Investigations on magnetic particles prepared by cyclic growth

Magnetic iron oxide nanoparticles are promising tools for medical applications like hyperthermia or magnetic drug targeting. The relevant properties in these applications strongly depend on particle size and size distribution. In order to investigate the influence of mean size as well as size distribution, iron oxide powders from particles by a cyclic method based on “conventional” precipitation from Fe-salt solution were prepared. Increasing mean particle size with increasing number of cycles is confirmed by XRD. Magnetic parameters of the saturation hysteresis loop, hysteresis losses calculated from minor loops and switching field distributions are shown. A fractionation experiment on a fluid sample of particles prepared by 3 cycles was carried out in order to improve the hysteresis losses.     

The influence of a demagnetizing field on hysteresis losses in a dense assembly of superparamagnetic nanoparticles

The electrodynamic method is used to measure the hysteresis losses of a dense assembly of magnetite nanoparticles with an average diameter D=25 nm in the frequency range f=10–150 kHz and for magnetic field amplitudes H₀=100–300 Oe. It is found that the specific loss power is determined by a demagnetizing factor of a whole sample. It diminishes approximately 4.5 times when the sample aspect ratio decreases from L/d=11.4 to L/d≈1, where L and d are the sample length and diameter, respectively. For H₀≤300 Oe the maximal specific loss power 120 W/g is obtained for the sample with L/d=11.4 at f=120 kHz. For comparison, the assembly specific absorption rate has been determined also by means of direct measurement of the temperature difference between the inner and outer surfaces of a flat cuvette containing magnetic nanoparticles. For both methods of measurement close values for the specific absorption rate are obtained for samples with similar demagnetizing factors.     

Effects of size and interactions on the magnetic behaviour of elliptical (0 0 1)Fe nanoparticles

Arrays of elliptical particles with aspect ratio 1:3 and short axes 50, 100 and 150 nm were prepared by electron-beam lithography and ion-beam milling of epitaxial (0 0 1)Fe films of thicknesses 10 and 20 nm. The domain state of an individual particle imaged by magnetic force microscopy in zero field after demagnetization was observed to change from being bi-domain or multidomain (MD) to stable single domains (SD) as the lateral size and film thickness were decreased. The critical size for SD formation was found to be close to the actual lateral sizes of 100 nm×300 nm and 150 nm×450 nm for the thicknesses of 20 and 10 nm, respectively. Only in the 10 nm thick ellipses of lateral size 100 nm×300 nm, the magnetization reversal may take place through coherent rotation. For all other investigated samples, the experimental switching field is lower than what would be required for this process.     

Sol-gel derived nanoparticles of Zn substituted lithium ferrite (Li0.32Zn0.36Fe2.32O4): magnetic and Mössbauer effect measurements and their theoretical analysis

Nanoparticles of Zn substituted lithium ferrite (Li_0.32Zn_0.36Fe_2.32O₄) have been prepared by a sol-gel method where the ultra-sonication technique has been adopted to reduce the agglomeration effect among the nanoparticles. The samples were heat-treated at three different temperatures and the formation of the nanocrystalline phase was confirmed by X-ray diffractograms (XRD). The average particle size of each sample has been estimated from the (311) peak of the XRD pattern using the Debye-Scherrer formula and the average sizes are in the range of 10-21 nm. The average particle size, crystallographic phase, etc. of some selected samples obtained from the high-resolution transmission electron microscopy are in agreement with those estimated from the XRD patterns. Static magnetic measurements viz., hysteresis loops, field cooled and zero field cooled magnetization versus temperature curves of some samples carried out by SQUID in the temperature range of 300 to 5 K clearly indicate the presence of superparamagnetic (SPM) relaxation of the nanoparticles in the samples. The maximum magnetization of the SPM sample annealed at 500 °C is quite high (68 Am²/Kg) and the hysteresis loops are almost square shaped with very low value of coercive field at room temperature (827.8 A/m). The particle size, magneto-crystalline anisotropy, etc. have been estimated from the detailed theoretical analysis of the static magnetic data. The dynamic magnetic behavior of the samples was also investigated by observing the ac hysteresis loops and magnetization versus field curves with different time windows at room temperatures. The different soft magnetic quantities viz., coercive field, magnetization, remanance, hysteresis losses, etc. were extracted from dynamic measurements. Dynamic measurements confirmed that the samples are in their mixed state of SPM and ordered ferrimagnetic particles, which is in good agreement with the results of static magnetic measurements. Mössbauer spectra of the samples recorded at room temperature (300 K) and at different temperatures down to 20 K confirmed the presence of the SPM relaxation of the nanoparticles of the samples.     

