The structure and magnetic properties of Zn1−xNixFe2O4 ferrite nanoparticles prepared by sol–gel auto-combustion

Zn_1−xNi_xFe₂O₄ ferrite nanoparticles were prepared by sol–gel auto-combustion and then annealed at 700 °C for 4 h. The results of differential thermal analysis indicate that the thermal decomposition temperature is about 210 °C and Ni–Zn ferrite nanoparticles could be synthesized in the self-propagating combustion process. The microstructure and magnetic properties were investigated by means of X-ray diffraction, scanning electron microscope, and Vibrating sample magnetometer. It is observed that all the spherical nanoparticles with an average grain size of about 35 nm are of pure spinel cubic structure. The crystal lattice constant declines gradually with increasing x from 0.8435 nm (x=0.20) to 0.8352 nm (x=1.00). Different from the composition of Zn_0.5Ni_0.5Fe₂O₄ for the bulk, the maximum M_s is found in the composition of Zn_0.3Ni_0.7Fe₂O₄ for nanoparticles. The H_c of samples is much larger than the bulk ferrites and increases with the enlarging x. The results of Zn_0.3Ni_0.7Fe₂O₄ annealed at different temperatures indicate that the maximum M_s (83.2 emu/g) appears in the sample annealed at 900 °C. The H_c of Zn_0.3Ni_0.7Fe₂O₄ firstly increases slightly as the grain size increases, and presents a maximum value of 115 Oe when the grains grow up to about 30 nm, and then declines rapidly with the grains further growing. The critical diameter (under the critical diameter, the grain is of single domain) of Zn_0.3Ni_0.7Fe₂O₄ nanoparticles is found to be about 30 nm.     

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.     

Microstructure and magnetic properties of nanocomposite FePt/MgO and FePt/Ag films

An in-plane magnetic anisotropy of FePt film is obtained in the MgO 5 nm/FePt t nm/MgO 5 nm films (where t=5, 10 and 20 nm). Both the in-plane coercivity (H_c∥) and the perpendicular magnetic anisotropy of FePt films are increased when introducing an Ag-capped layer instead of MgO-capped layer. An in-plane coercivity is 3154 Oe for the MgO 5 nm/FePt 10 nm/MgO 5 nm film, and it can be increased to 4846 Oe as a 5 nm Ag-capped layer instead of MgO-capped layer. The transmission electron microscopy (TEM)-energy disperse spectrum (EDS) analysis shows that the Ag mainly distributed at the grain boundary of FePt, that leads the increase of the grain boundary energy, which will enhance coercivity and perpendicular magnetic anisotropy of FePt film.     

Exchange bias in (1 1 0)-orientated Bi0.9La0.1FeO3/La0.5Ca0.5MnO3 films

We performed a systematic study on the exchange bias in (1 1 0)-orientated Bi_0.9La_0.1FeO₃/La_0.5Ca_0.5MnO₃ (BLFO/LCMO) heterostructure with a fixed BLFO film thickness of 600 nm and different LCMO layers ranging from t=0 to 30 nm. The LCMO is found to be weakly ferromagnetic, with the Curie temperature descending from ∼225 K to 0 as the layer thickness decreases from 30 nm to 3 nm. The main magnetic contributions come from the BLFO film, and the areal magnetization ratio is 1:0.07 for t=5 nm and 1:0.82 for t=30 nm for BLFO to LCMO at the temperature of 5 K. Further experiments show the presence of significant exchange bias, and it is, at the temperature of 10 K, ∼40 Oe for t=0 and ∼260 Oe for t=30 nm. The exchange bias reduces dramatically upon warming and disappears above the blocking temperature of the spin-glasslike behavior observed in the samples. The possible origin for exchange bias is discussed.     

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.     

