Size-dependent Magnetization Research Articles

The SLP estimation of the nanoparticle systems using size-dependent magnetic properties for the magnetic hyperthermia therapy

Magnetic Nanoparticle hyperthermia (MNH) is one of the hyperthermia therapy in which temperature gain is achieved through magnetic nanoparticles in the tumor tissue/targeted region. The key parameters for efficient heating in MNH are magnetic nanoparticle (MNPs) properties, its size & materials, and magnetic field parameters (amplitude and frequency). Thus, the present study investigates the effects of these parameters on heat generation during MNH. In this study, a comparison of heat generation by three nanoparticle systems (CoFe2O4, Fe3O4, and MnFe2O4) and its dependency on size & magnetic field parameters is theoretically investigated. The quantification of specific loss power (SLP) of nanoparticle systems depending on its size-dependent saturation magnetization (MS) and the anisotropy constant (Keff). of nanoparticles has been done in this work. Correlations for MS and Keff based on the previously reported experimental data have been established for considered MNP systems. The comparisons show that SLP estimation using MNP size-dependent saturation magnetization (MS) (Fig. 4) are closer to the experimentally reported values of SLP for all considered MNP systems (CoFe2O4, Fe3O4, MnFe2O4) in comparison to SLP estimated by fixed values of saturation magnetization (MS) and anisotropy constant (Keff). Results show that dissipated power reaches a peak at a certain particle size (DO). However, the (DO) varies with the magnetic field frequency and amplitude. It decreases as the frequency increases at a particular field amplitude. In addition, the SLP increment ceases after a certain frequency (fO) that is around 500 kHz for considered MNP systems. On the other hand, the SLP continuously increases with an increase in field amplitude. Thus this work incorporates the empirical correlations as well as experimental data to predict the SLP for three magnetic nanoparticle systems generally utilized in the MNH.

Size tunable biosynthesis and luminescence quenching of nanostructured hematite (α-Fe2O3) for catalytic degradation of organic pollutants

In the present study, an environmentally benign protocol has been proposed for surfactant-free and template-free aqueous biosynthesis of hematite nanoparticles without complicated heat treatment procedures. Nanostructured hematite of different sizes has been synthesized, exploiting the capping potential of polyphenolic tannin rich leaf broth of Psidium guajava plant, by optimizing the precursor concentration and quantity of broth. The synthesized nanostructures are characterized by XRD, SEM, TEM, HRTEM, EDX and FTIR techniques to confirm their phase purity, size distribution and chemical environment. In order to determine the proper annealing temperature, thermal analysis of the as synthesized product has been carried out. The absorption and reflection characteristics of the nanoparticles in the UV–vis-IR region confirm the existence of energy band gap of 2.1 eV in the samples. Magnetic characteristics of the samples have been investigated using Vibrating Sample Magnetometer to show weak ferromagnetism at room temperature, tendency of a superparamagnetic behavior at lower temperatures and size dependent coercivity and saturation magnetization. The relatively small difference between the irreversibility temperature and blocking temperature observed in FC and ZFC curves supports the fact that the nanoparticles are distributed in a small size range, which is in agreement with SEM and TEM result. The synthesized nanoparticles are found to show luminescent emission in the visible region at 575 nm on excitation by a wavelength of 384 nm. The capping of acidic tannin molecules could effectively quench the luminescence of the synthesized particles, thereby enhancing their catalytic efficiency. The biosynthesized nanoparticles could effectively catalyse degradation of the organic pollutants methyl red, methyl orange, eosin yellowish and methylene blue which recommends the possible application in the treatment of industrial waste water.

Voltage pulse induced repeated magnetization reversal in strain-mediated multiferroic nanomagnets: a size- and material-dependent micromagnetic study

Realization of complete magnetization reversal is a major challenge for electrical control of magnetism. This paper focuses on voltage pulse induced magnetization reversal which is mediated by localized strain generated in a two-phase magnetostrictive/piezoelectric multiferroic heterostructure. This paradigm takes advantage of the damping and precessional property of magnetization dynamics, of which the magnetization reversal needs precise control of the voltage pulse. We have performed a micromagnetic study of varied-size elliptical single-domain Ni and Terfenol-D nanomagnets at T = 300 K. By using the actual ground states of the nanomagnets at T = 300 K as the initial states, we obtained the key parameters of the voltage pulse which can induce repeated magnetization reversal. Then we uncovered the mechanism of the size-dependent incoherent magnetization switching of Terfenol-D nanomagnets by a quantitative analysis of the interaction between the exchange energy, demagnetization energy and stress anisotropy energy. These results lay the foundation for the application of strain-mediated magnetization switching technology, and also provide a set of guidelines for signal design and the choice of nanomagnets with proper material and size in the design of specific straintronic circuits.

