Influence of Nb substitution for Fe on the magnetic and magneto-impedance properties of amorphous and annealed Fe76.5−xSi13.5B9Cu1Nbx (x=0–7) ribbons

We report a systematic study of the influence of Nb substitution for Fe on the magnetic properties and magneto-impedance (MI) effect in amorphous and annealed Fe_76.5−xSi_13.5B₉Cu₁Nb_x (x=0, 1, 2, 3, 4, 5, 6, and 7) ribbons. The amorphous ribbons were annealed at different temperatures ranging from 530 to 560 °C in vacuum for different annealing times between 5 and 20 min. We have found that for the as-quenched amorphous ribbons, the substitution of Nb for Fe first increases the saturation magnetization (M_s) and decreases the coercivity (H_c) until x=3, for which the largest M_s∼152 emu/g and the smallest H_c∼1.3 Oe are obtained, then an opposite trend is found for x>3. The largest MI ratio (ΔZ/Z∼38% at f=6 MHz) is achieved in the amorphous ribbon with x=3. A similar trend has been observed for the annealed ribbons. The most desirable magnetic properties (M_s∼156 emu/g and H_c∼1.8 Oe) and the largest MI ratio (ΔZ/Z∼221% at f=6 MHz) are achieved for the x=3 sample annealed at 540 °C for 15 min. A correlation between the microstructure, magnetic properties, and MI effect in the annealed ribbons has been established.     

Photoluminescence and magnetic properties of Fe-doped ZnS nano-particles synthesized by chemical co-precipitation

This paper is focusing on the synthesis of Zn_1−xFe_xS nano-particles with x = 0, 0.1 and 0.2 by chemical co-precipitation method, the prepared of which are characterized by XRD, EDS, TEM, PL, magnetization versus field behavior and M-T curve. In the XRD patterns, Zn_1−xFe_xS nano-particles are shown of cubic zinc blende structure, and the broadening diffraction peaks consistent with the small-size characteristic of nano-materials. The diameter of nano-particles is between 3.3 and 5.5 nm according to the HR-TEM images. The EDS data confirm the existence of Fe ions in Fe-doped ZnS nanoparticles. There we found that Fe-doping did not import new energy bands or defect states, but reduced the intensity of PL peaks. The magnetization versus field behaviors were illustrated by the M-H curves at both 5 K and 300 K, respectively, where no remanence or coercive force was observed. This phenomenon indicates that the Zn_1−xFe_xS (x = 0.1) nano-particles are superparamagnetic. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves further reveal that the blocking temperature (T_B) of the superparamagnetic behavior might be below 5 K.     

Synthesis and characterization of Ni-doped ZnO: A transparent magnetic semiconductor

We report synthesis of a transparent magnetic semiconductor by incorporating Ni in zinc oxide (ZnO) matrix. ZnO and nickel-doped zinc oxide (ZnO:Ni) thin films (∼60 nm) are prepared by fast atom beam (FAB) sputtering. Both undoped and doped films show the presence of ZnO phase only. The Ni concentration (in at%) as determined by energy dispersive X-ray (EDX) technique is ∼12±2%. Magnetisation measurement using a SQUID magnetometer shows that the Ni-doped films are ferromagnetic, having coercivity (H_c) values 192, 310 and 100 Oe and saturation magnetization (M_s) values of 6.22, 5.32 and 4.73 emu/g at 5, 15 and 300 K, respectively. The Ni-doped film is transparent (>80%) across visible wavelength range. Resistivity of the ZnO:Ni film is ∼2.5×10⁻³ Ω cm, which is almost two orders of magnitude lower than the resistivity (∼4.5×10⁻¹ Ω cm) of its undoped counterpart. Impurity d-band splitting is considered to be the cause of increase in conductivity. Interaction between free charges generated by doping and localized d spins of Ni is discussed as the reason for ferromagnetism in the ZnO:Ni film.     

Near room temperature magnetocaloric properties of melt-spun Gd100−xBx (x=0, 5, 10, 15, and 20 at%) alloys

The magnetocaloric properties of melt-spun Gd-B alloys were examined with the aim to explore their potential application as magnetic refrigerants near room temperature. A series of Gd_100−xB_x (x=0, 5, 10, 15, and 20 at%) alloys were prepared by melt spinning. With the decrease in Gd/B ratio, Curie temperature (T_C) remains constant at ∼293 K, and saturation magnetization, at 275 K, decreases from ∼100 to ∼78 emu/g. Negligible magnetic hysteresis was observed in these alloys. The peak value of magnetic entropy change, (−ΔS_M)_max, decreased from ∼9.9 J/kg K (0-5 T) and ∼5.5 J/kg K (0-2 T) for melt-spun Gd to ∼7.7 J/kg K (0-5 T) and ∼4.0 J/kg K (0-2 T), respectively for melt-spun Gd₈₅B₁₅ and Gd₈₀B₂₀ alloys. Similarly, the refrigeration capacity (q) decreased monotonously from ∼430 J/kg (0-5 T) for melt-spun Gd to ∼330 J/kg (0-5 T) for melt-spun Gd₈₀B₂₀ alloy. The near room temperature magnetocaloric properties of melt-spun Gd_100−xB_x (0≤x≤20) alloys were found to be comparable to few first-order transition based magnetic refrigerants.     

