Magnetic behavior of MnAs precipitates in Ga1−xMnxAs diluted magnetic semiconductor

Ferromagnetic Ga_1−xMn_xAs layers (where x≈4.7–5.5%) were grown on (1 0 0) GaAs substrates by molecular beam epitaxy. These p-type (Ga,Mn)As films were revealed to have a ferromagnetic structure and ferromagnetism is observed up to a Curie temperature of 318 K, which is ascribed to the presence of MnAs secondary magnetic phases within the film. It is highly likely that the phase segregation occurs due to the high Mn cell temperature around 890–920 °C, as it is well established that GaMnAs is unstable at such a high temperature. The MnAs precipitate in the samples with x≈4.7–5.5% has a Curie temperature T_c≈318 K, which was characterized from field-cooled and zero-field-cooled magnetization curves.     

Structural and magnetic properties of ferromagnetic In1−xMnxAs1−yPy layers

The structural and magnetic properties of epitaxial In_1−xMn_xAs_1−yP_y quaternary layers with Mn content ranging from 0.01 to 0.04 and phosphorous content ranging from 0.11 to 0.21 were studied. X-ray diffraction indicated that the films were two phase consisting of an InMnAsP solid solution and hexagonal MnAs nanoprecipitates. Addition of phosphorus promoted precipitate formation. Films were ferromagnetic showing hysteretic behavior in the field dependence of magnetization at 5 and 298 K. From field-cooled magnetization measurements ferromagnetic transitions were observed at 280 and 325 K. The zero field-cooled magnetization versus temperature measurements showed irreversibility for T<300 K that was attributed to the presence of MnAs nanoprecipitates. The calculated coercivity using the Neel model was 1380 G compared to the experimental value of 380 G at 5 K. The difference was attributed to a strong inter-cluster exchange that stabilizes the ferromagnetic state.     

Magnetic behavior of the II-V-diluted magnetic semiconductor Cd1−xMnxSb

Polycrystalline samples of the II-V-diluted magnetic semiconductor Cd_1−xMn_xSb (x=0.05-0.20) were synthesized. Standard high temperature ceramic methods under an inert atmosphere were utilized for sample fabrication. Structural characterization was done using X-ray diffractometry (XRD), which indicated that a simple substitution of Mn for Cd is probably not occurring. Hysteresis, ac susceptibility, dc magnetization, and spontaneous magnetization measurements were performed for Cd_0.90Mn_0.10Sb. The hysteresis data indicated the presence of a ferromagnetic component. Ferromagnetism in the Cd_0.90Mn_0.10Sb system is likely due to two sources: Mn spins in small Mn-rich regions and a small amount of MnSb in a minority phase. Analysis of the spontaneous magnetization as a function of temperature for Cd_0.90Mn_0.10Sb yielded the value 0.172 for the critical exponent β. In MnSb, β was found to have the value 0.379, which is close to the theoretical value for 3D-Heisenberg systems. Thus, in Cd_0.90Mn_0.10Sb, the ferromagnetism is not of the 3D-Heisenberg type; rather, it is closer to 2D Ising behavior, indicating reduced effective dimensionality.     

Analysis of Raman scattering of Ga1−xMnxAs dilute magnetic semiconductor

Ferromagnetic Ga_1−xMn_xAs layers (where x=1.4-3.0%) grown on (1 0 0) GaAs substrates by molecular beam epitaxy were characterized using Raman spectroscopy. As Mn is introduced into GaAs, a marked increase in disorder in the material occurs, as indicated by the growth of the disorder-allowed transverse-optical Raman line. Another important result is that as the Mn concentration in Ga_1−xMn_xAs increases further beyond ca. 2%, Raman-active coupled-plasmon-longitudinal-optical phonon modes arise, which signals the increasing presence of holes, and thus provides a useful tool for determining their concentration. Using the depletion-layer approach from the Raman spectroscopy data, we determined the carrier concentration for samples with x=2.2% and 3.0% was to be 7.2×10¹⁹ and 8.3×10²⁰ cm⁻³, respectively.     

First-principles study of structural, electronic and magnetic properties in Cd1−xFexS diluted magnetic semiconductors

In this paper, we report theoretical investigations of structural, electronic and magnetic properties of ordered dilute ferromagnetic semiconductors Cd_1−xFe_xS with x=0.25, 0.5 and 0.75 in zinc blende (B3) phase using all-electron full-potential linear muffin tin orbital (FP-LMTO) calculations within the density functional theory and the generalized gradient approximation. The analysis of band structures, density of states, total energy, exchange interactions and magnetic moments reveals that both the alloys may exhibit a half-metallic ferromagnetism character. The value of calculated magnetic moment per Fe impurity atom is found to be 4 μ_B. Moreover, we found that p-d hybridization reduces the local magnetic moment of Fe from its free space charge value of 4 μ_B and produces small local magnetic moments on Cd and S sites.     

