spin-flip relaxation in ZnTe and the spin mixing between Zn_1–x Mn _x Te and ZnTe layers.     

Cyclotron Resonance of Electrons and Holes in Paramagnetic and Ferromagnetic InMnAs-Based Films and Heterostructures

We have studied the cyclotron resonance of electrons and holes in various types of InMnAs-based structures at ultrahigh magnetic fields. Our observations, in conjunction with an eight-band effective mass model including the s–d and p–d exchange interactions with Mn d-electrons, unambiguously suggest the existence of s-like and p-like delocalized carriers in all samples studied. The samples studied include Paramagnetic n-type In_1–x Mn _x As films (x 0.12) grown on GaAs, ferromagnetic p-type In_1–x Mn _x As films (x 0.025) grown on GaAs with Curie temperatures (T _C) > 5 K, paramagnetic n-type In_1–x Mn _x As/InAs superlattices, ferromagnetic p-type In_1–x Mn _x As/GaSb heterostructures (x 0.09) with T _C = 30-60 K, and ferromagnetic (In_0.53Ga_0.47)_1–x Mn _x As/In_0.53Ga_0.47As heterostructures (x 0.05) grown on InP with T _C up to 120 K.     

Finite Ferromagnetic Clusters La1?xZnxMnO3 (x = 0.05 and 0.10)

Detailed magnetic and nonlinear susceptibility measurements of the La_1–x Zn _x MnO₃ system are reported. For x = 0.05 and 0.1, a typical ferromagnetic (FM) signal in the field-cooled curve is observed at 38 K. However, for the insulating antiferromagnetic (AFM) parent compound LaMnO₃ and for the x = 0.2 and 0.33 samples, the magnetization curves in the low temperature range, are smooth and flat. This observation confirms the coexistence of finite FM clusters in the insulating AFM region below the critical concentration (x _c = 0.16) for percolation, predicted by the percolation model.     

The crystal structure,magnetic and magnetocaloric properties of Mn8−xCrxGa5

Mn₈Ga₅ alloy has certain significance in the study of materials for magnetic refrigeration applications due to its low coercivity. In this paper, the structure, magnetic properties and magnetocaloric effects (MCE) of Mn_8?xCr_xGa₅ (x?=?1.3, 1.7) polycrystalline alloys were first studied by using X-ray diffraction (XRD) technique, scanning electron microscope (SEM), energy-dispersion (EDS) analysis and magnetization measurement. The two samples crystallized in a cubic (Zn₈Cu₅)-type structure with space group of I-43 m (No. 217). The lattice constants and crystallite sizes of Mn_8?xCr_xGa₅ increase with the increase of Cr concentration. As the Cr content increases, the Curie temperature (T_c) of Mn_8?xCr_xGa₅ (x?=?1.3, 1.7) is increased from 145 to 187 K. The saturation magnetic moments of Mn_8?xCr_xGa₅ are 33.8 (x?=?1.3) and 36.6 (x?=?1.7) emu/g under the magnetic field of 16 kOe at 80 K, respectively. A second-order magnetic phase transition occurred in Mn_8?xCr_xGa₅ alloy samples. The maximum magnetic entropy changes (|?S_M|) of Mn_8?xCr_xGa₅ are 0.9 (x?=?1.3) and 0.8 J·kg^?1·K^?1 (x?=?1.7), and the relative cooling power (RCP) values are 25 (x?=?1.3) and 17 J/kg (x?=?1.7) under 15 kOe magnetic field.