VSM and Mössbauer study of nanostructured hematite

In the present work, we have synthesized nanostructured hematite samples using chemical precipitation method. The crystal structure and the grain size of the samples were studied using XRD. The zero field cooled and field cooled magnetization curves of the samples were recorded in the temperature range from 300 to 10 K. The variations of Morin transition temperature and blocking temperature with the grain size of the samples were investigated. The hysterics curves of the samples were recorded and the samples showed a superparamagnetic nature at room temperature whereas, at 10 K the samples showed open hysteresis curves. The sample with smaller grain size showed higher value of coercivity compared to samples with larger grain size. Mössbauer spectra of the samples were recorded and the grain size dependence on Mössbauer parameters was investigated.     

Nitrogen containing barium titanate: Preparation and characterisation

The purpose of this work is to study the incorporation of nitrogen into barium titanate at oxygen sites by ammonolysis. The nitrogen content of the resulting barium titanate powders strongly depends on the flow rate of ammonia and the ratio Ba/Ti. The amount of resulting oxygen vacancies is lowered by co-doping with fluoride and tantalum.By avoiding an excess of BaO and exchanging 10 mol% Ti by Ta it is possible to produce powders with nitrogen content of more than 1 wt%. Partial substitution of barium oxide by barium fluoride also increases the nitrogen content. The nitrogen containing powders are colored and show an additional absorption at 500 nm measured in the UV/vis reflectance spectrum. The concentrations of nitrogen and fluorine in the samples were determined using ion-sensitive electrodes. X-ray powder diffraction shows the composition of the powders. The thermal stability was investigated by thermogravimetric analysis, and in order to calculate the specific surface area the three-point BET method was used.     

Study on electro-magnetic properties of La substituted Ni–Cu–Zn ferrite synthesized by auto-combustion method

(Ni_0.25Cu_0.20Zn_0.55)La_xFe_2−xO₄ ferrite with x=0.00, 0.025, 0.050 and 0.075 compositions were synthesized through nitrate–citrate auto-combustion method. Crystalline spinel ferrite phase with about 16–19 nm crystallite size was present in the as-burnt ferrite powder. These powders were calcined, compacted and sintered at 950 °C for 4 h. Initial permeability, magnetic loss and AC resistivity of different compositions were measured in the frequency range from 10 Hz to 10 MHz. Saturation magnetization and hysteresis parameters were measured at room temperature with a maximum magnetic field of 10 kOe. Permeability and AC resistivity were found to increase and magnetic loss decreased with La substitution for Fe, up to x=0.025. Saturation magnetization and coercive field also increases up to that limit. The electromagnetic properties were found best in the ferrite composition of x=0.025, which would be better for more miniaturized multi layer chip inductor.     

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.     

Dependence of the power losses of a non-oriented 3% Si-steel on frequency and gauge

The power losses of a non-oriented 3% Si-steel rolled to gauges between 0.05 and 2 mm and heat-treated thereafter have been measured under sinusoidal polarizations at frequencies between 15 Hz and 10 kHz. The losses were analysed using a loss separation model based on statistical theory. For the thick samples the skin effect caused the model to fail above a certain frequency, while for the very thin samples the model seems to describe the losses well at all frequencies studied.     

Synthesis and magnetic properties of NiFe2−xAlxO4 nanoparticles

Nanocrystalline Al-doped nickel ferrite powders have been synthesized by sol–gel auto-ignition method and the effect of non-magnetic aluminum content on the structural and magnetic properties has been studied. The X-ray diffraction (XRD) revealed that the powders obtained are single phase with inverse spinel structure. The calculated grain sizes from XRD data have been verified using transmission electron microscopy (TEM). TEM photographs show that the powders consist of nanometer-sized grains. It was observed that the characteristic grain size decreases from 29 to 6 nm as the non-magnetic Al content increases, which was attributed to the influence of non-magnetic Al concentration on the grain size. Magnetic hysteresis loops were measured at room temperature with a maximum applied magnetic field of ≈1 T. As aluminum content increases, the measured magnetic hysteresis curves become more and more narrow and the saturation magnetization and remanent magnetization both decreased. The reduction of magnetization compared to bulk is a consequence of spin non-collinearity. Further reduction of magnetization with increase of aluminum content is caused by non-magnetic Al³⁺ ions and weakened interaction between sublattices. This, as well as the decrease in hysteresis was understood in terms of the decrease in particle size.     

