Strongly interacting superspins in Fe3O4 nanoparticles

In this paper we report structural and magnetic properties of Fe₃O₄ nanoparticles synthesized by thermal decomposition of ball milled iron nitrate and citric acid in N₂ and air ambient. The XRD pattern of samples which are prepared in air shows some impurity phases, while the samples synthesized in the N₂ atmosphere are almost pure Fe₃O₄ phase. The result shows that by increasing the particle size, the magnetization of the samples increases. The increase of magnetization by increasing the particle size could be attributed to the lower surface spin canting and surface spin disorder of the larger magnetic nanoparticles. The results of ac magnetic susceptibility measurements show that the susceptibility data are not in accordance with the Néel -Brown model for superparamagnetic relaxation, but fit well with conventional critical slowing down model which indicates that the dipole-dipole interactions are strong enough to cause superspin-glass like phase in these samples.     

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.     

Magnetoresistance and magnetic properties of Fe3O4 nanoparticle compacts

In this paper,we report on the magnetic properties of Fe3O4 nanoparticles with different grain sizes under different pressures.In all the samples,the saturated magnetization Ms shows a linear decrease with increasing pressure.The thickness of the magnetic dead layer on the nanoparticle surface nuder different pressures was roughly estimated,which also increases with increasing pressure.The transport measurements of the nanoparticle Fe3O4 compacts show that the low-field magnetoresistance (MR) value is insensitive to the grain size in a wide temperature range;however,the high-field MR value is dependent on grain size,especially at low temperatures.These experimental results can be attributed to the different surface states of the nanoparticles.     

Preparation of nanocrystalline MnFe2O4 by doping with Ti ions using solid-state reaction route

Nanostructured manganese ferrites (MnFe₂O₄) with diameters in the range of 45–30 nm were synthesized by Ti⁴⁺ ion doping, using conventional solid-state reaction route. The substitution of Ti⁴⁺ ions created vacancies at Mn²⁺ sites and the coupling of ferrimagnetically active oxygen polyhedra was broken. This created nanoscale regions of ferrites. A reduction of magnetization for decreasing particle size was observed. Coercivity showed an increasing trend. This was explained as arising due to multidomain/monodomain magnetic behaviour of magnetic nanoparticles. DC resistivities of the doped specimens indicated the presence of an interfacial amorphous phase formed by the nanoparticles. Zero-field cooled and field-cooled curves from 30 nm sized particles showed a peak at T_B (∼125 K), typical of superparamagnetic blocking temperature.     

The effect of grinding on magnetic properties of agglomereted MnFe2O4 nanoparticles

The effects of grinding on interparticle magnetic interactions for an ensemble of agglomerated MnFe₂O₄ nanoparticles have been studied. Structural analyses showed that by grinding the samples, a small variation in size of crystallites and lattice strain will occur. ac Magnetic susceptibility measurements under different conditions and spin dynamics analysis suggest that freezing temperature is frequency dependent and it is in good agreement with critical slowing down model. This is an indication that these nanoparticles have superspin glass behavior. The estimated zυ and τ₀ parameters using critical slowing down model show that by increasing the grinding time the interaction between nanoparticles decreases. ac Susceptibility measurements in cooling and heating process show a thermal hysteresis. The thermal hysteresis decreased by increasing the grinding time. Also, the thermal hysteresis is frequency dependent and it increased as frequency decreased. These results showed that interparticle interactions such as dipole-dipole and exchange interactions between nanoparticles become weaker by grinding.     

Saturation Magnetization and Law of Approach to Saturation for Self-formed Ionic Ferrofluids Based on MnFe2O2 Nanoparticles

测量了MnFe₂O₄纳米微粒及其磁性液体在室温下的磁化曲线.微粒的中值粒径为13.67 nm. 磁性液体的比饱和磁化强度小于理论值.在高场范围(5～10 kOe)下,磁性液体趋于饱和时,其体积分数越大,磁化曲线的斜率越大. 这种饱和磁化强度性质和趋饱和律分别源自于无场时的环状自组装团聚体和场致团聚体. 场致团聚体是耗散结构,以致于其趋饱和磁化律不同于顺磁理论所描述的趋饱和律. 磁性液体中的大微粒导致了表观磁滞现象.     

