Nanostructural,magnetic and Mössbauer studies of nanosized Co1−xZnxFe2O4 synthesized by co-precipitation

This work presents a systematic investigation on the structural and magnetic properties of Co_1−xZn_xFe₂O₄ (0.5<x<0.75) nanoparticles synthesized by the chemical co-precipitation method. The X-ray diffraction analysis, the Fourier Transform Infrared (FTIR) and the Vibrating Sample Magnetometer were carried out at room temperature to study the micro-structural and magnetic properties. The X-ray measurements revealed the production of a broad single cubic phase with the crystallite size within the range of 6–10 nm. The FTIR measurements between 400 and 4000 cm⁻¹ confirmed the intrinsic cation vibrations of the spinel structure. The magnetic measurements show that the saturation magnetization and coercivity decrease by increasing the zinc content. Furthermore, the results reveal that the sample with a chemical composition of Co_0.3Zn_0.7Fe₂O₄ exhibits the super-paramagnetic behavior and the Curie point of 97 °C.     

Heat generation of aqueously dispersed CoFe2O4 nanoparticles as heating agents for magnetically activated drug delivery and hyperthermia

Self-heating from magnetic nanoparticles under AC magnetic field can be used either for hyperthermia or to trigger the release of an anti-cancer drug, using thermo-responsive polymers. The heat generated by applying an AC magnetic field depends on the properties of magnetic nanoparticles (composition, size, crystal structure) as well as the frequency and amplitude of the magnetic field. Before these systems can be efficiently applied for in vitro or in vivo studies, a thorough analysis of the magnetically induced heating is required. In this study, CoFe₂O₄ nanoparticles were synthesized, dispersed in water, and investigated as heating agents for magnetic thermo-drug delivery and hyperthermia. The temperature profiles and infrared (IR) camera images of heat generation of CoFe₂O₄ nanoparticles under various AC magnetic fields of 127–700 Oe at 195, 231, and 266 kHz were measured using an IR thermacam, excluding the external AC magnetic field interruption. The CoFe₂O₄ nanoparticles were successfully dispersed in water using an 11-mercaptoundecanoic acid ligand exchange method to exchange the solvent used for synthesis of hexane for water. During the heating experiments, each of CoFe₂O₄ nanoparticle solutions reached a steady state where the temperature rose between 0.1 and 42.9 °C above ambient conditions when a magnetic field of 127–634 Oe was applied at 231 or 266 kHz. The heat generation was found to be dependent on the intensity of AC magnetic field and applied frequency. Therefore, the desired heating for magnetically triggered drug delivery or hyperthermia could be achieved in water-dispersed CoFe₂O₄ nanoparticles by adjusting the AC magnetic field and frequency.     

Magnetic properties of nanocrystalline CoFe2O4 synthesized by modified citrate-gel method

Nanocrystalline CoFe₂O₄ with an average grain size of about 40 nm was successfully prepared by a modified citrate-gel method. At temperatures of 3 and 300 K, the measured coercive fields are 0.43 and 0.07 T and the magnetizations at 7 T are 89 and 83 emu/g, respectively. At room temperature, the longitudinal and transversal magnetostriction values are −130 and 70 ppm, respectively. The contribution of a disordered magnetic phase was detected by the occurrence of a peak in the ac-susceptibilities curves at around 250 K. The temperature dependence of the field-cooled and zero field-cooled low-field magnetization showed a larger irreversibility below this temperature. This disordered phase behaves like a spin-glass, which is coexisting with the ferrimagnetically ordered main phase     

Hydrothermal synthesis and magnetic properties of gadolinium-doped CoFe2O4 nanoparticles

CoFe_2−xGd_xO₄ (x=0-0.25) nanoparticles were synthesized via a simple hydrothermal process at 200 °C for 16 h without the assistance of surfactant. The as-synthesized powders were characterized by X-ray diffraction, transmission electron microscopy, and a vibrating sample magnetometer. The X-ray diffraction results showed that the as-synthesized powders were in the pure phase with a doping amount of ≤0.25, and the peaks could be readily indexed to the cubic spinel cobalt ferrite. Transmission electron microscopy and high resolution transmission electron microscopy observations revealed that the gadolinium-doped cobalt ferrite nanoparticles were single crystal, roughly spherical, uniformly distributed, and not highly agglomerated. The room temperature magnetic field versus magnetization measurements confirmed a strong influence of gadolinium doping on the saturation magnetization and coercivity due to large lattice distortion and grain growth of small particles.     

