Synthesis and magnetic properties of Au-coated amorphous Fe20Ni80 nanoparticles

Gold-coated nanoparticles of Fe₂₀Ni₈₀ (permalloy) have been synthesized by a microemulsion process. The as-prepared samples consist of ∼5 nm diameter particles of amorphous Fe₂₀Ni₈₀ that are likely encapsulated in B₂O₃. One or more Fe₂₀Ni₈₀@B₂O₃ particles are subsequently encapsulated in 8-20 nm gold nanospheres, as determined by TEM and X-ray powder diffraction (XRD) line broadening. The gold shells were found to be under expansive strain. Magnetic data confirm the existence of a superparamagnetic phase with a blocking temperature, T_B, of ∼33 K. The saturation magnetization, M_S, of the as-prepared, Au-coated sample is ∼65 emu g⁻¹ at 5 K and ∼16 emu g⁻¹ at 300 K. The coercivity, H_C, is ∼280 Oe at 5 K.     

Sintering temperature dependence of room temperature magnetic and dielectric properties of Co0.5Zn0.5Fe2O4 prepared using mechanically alloyed nanoparticles

Co_0.5Zn_0.5Fe₂O₄ nanoparticles were prepared using mechanical alloying (MA) and sintering. The crystallite size, coercivity, retentivity and saturation magnetization were also measured. The frequency dependence of dielectric and the magnetic parameters, namely, real permittivity ε′, loss tanget tan δ, real permeability μ′ and loss factor μ″ were measured at room temperature for samples sintered from 600 to 1000 °C, in the frequency range 10 MHz to 1.0 GHz. The results show that the crystallite size of the resulting products ranges between 16 and 67 nm for as-milled sample and the sample sintered at 1000 °C, respectively. The sample sintered at 1000 °C, measured at room temperature exhibited a saturation magnetization of 37 emu g⁻¹. The values of permittivity remain constant within the measured frequency, but vary with sintering temperature. The permeability values, on the other hand however vary with both the sintering temperature and the frequency, thus, the absolute value of the permeability decreased after the natural resonance frequency.     

Spin-glass-like behaviour and magnetic order in Mn[Cr0.5Ga1.5]S4 spinels

Magnetization and susceptibility were investigated as a function of temperature and magnetic field in polycrystalline Mn[Cr_0.5Ga_1.5]S₄ spinel. The dc susceptibility measurements at 919 Oe showed a disordered ferrimagnetic behaviour with a Curie-Weiss temperature θ_CW=−55 K and an effective magnetic moment of 5.96 μ_B close to the spin-only value of 6.52 μ_B for Cr³⁺ and Mn²⁺ ions in the 3d³ and 3d⁵ configurations, respectively. The magnetization measured at 100 Oe revealed the multiple magnetic transitions with a sharp maximum at the Néel temperature T_N=3.9 K, a minimum at the Yafet-Kittel temperature T_YK=5 K, a broad maximum at the freezing temperature T_f=7.9 K, and an inflection point at the Curie temperature T_C=48 K indicating a transition to paramagnetic phase. A large splitting between the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations at a temperature smaller than T_C suggests the presence of spin-glass-like behaviour. This behaviour is considered in a framework of competing interactions between the antiferromagnetic ordering of the A(Mn) sublattice and the ferromagnetic ordering of the B(Cr) sublattice.     

Structural and magnetic ordering in the VxNb1+yS2 system

The influence of composition on the structural ordering and magnetism in the V_xNb_1+yS₂ system has been investigated by X-ray diffraction and magnetic measurements. Stoichiometric V_1/3NbS₂ did not exhibit the structural ordering of vanadium between the NbS₂ layers. In the ordered structure, the vanadium composition deviated from the ideal value of to both higher and lower values, while the niobium composition was in the range of 0.05?y?0.18. Excess niobium, y>0, is thought to play an essential role in the structural ordering in this system. For samples with excess niobium and ordered structures, a magnetic transition was observed at 20-50 K, depending on the composition. The spontaneous magnetization of 3-5×10⁻³ μ_B/V atom is thought to be intrinsic to this system. The magnetization curves consisted of a constant and a proportional parts of the magnetic field, which correspond to the spontaneous magnetization and high-field susceptibility, respectively. The magnetization curves and the temperature dependencies of the high-field susceptibility were quite similar to those of the canted antiferromagnetic NiS₂. A correlation between the structural and magnetic ordering is suggested.     

