Formation of room-temperature ferromagnetic Zn1−xCoxO nanocrystals

Optical and magnetic properties of Co²⁺-doped ZnO nanocrystals were studied. Optical measurements confirm the incorporation of Co²⁺ in ZnO lattice with tetrahedral geometry. Optical absorption spectra also reveal the partial bleaching of the excitonic feature attributable to an increase in electron concentration. Magnetization measurements indicate the ferromagnetic ordering in Co²⁺-doped ZnO nanocrystals with saturation magnetization . No structural changes were observed in lightly doped ZnO nanocrystals. The present investigations are important in obtaining the ferromagnetic Zn_1−xCo_xO nanocrystals.     

Magnetic and magnetocaloric properties of Y bMn6Sn6−xInx

We have synthesized three Y bMn₆Sn_6−xIn_x representatives (x=0.45, 0.80, 1.20), the first pseudo-ternary RMn₆Sn_6−xX′_x compounds involving a divalent R metal. The crystal structure is found to evolve with the In concentration without modification of the Yb valency: Y bMn₆Sn_5.55In_0.45 is isotypic with HoFe₆Sn₆ (Immm) while Y bMn₆Sn_5.20In_0.80 and Y bMn₆Sn_4.80In_1.20 crystallize in the TbFe₆Sn₆-type (Cmcm). The In content is also determining as regards the magnetic and magnetocaloric properties: Y bMn₆Sn_5.55In_0.45 () almost behaves like a simple ferromagnet while Y bMn₆Sn_5.20In_0.80 and Y bMn₆Sn_4.80In_1.20 also order ferromagnetically but at significantly lower temperatures ( and 129 K, respectively) and are further characterized by the interference of low temperature antiferromagnetic interactions. The results are discussed and compared to previously published data.     

Structural, electrical, optical and magnetic properties of Co0.2AlxZn0.8−xO films

Co_0.2Al_xZn_0.8−xO films prepared with different molar ratio of aluminum nitrate to zinc acetate were deposited on substrates by the sol-gel technique. X-ray diffraction, photoluminescence and ferromagnetism measurements were used to characterize the Co_0.2Al_xZn_0.8−xO diluted magnetic semiconductors. The authors found that the intensity of the acceptor-related photoluminescence increased with increasing aluminum concentration and an increase in the number of the acceptor-like defects (zinc vacancies especially) in the Co_0.2Al_xZn_0.8−xO film might lead to the enhancement of the magnetic properties. This implies that controls of the aluminum concentration and the number of the acceptor-like defects are important factors to produce strong ferromagnetism Co_0.2Al_xZn_0.8−xO films prepared by the sol-gel method.     

Spin polarization of Fe-rich ferromagnetic compounds in Ru2−xFexCrSi Heusler alloys

We report spin polarization P of Ru_2−xFe_xCrSi Heusler alloys by the Andreev reflection technique. Ru_2−xFe_xCrSi with L2₁-type structure and saturation magnetic moment of per formula unit is theoretically predicted to be half-metals in the wide range of the composition x. We had clarified that the experimental results of saturation magnetic moment in Fe-rich compounds had coincided with the theoretical prediction. Therefore, we have measured the differential conductance of Ru_2−xFe_xCrSi/Pb planar-type junctions. The P value of Ru_2−xFe_xCrSi was determined by fitting the differential conductance with the modified Blonder-Tinkham-Klapwijk theory. We have found that the behavior of P for Ru_2−xFe_xCrSi was independent of the composition x in the Fe-rich region; P=0.53 for both of x=1.5 and 1.7. The spin polarization is the similar value to Co-based Heusler alloys.     

Surface defect states of CdTexS1 − x quantum dots

Properties of surface defect states of CdTe_xS_1 − x quantum dots with an average diameter of 7 nm are investigated experimentally. The stoichiometric ratio is found to be for by use of the energy dispersive analysis of x-ray. The photoluminescence spectrum, the photoluminescence excitation spectrum, and the surface passivation are adopted to characterize the properties of surface defect states. The energy levels of surface defect states of CdTe_xS_1 − x quantum dots are also determined.     