Thermomagnetic determination of Fe3O4 magnetic nanoparticle diameters for biomedical applications

The utility and promise of magnetic nanoparticles (MagNPs) for biomedicine rely heavily on accurate determination of the particle diameter attributes. While the average functional size and size distribution of the magnetic nanoparticles directly impact the implementation and optimization of nanobiotechnology applications in which they are employed, the determination of these attributes using electron microscopy techniques can be time-consuming and misrepresentative of the full nanoparticle population. In this work the average particle diameter and distribution of an ensemble of Fe₃O₄ ferrimagnetic nanoparticles are determined solely from temperature-dependent magnetization measurements; the results compare favorably to those obtained from extensive electron microscopy observations. The attributes of a population of biocompatible Fe₃O₄ nanoparticles synthesized by a thermal decomposition method are obtained from quantitative evaluation of a model that incorporates the distribution of superparamagnetic blocking temperatures represented through thermomagnetization data. The average size and size distributions are determined from magnetization data via temperature-dependent zero-field-cooled magnetization. The current work is unique from existing approaches based on magnetic measurement for the characterization of a nanoparticle ensemble as it provides both the average particle size as well as the particle size distribution.     

Suppression of low-temperature magnetic states in Mn3O4 nanoparticles

We have investigated the low-temperature magnetic properties of Mn₃O₄ nanoparticles using thermodynamic and magnetic measurements. While bulk Mn₃O₄ exhibits three magnetic transitions close to 42, 40 and 34 K, the two lower temperature transitions appear to be absent above 15 K in Mn₃O₄ nanoparticles. The magnetization and spin entropy associated with the ferrimagnetic transition at 42 K is smaller in the Mn₃O₄ nanoparticles than bulk Mn₃O₄, which is consistent with roughly 30-50% of the spins not contributing to the magnetic order. We tentatively attribute this suppression of the lower temperature transitions to a combination of finite size effects and effects arising from amorphous surface spins on the nanoparticles.     

低温固相反应法制备的NiFe2O4纳米颗粒的结构与磁性

采用低温固相反应法制备了晶粒尺寸在8—47nm之间的NiFe₂O₄纳米颗粒系列样品，用X射线衍射仪(XRD)、高分辨中子粉末衍射谱仪、振动样品磁强计和超导量子干涉仪等对样品的晶体结构、宏观磁性和纳米颗粒的表面各向异性进行了分析研究.XRD和中子衍射测量结果显示纳米颗粒的晶格常数略高于块体材料，样品的氧参量表明纳米颗粒的晶格畸变程度没有块体材料严重.相对块体材料，纳米颗粒具有较小的磁化强度、较大的矫顽力和各向异性能密度.纳米颗粒从多畴转变为单畴的临界尺寸约为40nm，超顺磁性临界尺寸约为16nm.     

Synthesis and magnetic characterization of Zn0.6Ni0.4Fe2O4 nanoparticles via a polyethylene glycol-assisted hydrothermal route

Zn-doped nickel ferrite nanoparticles (Zn_0.6Ni_0.4Fe₂O₄) have been prepared via a surfactant, polyethylene glycol assisted hydrothermal route. X-ray powder diffractometry (XRD), Fourier transform infrared spectroscopy, transmission electron microscopy (TEM), and vibrating scanning magnetometry (VSM) were used for the structural, morphological, and magnetic characterizations of the product, respectively. TEM analysis revealed that the nanoparticles have a narrow size distribution, with average particle size of 15±1 nm, which agrees well with the XRD based estimate of 14±2 nm. The absence of saturation and remanent magnetization, and coercivity in the high temperature region of the M-H curve and non-zero magnetic moments indicate superparamagnetism of the nanoparticles with a canted spin structure. The appearance of a peak on the temperature-dependent zero-field cooling magnetization curve at ∼190 K indicates the blocking temperature of the sample.     