Spin-polarized structural, electronic and magnetic properties of diluted magnetic semiconductors Cd1−xMnxTe in zinc blende phase

We have investigated the structural, electronic and magnetic properties of the diluted magnetic semiconductor (DMS) Cd_1−xMn_xTe (for x=0.75 and 1.0) in the zinc blende (B3) phase by employing the ab-initio method. Calculations were performed by using the full potential linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method within the frame work of spin-polarized density functional theory (SP-DFT). The electronic exchange-correlation energy is described by generalized gradient approximation (GGA). We have calculated the lattice parameters, bulk modulii and the first pressure derivatives of the bulk modulii, spin-polarized band structures, and total and local densities of states. We estimated the spin-exchange splitting energies Δ_x(d) and Δ_x(pd) produced by the Mn3d states, and we found that the effective potential for the minority spin is more attractive than that of the majority spin. We determine the s-d exchange constant N₀α (conduction band) and p-d exchange constant N₀β (valence band) and these somewhat agree with a typical magneto-optical experiment. The value of calculated magnetic moment per Mn impurity atom is found to be 4.08 μ_B for Cd_0.25Mn_0.75Te and 4.09 μ_B for Cd_0.0Mn_1.0Te. Moreover, we found that p-d hybridization reduces the local magnetic moment of Mn from its free space charge value of 5.0 μ_B and produces small local magnetic moments on the nonmagnetic Cd and Te sites.     

Magnetic and transport properties in Sr1−xLaxFe1−xMnxO3

The magnetic and transport properties in the perovskite Sr_1−xLa_xFe_1−xMn_xO₃ have been explored. As x rises, the systemic ferromagnetism increases gradually and cluster-spin-glass state occurs in the low-temperature region. For 0.3?x?0.7, the ferromagnetic phase separation from the paramagnetic phase was observed from the results of electron-spin-resonance measurement. Although all samples show a semiconducting behavior, their transport properties are dominated by two different mechanisms, namely, the electronic transport of x?0.5 samples is realized by thermal activation but the variable-range hopping is applied in x?0.7 ones. The different transport mechanism can be understood from the Mn/Fe ions interaction.     

Structural, thermal and magnetic properties of Ni1−xMnxFe2O4 nanoferrites

In this paper, the structural, thermal and magnetic properties of Ni_1−xMn_xFe₂O₄ are presented. It is observed that high concentration of Mn²⁺ ions into NiFe₂O₄ tends to reduce the particle size. Calcination at 500 °C has resulted in the growth of Ni_1−xMn_xFe₂O₄ nanoparticles, but the calcination at 900 °C has led to the evaporation of the majorities of the polyvinyl alcohol. After calcination at 900 °C, crystallographically oriented NiMnFe₂O₄ nanoparticles are formed. These Ni_1−xMn_xFe₂O₄ nanoparticles show hysteresis behaviour upon magnetization. On the other hand, saturation magnetization (Ms) values decreases with increasing Mn content in ferrite due to the influence of Mn²⁺ ion in the sub lattice.     

Structural and magnetic studies on mechanosynthesized BaFe12−xMnxO19

Barium hexaferrite powders with manganese substitution were prepared by mechanosynthesis. The structural and magnetic properties were characterized by X-ray diffractometer and vibration sample magnetometer, respectively. XRD patterns were refined by Rietveld method. Preferential site occupation of manganese ion was investigated by room temperature (RT) Mössbauer measurements. XRD results showed a single-phase barium hexaferrite with some residual hematite. Crystallite size was observed to decrease with substitution amount. Lower saturation magnetization and increased coercivity is observed in substituted samples. RT Mössbauer measurements showed that manganese ions preferentially occupy 12k, 4f₂, and 2a sites.     