Magnetic properties of nanocrystalline Co1−xZnxFe2O4 prepared by forced hydrolysis method

Nanocrystalline zinc-substituted cobalt ferrite powders, Co_1−xZn_xFe₂O₄ (x=0, 0.2, 0.4), were for the first time prepared by forced hydrolysis method. Magnetic and structural properties in these specimens were investigated. The average crystallite size is about 3.0 nm. When the zinc substitution increases from x=0 to x=0.4, at 4.2 K, the saturation magnetization increases from 72.1 to 99.7 emu/g and the coercive field decreases from 1.22 to 0.71 T. All samples are superparamagnetic at room temperature and ferrimagnetic at temperatures below the blocking temperature. The high value of the saturation magnetization and the very thin thickness of the disorder surface layer of all samples suggests that this forced hydrolysis method is suitable not only for preparing two metal element systems but also for three or more ones.     

Temperature dependence of magnetic properties of NiFe2O4 nanoparticles embeded in SiO2 matrix

Spinel ferrite NiFe₂O₄ nanoparticles (?25 nm) in SiO₂ matrix were prepared by sol–gel method. The phase and average crystallite size of the samples were determined by X-ray diffraction method and the particle size distributions were studied by a transmission electron microscope. Magnetic properties of the samples were investigated with different ferrite particle sizes and at various temperatures down to 10 K. Superparamagnetic properties were observed at room temperature when the particle size is less than 10 nm.In superparamagnetic state, the field dependence of magnetization follows Langevin function which was originally developed for paramagnetism. The effective anisotropy constant K_eff is found to increase significantly with the decrease in particle volume and an order of magnitude higher than that of the bulk samples when the particle size is below 5 nm due to the dominance of surface anisotropy. In case of nanosized systems, the effect of size reduction on the law of approach to saturation has also been studied in detail.     

Fabrication and characterization of magnetic Fe3O4-CNT composites

Carbon nanotubes (CNTs) decorated with magnetite nanoparticles on their external surface have been fabricated by in situ solvothermal method, which was conducted in benzene at 500 °C with ferrocene and CNTs as starting reagents. The as-prepared composites were characterized using XRD, FTIR, SEM and TEM. It has been found that the amount of magnetite nanoparticles deposited on the CNTs can be controlled by adjusting the initial mass ratio of ferrocene to CNTs. The Fe₃O₄-CNT composites display good ferromagnetic property at room temperature, with a saturation magnetization value (M_s) of 32.5 emu g⁻¹ and a coercivity (H_c) of 110 Oe.     

Synthesis and characterization of room-temperature ferromagnetism in Fe- and Ni-co-doped In2O3

Observation of room-temperature ferromagnetism in Fe- and Ni-co-doped In₂O₃ samples (In_0.9Fe_0.1−xNi_x)₂O₃ (0?x?0.1) prepared by citric acid sol-gel auto-igniting method is reported. All of the samples with intermediate x values are ferromagnetic at room-temperature. The highest saturation magnetization (0.453 μ_B/Fe+Ni ions) moment is reached in the sample with x=0.04. The highest solubility of Fe and Ni ions in the In₂O₃ lattice is around 10 and 4 at%, respectively. The 10 at% Fe-doped sample is found to be weakly ferromagnetic, while the 10 at% Ni-doped sample is paramagnetic. Extensive structure including Extended X-ray absorption fine structure (EXAFS), magnetic and magneto-transport including Hall effects studies on the samples indicate the observed ferromagnetism is intrinsic rather than from the secondary impurity phases.     