The size effect of Ba0.6Sr0.4TiO3 thin films on the ferroelectric properties

Barium strontium titanate (BST) thin films were prepared by RF magnetron sputtering. The dielectric constant-voltage curves and the hysteresis loops of BST thin films with different grain sizes and film thicknesses were investigated. When the grain size increases from 12 nm to 35 nm, remarkable increases in dielectric constant and tunability were observed. Above 12 nm, the BST films exhibited size effects, i.e. a decrease in maximal polarization (P_m) and an increase in coercive electric field (E_c) with reduction in grain size. In our investigation, the dielectric constant, tunability and maximal polarization increased as the film thickness increased. Furthermore, the size dependence of the dielectric constant and tunability of Ba_0.6Sr_0.4TiO₃ thin films is determined by that of the maximal polarization and the coercive electric field.     

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.     

Influence of milling time on the structural, microstructural and magnetic properties of mechanically alloyed Ni58Fe12Zr10Hf10B10 nanostructured/amorphous powders

This paper investigates structural, microstructural and magnetic properties of amorphous/nanocrystalline Ni₅₈Fe₁₂Zr₁₀Hf₁₀B₁₀ powders prepared by high energy milling. Ball milling of Ni, Fe, Zr, Hf and B leads to alloying of the element powders at 120 h. The results show that at 190 h the amorphous content is at the highest level and the grain size is about 2 nm. The magnetic measurements reveal that the coercivity and the saturation magnetization reach about 20 Oe and 30 emu/g at 190 h and become approximately 5 Oe and 40 emu/g after a suitable heat treatment, respectively.     

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.     

Magnetic and magnetoelastic properties of Zn-doped cobalt-ferrites—CoFe2−xZnxO4 (x=0, 0.1, 0.2, and 0.3)

Cobalt-ferrite (CoFe₂O₄) based materials are suitable candidates for magnetomechanical sensor applications owing to a strong sensitivity of their magnetostriction to an applied magnetic field. Zn-doped cobalt-ferrites, with nominal compositions CoFe_2−xZn_xO₄ (x=0–0.3), were synthesized by auto-combustion technique using Co- , Fe- , and Zn-nitrate as precursors. X-ray spectra analysis and Transmission electron microscopy studies revealed that the as-prepared powders were comprised of nano-crystalline (∼25–30 nm) cubic-spinel phase with irregularly-shaped grains morphology along with minor impurity phases. Calcination (800 °C for 3 h) of the precursor followed by sintering (1300 °C for 12 h) resulted in a single phase cubic-spinel structure with average grain size ∼2–4 μm, as revealed from scanning electron micrographs. The magnitude of coercive field decreases from ∼540 Oe for x=0 to 105 Oe for x=0.30. Saturation magnetization initially increases and peaks to ∼87 emu/g for x=0.2 and then decreases. The peak value of magnetostriction monotonically decreases with increasing Zn content in the range 0.0–0.3; however the piezomagnetic coefficient (dλ/dH) reaches a maximum value of 105×10⁻⁹ Oe⁻¹ for x=0.1. The observed variation in piezomagnetic coefficient in the Zn substituted cobalt ferrite is related to the reduced anisotropy of the system. The Zn-doped cobalt-ferrite (x=0.1) having high strain derivative could be a potential material for stress sensor application.     

The role of the dopant in the magnetism of Fe-doped SnO2 films

Transparent pure and Fe-doped SnO₂ thin films were grown by pulsed laser deposition technique on LaAlO₃ substrates. X-ray diffraction shows that the films are polycrystalline and have the rutile structure. Surprisingly, the pure film presents magnetic-like behavior at room temperature with a saturated magnetization of almost one-third of the doped film (∼3.6 and 11.3 emu/g, respectively) and its magnetization could not be attributed to any impurity phase. Taking into account the magnetic moment measured in the pure film, the effective contribution of the impurity in the doped one can be inferred to be ∼2 μ_B per Fe atom. A large magnetic moment was also predicted by an ab initio calculation in the doped system, which increases if an oxygen vacancy is present near the Fe impurity.     