Magnetic properties in Fe-doped NiO synthesized by co-precipitation

The magnetic properties of 1.5 at% Fe-doped NiO bulk samples were investigated. The samples were prepared by sintering the corresponding precursor in air at temperatures between 400 and 800 °C for 6 h. The synthesis was by a chemical co-precipitation and post-thermal decomposition method. In order to allow a comparison, a NiO/0.76 at% NiFe₂O₄ mixture was also prepared. The X-ray diffraction pattern shows that the samples that were sintered at 400 and 600 °C remain single phase. As the sintering temperature increased to 800 °C, however, the sample becomes a mixture of NiO and NiFe₂O₄ ferrite phases. The samples were investigated by measuring their magnetization as a function of magnetic field. The samples sintered between 400 and 800 °C and the one mixed directly with NiFe₂O₄ nanoparticles show a coercivity value of H_c≈200, 325, 350 and 110 Oe, respectively. The magnetic properties of the samples depend strongly on the sintering temperature. Simultaneously, the field-cooling hysteresis loop shift also observed after cooling the sample sintered at 600 °C to low temperature suggests the possibility of the existence of a ferromagnetic/antiferromagnetic exchange coupling.     

High-frequency properties of thin amorphous ribbons

Magnetic energy losses have been investigated in Co-based near-zero-magnetostriction amorphous ribbons from DC to 10 MHz. Attention has been devoted to the properties of field-annealed ribbons thinned down to 5.8 μm and their behavior at high frequencies. A rationale is provided for the frequency dependence of the magnetic losses over the investigated many-decade range through analysis of the loss components. Ribbons annealed under transverse field benefit by limited irreversible domain wall activity and correspondingly reduced hysteresis and excess losses. Based on the near-linear response of the material and the permeability–energy loss relationship, the separate contributions of domain wall displacements and rotations to the magnetization process and the related dissipation effects are singled out at all frequencies. Very thin amorphous ribbons are shown to display lower loss and higher permeability (i.e. higher Snoek's product) than Mn–Zn ferrites at all frequencies.     

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.     

Hysteresis loss subdivision

Assuming that different energy dissipation mechanisms are at work along hysteresis, a hysteresis loss subdivision procedure has been proposed, using the induction at maximum permeability (around 0.8 T, in electrical steels) as the boundary between the “low-induction” and the “high-induction” regions. This paper reviews the most important results obtained in 10 years of investigation of the effect of microstructure on these components of the hysteresis loss. As maximum induction increases, the “low-induction loss” increases linearly up to 1.2 T, while the “high-induction loss” is zero up to 0.7 T and then increases as a power law with n=5. Low-induction loss behavior is linearly related to H_c between 0.4 and 1.2 T. Grain size has a larger influence on low-induction losses than on high-induction losses. Texture has a much stronger influence on high loss than on low-induction loss, and it is related to the average magnetocrystalline energy. 6.5%Si steel shows smaller hysteresis loss at 1.5 T than 3.5%Si steel only because of its smaler high-induction component. The abrupt increase in hysteresis loss due to very small plastic deformation is strongly related to the high-induction loss component. These results are discussed in terms of energy dissipation mechanisms such as domain wall movement, irreversible rotation and domain wall energy dissipation at domain nucleation and annihilation.     

Experimental analysis of the fast magnetization dynamics in high perpendicular anisotropy Co/Pt nanostructures

We present preliminary results for magnetization dynamics in high perpendicular anisotropy Co/Pt cross-shaped nanostructures submitted to a single/train out-of-plane field pulses. For this, a fabrication process has been developed to obtain high-quality microwave test structures. Using extraordinary Hall effect, we measure quasistatic hysteresis loops while applying either a single or a train of field pulses of given amplitude (32 mT at maximum), variable duration (0.2–10 ns) and repetition frequency (1–100 kHz). We get that field pulses affect only one side of the hysteresis loops without any clear change in the squareness of the loop. Such behavior can be most probably attributed to pulse-induced nucleation of reverse domains in the cross area. The measured shift in coercive field decreases linearly with the logarithm of the pulse duration and two switching regimes that get connected for pulse duration around 1 ns have been observed. Switching probability with single pulse showed that field pulse of 10 ns and 32 mT of magnitude has the same effect of 2 mT static fields.     