Spin-dependent shot noise in diluted magnetic semiconductor/semiconductor heterostructures with a nonmagnetic barrier

We investigate quantum size effect on the spin-dependent shot noise in the diluted magnetic semiconductor (DMS)/semiconductor heterostructure with a nonmagnetic semiconductor (NMS) barrier in the presence of external magnetic and electric fields. The results demonstrate that the NMS barrier plays a quite different role from the DMS layer in the electron transport process. It is found that spin-down shot noise shows relatively regular oscillations as the width of DMS layer increases, while the spin-up shot noise deceases monotonically. However, as the width of NMS layer increases, the spin-down shot noise displays irregular oscillations at first and then decreases while the spin-up shot noise decreases at a quite different rate. The results indicate that the shot noise can be used as a sensitive probe in detecting material type and its size.     

Structural and magnetic properties of Mg(In2−xMnx)O4 system

We synthesized the Mn-doped Mg(In_2−xMn_x)O₄ oxides with 0.03?x?0.55 using a solid-state reaction method. The X-ray diffraction patterns of the samples were in a good agreement with that of a distorted orthorhombic spinel phase. Their lattice parameters and unit-cell volumes decrease with x due to the substitution of the smaller Mn³⁺ ions to the larger In³⁺ ions. The undoped MgIn₂O₄ oxide presents diamagnetic signals for 5 K?T?300 K. The M(H) at T=300 K reveals a fairly negative-sloped linear relationship. Neither magnetic hysteresis nor saturation behavior was observed in this parent sample. For the Mn-doped samples, however, positive magnetization were observed between 5 and 300 K even if the x value is as low as 0.03. The mass susceptibility enhances with Mn content and it reaches the highest value of 1.4×10⁻³ emu/g Oe (at T=300 K) at x=0.45. Furthermore, the Mn-doped oxides with x=0.06 and 0.2, respectively, exhibit nonlinear magnetization curves and small hysteretic loops in low magnetic fields. Susceptibilities of the Mn-doped samples are much higher than those of MnO₂, Mn₂O₃ oxides, and Mn metals. These results show that the oxides have potential to be magnetic semiconductors.     

Tailoring the morphology of CoxPt1−x magnetic nanostructures

Co_xPt_1−x nanostructures with varying composition and controllable morphologies have been synthesized through the thermal decomposition of appropriate platinum and cobalt precursors in organic solvents. The employment of several different surfactants facilitated the production of nanostructures with various sizes and shapes including nanowires, flower-like structures and spherical particles. The composition of the as-prepared nanomaterials ranged between Pt-rich and stoichiometric CoPt alloy, mainly depending on the starting ratio of the precursors. Three-dimensional structures such as the ‘flower-like’ ones showed a net ferromagnetic behavior, even at room temperature. In certain cases, the alloy nanostructures were annealed in order to obtain the ‘hard’ fct-CoPt phase, which displayed high coercivity values.     

Structural, chemical and magnetic investigations of polycrystalline Zn1−xMnxO

Present study reports the detailed structural and magnetic, as well as chemical analysis of polycrystalline Zn_1−xMn_xO (where x=0, 0.005, 0.01, 0.03, 0.05 and 0.1) samples synthesized by the high-temperature solid state reaction route. X-ray diffraction studies reveal the presence of secondary phase for higher Mn-doping concentrations (x≥0.03). Secondary phase formation having spinel structure is confirmed and reported as an evidence for the first time using transmission electron microscopy study. Chemical investigations using X-ray photoelectron spectroscopy showed the presence of Mn in two valence states. From the observed results we are of the opinion that Zn²⁺ ions, mainly present at or near grain boundaries, diffuse into manganese oxide to form a stable spinel phase Zn_xMn_3−xO₄ at or near the grain boundaries of ZnO/Zn_1−xMn_xO. Magnetization measurements did not show any magnetic transition down to 5 K.     

Magnetism on Mg1−xZnxCyNi3

The magnetic phase diagram for Mg_1−xZn_xC_yNi₃ has been tentatively constructed based on magnetization and muon spin relaxation (μSR) measurements. The superconducting phase was observed to fade as x (y) increases (decreases). The low y samples show early stages of long-range ferromagnetism, or complete long-range ferromagnetism. In the phase diagram, the ferromagnetic phase exists in addition to the superconducting phase, suggesting that there is some correlation between superconductivity and ferromagnetism, even though the coexistence of ferromagnetism and superconductivity is not observed from the μSR measurements down to 20 mK for the superconducting sample (T_c=2.5 K, (x, y)=(0, 0.9)).     