Incoherent magnetization dynamics in strain mediated switching of magnetostrictive nanomagnets

Micromagnetic studies of the magnetization change in magnetostrictive nanomagnets subjected to stress are performed for nanomagnets of different sizes. The interplay between demagnetization, exchange and stress anisotropy energies is used to explain the rich physics of size-dependent magnetization dynamics induced by modulating stress anisotropy in planar nanomagnets. These studies have important implications for strain mediated ultralow energy magnetization control in nanomagnets and its application in energy-efficient nanomagnetic computing devices.

Magneto-optical and catalytic properties of Fe3O4@HA@Ag magnetic nanocomposite

Fe3O4@HA@Ag magnetic nanocomposites (MNCs) were successfully synthesized by the simple reflux method for the removal of azo dyes from the industrial aqueous media. Fe3O4@HA@AgMNCs exhibited high catalytic activity to reduce MB within 20min from the waste water. The obtained materials were characterized by the means of different techniques. Powder X-ray diffraction (XRD) analysis confirmed the single-phase of Fe3O4 spinel structure. SEM and TEM analysis indicated that Fe3O4@HA@AgMNCs were nanoparticles like structure with small agglomeration. TG result showed that the products contained 9% of HA. The characteristic peaks of HA at 1601cm−1 and 1703cm−1 was observed by the means of FT-IR spectra of Fe3O4@HA@AgMNCs. The hysteresis (σ–H) curves revealed Fe3O4@HA@Ag MNCs exhibit a typical superparamagnetic characteristic with a saturation magnetization of 59.11emu/g and measured magnetic moment is 2.45µB. The average magnetic particle dimension (Dmag) is 13.25nm. In accordance, the average crystallite and particle dimensions were obtained as 11.50nm and 13.10nm from XRD and TEM measurements, respectively. Magnetocrystalline anisotropy was offered as uniaxial and calculated effective anisotropy constant (Keff) is 2.96×105Erg/g. The blocking temperature was estimated as 522K. The size-dependent saturation magnetization suggests the existence of a magnetically dead layer as 0.793nm for Fe3O4@HA@Ag MNCs. The UV–vis diffuse reflectance spectroscopy (DRS) and Kubelka–Munk theory were applied to determine the optical properties of powder samples. The direct optical energy band gap (Eg) values were estimated from Tauc plots between 1.62eV and 2.12eV.

Synthesis and characterization of oleylamine capped MnxFe1-xFe2O4 nanocomposite: Magneto-optical properties, cation distribution and hyperfine interactions

Pure Fe3O4 NPs and Oleylamin (OAm) capped MnxFe1-xFe2O4 (MnxFe1-xFe2O4@OAm) (0.2 ≤ x ≤ 1.0) nanocomposites (NCs) were synthesized by the polyol route. Lattice parameter increases with increasing Mn2+ concentration, due to the respective larger ionic radius of Mn2+ ion compared with the Fe2+ ion. The VSM analyses revealed superparamagnetic characteristics of all samples. The extrapolated specific saturation magnetization (σs) values decreased from maximum 50.74 emu/g to minimum 15.34 emu/g by increasing Mn content. The particle size dependent Langevin function was applied to determine the magnetic particle dimensions (Dmag) between 9.10 nm and 21.50 nm. The observed magnetic moments of NPs and NCs are in range of (0.64–2.10) μB and significantly less than 4 μB of bulk Fe3O4. Magnetic anisotropy was determined as uniaxial and calculated effective anisotropy constants (Keff) are between 32.14 × 104 Erg/g and 8.71 × 104 Erg/g. The size dependent saturation magnetization suggests the existence of a magnetically dead layers around the magnetic cores for NCs between 0.72 nm and 1.29 nm. From 57Fe Mössbauer spectroscopy data, the variation in line width, isomer shift, quadrupole splitting and hyperfine magnetic field values on Mn2+ substitution have been determined. Although, the Mössbauer spectra for the sample x = 0.2 is composed of ferromagnetic sextets, paramagnetic doublet is also formed for other samples. The percent diffuse reflectance spectroscopy (DR %) and Kubelka-Munk theory were used to specify the optical properties. The estimated optical band gap (Eg) values from Tauc plots are between 1.50 eV and 2.05 eV. Increasing Mn content in NCs increased the band gap at different magnitudes.