Structural,optical and magnetic study of (1−x)ZnO–xMgO composites prepared through solid state reaction method

We report the study of structural, optical and magnetic properties of (1−x)ZnO–xMgO (x=0.35, 0.40, 0.45 and 0.50) composites prepared by solid state reaction method. X-ray diffraction pattern confirms the presence of both the phases associated with ZnO (hexagonal) and MgO (cubic), which is revealed through the existence of (1 1 1) and (2 0 0) peaks in addition to ZnO peaks. The lattice parameter c as calculated using X-ray analysis undergoes shrinkage with increasing content of MgO. Microstructural analysis suggests that there is no variation in spherical elongated shape of grains with increasing concentration of MgO, where the average grain size is found to be ∼600 nm. The band gap as calculated from optical absorption spectra obtained by diffuse reflectance method recorded at room temperature is tuned from 3.16 to 3.55 eV. Photoluminescence spectra consist of near band edge UV emission (389 nm) and defect level emission (503 nm). The increase of MgO concentration leads to blue shift of UV emission peaks. The magnetic measurements conducted using SQUID at 5 K temperature reveals ferromagnetism along with paramagnetic and superparamagnetic components. Saturation magnetisation (M_s) is observed to be enhanced with MgO doping.     

Structural, microstructural and magnetic properties of NiFe2O4, CoFe2O4 and MnFe2O4 nanoferrite thin films

The structural, microstructural and magnetic properties of nanoferrite NiFe₂O₄ (NF), CoFe₂O₄ (CF) and MnFe₂O₄ (MF) thin films have been studied. The coating solution of these ferrite films was prepared by a chemical synthesis route called sol-gel combined metallo-organic decomposition method. The solution was coated on Si substrate by spin coating and annealed at 700 °C for 3 h. X-ray diffraction pattern has been used to analyze the phase structure and lattice parameters. The scanning electron microscopy (SEM) and atomic force microscopy (AFM) have been used to show the nanostructural behavior of these ferrites. The values of average grain's size from SEM are 44, 60 and 74 nm, and from AFM are 46, 61 and 75 nm, respectively, measured for NF, CF and MF ferrites. At room temperature, the values of saturation magnetization, M_s∼50.60, 33.52 and 5.40 emu/cc, and remanent magnetization, M_r∼14.33, 15.50 and 1.10 emu/cc, respectively, are observed for NF, CF and MF. At low temperature measurements of 10 K, the anisotropy of ferromagnetism is observed in these ferrite films. The superparamagnetic/paramagnetic behavior is also confirmed by χ′(T) curves of AC susceptibility by applying DC magnetizing field of 3 Oe. The temperature dependent magnetization measurements show the magnetic phase transition temperature.     

Growth temperature dependence of the hysteretic behavior of Ni0.5Zn0.5Fe2O4 thin films

Herein, a discussion of the effect of deposition temperature on the magnetic behavior of Ni_0.5Zn_0.5Fe₂O₄ thin films. The thin films were grown by r.f. sputtering technique on (1 0 0) MgO single-crystal substrates at deposition temperatures ranging between 400 and 800 °C. The grain boundary microstructure was analyzed via atomic force microscopy (AFM). AFM images show that grain size (φ∼70-112 nm) increases with increasing deposition temperature, according to a diffusion growth model. From magneto-optical Kerr effect (MOKE) measurements at room temperature, coercive fields, H_c, between 37and 131 Oe were measured. The coercive field, H_c, as a function of grain size, reaches a maximum value of 131 Oe for φ ∼93 nm, while the relative saturation magnetization exhibits a minimum value at this grain size. The behaviors observed were interpreted as the existence of a critical size for the transition from single- to multi-domain regime. The saturation magnetization (21 emu/g<M_s<60 emu/g) was employed to quantify the critical magnetic intergranular correlation length (L_c≈166 nm), where a single-grain to coupled-grain behavior transition occurs. Experimental hysteresis loops were fitted by the Jiles-Atherton model (JAM). The value of the k-parameter of the JAM fitted by means of this model (k/μ_o∼50 A m²) was correlated to the domain size from the behavior of k, we observed a maximum in the density of defects for the sample with φ∼93 nm.     