     

Ferromagnetic Properties of Mn-Doped III–V Semiconductor Quantum Wells

We investigate magnetic properties of In_1–xMn_xP and Ga_1–xMn_xN quantum wells in the mean-field approximation and show the difference between them. In the case of the In_1–xMn_xP, the dependence of the Curie temperature (T_c) on the hole density exhibits a step-like behavior, reflecting the effect of a two-dimensional Fermi (hole) gas, when the hole–hole exchange interaction is ignored. When we take into account the hole–hole exchange interaction, however, this behavior is broken by the appearance of peaks at the specific two-dimensional carrier densities, and T_c is substantially enhanced in this region. In the case of Ga_1–xMn_xN, the step-like behavior is obscure, and it appears that T_c increases rather continuously with the increasing two-dimensional (2D) carrier density. This shows very weak step-like behavior compared to other III-Mn-V DMS quantum wells, because the hole effective mass of Ga_1–xMn_xN is very large, and the large hole effective mass reduces the energy splitting due to the confinement effect. In a multi-heavy-valence-subband model, the Curie temperature of the In_1–xMn_xP quantum well is about 68 K with 6.5×10¹² holes per cm² and the Mn mole fraction x = 0.05 and the exchange constant J_pd = 0.15 eV nm³. The Curie temperature of the p-type Ga_1–xMn_xN quantum well can be above room temperature, unless the spin-exchange interaction integral is abnormally small.     

Origin of Spin-Glass Behavior of Zn1 ? xMnxO

Alternating current susceptibility has been studied for polycrystalline Zn_{1 – x} Mn _x O. Stoichiometric samples demonstrate Curie–Weiss behavior, which indicates mostly antiferromagnetic interactions. Magnetic susceptibility can be described by a diluted Heisenberg magnet model developed for semimagnetic semiconductors. High-pressure oxygen annealing induces spin-glass like behavior in Zn_{1 – x} Mn _x O by precipitation of ZnMnO₃ in the paramagnetic matrix.     

Correlation of Mn Lattice Location, Free Hole Concentration, and Curie Temperature in Ferromagnetic GaMnAs

We provide experimental evidence that the electrical and magnetic characteristics of Ga_1–x Mn _x As for a given x depend primarily on the distribution of Mn atoms over their different possible locations in the crystal lattice. Using combined channeling Rutherford backscattering and particle-induced X-ray emission, we show that optimal postgrowth annealing—which leads to an increase of the Curie temperature T _C and is accompanied by an increase of free hole concentration and saturation magnetization—is caused by the reduction in the number of Mn atoms occupying interstitial positions. On the other hand, when Ga_1–x Mn _x Asis additionally doped with Be, we observe that—while the hole concentration remains nearly constant—there occurs a strong decrease of T _C together with a dramatic increase in the concentration of Mn interstitials. These results indicate that there is a thermodynamic limit imposed on the maximum Curie temperature in Ga_1–x Mn _x As.     

Transport and Optical Properties of Diluted Magnetic Semiconductors

We investigated the transport and optical properties of diluted magnetic semiconductors theoretically by using a simple model where carriers move in a single band. In this model the carrier feels a nonmagnetic potential at a magnetic impurity site, and its spin interacts with the localized spins of the magnetic impurities through exchange interactions. The electronic states of a carrier were calculated by using the coherent potential approximation (CPA). The magnetism was investigated by minimizing the free-energy and the electrical conductivity was calculated by using the Kubo formula. We examined the results in several typical cases which correspond to (Ga_1–xMn_x)As with x = 0.05.     

Crystal growth and magnetic characterization of Zn1−x Mn x Cr2O4 single crystals

Single crystals of Zn_1–x Mn _x Cr₂O₄, where x=0–1, were grown by the chemical vapour transport (CVT) technique using chlorine as the transport agent. A thermodynamic study of both systems, (ZnCr₂O₄ + MnCr₂O₄-Cl₂ and Zn_1–x Mn _x Cr₂O₄-Cl₂, was done in order to investigate the effect of spurious species, i.e. MnCl_2(liq), on the transport of the mixed spinel, and structural investigations using X-ray diffraction were carried out. Susceptibility measurements as a function of temperature from 2–300 K for the different compositions are reported, and the magnetic phase diagram was determined as a function of composition and temperature.     

