Investigation of magneto-anisotropy field for system of particles with near superparamagnetic volume

We obtained the temperature dependence for low-field boundary of the anisotropy field distribution in a system of barium hexaferrite nanocrystals in the temperature range from 300 to 700 K. We treated the experimental data taking into account the influence of thermal fluctuations on the anisotropy field and the transition of particles into the paramagnetic state, stimulated by external magnetic field. We showed that the dependence under consideration is formed by particles of different volume, which increased from 3.5×10⁻¹⁸ to 40×10⁻¹⁸ cm³ while the particles lost their magnetic stability with the temperature growth.     

Electronic and magnetic properties and their response to pressure in Ni2MnB

The electronic structure and magnetic properties of Ni₂MnB upon pressure up to 20 GPa have been studied by using the density functional theory (DFT) method. The results indicate that ferromagnetic ordered Ni₂MnB in L2₁ structure is more stable than the nonmagnetic one. The magnetic moments of Ni and Mn atoms as well as the total magnetic moment of Ni₂MnB are found to decrease weakly with increasing pressure. The pressure derivative of the total magnetic moment is −3.07×10⁻³ GPa⁻¹. The equilibrium bulk modulus and its derivative from the Murnaghan equation of state (EOS) are B₀=247.7 GPa, B′=4.98.     

Characterizations of n-type ferromagnetic GaMnN thin film grown on GaN/Al2O3 (0 0 0 1) by metal-organic chemical vapor deposition

A high-quality ferromagnetic GaMnN (Mn=2.8 at%) film was deposited onto a GaN buffer/Al₂O₃(0 0 0 1) at 885 °C using the metal-organic chemical vapor deposition (MOCVD) process. The GaMnN film shows a highly c-axis-oriented hexagonal wurtzite structure, implying that Mn doping into GaN does not influence the crystallinity of the film. No Mn-related secondary phases were found in the GaMnN film by means of a high-flux X-ray diffraction analysis. The composition profiles of Ga, Mn, and N maintain nearly constant levels in depth profiles of the GaMnN film. The binding energy peak of the Mn 2p_3/2 orbital was observed at 642.3 eV corresponding to the Mn (III) oxidation state of MnN. The presence of metallic Mn clusters (binding energy: 640.9 eV) in the GaMnN film was excluded. A broad yellow emission around 2.2 eV as well as a relatively weak near-band-edge emission at 3.39 eV was observed in a Mn-doped GaN film, while the undoped GaN film only shows a near-band-edge emission at 3.37 eV. The Mn-doped GaN film showed n-type semiconducting characteristics; the electron carrier concentration was 1.2×10²¹/cm³ and the resistivity was 3.9×10⁻³ Ω cm. Ferromagnetic hysteresis loops were observed at 300 K with a magnetic field parallel and perpendicular to the ab plane. The zero-field-cooled and field-cooled curves at temperatures ranging from 10 to 350 K strongly indicate that the GaMnN film is ferromagnetic at least up to 350 K. A coercive field of 250 Oe and effective magnetic moment of 0.0003 μ_B/Mn were obtained. The n-type semiconducting behavior plays a role in inducing ferromagnetism in the GaMnN film, and the observed ferromagnetism is appropriately explained by a double exchange mechanism.     

Removal of chloridazon by natural and ammonium kerolite samples

The adsorption of chloridazon (5-amino-4-chloro-2-phenylpyridazin-3(2H)-one) on natural and ammonium kerolite samples from aqueous solution at 10, 25 and 40 °C has been studied by using batch experiments. The experimental data points were fitted to the Langmuir equation in order to calculate the adsorption capacities (X_m) of the samples; two straight lines were obtained, which indicates that the adsorption process takes place in two different stages. Values for X_m1 (first stage) ranged from 1.1 × 10⁻² mol kg⁻¹ for natural kerolite at 40 °C up to 5.1 × 10⁻² mol kg⁻¹ for ammonium kerolite at 10 °C and the values for X_m2 (second stage) ranged from 9.1 × 10⁻² mol kg⁻¹ for natural kerolite at 40 °C up to 14 × 10⁻² mol kg⁻¹ for natural kerolite at 10 °C. The adsorption experiments showed on the one hand, that the ammonium kerolite is more effective than natural kerolite to adsorb chloridazon in the range of temperature studied and on the other hand, that the lower temperature, the more effective the adsorption of chloridazon on the adsorbents studied.     