Effect of Zr:Sn ratio in the lead lanthanum zirconate stannate titanate ceramics on microstructure and electric properties

The effect of variations of the Zr:Sn ratio on the microstructure and electric properties of lead lanthanum zirconate stannate titanate (PLZST) antiferroelectric ceramics were investigated. The precursor powders were synthesized by the modified coprecipitation method and all the samples were pure perovskite phase in the XRD patterns. The ceramics sintered at 1100 °C exhibited the highest relative density. With the increasing of Sn⁴⁺ content, the grain size of the ceramics was decreased in the SEM and the maximum dielectric constant and the corresponding temperature were decreased. The P-E hysteresis loops indicated that it is helpful to steady the antiferroelectric phase by increasing Sn⁴⁺ content.     

The effect of La-substitution on magnetic properties of nanosized Sr1−xLaxTi0.05Zn0.2(Fe)11.75−δ(Fe)δO19 powders

A series of La-substituted M-type Sr hexaferrite powders Sr_1−xLa_xTi_0.05Zn_0.2Fe³⁺_11.75O₁₉, wherein x ranges from 0.1 to 0.5 with a step of 0.1, have been prepared by the conventional ceramic method and were then milled in a high energy mill to prepare nanosized powders. XRD investigation of the calcined and the milled powders shows that single phase hexaferrite structure has been formed after calcining and has not changed after milling. The lattice parameters and the mean crystallite sizes of the samples have been determined from the XRD data and Scherrer's formula. The results show that the lattice parameters (“а” and “c”) decrease with increase in La-substitution and the mean crystallite size of the milled powders is about 17 nm. Coercivities and magnetizations of the samples in a magnetic field of 16 kOe have been determined from the room temperature hysteresis loops. It was found that both parameters increase with La substitutions up to 0.3 and then decrease for higher substitutions. These variations were attributed to the enhancement of hyperfine field and spin-canting magnetic structure when La content increases. In addition, the magnetizations were smaller for the nanosized samples in comparison with those of bulk ones, which were discussed according to the core-shell model. Also the results show that annealing of the nanosized samples up to 500 °C can enhance coercivity and magnetization of the samples, which is discussed based on crystallite size growth.     

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.     

Mössbauer and magnetic studies in nickel ferrite nanoparticles: Effect of size distribution

The magnetic properties of nickel ferrite nanoparticles in the form of powders, prepared by the sol-gel process and subjected to different annealing temperatures, were investigated using both static and dynamic measurements namely hysteresis, zero field cooled-field cooled magnetization (ZFC-FC) measurements and Mössbauer spectroscopy. The Transmission Electron Microscopy (TEM) studies reveal particle sizes for the as-prepared particles which increases upto 52 nm with annealing. A bimodal distribution, upto an annealing temperature of was observed. ZFC-FC measurements for the as-prepared samples reveal twin peaks, indicative of the bimodal size distribution. ZFC-FC measurements performed for fields varying from 100 Oe to 3 kOe show a superparamagnetic phase with blocking temperatures between 320 and . Numerical simulations for the ZFC-FC studies indicate that the signature of the bimodal size distribution can be seen only at very low fields. The variation of coercivity with particle size, as determined from the hysteresis measurements, shows a transition from a single domain to a multi domain state for particle sizes larger than 35 nm. Mössbauer measurements performed at room temperature for the as-prepared sample shows a six finger pattern for the samples with higher particle size and a doublet pattern for the samples with smaller particle size, which is indicative of their superparamagnetic nature.     

Structural, electrical and magnetic characterizations of Ni-Cu-Zn ferrite synthesized by citrate precursor method

Fine powders of NiCuZn ferrite with composition Ni_(0.7−x)Cu_xZn_0.3Fe₂O₄ (where x=0, 0.2, 0.4 and 0.6) were prepared by the citrate precursor method. X-ray diffraction measurements confirm the formation of single-phase cubic spinel structure. The grain size was estimated by SEM micrograph which increases with Cu content. Dielectric constant (?) and loss tangent (tan δ) were measured as a function of frequency. The ? and tan δ show a decreasing trend with increase of frequency for all the samples. The DC resistivity was measured as a function of temperature. The temperature-dependent DC resistivity measurements show that the room-temperature DC resistivity of NiCuZn ferrite with x=0.2 is of the order of 10⁹ Ω cm. The AC conductivity (σ_AC) was studied as a function of frequency. The hysteresis data indicate that the maximum saturation magnetization of 38.66 emu/g is obtained for the composition with x=0.2.