Magnetic Specific Heat of Heavily p-Type Doped (Zn,Mn)Te:P

The magnetic contribution to the specific heat of bulk crystals of Zn_1–x Mn _x Te ( x = 0.03) heavily (up to 10¹⁹ cm^–3) p-type doped with P is studied over the temperature range 0.5–15 K and magnetic field range 0–3 T. The magnetic specific heat observed at zero magnetic field indicates that a substantial part of the magnetic ions has the degeneracy of their magnetic ground state lifted by d–d and p–d exchange interactions. The effect increases for doped and annealed samples with higher concentration of conducting holes. We have also carried out a theoretical analysis that takes into account the contributions due to small magnetic clusters, single magnetic ions in crystal field of distorted crystal lattice, and low energy excitations of the p–d exchange-coupled system of local moments and carriers.     

Low Temperature Annealing Studies of Ga1?xMnxAs

High and low field magnetotransport measurements, as well as SQUID measurements of magnetization, were carried out on Ga_1–x Mn _x As epilayers grown by low temperature molecular beam epitaxy, and subsequently annealed under various conditions. We observe a large enhancement of ferromagnetism when the samples are annealed at an optimal temperature, typically about 280^°C. Such optimal annealing leads to an increase of Curie temperature, accompanied by an increase of both the conductivity and the saturation magnetization. A decrease of the coercive field and of magnetoresistivity is also observed for Ga_1–x Mn _x As annealed at optimal conditions. We suggest that the experimental results reported in this paper are related to changes in the domain structure of Ga_1–x Mn _x As.     

Influence of Codoping on the Magnetoresistance of Paramagnetic (Ga,Mn)As

We studied the correlation between magnetoresistance (MR) effects and the type of conducti- vity in paramagnetic Ga_1–x Mn _x As with x < 0.5%. Series of samples of (Ga,Mn)As with different codoping levels of Te have been prepared by metal-organic vapor-phase epitaxy (MOVPE). With increasing Mn-content from doping levels to x = 0.5% in paramagnetic Ga_1–x Mn _x As, the MR at 1.6 K changes continuously from positive to strongly negative. This manifests the complex interplay between p–d exchange induced valence band splitting and the metal–insulator transition. Codoping with Te leads eventually to a dominant conduction band transport. It is characterized by a small negative contribution at low magnetic fields to the parabolic MR similar to that found in highly n-doped diamagnetic semiconductors. It shows that the Mn-induced conduction band splitting is much smaller than the valence band splitting, i.e., |N₀| |N₀|.     

Observation of Combined Ferromagnetic/Paramagnetic Phase in Ga1?xMnxAs by Magnetic Circular Dichroism

Magnetic properties of a series of Ga_1–xMn_xAs layers with different Mn and hole concentrations has been studied by measuring magneto-absorption and magnetic circular dichroism (MCD). We first focus on comparing the MCD spectra of samples with unambiguous ferromagnetic or paramagnetic magnetization. We then investigate MCD in a sample with parameters between these two extremes, and interpret the observed behavior in terms of the coexistence of ferromagnetic domains and paramagnetic regions that may result, for example, from inhomogeneities in the sample caused, e.g., by compositional or doping fluctuations.     

Magnetic Behaviors of Ferromagnetic Semiconductor Zn1?xCrxTe Grown by MBE

The magnetic and structural properties of MBE-grown films of Zn_1–xCr_xTe were investigated. The magnetization versus magnetic field (M–H) measurement of Zn_1–xCr_xTe (x = 0.01–0.17) showed clear hysteresis loop at low temperatures. The ferromagnetic transition temperature (T_C) estimated from the Arrott-plot analysis increased almost linearly with the Cr composition (x) up to 275 K at x = 0.17. However, in the magnetization versus temperature (M–T) measurement, the irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) processes was observed. This is typically observed in the magnetic random system such as spin-glass or superparamagnetic phase. In the high resolution transmission microscopy (HRTEM) observations, structural defects such as stacking faults and polycrystalline-like structure were observed at high Cr compositions, whereas any apparent precipitates of different phases were not seen in all the range of Cr compositions examined. The correlation of the observed magnetic randomness with the local structural defects was discussed.     