Direct observation of room temperature magnetism in (In,Mn)As thin films by magnetic force microscopy

Variable-temperature magnetic force microscopy (MFM) has been performed over the temperature range of 298-348 K on ferromagnetic (In,Mn)As thin films deposited by metal-organic vapor phase epitaxy (MOVPE). Ferromagnetic domains were observed with submicron resolution in both single and two phase (In,Mn)As films, persisting up to 328 K. Isolated cylindrical domains ranging from 100 to 350 nm in diameter with densities of 2-5 × 10⁸ cm⁻² were observed in phase pure films. Longer range magnetic order, in the form of ribbon-like domains up to 1 μm in length, are present in the regions between the cylindrical domains. Two phase (In,Mn)As films produced a well-resolved complex domain structure consisting of 180° parallel and antiparallel domains. Excellent agreement between the temperature dependence of the relative magnetization obtained by MFM and superconducting quantum interference device measurements was observed.     

Ferromagnetism in amorphous Ge1−xMnx grown by low temperature vapor deposition

Magnetic properties of amorphous Ge_1−xMn_x thin films were investigated. The thin films were grown at 373 K on (100) Si wafers by using a thermal evaporator. Growth rate was ∼35 nm/min and average film thickness was around 500 nm. The electrical resistivities of Ge_1−xMn_x thin films are 5.0×10⁻⁴∼100 Ω cm at room temperature and decrease with increasing Mn concentration. Low temperature magnetization characteristics and magnetic hysteresis loops measured at various temperatures show that the amorphous Ge_1−xMn_x thin films are ferromagnetic but the ferromagnetic magnetizations are changing gradually into paramagnetic as increasing temperature. Curie temperature and saturation magnetization vary with Mn concentration. Curie temperature of the deposited films is 80-160 K, and saturation magnetization is 35-100 emu/cc at 5 K. Hall effect measurement at room temperature shows the amorphous Ge_1−xMn_x thin films have p-type carrier and hole densities are in the range from 7×10¹⁷ to 2×10²² cm⁻³.     

Propagation regimes of intense circularly polarized laser beam in magnetoactive plasma

This paper presents an analytical and numerical investigation of an intense circularly polarized wave propagating along the static magnetic field parallel to oscillating magnetic field in magnetoactive plasma. In the relativistic regime such a magnetic field is created by pulse itself. The authors have studied different regimes of propagation with relativistic electron mass effect for magnetized plasma. An appropriate expression for dielectric tensor in relativistic magnetoactive plasma has been evaluated under paraxial theory. Two modes of propagation as extraordinary and ordinary exist; because of the relativistic effect, ultra-strong magnetic fields are generated which significantly influence the propagation of laser beam in plasma. The nature of propagation is characterized through the critical-divider curves in the normalized beam width with power plane For given values of normalized density (ω_p/ω) and magnetic field (ω_c/ω) the regions are namely steady divergence (SD), oscillatory divergence (OD) and self-focusing (SF). Numerical computations are performed for typical parameters of relativistic laser-plasma interaction: magnetic field B = 10-100 MG; intensity I = 10¹⁶ to 10²⁰ W/cm²; laser frequency ω = 1.1 × 10¹⁵ s⁻¹; cyclotron frequency ω_c = 1.7 × 10¹³ s⁻¹; electron density n_e = 2.18 × 10²⁰ cm⁻³. From the calculations, we confirm that a circularly polarized wave can propagate in different regimes for both the modes, and explicitly indicating enhancement in wave propagation, beam focusing/self-guiding and penetration of E-mode in presence of magnetic field.     