Synthesis,characterization and magnetic properties of MFe2O4 (M=Co,Mg, Mn,Ni) nanoparticles using ricin oil as capping agent

We focused on obtaining MFe₂O₄ nanoparticles using ricin oil solution as surfactant and on their structural characterization and magnetic properties. The annealed samples at 500 °C in air for 6 h were analyzed for the crystal phase identification by powder X-ray diffraction using CuKα radiation. The particle size, the chemical composition and the morphology of the calcinated powders were characterized by scanning electron microscopy. All sintered samples contain only one phase, which has a cubic structure with crystallite sizes of 12–21 nm. From the infrared spectra of all samples were observed two strong bands around 600 and 400 cm⁻¹, which correspond to the intrinsic lattice vibrations of octahedral and tetrahedral sites of the spinel structure, respectively, and characteristic vibration for capping agent. The magnetic properties of fine powders were investigated at room temperature by using a vibrating sample magnetometer. The room temperature M–H hysteresis loops show ferromagnetic behavior of the calcined samples, with specific saturation magnetization (M_s) values ranging between 11 and 53 emu/g.     

Monodispersed nanocrystalline Co1–xZnxFe2O4 particles by forced hydrolysis: Synthesis and characterization

Zinc-substituted cobalt ferrites, Co_1–xZn_xFe₂O₄, were for the first time successfully prepared by forced hydrolysis method. The obtained materials are single phase, monodispersed nanocrystalline with an average grain size of about 3 nm. These materials are superparamagnetic at room temperature and ferrimagnetic at temperature lower than the blocking temperature. When the zinc substitution increases from x=0 to 0.4, at 4.2 K, the saturation magnetization increases from 72.1 to 99.7 emu/g. The high saturation magnetization of these samples suggests that this method is suitable for preparing high-quality nanocrystalline magnetic ferrites for practical applications.     

Magnetic properties of prussian blue modified Fe3O4 nanocubes

Size controlled cubic Fe₃O₄ nanoparticles in the size range 90–10 nm were synthesized by varying the ferric ion concentration using the oxidation method. A bimodal size distribution was found without ferric ion concentration and the monodispersity increased with higher concentration. The saturation magnetization decreased from 90 to 62 emu/g when the particle size is reduced to 10 nm. The Fe₃O₄ nanoparticles with average particle sizes 10 and 90 nm were surface modified with prussian blue. The attachment of prussian blue with Fe₃O₄ was found to depend on the concentration of HCl and the particle size. The saturation magnetization of prussian blue modified Fe₃O₄ varied from 10 to 80 emu/g depending on the particle size. The increased tendency for the attachment of prussian blue with smaller particle size was explained based on the surface charge. The prussian blue modified magnetite nanoparticles could be used as a radiotoxin remover in detoxification applications.     

Comparative study of structural and magnetic properties of nano-crystalline Li0.5Fe2.5O4 prepared by various methods

Lithium ferrite has been considered as one of the highly strategic magnetic material. Nano-crystalline Li_0.5Fe_2.5O₄ was prepared by four different techniques and characterized by X-ray diffraction, vibrating sample magnetometer (VSM), transmission electron microscope (TEM) and Fourier transform infrareds (FTIR). The effect of annealing temperature (700, 900 and 1050 °C) on microstructure has been correlated to the magnetic properties. From X-ray diffraction patterns, it is confirmed that the pure phase of lithium ferrite began to form at 900 °C annealing. The particle size of as-prepared lithium ferrite was observed around 40, 31, 22 and 93 nm prepared by flash combustion, sol-gel, citrate precursor and standard ceramic technique, respectively. Lithium ferrite prepared by citrate precursor method shows a maximum saturation magnetization 67.6 emu/g at 5 KOe.     