Structural, optical and magnetic properties of (ZnO)1−x(MnO2)x thin films deposited at room temperature

The structural, magnetic and optical properties of (ZnO)_1−x(MnO₂)_x (with x = 0.03 and 0.05) thin films deposited by pulsed laser deposition (PLD) were studied. The pellets used as target, sintered at different temperatures ranging from 500 °C to 900 °C, were prepared by conventional solid state method using ZnO and MnO₂ powders. The observation of non-monotonic shift in peak position of most preferred (1 0 1) ZnO diffraction plane in XRD spectra of pellets confirmed the substitution of Mn ions in ZnO lattice of the sintered targets. The as-deposited thin film samples are found to be polycrystalline with the preferred orientation mostly along (1 1 0) diffraction plane. The UV-vis spectroscopy of the thin films revealed that the energy band gap exhibit blue shift with increasing Mn content which could be attributed to Burstein-Moss shift caused by Mn doping of the ZnO. The deposited thin films exhibit room temperature ferromagnetism having effective magnetic moment per Mn atom in the range of 0.9-1.4μ_B for both compositions.     

Optical and electronic properties of nano-Cd1−xMnxTe alloy

Cd_1−xMn_xTe thin films were fabricated by thermal interdiffusion of multilayers of sputtered compound semiconductors as well as thermally evaporated elements. Electron microscopy revealed their nanostructures. The alloys have been investigated for evaluation of optical and electronic parameters. Spectrophotometry helped to find out the bandgap and composition; photoluminescence was used for observing relative transition probabilities at room temperature. Photoresponse showed the light dependence of the resistance of the alloy films. Hall measurements and four-probe tests indicated the influence of manganese on the room-temperature electronic properties of the alloy.     

Synthesis and magnetic properties of NiFe2−xAlxO4 nanoparticles

Nanocrystalline Al-doped nickel ferrite powders have been synthesized by sol–gel auto-ignition method and the effect of non-magnetic aluminum content on the structural and magnetic properties has been studied. The X-ray diffraction (XRD) revealed that the powders obtained are single phase with inverse spinel structure. The calculated grain sizes from XRD data have been verified using transmission electron microscopy (TEM). TEM photographs show that the powders consist of nanometer-sized grains. It was observed that the characteristic grain size decreases from 29 to 6 nm as the non-magnetic Al content increases, which was attributed to the influence of non-magnetic Al concentration on the grain size. Magnetic hysteresis loops were measured at room temperature with a maximum applied magnetic field of ≈1 T. As aluminum content increases, the measured magnetic hysteresis curves become more and more narrow and the saturation magnetization and remanent magnetization both decreased. The reduction of magnetization compared to bulk is a consequence of spin non-collinearity. Further reduction of magnetization with increase of aluminum content is caused by non-magnetic Al³⁺ ions and weakened interaction between sublattices. This, as well as the decrease in hysteresis was understood in terms of the decrease in particle size.     

Optical investigation of ferromagnetic Ga1−xMnxN layers grown on sapphire (0 0 0 1) substrates

We have investigated the temperature-dependent photoluminescence (PL) spectra in Ga_1−xMn_xN layers (where x≈0.1–0.8%) grown on sapphire (0 0 0 1) substrates using the plasma-enhanced molecular beam epitaxy technique. All the layers doped with manganese exhibited n-type conductivity with Curie temperature over 350 K. The efficient PL are peaked in the red (1.86 eV), yellow (2.34 eV), and blue (3.29 eV) spectral range. It was found that the blue band at 3.29 eV is mostly associated with the formation complexes between donors (e.g., N vacancy) and Mn acceptors, which results in forming donor levels at 0.23 eV below the conduction band edge. The yellow band is attributed to intrinsic gallium defects. The broad band at 1.86 eV is attributed to inner ⁵D state transition (T₂ to E) of Mn ions.     

Oxygen vacancy doping effect on the electrical and magnetic behavior of Ba5−xLaxNb4−xTixO15

We report electric and magnetic properties of oxygen deficient Ba_5−xLa_xNb_4−xTi_xO_15−δ phases, which have been prepared by solid-state reaction method followed by a controlled reduction process under hydrogen atmosphere. The extra electrons added by the formation of the oxygen vacancies (δ) introduce localized spins and the magnetic susceptibility can be described by a temperature-independent contribution and a Curie-Weiss term associated to the Ti³⁺ ion formation. Besides, the experimental resistivity (ρ) data of these four reduced compounds are well described in a wide temperature range with the equation , which suggests the presence of small polarons in the system. Although, all samples present electrical insulating behavior, the electrical resistivity decreases four orders of magnitude for intermediate x values. We interpreted this fact as a consequence of the mix between the localized bands of the Nb and Ti ions, which favors the promotion of carriers due to reduction of the band gap.