Quadratic temperature dependence of electron-phonon scattering in disordered V1−xPdx alloys

We report the results of a comprehensive study of weak localization and electron-electron interaction effects in disordered V_1−xPd_x alloys whose compositions are close to the (low T_c) A15 V₃Pd compound. Magnetoresistivity and zero field resistivity have been measured within the temperature range 1.5≤T≤300 K. The low-temperature resistivity obeys a law, which is explained by electron-electron interaction. We have determined the electron-phonon scattering time (τ_e-ph) for V_1−xPd_x alloys. Our results indicate an anomalous electron-phonon scattering rate obeying quadratic temperature dependence. This observation is interpreted by the existing theories of electron-phonon interactions.     

Magnetocaloric properties of DyCo2−xGax alloys

The influence of the substitution of Ga atoms for Co atoms in DyCo₂ compounds on magnetocaloric properties has been investigated. A series of DyCo_2−xGa_x alloys with x=0, 0.03, 0.06, 0.1, 0.15, and 0.2 was prepared by the arc-melting method for this investigation. Experimental results revealed that the Ga substitution for Co in DyCo₂ can form a single phase with the cubic Laves phase structure up to x=0.2. As the Ga content x increases, the lattice parameter and the Curie temperature T_c increases from 143 to 196 K linearly. The maximum magnetic entropy changes in a low field change of 0-1.5 T, increasing from 8.24 to 10.61 J/K kg when the Ga content x increases from 0 to 0.03, but decreasing gradually to 3.51 J/K kg as the Ga content further increases to x=0.2. All the samples show a relatively large magnetic entropy change with very small hysteresis loss.     

STM spectroscopy on superconducting FeSe1−xTex single crystals at 300 mK

We report cryogenic scanning tunneling spectroscopy measurements on single crystals of superconducting FeSe_1−xTe_x, at doping levels of x=0.5 and 0.7, with critical temperatures . Atomically resolved topographic images were obtained, showing large-scale density-of-state clustering which appears to have no periodicity and to vary with the doping. Conductance spectra taken at 300 mK showed a generally asymmetric V-shaped background, along with a sharp dip structure within . These spectra appeared to vary over ∼nm length scale, and not correlated with the topography. The overall spectral evolution versus temperature is consistent with the dip structure arising from a superconducting energy gap which closes above T_c, and with the spectral background having a non-superconducting origin. The persistence of finite zero-bias conductance down to 300 mK, well below T_c, indicates the presence of low-energy quasiparticles on parts of the Fermi surface. We discuss our data in light of some other recent spectroscopic measurements of FeSe_1−xTe_x, and in terms of its characteristic band structure.     

Ordered zinc-vacancy induced Zn0.75Ox nanophase structure

Using transmission electron microscopy, a new nano-phase structure of Zn_0.75O_x induced by Zn-vacancy has been discovered to grow on wurtzite ZnO nanobelts. The superstructure grows epitaxial from the surface of the wurtzite ZnO nanobelts and can be fitted as an orthorhombic structure, with lattice parameters a′=2a, and c′=c, where a and c are the lattice parameters of ZnO. The superstructured phase is resulted from high-density Zn vacancies orderly distributed in the ZnO matrix. This study provides direct observation about the existence of Zn-vacancies in ZnO.     

A Hall coefficient investigation of ferromagnetic Ga1−xMnxAs layers on (100) GaAs substrates

Ferromagnetic Ga_1−xMn_xAs epilayers with Mn mole fraction in the range of x≈2.2-4.4% were grown on semi-insulating (100) GaAs substrates using the molecular beam epitaxy technique. The transport properties of these epilayers were investigated through Hall effect measurements. The measured hole concentration of Ga_1−xMn_xAs layers varied from 4.4×10¹⁹ to 3.4×10¹⁹ cm⁻³ in the range of x≈2.2-4.4% at room temperature. From temperature dependent resisitivity data, the sample with x≈4.4% shows typical behavior for insulator Ga_1−xMn_xAs and the samples with x≈2.2 and 3.7% show typical behavior for metallic Ga_1−xMn_xAs. The Hall coefficient for the samples with x≈2.2 and 4.4% was fitted assuming a magnetic susceptibility given by Curie-Weiss law in a paramagnetic region. This model provides good fits to the measured data up to and the Curie temperature T_c was estimated to be 65, 83 K and hole concentration p was estimated to be 5.1×10¹⁹, 4.6×10¹⁹ cm⁻³ for the samples with x≈2.2 and 4.4%, respectively, confirming the existence of an anomalous Hall effect for metallic and insulating samples.     