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.     

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.     

Magnetic Behaviour and Heating Effect of Fe3O4 Ferrofluids Composed of Monodisperse Nanoparticles

Fe3O4 ferrofluids containing monodisperse Fe3O4 nanoparticles with different diameters of 8, 12, 16 and 18nm are prepared by using high-temperature solution phase reaction. The particles have single crystal structures with narrow size distributions. At room temperature, the 8-nm ferrofluid shows superparamagnetic behaviour, whereas the others display hysteresis properties and the coercivity increases with the increasing particle size. The spin glass-like behaviour and cusps near 190K are observed on all ferrofluids according to the temperature variation of field-cooled （FC） and zero-field-cooled （ZFC） magnetization measurements. The cusps are found to be associated with the freezing point of the solvent. As a comparison, the ferrofluids are dried and the FC and ZFC magnetization curves of powdery samples are also investigated. It is found that the blocking temperatures for the powdery samples are higher than those for their corresponding ferrofluids. Moreover, the size dependent heating effect of the ferrofluids is also investigated in ac magnetic field with a frequency of 55 kHz and amplitude of 200 Oe.     

T1 and T2 relaxivities of succimer-coated MFe2O4 (M=Mn, Fe and Co) inverse spinel ferrites for potential use as phase-contrast agents in medical MRI

Superparamagnetic MFe₂³⁺O₄ (M=Mn²⁺, Fe²⁺ and Co²⁺) inverse spinel ferrite (ISF) nanoparticles with narrow size distribution having average diameters of 6-8 nm were synthesized by a diol reduction of organic metals and the surface was modified to be hydrophilic by coating with succimer. Magnetic resonance imaging (MRI) contrast enhancement by dipolar coupling defined interactions between the synthesized ISFs and protons in the bulk water was investigated with initial susceptibility, magnetization and anisotropy of the succimer-coated ISFs. The relaxivity ratios, r₂/r₁, for MnFe₂O₄, Fe₃O₄ and CoFe₂O₄ were measured to be 12.2, 23.1 and 62.3, respectively, which demonstrate the potential usefulness of these magnetic nanoparticles as T₂ contrast agents for MRI.     

Sol-gel synthesis and magnetization study of Mn1−xCuxFe2O4 (x=0, 0.2) nanocrystallites

The magnetic nanoparticles of Mn_1−xCu_xFe₂O₄ (x=0, 0.2) were prepared by using a sol-gel method. It is proved that both the MnFe₂O₄ and Mn_0.8Cu_0.2Fe₂O₄ nanoparticle samples have superparamagnetic feature. Although the particle sizes are the same, substitution of a small fraction Cu for Mn results in the increase of magnetocrystallite anisotropy energy, thus enhances the blocking temperature from 130 K for MnFe₂O₄ to 260 K for Mn_0.8Cu_0.2Fe₂O₄. Mössbauer spectroscopy confirms that the anisotropy constant K of the Mn_0.8Cu_0.2Fe₂O₄ material is distinctly higher than that of the MnFe₂O₄ compound. Increase of the blocking temperature suggests that the approach we employed is effective to tackle the ‘superparamagnetic limit’ problem.     

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.     