Size-induced effect on nano-crystalline CoFe2O4

Cobalt ferrite (CoFe₂O₄) nano-particles have been synthesized successfully and we studied the effect of temperature on them. The particles have been annealed at different temperatures ranging from 373 to 1173 K. Significant effect on the physical parameters like crystalline phase, crystallite size, particle size, lattice strain and magnetic properties of the nano-particles has been investigated. The studies have been carried out using a powder X-ray diffractometer (XRD), a transmission electron microscope (TEM) and a vibrating sample magnetometer (VSM). A thorough study of the variation of specific surface area and particle size with annealing is presented here, with their effects on saturation magnetization.     

Effect of Li2O doping on electrical properties of CoFe2O4

The solid–solid interactions between cobalt and ferric oxides to produce CoFe₂O₄ were followed up using XRD investigation. The effect of Li₂O-doping on the ferrite formation was also studied. The electrical and dielectric parameters of pure and doped mixed solids precalcined at 1273 K were measured using d.c and a.c instruments.The dopant concentration was varied between 0.5 and 6 mol% Li₂O. The results obtained revealed that Li₂O doping much enhanced the ferrite formation due to an increase in the mobility of the reacting species.

The addition of the smallest amount of Li₂O (0.5 mol%) resulted in measurable variations in the electrical constants (ρ, E_a, ′, ″ and tan δ). Resistivity increased upon increasing the dopant concentration up to 1.5 mol% exceeding the values measured for the undoped sample. Furthermore, the presence of 6 mol% Li₂O brought about a significant decrease of electrical resistivity. Also, the activation energy decreased with increasing the dopant concentration. The dielectric constant behaves according to ε=const. 1/ρ^1/2.

The Li₂O-doping modified the values of different dielectric constants, the change in these constants was found to be strongly dependent on the amount of Li₂O added.These results have been discussed in terms of the potentiality of Li₂O in increasing the mobility of the reacting species involved in the ferrite formation.     

Synthesis of nanocrystalline spinel CoFe2O4 via a polymer-pyrolysis route

Nanocrystalline CoFe₂O₄ spinel ferrites were synthesized via the pyrolysis of polyacrylate salt precursors prepared by in situ polymerization of metal salts and acrylic acid. The pyrolytic behaviors of the polymeric precursors were analyzed by use of simultaneous thermogravimetric and differential thermal analysis (TG-DTA). The structural characteristics of the calcined products were obtained by powder X-ray diffraction (XRD), infrared spectroscopy (IR) and transmission electron microscope (TEM). The results revealed that cobalt ferrites had nano-sized morphology and good crystallinity even if calcined at moderate temperature like 500 °C for 3 h. The average size of nanocrystalline cobalt ferrites ranged from 20 to 30 nm with a narrow size distribution, while the particle size increased with the increase of the calcination temperature. Magnetic properties were obtained at room temperature using a vibrating sample magnetometer. The samples exhibited hysteresis loop typical of magnetic behaviors, indicating that the presence of an ordered magnetic structure could exist in the mixed spinel system. The as-calcined cobalt ferrites at 500 °C exhibited the highest magnetization value of 77.4 emu/g at 10 kOe, while the highest remanence and coercivity of 35.6 emu/g and 1445 Oe, respectively, for those calcined at 700 °C were obtained.     

X-ray diffraction studies on crystallite size evolution of CoFe2O4 nanoparticles prepared using mechanical alloying and sintering

Nanosized cobalt ferrite spinel particles have been prepared by using mechanically alloyed nanoparticles. The effects of various preparation parameters on the crystallite size of cobalt ferrite which includes milling time; ball-to powder weight ratio (BPR) and sintering temperature, were studied using X-ray diffractometer (XRD). Scherrer's equation was used to study the crystallite size evolution of the as-prepared materials. The results of the as-milled sample revealed that both milling time and BPR plays a role in determining the crystallite size of the milled powder. However, where sintering is involved, the sintering temperature results in grain growth, and thus plays a dominant role in determining the final crystallite size of the samples sintered at higher temperature (above 900 °C). From the vibrating-sample magnetometer (VSM) measurement it was observed that the coercivity of the as-milled samples without sintering is almost negligible, which is a type characteristic of superparamagnetic material. However, for the sintered samples, the saturation increases while coercivity decreases with increases sintering temperature.     