Magnetoelectric characterization of xNi0.75Co0.25Fe2O4-(1−x)BiFeO3 nanocomposites

Magnetoelectric (ME) nanocomposites containing Ni_0.75Co_0.25Fe₂O₄-BiFeO₃ phases were prepared by citrate sol-gel process. X-ray diffraction (XRD) analysis showed phase formation of xNi_0.75Co_0.25Fe₂O₄-(1−x)BiFeO₃ (x=0.1, 0.2, 0.3 and 0.4) composites on heating at 700 °C. Transmission electron microscopy revealed the formation of powders of nano order size and the crystal size was found to vary from 30 to 85 nm. Dispersion in dielectric constant (ε) and dielectric loss (tan δ) in the low-frequency range have been observed. It is seen that nanocomposites exhibit strong magnetic properties and a large ME effect. On increasing Ni_0.75Co_0.25Fe₂O₄ contents in the nanocomposites, the saturation magnetization (M_S) and coercivity (H_C) increased after annealing at 700 °C. The large ME output in the nanocomposites exhibits strong dependence on magnetic bias and magnetic field frequency. The large value of ME output can be attributed to small grain size of ferrite phase of nanocomposite being prepared by citrate precursor process.     

Synthesis, structural and magnetic properties of the nanocomposite Fe63B23Nd7Y3Nb3Cr1 magnets

The Fe₆₃B₂₃Nd₇Y₃Nb₃Cr₁ nanocomposite magnets in the form of sheets have been prepared by copper mold casting technique. The phase evolution, crystal structure, microstructural and magnetic properties have been investigated in the as-cast and annealed states. The as-cast sheets show magnetically soft behaviors which become magnetically hard by thermal annealing. The optimal annealed microstructure was composed of nanosize soft magnetic α-Fe (19-29 nm) and hard magnetic Nd₂Fe₁₄B (45-55 nm) grains. The best hard magnetic properties such as intrinsic coercivity, _jH_c of 1119 kA/m, remanence, B_r of 0.44 T, magnetic induction to saturation magnetization ratio, M_r/M_s=0.61 and maximum energy product, (BH)_max of 55 kJ/m³ was obtained after annealing at 680 °C for 15 min. The annealing treatment above 680 °C results in non-ideal phase grains growth, which degrade the magnetic properties.     

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.     

Particles size effects of single domain CoFe2O4 on suspensions stability

Single domain magnetic CoFe₂O₄ nanoparticles with spinel structure were prepared by the coprecipitation method. Particles with size of 16, 20, 40 and 60 nm were synthesized by sintering the precursor at 500, 600, 800 and 900 °C, respectively. The magnetic hysteresis measurement of CoFe₂O₄ particles showed that particles were single domain particles with similar saturation magnetization (∼300 emu/cm³) at room temperature. The zeta potential study of suspensions (CoFe₂O₄-acetylacetone system) with various particle sizes showed the suspension systems had similar zeta potential values (∼40 mV). The effects of magnetic particle size on the suspension stability characterized by electrophoretic deposition yields and sediment volumes were studied. The suspension stability decreased with an increase in particle size and a flocculation threshold of particle radius a was found at 30 nm. A suspension stability theory approaching to the phenomenon was established. The theory based on the DLVO theory was developed by introducing an extra magnetic interaction force. Dormann model was adopted, in which the magnetic interactions of two spherical nanoparticles were investigated in terms of dipole-dipole interactions. Compared to DLVO, suspension's physical parameters not only zeta potential ζ and the Debye length 1/κ, but also particles' radius a brought about stable to flocculation transition in the theory.     

Simultaneous variations of microstructure and magnetic properties of Ni58Fe12Zr20B10 nanostructure/amorphous alloy prepared by mechanical alloying

This study aims to evaluate magnetic and micro-structural properties of amorphous/nanocrystalline mechanically alloyed Ni₅₈Fe₁₂Zr₂₀B₁₀ powders with ball-milling time up to 190 h. Structural, micro-structural and thermal evaluations of the milled powders were carried out by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and differential scanning calorimetry (DSC) methods. Magnetic properties were also measured by a vibrating sample magnetometer (VSM) instrument. Results showed that the amorphous phase reached maximum value of 95% and the crystallite size was about 3 nm at the end of the milling. Magnetization saturation (M_s) decreased slightly and coercivity (H_c) reached to the highest value at 72 h of the milling time. At the 190 h of milling, the coercivity and saturation magnetization reached 18 Oe and 20 emu/g, respectively. While, after an appropriate amount of heat treatment, these two variables became approximately 2 Oe and 32 emu/g.     