Structure, magnetic, and transport properties of the Co-doped manganites La0.9Te0.1Mn1−xCoxO3 (0≤x≤0.25)

The effect of Co doping at Mn-site on the structural, magnetic and electrical transport properties in electron-doped manganties La_0.9Te_0.1Mn_1−xCoxO₃ (0≤x≤0.25) has been investigated. The room temperature structural transition from rhombohedra to orthorhombic (Pbnm) symmetry is found in these samples with x≥0.20 by the Rietveld refinement of X-ray powder diffraction patterns. All samples undergo the paramagnetic-ferromagnetic (PM-FM) phase transition. The Curie temperature T_C of these samples decreases and the transition becomes broader with increasing Co-doping level. The magnetization magnitude of Co-doping samples increases at low temperatures with increasing Co-doping level for x≤0.15 and decreases with increasing Co-doping content further. The metal-insulator (M-I) transitions observed in the sample with x=0 are completely suppressed with Co doping, and the resistivity displays semiconducting behavior within the measured temperature region for these samples with x>0. All results are discussed according to the changes of the structure parameters and magnetic exchange interaction caused by Co-doping. In addition, the different effects between the Co doping and Cu doping in the Mn site for the electron-doped manganites are also discussed.     

Observation of optical properties related to room-temperature ferromagnetism in co-sputtered Zn1−xCoxO thin films

We report on the analysis of optical transmittance spectra and the resulting ferromagnetic characteristics of sputtered Zn_1−xCo_xO films. Zn_1−xCo_xO films were prepared on (0001)-oriented Al₂O₃ substrates by the radio-frequency (rf) magnetron co-sputtering method. The XRD results showed that the crystallinity of films was properly maintained up to x=0.30 and no second phase peaks were detected up to x=0.40. The transmittance spectra showed both the increase of the absorption band intensity and the red shift of the absorption peak as well as the band edge with increasing x. We have proved experimentally that these changes depend on Co concentration. These optical properties suggest that sp-d exchange interactions and typical d-d transitions become activated with increasing x, which leads to the enhancement of ferromagnetic properties in Zn_1−xCo_xO films as shown in the AGM results. Therefore, it is concluded that the ferromagnetism derives from the substitution of Co²⁺ for Zn²⁺ without changing the wurtzite structure.     

Optical properties of bulk Zn1−xCoxO magnetic semiconductors

Polycrystalline Zn_1−xCo_xO (x=0, 0.02, 0.05, 0.10 and 0.15) oxides have been synthesized by solid state reaction via sintering ZnO and Co powders in open air. X-ray diffraction analyses using Rietveld refinement indicate that a stoichiometric single phase with a wurtzite-like structure was found in Zn_1−xCo_xO samples with x up to 0.10. The elemental mapping using energy dispersive X-ray spectroscopic analyses presents a uniform distribution of Co. Optical transmittance measurements show that several extra absorption bands appear in the Co-doped ZnO, which is due to the transitions between the crystal-field-split 3d levels of tetrahedral Co²⁺ substituting Zn²⁺ ions. Raman measurements show that limited host lattice defects are induced by Co doping. Magnetization measurements reveal that the Co-doped ZnO samples are paramagnetic due to the absence of free carriers and in low temperature the dominant magnetic interaction is nearest-neighbor antiferromagnetic.     