Microstructure and magnetic properties of Ni-rich Ni54Mn25.7Ga20.3 ferromagnetic shape memory alloy thin film

A Ni₅₄Mn_25.7Ga_20.3 ferromagnetic shape memory alloy thin film has been fabricated by using the RF magnetron-sputtering technique. The structure and magnetic properties of the film were systematically investigated. The results show that the film is in ferromagnetic martensite state at room temperature with the Curie temperature (T_c) of about 370 K. The saturation magnetization (M_s) of the film reaches 45 emu/g at 300 K, which is about 80% as large as that of Ni–Mn–Ga bulk material. The magnetization hysteresis loops significantly depend on temperatures. The residual magnetization (M_r) and the coercive force (H_c) increase with decreasing temperatures. The grains homogeneously distribute in the film. The microstructure of the film consists of martensite plates. The interface between the martensite variants is clear and straight, indicating a good mobility.     

Synthesis of α-Fe2O3 nanobelts and nanoflakes by thermal oxidation and study to their magnetic properties

α-Fe₂O₃ nanobelts and nanoflakes have been successfully synthesized by oxidation of iron-coated ITO glass in air. The X-ray diffraction, Raman spectrum and scanning electron microscopy are carried out to characterize the nanobelts and nanoflakes. The formation mechanism has been presented. Significantly, the magnetic investigations show that the magnetic properties are strongly shape-dependent. The magnetization measurements of belt-like and flake-like α-Fe₂O₃ in perpendicular exhibit ferromagnetic feature with the coercivity (H_c) and saturation magnetization (M_s) of 334.5 Oe and 1.35 emu/g, 239.5 Oe and 0.12 emu/g, respectively. For the parallel, belt-like and flake-like α-Fe₂O₃ also exhibit ferromagnetic feature with the H_c and M_s of 205.5 Oe and 1.44 emu/g, 159.6 Oe and 0.15 emu/g, respectively.     

Motion of positively charged muonium in ZnO

We report on a study of the motional characteristics of positively charged muonium defect centers in ZnO as an analog for H⁺ behavior. Muon spin depolarization measurements at zero applied magnetic field were completed from 20 K to 400 K, with preliminary results to 750 K. Results at the lower temperatures imply that Mu⁺ occupied two sites, and indicate local motion as thermally assisted tunneling with a characteristic energy of ∼60 meV, as well as a site change transition above 200 K with barrier energy ∼440 meV. Based on theoretical results, we have tentatively assigned these features to tunneling among three equivalent oxygen anti-bonding sites (AB_⊥) and a transition to a lower-energy bond-centered site (BC_‖) oriented along the c-axis. Preliminary fits suggest that global diffusion of muonium occurs above 400 K, with a diffusion barrier energy of ∼0.7 eV.     

High temperature thermoelectric properties of Nb-doped ZnO ceramics

Solid-state reaction processing technique was used to prepare Zn_xNb_1−xO (0≤x≤0.02) polycrystalline bulk samples. In the present study, we find that their lattice parameters a and c tend to decrease with increasing amount of Nb additive. The electrical conductivity of all the Zn_1−xNb_xO samples increased with increasing temperature, indicating a semiconducting behavior in the measured temperature range. The addition of Nb₂O₅ to ZnO led to an increase in the electrical conductivity and a decrease in the absolute value of the Seebeck coefficient. The best performance at 1000 K has been observed for nominal 0.5 at% Nb-doped ZnO, with an electrical resistivity of about 73.13 (S cm⁻¹) and Seebeck coefficient of ∼257.36 μV K⁻¹, corresponding to a power factor (S²σ) of 4.84×10⁻⁴ Wm⁻¹ K⁻². The thermal conductivity, κ, of the oxide decreased as compared to pure ZnO. The figure of merit ZT values of ZnO-doped Nb₂O₅ samples are higher than the ZnO pure sample, demonstrating that the Nb₂O₅ addition is fairly effective for enhancing thermoelectric properties.     