Effect of Co underlayer on soft magnetic properties and microstructure of FeCo thin films

High saturation magnetization soft magnetic FeCo (=Fe₆₅Co₃₅) films were prepared using a thin Co underlayer. The FeCo/Co films exhibited a well-defined in-plane uniaxial anisotropy with easy axis coercivity (H_ce) of 10 Oe and hard axis coercivity (H_ch) of 3 Oe, and a half reduction of H_c with H_ce=4.8 Oe and H_ch=1.0 Oe was obtained when the composition was adjusted to 25 at% Co. The effective permeability of the films remains flat around 250 to 800 MHz. The saturation magnetostriction was 5.2×10⁻⁵ and the intrinsic stress was 0.8 GPa in FeCo single layer, both were slightly reduced by Co underlayer. The Co underlayer changed the preferred orientation of the FeCo films from (2 0 0) to (1 1 0) but more significantly, reduced the average grain size from ∼74 to ∼8.2 nm. It also reduced the surface roughness from 2.351 to 0.751 nm. The initial stage and interface diffusion properties were examined by TEM and XPS.     

Magnetic properties of strontium hexaferrite films prepared by pulsed laser deposition

The magnetic properties of strontium hexaferrite (SrFe₁₂O₁₉) films fabricated by pulsed laser deposition on the Si(100) substrate with Pt(111) underlayer have been studied as a function of film thickness (50–700 nm). X-ray diffraction patterns confirm that the films have c-axis perpendicular orientation. The coercivities in perpendicular direction are higher than those for in-plane direction which indicates the films have perpendicular magnetic anisotropy. The coercivity was found to decrease with increasing of thickness, due to the increasing of the grain size and relaxation in lattice strain. The 200 nm thick film exhibits hexagonal shape grains of 150 nm and optimum magnetic properties of M_s=298 emu/cm³ and H_c=2540 Oe.     

Growth of superconducting FeSe0.5Te0.5 thin films by pulsed-laser deposition

FeSe_0.5Te_0.5 thin films with PbO-type structure are successfully grown on MgO(1 0 0) and LaSrAlO₄(0 0 1) substrates from FeSe_0.5Te_0.5 or FeSe_0.5Te_0.75 polycrystalline targets by pulsed-laser deposition. The film deposited on the MgO substrate (film thickness ∼ 55 nm) shows superconductivity at 10.6 K (onset) and 9.2 K (zero resistivity). On the other hand, the film deposited on the LaSrAlO₄ substrate (film thickness ∼ 250 nm) exhibits superconductivity at 5.4 K (onset) and 2.7 K (zero resistivity). This suggests the strong influence of substrate materials and/or the c-axis length to superconducting properties of FeSe_0.5Te_0.5 thin films.     

Effect of surface-topology on magnetic properties in (La0.6Eu0.4)0.67Ca0.33MnO3 of anisotropic shape of nanoplates

(La_0.6Eu_0.4)_0.67Ca_0.33MnO₃ has been prepared in the shape of nanoplates of single crystallites (an orthorhombic structure) through polymer templates. HRTEM images reveal 18, 25, and 30 nm thicknesses of plates after heating a precursor powder at 873, 1073, and 1273 K in air for 2 h. These values present average crystallite size determined from broadening of the X-ray diffraction peaks. A spin-glass-like surface (GS) overlayer (3-5 nm thickness) in such plates facilitates a ferromagnetic→ferrimagnetic reordering with markedly suppressed Curie point T_C, i.e., as small as 90 K in a 873 K heated sample, from the parent value 268 K. The T_C point increases to 103 K (or 120 K) when heating at higher temperature 1073 (or 1273 K), during which the core grows at the expense of the overlayer. The GS tailors as high coercivity H_c as 617 Oe in the zero field cooled (ZFC) sample that is decreased to 500 Oe in the field cooled (FC) sample in the surface spin-freezing along the field direction. The H_c-value (ZFC) that steps down successively to 252 Oe on the overlayer is thinned down by heating at 1273 K. Samples heated at 873, 1073, or 1273 K have regularly increased saturation magnetization 35.3, 63.9, or 69.6 emu/g in ZFC, while 43.7, 70.2, or 75.5 emu/g in FC measured at 10 K. The ferrimagnetic reordering are described based on the scenario of an antiferromagnetic exchange coupling between the Eu³⁺ and Mn³⁺ (or Mn⁴⁺) sublattices.     