Towards beyond 1 GHz NMR: Mechanism of the long-term drift of screening current-induced magnetic field in a Bi-2223 coil

The screening current-induced magnetic field in the (Bi,Pb)₂Sr₂Ca₂Cu₃O_x (Bi-2223) insert coil proposed for a beyond 1 GHz nuclear magnetic resonance (NMR) spectrometer may generate a long-term field drift, resulting in a loss of field-frequency lock operation and an inability to make high resolution NMR measurements. The measured screening current-induced magnetic field of a Bi-2223 double-pancake coil exhibits a hysteresis effect at 4.2 K that is reproduced by a numerical simulation based on a finite thickness rectangular superconductor bar model. The screening current-induced field at the coil center is of opposite polarity to that generated by the coil current, and thus the apparent field intensity shows a positive drift with time. On the contrary, the field at a coil end is of the same polarity as the coil field, and the apparent field intensity decreases with time. If we wait for ∼1000 h after coil excitation, the field drift rate approaches the field decay rate of the persistent current of 10⁻⁸ h⁻¹, suitable for a long-term NMR measurement in a beyond 1 GHz NMR spectrometer.     

Effect of Mn-site vacancies on the magnetic entropy change and the Curie temperature of La0.67Ca0.33Mn1−xO3 perovskite

Single-phase polycrystalline samples of La_0.67Ca_0.33Mn_1−xO₃ (x=0.00, 0.02, 0.04, 0.06) have been prepared using the sol-gel method. The structure, magnetocaloric properties and the Curie temperature of the samples with different Mn vacancy concentrations have been investigated. The experimental results show that vacancy doping at the Mn-sites has a significant influence on the magnetic properties of La_0.67Ca_0.33Mn_1−xO₃. The Curie temperature decreases monotonically with increasing the Mn-site vacancy concentration x. A remarkable enhancement of the magnetic entropy change has been obtained in the La_0.67Ca_0.33Mn_0.98O₃ sample. The entropy change reaches |ΔS_M|=3.10 J kg⁻¹ K⁻¹ at its Curie temperature (264 K) under an applied magnetic field H=10 kOe, which is almost the same value as that of pure Gd.     

Synthesis, structural and magnetic study of polycrystalline LaNi1−xMnxO3 films

Polycrystalline double perovskite LaNi_1−xMn_xO₃ (x=0.3, 0.4, 0.5 and 0.7, which is defined as Mn03, Mn04, Mn05 and Mn07, respectively) thin films are successfully deposited on Si (1 0 0) substrates via chemical solution deposition method. Their structural and magnetic properties are measured. All the thin films are of single phase. Raman spectra indicate that relative intensity of Mn05 is stronger than that of others that can be attributed to the higher degree of B-site ordering. The low temperature magnetic moment of Mn05 is about 500 emu/cm³, which is obviously larger than that of Mn03 and Mn07 because of the long-range ordering of Mn and Ni ions in Mn05.     

Magnetic properties of Cr-Fe-Mn alloys

The magnetic behaviour of a Cr_80−xFe₂₀Mn_x alloy system with x=2, 7, 10, 13 and 22 has been investigated in the temperature range 2-400 K through measurements of magnetization, electrical resistivity, magnetoresistivity, specific heat and thermal expansion. The temperature vs. Mn concentration magnetic phase diagram of the system is rich in magnetic behaviour with ferromagnetic (FM), antiferromagnetic (AFM) and paramagnetic phase regions and a spin-glass (SG) region at the lowest temperatures. Phase transition temperatures amongst these different magnetic phases could be identified from well-defined anomalies of magnetic origin that are displayed by graphs of the above-mentioned physical properties as a function of temperature. The time relaxation of the thermoremanent, isothermal remanent and field-cooled magnetizations below and above the SG freezing temperature show unusual aspects. These relaxations do not follow the usual superposition principle that is expected for typical SG materials. Negative giant magetoresistance (GMR) is observed in the alloys at 4 K. The GMR initially increases sharply on increasing the Mn content in the alloy system, followed by a tendency towards a saturation negative value for concentrations of more than about 10 at% Mn. Low-temperature plots of C_p/T vs. T², where C_p is the specific heat, present anomalous behaviour for alloys with x=2, 10 and 22. For x=2 the plot shows an upturn at the lowest temperatures that changes over to a prominent downturn for x=10 and 22. This behaviour is attributed to Fe concentration fluctuations in the alloys, confirming the theoretical model of Matthews.     