Effect of Cr100−xTix (x = 0-19 at.%) alloy underlayers on the high coercivity of FePt thin film below the substrate temperature of 250 °C

The effect of Cr_100−xTi_x underlayer on orderd-L1₀ FePt films was investigated. A low-temperature ordering of FePt films could be attained through changing the Ti content of Cr_100−xTi_x underlayer. The ordering temperature of the 30 nm FePt film grown on 20 nm Cr₉₀Ti₁₀ underlayer was reduced to 250 °C which is practical manufacture process temperature. An in-plane coercivity was very high to 6000 Oe and a ratio of remnant magnetization (M_r) to saturation magnetization (M_s) was as large as 0.85. This result indicates that the coercivity obtained at 250 °C by the effect of CrTi underlayer is significantly higher than those obtained at 250-275 °C by the effect of underlayers in other conventional studies. The prominent improvement of the magnetic properties of ordered FePt thin films at low temperature of 250 °C could be understood with considering the strain-induced ordering phase transformation associated with lattice mismatch between Cr underlayer and FePt magnetic layer due to an addition of Ti content.     

Synthesis, structure and magnetic properties of the new double perovskite Ca2CrSbO6

The title double perovskite has been synthesized by solid-state reaction in air. The crystal structure has been studied from powder X-ray diffraction data. Rietveld fits to the pattern show that this compound has a monoclinic symmetry [a=5.4932(3) Å, b=5.4081(3) Å, c=7.6901(5) Å, β=90.0022(1)°, at 300 K] defined in the space group P2₁/n, where the Cr and Sb cations are almost completely ordered in the B-sublattice of the perovskite structure. Magnetic susceptibility and magnetization measurements show that this compound behaves as a Curie-Weiss paramagnet at high temperatures with μ_eff=3.53(1) μ_B and θ_P=8 K, and exhibits a robust ferromagnetic component below the ordering temperature of T_C=13 K, with a saturation magnetization of 2.36 μ_B/f.u. at 5 K. To our knowledge, this is the first example of a ferromagnetic double perovskite containing a non-magnetic element, such as Sb, occupying one half of the B positions of the perovskite structure.     

Understanding the magnetic ground state of rare-earth intermetallic compound Ce4Sb3: Magnetization and neutron diffraction studies

Magnetization and neutron diffraction studies have been performed on Ce₄Sb₃ compound (cubic Th₃P₄-type, space group I4¯3d, no. 220). Magnetization of Ce₄Sb₃ reveals a ferromagnetic transition at ∼5 K, the temperature below which the zero-field-cooled and field-cooled magnetization bifurcate in low applied fields. However, a saturation magnetization (M_S) value of only ∼0.93μ_B/Ce³⁺ is observed at 1.8 K, suggesting possible presence of crystal field effects and a paramagnetic/antiferromagnetic Ce³⁺ moment. Magnetocaloric effect in this compound has been computed using the magnetization vs. field data obtained in the vicinity of the magnetic transition, and a maximum magnetic entropy change, −ΔS_M, of ∼8.9 J/kg/K is obtained at 5 K for a field change of 5 T. Inverse magnetocaloric effect occurs at ∼2 K in 5 T indicating the presence of antiferromagnetic component. This has been further confirmed by the neutron diffraction study that evidences commensurate antiferromagnetic ordering at 2 K in zero magnetic field. A magnetic moment of ∼1.24μ_B/Ce³⁺ is obtained at 2 K and the magnetic moments are directed along Z-axis.