Ni80Fe20 permalloy nanoparticles: Wet chemical preparation, size control and their dynamic permeability characteristics when composited with Fe micron particles

Ni₈₀Fe₂₀ permalloy nanoparticles (NPs) have been prepared by the polyol processing at 180 °C for 2 h and their particle sizes can be precisely controlled in the size range of 20-440 nm by proper addition of K₂PtCl₄ agent. X-ray diffraction results show that the Ni-Fe NPs are of FCC structure, and a homogeneous composition and a narrow size distribution of these NPs have been confirmed by scanning electron microscopy assisted with energy dispersion spectroscopy of X-ray (SEM-EDX). The saturation magnetization of ~440nm NPs is 80.8 emu/g that is comparable to that of bulk Ni₈₀Fe₂₀ alloys, but it decreases to 28.7 emu/g for ~20 nm NPs. The coercive force decreases from 90 to 3 Oe with decreasing NP size. The wide range of particle size is exploited to seek for high permeability composite particles. The planar type samples composed of the NiFe NPs exhibit low initial permeability due to the deteriorated magnetic softness and low packing density. However, when they are mixed with Fe micron particles, the initial permeability significantly increases depending on the mixing ratio and the NiFe NP size. A maximum initial permeability is achieved to be ~9.1 at 1 GHz for the Fe-10 vol%NiFe (~20 nmΦ), which is about three times that of pure Fe micron particles. The effects of Ni-Fe particle size, volume percentage and solvent on the static and dynamic permeability are discussed.     

Acquisition of vector hysteresis loops from micro-arrays of nano-magnets

A modified scanning Kerr microscope has been used as a static Kerr magnetometer to acquire in-plane vector hysteresis loops from square Si/Ta(50 Å)/Co₈₀Fe₂₀(40 Å)/Ni₈₈Fe₁₂(108 Å)/Ta(100 Å) elements with size ranging from 123 nm to 10 μm. The nanoscale elements were arranged in square arrays of 4 μm size. The laser beam was focused to a sub-micron spot, while polarization changes were recorded with an optical bridge detector containing a beam-splitting polarizer and two quadrant photodiodes. The coercive field exhibited a non-monotonic increase from 11 Oe in the 10 μm element to 170 Oe in the 123 nm elements. Loops acquired with the field applied parallel to the easy and hard in-plane uniaxial anisotropy axes were observed to become more similar in shape as the element size decreased.     

Simple recipe to synthesize single-domain BaFe12O19 with high saturation magnetization

We report a new synthesis route for preparation of single-domain barium hexaferrite (BaFe₁₂O₁₉) particles with high saturation magnetization. Nitric acid, known as a good oxidizer, is used as a mixing medium during the synthesis. It is shown that formation of BaFe₁₂O₁₉ phase starts at 800 °C, which is considerably lower than the typical ceramic process and develops with increasing temperature. Both magnetization measurements and scanning electron microscope micrographs reveal that the particles are single domain up to 1000 °C at which the highest coercive field of 3.6 kOe was obtained. The best saturation magnetization of ≈60 emu/g at 1.5 T was achieved by sintering for 2 h at 1200 °C. Annealing at temperatures higher than 1000 °C increased the saturation magnetization, on the other hand, decreased the coercive field which was due to the formation of multi-domain particles with larger grain sizes. It is shown that the best sintering to obtain fine particles of BaFe₁₂O₁₉ occurs at temperatures 900-1000 °C. Finally, magnetic interactions between the hard BaFe₁₂O₁₉ phase and impurity phases were investigated using the Stoner-Wohlfarth model.     

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.     

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.