Magnetic properties of (Dy1−xGdx)3Ga5O12 garnet single crystals

Magnetic properties of Dy_1–x Gd _x )₃Ga₅O₁₂ (DGGG) garnet single crystals were calculated using the Weiss molecular field theory and also measured using the vibrating sample magnetometer in the temperature range 4.2–40 K in the effective magnetic field from 0–7 T. The magnetic properties of DGGG single crystals are distributed between those of Dy₃Ga₅O₁₂ (DGG) and Gd₃Ga₅O₁₂ (GGG) single crystals, but are considerably closer to those of DGG. Based on the magnetic properties, the magnetic entropy change, S_M, was evaluated in the temperature range below 15 K. DGGG single crystals are a prospective material for magnetic refrigeration below 15 K.     

Giant Zeeman Effects and Spin Dynamics of Excitons in Dense Self-Organized Quantum Dots of CdSe and Cd1?xMnxSe

Giant Zeeman effects and spin dynamics of excitons are studied in dense self-organized quantum dots (QDs) of CdSe and Cd_1–xMn_xSe. Microphotoluminescence (PL) measurements for each individual dot reveal the typical dot diameter of 3.5 ± 0.2 nm and the density of 5000 m^–2 in the CdSe QDs. The exciton lifetime is shorter in smaller dots with higher energies, indicating energy transfer and tunneling processes among the dots. Circular polarization of excitonic PL is observed at 0 T with an opposite sign to that of the excited light and with the rise time of 50 ps. The CdSe QDs coupled with a Zn_1–xMn_xSe layer show the giant Zeeman shift of exciton, arising from overlapping of exciton wavefunctions in the dots with Mn ions. Spin polarization dynamics in the coupled QDs is also studied.     

Electron Magnetic Resonance,Neutron Diffraction and ac Susceptibility Study of CaMn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ru{<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> (x ≤ 0.40) Manganite System

Electron magnetic resonance (EMR), neutron powder diffraction (NPD) and ac susceptibility techniques were employed for studying the crystallographic structure and magnetic ordering in CaMn_1−xRu_xO₃ (x ≤ 0.40) manganite system. EMR measurements were done on polycrystalline samples at 120 ≤ T ≤ 500 K. High temperature EMR spectra of pristine antiferromagnetic (AFM) CaMnO₃ show a singlet Lorentzian-like line, whose intensity diminishes, zeroing at Neel temperature T _N=120 K. Strong broadening of paramagnetic (PM) lines with increase of Ru-content (Δ H_pp ∼ 1 T for x=0.10) was found. Upon cooling low-doped (x ≤ 0.06) samples remain AFM, whereas higher doped ones (0.10 ≤ x ≤ 0.40) clearly show progressive appearance of ferromagnetic (FM) phases. Thus, EMR evidences that Ru-doping modifies both PM and AFM states and creates an inhomogeneous phase separated FM and AFM ground states at x0.06. Complementary measurements of NPD and ac susceptibility corroborate the complex character of magnetic ordering, revealed by EMR. The changes of the magnetic ordering in CaMn_1−xRu_xO₃ supposed to be solely determined by doping of Mn-sites with Ru.     

Regular Arrays of (Zn,Mn)S Quantum Wires with Well-Defined Diameters in the Nanometer Range

Zn_1–x Mn _x S, with x varying between 0.01 and 0.30, were formed inside the ordered pore systems of different mesoporous SiO₂ matrices. Because of the highly ordered structure of the hosts, regular arrays of Zn_1–x Mn _x S quantum wires with lateral dimensions of 3 and 5.5 nm, respectively, separated by 2-nm SiO₂ barriers were obtained. The wires were characterized using photoluminescence (PL) and PL excitation (PLE) spectroscopy at liquid Helium temperatures. The PL of the wires is dominated by the ⁴T₁ ⁶A₁ internal transition of the Mn²⁺(3d⁵) ions. The corresponding PLE spectra show higher internal Mn transitions as well as the band to band transition. The energies of the internal Mn transitions are typical for Mn²⁺ on a cation site of (II,Mn)VI semiconductors. Because of the comparable bandgaps of the SiO₂ and the Zn_1–x Mn_xS as well as the small exciton Bohr radius in (Zn,Mn)S quantum confinement effects in the wires are less than about 150 meV.