Magnetoelectric properties of particulate and bi-layer PMN-PT/CoFe2O4 composites

Our studies comprise electrical dielectric and magnetoelectric properties of CoFe₂O₄ (CFO) and Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ [PMN-PT] magnetoelectric composites. The individual phases were prepared by conventional ceramic method. The particulate composites of ferrite and ferroelectric phases were prepared in ferroelectric rich region. Presence of both the phases in the composites was confirmed using X-ray diffraction techniques. The scanning electron microscopic images recorded in backscattered mode were used to study the microstructure of composites. Lattice constant, dielectric constant, electrical resistivity, ferroelectric, and magnetic properties of individual as well as particulate composites were studied. Further the bi-layer composites were made using the discs obtained from the powders of individual phases where hot press technique was employed to obtain disc of individual phases. CFO phase used in bi-layer composites was obtained using chemical co-precipitation technique. Magnetoelectric (ME) measurements were carried out on both, particulate and layered magnetoelectric composites. Comparison of ME signal obtained from particulate and layered composites revealed that the layered composites gives superior magnetoelectric signal. ME data obtained for layered composites show good agreement with the theoretical model.     

Magnetic response of NiFe2O4 nanoparticles in polymer matrix

We report the magnetic properties of magnetic nano-composite, consisting of different quantity of NiFe₂O₄ nanoparticles in polymer matrix. The nanoparticles exhibited a typical magnetization blocking, which is sensitive on the variation of magnetic field, mode of zero-field-cooled/field-cooled experiments and particle quantity in the matrix. The samples with lower particle quantity showed an upturn of magnetization down to 5 K, whereas the blocking of magnetization dominates at lower temperatures as the particle quantity increases in the polymer. We examine such magnetic behaviour in terms of the competitive magnetic ordering between core and surface spins of nanoparticles, taking into account the effect of inter-particle (dipole-dipole) interactions on nanoparticle magnetic dynamics.     

Synthesis, structural, magnetic and optical properties of nanocrystalline ZnFe2O4

Nanocrystalline zinc ferrite (ZnFe₂O₄) is synthesized by high-energy ball-milling after 12 h from a powders mixture of zinc oxide (ZnO) and hematite (α-Fe₂O₃) with balls to powders mass ratio of 20:1. X-ray diffraction, vibrating sample magnetometer (VSM), the Mössbauer spectrometry and photoluminescence (PL) are used to characterize the samples. Rietveld analysis and VSM measurements show that the powder has an average crystallites size of 10 nm and a ferrimagnetic behavior with a saturation magnetization of 30 emu/g. After annealing at 700 °C, the lattice parameter reduces from 8.448 to 8.427 Å and the sample transforms into a superparamagnetic behavior, which was confirmed as well by the room temperature Mössbauer spectrometry. Different mechanisms to explain the obtained results and the correlation between magnetism and structure are discussed. Finally, the broadband visible emission band is observed in the entire PL spectrum and the estimated energy band gap is about 2.13 eV.     

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.     

α-Fe2O3-In2O3 mixed oxide nanoparticles synthesized under hydrothermal supercritical conditions

α-Fe₂O₃-In₂O₃ mixed oxide nanoparticles system has been synthesized by hydrothermal supercritical and postannealing route, starting with (1−x)Fe(NO₃)₃·9H₂O·xIn(NO₃)₃·5H₂O aqueous solution (x=0-1). X-ray diffraction and Mössbauer spectroscopy have been used to study the phase structure and substitutions in the nanosized samples. The concentration regions for the existence of the solid solutions in the α-Fe₂O₃-In₂O₃ nanoparticle system together with the solubility limits of In³⁺ ions in the hematite lattice and of Fe³⁺ ions in the cubic In₂O₃ structure have been evidenced. In general, the substitution level is considerably lower than the nominal concentration x. A justification of the processes leading to the formation of iron and indium phases in the investigated supercritical hydrothermal system has been given.     

Effects of synthesis variables on the magnetic properties of CoFe2O4 nanoparticles

Cobalt ferrite nanoparticles (CoFe₂O₄) have been synthesized using precipitation in water solution with polyethylene glycol as surfactant. Influence of various synthesis variables included pH, reaction time and annealing temperature on the magnetic properties and particle sizes has also been studied. Structural identification of the samples was carried out using Thermogravimetric and Differential thermal analysis, X-ray diffraction, Fourier transform infrared spectroscopy, Scanning electron microscopy, High resolution transmission electron microscopy. Vibrating sample magnetometer was used for the magnetic investigation of the samples. Magnetic properties of nanoparticles show strong dependence on the particle size. The magnetic properties increase with pH of the precipitating medium and annealing temperature while the coercivity goes through a maximum, peaking at around 25 nm.     