Improvement to the exchange field acting upon Tb ions in TbIG

The spin fluctuations of the magnetic ions play an important role on the magnetic properties of the crystals and lead to a new mechanism for the Curie-Weiss susceptibility. The exchange field H_exch acting on the rare-earth ions in Tb:YIG is improved based on the temperature dependence of the spin fluctuations, which is expressed as H_exch=n₀(1+γT+βT⁻²)M_YIG. By means of the improved exchange field, the magnetic and magneto-optical properties of Tb³⁺ ions in Tb:YIG are calculated. The calculated results are in good agreement with the measured data in the temperature range from 40 to 300 K.     

Influence of vapor transport equilibration on Raman spectra of Er:LiNbO3 crystals

Raman spectra of as-grown and vapor transport equilibration (VTE) treated Er:LiNbO₃ crystals, which have different cut orientations (X-cut and Z-cut), different Er-doping levels (Er:(0.2, 0.4 and 2.0 mol%)LiNbO₃) and different VTE durations (80, 120, 150 and 180 h), were recorded at room temperature in the wavenumber range 50-1000 cm⁻¹ by using backward scattering geometry. The spectra were attributed on the basis of their spectral features and the previous experimental work and the most recent theoretical progress in lattice dynamics on pure LiNbO₃. In comparison with the pure crystal the most remarkable effect of Er-doping on the Raman spectrum is observed for the E(TO₉) mode. It does not appear at 610 cm⁻¹ as the pure crystal, but locates at 633 cm⁻¹. In addition, the doping also results in the lowering of the Raman phonon frequency, the broadening of the Raman linewidth and the changes of the relative Raman intensity of some peaks. The VTE treatment results in the narrowing of the linewidth, the recovery of the lowered phonon frequency and the further changes of relative Raman intensity. The narrowing of Raman linewidth indicates that the VTE processing has brought these crystals closer to stoichiometric composition. The VTE treatment has induced the formation of a precipitate ErNbO₄ in the high-doped Er(2.0%):LiNbO₃ crystals whether X- or Z-cut. For these precipitated crystals, besides above linewidth and phonon frequency features, they also display more significant Raman intensity changes compared with those not precipitated crystals. In addition, a slight mixing between A₁(TO) and E(TO) spectra is also observed for these precipitated crystals. Above doping and VTE effects on Raman spectra were quantitatively or qualitatively correlated with the characteristics of the crystal structure and phonon vibrational system.     

The important role of Mn in the room-temperature ferromagnetism of Mn-doped GaN films

Mn-doped GaN films (Ga_1−xMn_xN) were grown on sapphire (0 0 0 1) using Laser assisted Molecular Beam Epitaxy (LMBE). High-quality nanocrystalline Ga_1−xMn_xN films with different Mn concentration were then obtained by thermal annealing treatment for 30 min in the ammonia atmosphere. Mn ions were incorporated into the wurtzite structure of the host lattice by substituting the Ga sites with Mn³⁺ due to the thermal treatment. Mn³⁺, which is confirmed by XPS analysis, is believed to be the decisive factor in the origin of room-temperature ferromagnetism. The better room-temperature ferromagnetism is given with the higher Mn³⁺ concentration. The bound magnetic polarons (BMP) theory can be used to prove our room-temperature ferromagnetic properties. The film with the maximum concentration of Mn³⁺ presents strongest ferromagnetic signal at annealing temperature 950 °C. Higher annealing temperature (such as 1150 °C) is not proper because of the second phase Mn_xGa_y formation.     

On-chip free-flow magnetophoresis: Separation and detection of mixtures of magnetic particles in continuous flow

The complete separation of mixtures of magnetic particles was achieved by on-chip free-flow magnetophoresis. In continuous flow, magnetic particles were deflected from the direction of laminar flow by a perpendicular magnetic field depending on their magnetic susceptibility and size and on the flow rate. 2.8 and 4.5 μm superparamagnetic particles with magnetic susceptibilities of 1.1×10⁻⁴ and 1.6×10⁻⁴ m³ kg⁻¹, respectively, could be completely separated from each other reproducibly. The separated particles were detected by video observation and also by on-chip laser light scattering. Potential applications of this separation method include sorting of magnetic micro- and nanoparticles as well as magnetically labelled cells.     