Cu1.5[Cr(CN)6]⋅6.5H2O nanoparticles: synthesis, characterization, and magnetic properties

We have synthesized nanoparticles of Cu_1.5[Cr(CN)₆]⋅6.5H₂O of varying size by using poly(vinylpyrrolidone) (PVP) as a protecting polymer. The particle size variation has been achieved by varying the amount of the PVP surfactant with the reactants. The prepared nanoparticles have been investigated by using X-ray diffraction, transmission electron microscopy, and direct-current magnetization techniques. The nanoparticles crystallize in a face centred cubic structure (space group: Fm3m). The approximate particle sizes for the three samples are 18, 9, and 5 nm, respectively. Non-PVP nanoparticles (18 nm) show a magnetic ordering temperature of 65 K. A decrease in the magnetic ordering temperature was observed with decreasing particle size. These nanoparticles are magnetically very soft, showing negligibly small values of the coercivity and remanent magnetization. The maximum magnetization and spontaneous magnetization values at 5 K are found to decrease with decreasing particle size. The observed magnetization behaviour of the nanoparticles has been attributed to the increasing surface spin disorder with decreasing particle size.     

Structural and magnetic properties of size-controlled Mn<Subscript>0.5</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles and magnetic fluids

Mn_0.5Zn_0.5Fe₂O₄ ferrite nanoparticles with tunable Curie temperature and saturation magnetization are synthesized using hydrothermal co-precipitation method. Particle size is controlled in the range of 54 to 135 Å by pH and incubation time of the reaction. All the particles exhibit super-paramagnetic behaviour at room temperature. Langevin’s theory incorporating the interparticle interaction was used to fit the virgin curve of particle magnetization. The low-temperature magnetization follows Bloch spin wave theory. Curie temperature derived from magnetic thermogravimetric analysis shows that Curie temperature increases with increasing particle size. Using these particles magnetic fluid is synthesized and magnetic characterization is reported. The monolayer coating of surfactant on particle surface is confirmed using thermogravimetric measurement. The same technique can be extended to study the magnetic phase transition. The Curie temperature derived using this measurement complies with the low-temperature magnetic measurement. The room-temperature and high-temperature magnetization measurements are also studied for magnetic fluid systems. The magnetic parameters derived for fluid are in good agreement with those obtained for the particle system.     

Mössbauer and magnetic studies of MFe2O4(M = Co, Ni) nanoparticles

Nanocrystalline MFe₂O₄ (M?=?Co, Ni) particles are synthesized by citrate precursor technique. Mössbauer and magnetic studies are carried out with the CoFe₂O₄ samples having particle sizes of 9, 14 and 30 nm and the NiFe₂O₄ samples having particle sizes of 9, 21 and 30 nm. The intrinsic magnetic parameters are found to vary with the particle size. The magnetic interactions and cation distribution present in these systems influence the room temperature Mössbauer parameters. Ferrimagnetic sextets are observed for all the different particle sizes. The observed reduction of the magnetic hyperfine field values with the decrease in the size of MFe₂O₄ particles are attributed to the intrinsic size effect and the canted spin structure at the surface of the nanoparticles.     

Magnetic and rheological properties of monodisperse Fe3O4 nanoparticle/organic hybrid

Fe₃O₄ nanoparticle/organic hybrids were synthesized via hydrolysis using iron (III) acetylacetonate at ∼80 °C. The synthesis of Fe₃O₄ was confirmed by X-ray diffraction, selected-area diffraction, and X-ray photoelectron spectroscopy. Fe₃O₄ nanoparticles in the organic matrix had diameters ranging from 7 to 13 nm depending on the conditions of hydrolysis. The saturation magnetization of the hybrid increased with an increase in the particle size. When the hybrid contained Fe₃O₄ particles with a size of less than 10 nm, it exhibited superparamagnetic behavior. The blocking temperature of the hybrid containing Fe₃O₄ particles with a size of 7.3 nm was 200 K, and it increased to 310 K as the particle size increased to 9.1 nm. A hybrid containing Fe₃O₄ particles of size greater than 10 nm was ferrimagnetic, and underwent Verwey transition at 130 K. Under a magnetic field, a suspension of the hybrid in silicone oil revealed the magnetorheological effect. The yield stress of the fluid was dependent on the saturation magnetization of Fe₃O₄ nanoparticles in the hybrid, the strength of the magnetic field, and the amount of the hybrid.