Structural and magnetic properties of Co1-xZnxFe2O4 nanoparticles

Co1-xZnxFe2O4 nanoparticles, prepared by the polyvinyl alcohol sol-gel method, have been investigated by x-ray diffraction and MSssbauer spectroscopy. These results are compared with those for the bulk material. The lattice parameters of CoZn ferrite nanoparticles are larger than those of the bulk material. Thermal scanning of MSssbauer measurement shows that the transition temperatures for nanoparticles are higher than those of the bulk material except for the sample CoFe2O4.     

Effects of substrate temperature and oxygen pressure on the magnetic properties and structures of CoFe2O4 thin films prepared by pulsed-laser deposition

CoFe₂O₄ thin films were grown on silicon substrates by pulsed-laser deposition techniques at various temperatures from 350 °C to 700 °C and different pressures from 0.1 Pa to 10 Pa. The CoFe₂O₄ films with highly (1 1 1)-preferred orientation and smooth surfaces were obtained. The high coercivities of the films were attributed to the residual stress in the films, and the saturation magnetizations were mainly dependent on the oxygen pressure. Higher oxygen pressure could decrease the oxygen deficiencies in the films. Sufficient oxygen ions in the films enhanced the exchange interactions between the magnetic ions, as a result, increasing the saturation magnetization.     

Finite size effects on the structural and magnetic properties of sol–gel synthesized NiFe2O4 powders

Nanoparticles of nickel ferrite have been synthesized by the sol–gel method and the effect of grain size on its structural and magnetic properties have been studied in detail. X-ray diffraction (XRD) studies revealed that all the samples are single phasic possessing the inverse spinel structure. Grain size of the sol–gel synthesized powders has been determined from the XRD data and the strain graph. A grain size of 9 nm was observed for the as prepared powders of NiFe₂O₄ obtained through the sol–gel method. It was also observed that strain was induced during the firing process. Magnetization measurements have been carried out on all the samples prepared in the present series. It was found that the specific magnetization of the nanosized NiFe₂O₄ powders was lower than that of the corresponding coarse-grained counterparts and decreased with a decrease in grain size. The coercivity of the sol–gel synthesized NiFe₂O₄ nanoparticles attained a maximum value when the grain size was 15 nm and then decreased as the grain size was increased further.     

Anisotropy and relaxation processes of uniaxially oriented CoFe2O4 nanoparticles dispersed in PDMS

When a uniaxial magnetic field is applied to a non-magnetic dispersive medium filled with magnetic nanoparticles, they auto-assemble into thin needles parallel to the field direction, due to the strong dipolar interaction among them. We have prepared in this way magnetically oriented nanocomposites of nanometer-size CoFe₂O₄ particles in a polydimethylsiloxane polymer matrix, with 10% w/w of magnetic particles. We present the characteristic magnetic relaxation curves measured after the application of a magnetic field forming an angle α with respect to the needle direction. We show that the magnetic viscosity (calculated from the logarithmic relaxation curves) as a function of α presents a minimum at α=0, indicating slower relaxation processes associated with this configuration of fields. The results seems to point out that the local magnetic anisotropy of the nanoparticles is oriented along the needles, resulting in the macroscopic magnetic anisotropy observed in our measurements.     

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.     

The effect of cation distribution on magnetization of ZnFe2O4 nanoparticles

In this work zinc ferrite (ZnFe₂O₄) nanoparticles have been prepared by sol-gel method in two different media, one acidic and another one basic and then annealed at different temperatures from 350 to 800 °C. XRD investigations show that both samples have a single phase spinel structure. Mean crystallite sizes of the samples were calculated, using Scherrer’s formula, which are 13 and 16 nm for the samples prepared in acidic and basic media, respectively. The variation of cation distribution in the samples was estimated by the ratio of (2 2 0) and (2 2 2) intensity diffraction peaks and the results show that as-prepared nanoparticles have different ionic distributions in comparison with that of bulk zinc ferrite. Also the results show that by increasing annealing temperature the ionic distribution of the zinc ferrite nanoparticles tends to that of bulk sample. The magnetic properties of the samples were studied by VSM and the results show that zinc ferrite nanoparticles have a ferrimagnetic behavior. Also the morphology of the powders was examined by TEM.