Specific heat of (Ca1−xSrx)3Ru2O7 single crystals

We have measured the specific heat of crystals of (Ca_1−xSr_x)₃Ru₂O₇ using ac- and relaxation-time calorimetry. Special emphasis was placed on the characterization of the Néel () and structural () phase transitions in the pure, x=0 material. While the latter is believed to be first order, detailed measurements under different experimental conditions suggest that all the latent heat (with L∼0.3R) is being captured in a broadened peak in the effective heat capacity. The specific heat has a mean-field-like step at T_N, but its magnitude () is too large to be associated with a conventional itinerant electron (e.g. spin-density-wave) antiferromagnetic transition, while its entropy is too small to be associated with the full ordering of localized spins. The T_N transition broadens with Sr substitution while its magnitude decreases slowly. On the other hand, the entropy change associated with the T_c transition decreases rapidly with Sr substitution, and is not observable for our x=0.58 sample.     

Low temperature magnetization and magnetoresistance studies on the layered manganite system La1.2Sr1.8Mn2−xRuxO7 (x=0, 0.1, 0.5, 1.0)

We report the detailed results of magnetization and magnetoresistance measurements in the Ru doped layered manganite system La_1.2Sr_1.8Mn_2−xRu_xO₇ (x=0, 0.1, 0.5, 1.0). High-resolution measurements of magnetization and magnetoresistance were carried out as functions of temperature, magnetic field and time. We find evidence for the existence of competing ferromagnetic and antiferromagnetic interactions resulting in the formation of a frustrated spin-glass-like state at low temperatures. The time dependent magnetization follows the relation very well. We find that Ru doping enhances the coercive field and drives the system towards a magnetically mixed phase at low temperatures. Large negative magnetoresistance values are observed in all samples and at low temperatures the magnetoresistance varies as the square root of the applied magnetic field.     

Physical properties of BaMg1/3Nb2/3O3-BaCo1/3Nb2/3O3 solid solutions

Structural, electric and magnetic properties of Ba₃Mg_1−xCo_xNb₂O₉ based dielectric ceramic compounds have been studied. The samples, prepared by a solid state reaction method, were characterised by X-ray powder diffraction (XRPD), electron microscopy (SEM), dielectric (ε(T)) and magnetic measurements (χ⁻¹(T)). The XRPD analyses showed that the crystal structure of these compounds does change by the increase of substitution degree, passing from a superstructure hexagonal-type, (no. 164), space group (SG) to a simple structure cubic-type, (no. 221), SG. However, the evolution of the elementary unit cell lattice parameter can be followed and it exhibit a linear increasing tendency with increase in the substitution, indicating the existence of a solid solution through out the investigated range of substitution (0-1). The microstructure analysis shows a variation in the grain size and also the porosity of the samples with the degree of substitution. The results are in good agreement with that of dielectric measurements, which also showed that the dielectric constant (ε) increases with the increase of cobalt content. The magnetic characterization of cobalt substituted samples showed an antiferromagnetic type super-exchange interaction between these magnetic ions. At the same time, the values of effective magnetic momentum (μ_eff) are close to the value that corresponds to Co²⁺ free ions. The study highlights the possibility of modelling these materials by substitutions, in order to improve properties of negative-positive-zero (NPO) type dielectric applications.     

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.     

Magnetic phase transitions of antiperovskite Mn3−xFexSnC (0.5≤x≤1.3)

The magnetization and electrical resistivity of Mn_3−xFe_xSnC (0.5≤x≤1.3) were measured to investigate the behavior of the complicated magnetic phase transitions and electronic transport properties from 5 to 300 K. The results obtained demonstrate that Fe doping at the Mn sites of Mn₃SnC induces a more complicated magnetic phase transition than that in its parent phase Mn₃SnC from a paramagnetic (PM) state to a ferrimagnetic (FI) state consisting of antiferromagnetic (AFM) and ferromagnetic (FM) components, while, with the change of Fe-doped content and magnetic field, there is a competition between the AFM component and FM component in the FI state. Both the Curie temperature (T_C) and the saturated magnetization M_s increase with increasing x. The FM component region becomes broader with further increasing Fe-doped content x. The external magnetic field easily creates a saturated FM state (and increased T_C) when . Fe doping quenches the negative thermal expansion (NTE) behavior from 200 to 250 K reported in Mn₃SnC.