Mechanical and magnetic properties of ZnO/Fe2O3 ceramic varistors

Mechanical and magnetic properties of the ZnO/Fe₂O₃ ceramic varistors have been examined by using mechanical analyzer, digital microhardness tester and vibrating sample magnetometer. The initial stress–strain behavior is found to be linear (elastic) then becomes nonlinear (plastic deformation) without reaching the failure limit up to the maximum available stress (0.07 MPa). The compressive elastic modulus varies between 0.2 and 0.8 MPa with Fe addition up to 0.50. Furthermore, an approximately monotonically linear decrease in VHN with increasing Fe content up to 50% has been observed for all applied loads, which closely resembles the behavior of the true hardness and the surface energy. The magnetic measurements revealed an antiferromagnetic to paramagnetic to transition for all Fe doped samples. The Fe free sample showed paramagnetic behavior down to 2 K. The Neel temperature moderately increased from 18 K at 0.05% Fe to 25 K at 0.5% Fe. The magnetization (M) versus applied magnetic field (H) did not reach saturation for all samples up to 9 Tesla. The saturated magnetization (per Fe contents) is low and found to decreases linearly at a rate of (−35 emu/g-Fe) in a clear manifestation of the strengthening of the antiferromagnetic exchange interaction with increasing Fe contents.     

Microstructural,dielectric and magnetic behavior of spin-deposited nanocrystalline nickel–zinc ferrite thin films for microwave applications

Nanocrystalline nickel ferrite and zinc doped nickel ferrite thin films with general composition Ni_1−xZn_xFe₂O₄; x=0.0, 0.2 and 0.5 were fabricated by the spin-deposition technique. Citrate precursor method was adopted to prepare coating solution used for film deposition. This method resulted in single phase, transparent, homogeneous and crack-free nanocrystalline ferrite thin films at annealing temperature as low as 400 °C. The substrates used for film deposition were ITO-coated 7059 glass, fused quartz and Si (1 0 0). The thickness of films was found to be in the range ∼1000–5500 Å. The surface microstructure and morphology investigated by atomic force microscopy (AFM) confirmed the grain size of nickel–zinc ferrite films to be in nanometer range indicating nanocrystalline nature of the films. Dielectric properties such as the real (∈′) and imaginary parts (∈″) of complex permittivity were measured in the X-band microwave frequency region (8–12 GHz) by employing extended cavity perturbation technique. The M–H hysteresis measurements on the films annealed at 650 °C revealed narrow hysteresis curves with H_c and M_s varying for different compositions.     

Structural and magnetic properties of self-assembled Sm-Co spherical aggregates

Self-assembled Sm-Co nanoparticles in the form of spherical aggregates (referred as nanospheres) with diameter ranging from 50 to 180 nm were achieved by means of polyol technique. The size distribution of the Sm-Co nanospheres can be regulated close to ∼100 nm by controlling the molar ratio of Sm:Co precursor. The spherical aggregates exhibited Sm₂Co₇ phase as a major constituent; while the aggregates obtained at higher Co concentration showed co-existence of Co-phase with Sm₂Co₇ phase. Upon annealing, the biphasic nature of nanospheres (Sm₂Co₇/Co) transformed into Sm₂Co₁₇ structure. By varying the Sm:Co precursor ratio from 1:5 to 1:9, the coercivity (H_c) and magnetization (M_s) values of the as-synthesized nanospheres can be tuned from 336 to 140 Oe and from 63.7 to 108 emu/g, respectively, and these values significantly improved after annealing. Maximum values of H_c (1050 Oe) at the Sm:Co molar ratio of 1:5 and M_s of 184.6 emu/g at the Sm:Co molar ratio of 1:9 were achieved in the annealed samples.     

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.     

Synthesis and surface modified hard magnetic properties in Co0.5Pt0.5 nanocrystallites from a rheological liquid precursor

Small crystallites of a metastable phase Co_0.5Pt_0.5 are precipitated by heating a rheological liquid precursor of cobalt–hydrazine complex and platinum chloride H₂PtCl₆·xH₂O in polymer molecules of poly(vinylpyrrolidone) (PVP) in ethylene glycol. The hydrazine co-reduces nascent atoms from the Co²⁺ and Pt⁴⁺ that recombine and grow as Co_0.5Pt_0.5. The PVP molecules cap a growing Co_0.5Pt_0.5 as it achieves a critical size so that it stops growing further in given conditions. X-ray diffraction pattern of a recovered powder reveals a crystalline Co_0.5Pt_0.5 phase (average crystallite size D∼8 nm) of a well-known Fm3m-fcc crystal structure with the lattice parameter a=0.3916 nm (density ρ=14.09 g/cm³). A more ordered L1₀ phase (ρ=15.91 g/cm³) transforms (D≥25 nm) upon annealing the powder at temperature lesser than 700 °C (in vacuum). At room temperature, the virgin crystallites bear only a small saturation magnetization M_s=5.54 emu/g (D=8 nm) of a soft magnet and it hardly grows on bigger sizes (D≤31 nm) in a canted ferromagnetic structure. A rectangular hysteresis loop is markedly expanded on an optimally annealed L1₀ phase at 800 °C for 60 min, showing a surface modified coercivity H_c=7.781 kOe with remnant ratio M_r/M_s=0.5564, and M_s=39.75 emu/g. Crystallites self-assembled in an acicular shape tailor large H_c from ideal single domains and high magnetocrystalline anisotropy of a hard magnet L1₀ phase.     