Molecular dynamics and phase transitions in paramagnetic [Mn(H2O)6][SiF6] investigated by H NMR

The temperature dependences of ²H NMR spectra and spin-lattice relaxation time T₁ have been measured for paramagnetic [Mn(H₂O)₆][SiF₆]. The obtained ²H NMR spectra were simulated by considering the quadrupole interaction and paramagnetic shift. The variation of the spectra measured in phase III was explained by the 180° flip of water molecules. The activation energy E_a and the jumping rate at infinite temperature k₀ for the 180° flip of H₂O were obtained as 35 kJ mol⁻¹ and 4×10¹⁴ s⁻¹, respectively. The spectral change in phases I and II was ascribed to the reorientation of [Mn(H₂O)₆]²⁺ around the C₃ axis where the E_a and k₀ values were estimated as 45 kJ mol⁻¹ and 1×10¹³ s⁻¹, respectively. From the almost temperature independent and short T₁ value, the correlation time for electron-spin flip-flops, τ_e, and the exchange coupling constant J were obtained as 3.0×10⁻¹⁰ s and 2.9×10⁻³ cm⁻¹, respectively. The II-III phase transition can be caused by the onset of the jumping motion of [Mn(H₂O)₆]²⁺ around the C₃ axis.     

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.     

Recombination mechanism of photoluminescence in InN epilayers

We report an investigation of the recombination mechanism for photoluminescence (PL) in InN epilayers grown by molecular beam epitaxy and metal-organic chemical vapor deposition with a wide range of free electron concentrations from 3.5×10¹⁷-5×10¹⁹ cm⁻³. We found that the PL spectra are strongly blueshifted with increasing excitation intensity. For all the samples studied, the exponent of the relationship between the integrated PL intensity and the excitation intensity is very close to unity and independent of the temperature. By assuming Gaussian fluctuations of the random impurity potential, calculation based on the ‘free-to-bound’ recombination model can be used to interpret our results very well and it correctly reproduces the development of the total PL peak shift as a function of carrier concentration. It is concluded that the PL transition mechanism in InN epifilms can be characterized as the recombination of free electrons in the conduction band to nonequilibrium holes in the valence band tail.     

Sensitive determination of enoxacin by its enhancement effect on the fluorescence of terbium(III)-sodium dodecylbenzene sulfonate and its luminescence mechanism

The fluorescence system of enoxacin-Tb³⁺-sodium dodecylbenzene sulfonate (SDBS) was investigated in this paper. The experiments indicated that the fluorescence intensity of Tb³⁺-SDBS was greatly enhanced by enoxacin. Accordingly, a sensitive fluorimetric method for determining enoxacin was established. The fluorescence intensity was measured by a 1-cm quartz cell with an excitation wavelength of 290 nm and an emission wavelength of 545 nm. The enhanced fluorescence intensity of the system (ΔF) showed a good linear relationship with the concentration of enoxacin in the range of 5.0×10⁻⁹ to 2.0×10⁻⁶ mol L⁻¹, its correlation coefficient was 0.9992 and the detection limit (S/N=3) was 2.8×10⁻⁹ mol L⁻¹. The presented method was used to determine enoxacin in real pharmaceutical samples. The luminescence mechanism was also discussed in detail. In the fluorescence system of enoxacin-Tb³⁺-SDBS, SDBS not only acted as the surfactant but also acted as the energy donor.     

Optical and FTIR properties of Se93−XZn2Te5InX chalcogenide glasses

In this work, we report the optical properties of bulk Se_93−XZn₂Te₅In_X (X=0, 2, 4, 6 and 10) chalcogenide glasses. Refractive index, extinction coefficient, real dielectric constant (ε′), imaginary dielectric constant (ε″), absorption coefficient (α) and energy band gap were obtained from analysis of common range (250-1100 nm) UV/Visible transmittance spectrum. Besides, transmission percentages were obtained from FTIR spectra in wave number range 4000-400 cm⁻¹.The concentration dependence structural phenomena have been explained with help of average coordination 〈z〉.