Effect of sintering temperature on the structural, dielectric and ferroelectric properties of tungsten substituted SBT ceramics

Structural, dielectric and ferroelectric properties of tungsten (W) substituted SrBi₂(Ta_1−xW_x)₂O₉ (SBTW) [x=0.0, 0.025, 0.05, 0.075, 0.1 and 0.2] have been studied as a function of sintering temperature (1100-1250 °C). X-ray diffraction patterns confirm the single-phase layered perovskite structure formation up to x=0.05 at all sintering temperatures. The present study reveals an optimum sintering temperature of 1200 °C for the best properties of SBTW samples. Maximum T_c of ∼390 °C is observed for x=0.20 sample sintered at 1200 °C. Peak-dielectric constant (ε_r) increases from ∼270 to ∼700 on increasing x from 0.0 to 0.20 at 1200 °C sintering temperature. DC conductivity of the SBTW samples is nearly two to three orders lower than that of the pristine sample. Remnant polarization (P_r) increases with the W content up to x≤0.075. A maximum 2P_r (∼25 μC/cm²) is obtained with x=0.075 sample sintered at 1200 °C. The observed behavior is explained in terms of improved microstructural features, contribution from the oxygen and cationic vacancies in SBTW. Such tungsten substituted samples sintered at 1200 °C exhibiting enhanced dielectric and ferroelectric properties should be useful for memory applications.     

Novel hybrid nanostructured materials of magnetite nanoparticles and pectin

A novel hybrid nanostructured material comprising superparamagnetic magnetite nanoparticles (MNPs) and pectin was synthesized by crosslinking with Ca²⁺ ions to form spherical calcium pectinate nanostructures, referred as MCPs, which were typically found to be 100-150 nm in size in dried condition, confirmed from transmission electron microscopy and scanning electron microscopy. The uniform size distribution was revealed from dynamic light scattering measurement. In aqueous medium the MCPs showed swelling behavior with an average size of 400 nm. A mechanism of formation of spherical MCPs is outlined constituting a MNP-pectin interface encapsulated by calcium pectinate at the periphery, by using an array of characterization techniques like zeta potential, thermogravimetry, Fourier transformed infrared and X-ray photoelectron spectroscopy. The MCPs were stable in simulated gastrointestinal fluid and ensured minimal loss of magnetic material. They exhibited superparamagnetic behavior, confirmed from zero field cooled and field cooled profiles and showed high saturation magnetization (M_s) of 46.21 emu/g at 2.5 T and 300 K. M_s decreased with increasing precursor pectin concentrations, attributed to quenching of magnetic moments by formation of a magnetic dead layer on the MNPs.     

Sequence of structural and magnetic transitions in Ni48Co2Mn39Sn11 shape memory alloy

The structural and magnetic transitions in Ni₄₈Co₂Mn₃₉Sn₁₁ shape memory alloy were systematically investigated. During cooling, the paramagnetic austenite transforms into paramagnetic martensite at T_M∼375 K, followed by a gradual transition from paramagnetic to superparamagnetic martensite around T_S∼320 K. Upon further cooling through T_P∼100 K, the superparamagnetic clusters collectively freeze into a superspin glass state as corroborated by aging, rejuvenation, and memory effects. Consequently, the unique transition sequence of paramagnetic austenite→paramagnetic martensite→superparamagnetic martensite→superspin-glass martensite is disclosed.     

Characterization of Cu additive FePt-C granular films

FePt-C granular films doped with different Cu atomic fractions (x_Cu) were fabricated using facing-target sputtering at room temperature and subsequently annealed at 650 °C. Structural analyses reveal that the as-deposited films are in amorphous state. Appropriate Cu addition (x_Cu = 14) can improve the ordering of L1₀ FePt phase, and excessive Cu doping destroys the formation of ordered L1₀ phase with the appearance of Fe₃C and CuPt phases. Besides, preferential graphitization of amorphous carbon (a-C) occurs near large metal particles upon annealing. Annealing turns the as-deposited superparamagnetic films into ferromagnetic associated with coercivity peaks at x_Cu = 14, drops from ∼11.2 kOe at 5 K to ∼7.2 kOe at 300 K in a 50 kOe field.