A soft chemical route to the synthesis of BiFeO3 nanoparticles with enhanced magnetization

Utilization of BiFeO3 (BFO) into modern device applications is restricted by its very low magnetic moments and high leakage current. Enhancement in magnetization is a real challenge for multiferroic BFO in the context of device miniaturization and high density information storage system. In this study a significant improvement in magnetization has been recorded for BFO nanoparticles, exploiting the beneficial effect of size confinement. BFO nanoparticles with different size in the range of 21–68nm are synthesized via modified Pechini sol–gel approach followed by leaching with acetic acid. X-ray diffraction result confirms pure and well crystallized BFO annealed at temperature lower than 600°C, compared to more than 800°C for the traditional solid-state sintering process. A strong size-dependent magnetization which increases with decreasing particle size is confirmed with a value of 1.4emu/g for 40nm particles in contrast to 7.5emu/g for 21nm particles.

Preparation and characterization of SPION functionalized via caffeic acid

Caffeic acid coated superparamagnetic iron oxide nanoparticles (SPION-CFA) was synthesized by reflux method. The structural, spectroscopic and magnetic properties were studied by X-ray diffraction (XRD), Transmission electron microscopy (TEM), Scanning electron microscopy (SEM), and Vibrating sample magnetometer (VSM) techniques. Thermal gravimetric analysis (TG) and Fourier transform infrared spectroscopy (FT-IR) confirmed the presence of CA on the surface of SPION. The theoretical analyzes performed on recorded room temperature VSM spectrum confirmed the formation of superparamagnetic nature of SPION-CFA. The particle size dependent Langevin function was applied to determine the average magnetic particle dimension (Dmag) around 11.93nm. In accordance, the average crystallite and particle sizes were obtained as 11.40nm and ~12.00nm from XRD and TEM measurements. The extrapolated specific saturation magnetization (σs) is 44.11emu/g and measured magnetic moment is 1.83µB. These parameters assign small order of magnetization for NPs with respect to bulk Fe3O4. Magnetic anisotropy was offered as uniaxial and calculated effective anisotropy constant (Keff) is 34.82×104Erg/g. The size-dependent saturation magnetization suggests the existence of a magnetically inactive layer as 1.035nm for SPION-CFA.

Magneto-optical properties of Mn3+ substituted Fe3O4 nanoparticles

MnxFe3−xO4 (0.0≤x≤1.0) nanoparticles were synthesized by the polyol synthesis method and the effect of Mn3+ substitution on structural, magnetic and optical properties of Fe3O4 was studied. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV–visible spectroscopy and vibrating sample magnetometer (VSM) were used to study the physical properties. The crystallite (from XRD) and particle sizes (from TEM and SEM) are in close agreement with each other. Lattice parameter increases with increasing Mn3+ concentration, due to the respective larger ionic radius of Mn3+ ion compared with the Fe3+ ion. The magnetic hysteresis (M–H) curves revealed superparamagnetic characteristics of the products. The extrapolated specific saturation magnetization (σs) values decreased from maximum value of 47.3emu/g to the minimum value of 25.6emu/g by increasing Mn composition. The particle size dependent Langevin function was applied to determine the magnetic particle dimensions (Dmag) around 15nm. The observed magnetic moments of NPs are in range of (1.06–1.96)µB and significantly less than 4µB of bulk Fe3O4. Magnetic anisotropy was offered as uniaxial and calculated effective anisotropy constants (Keff) are between 34.47×104Erg/g and 21.83×104Erg/g. The size-dependent saturation magnetization suggests the existence of a magnetically inactive layer as 1.638nm on FeMnxFe2−xO4 NPs. The UV–vis diffuse reflectance spectroscopy (DRS) and Kubelka−Munk theory were applied to determine the optical properties. The estimated optical band gap (Eg) values dropped almost linearly from 2.05eV to 1.17eV with increasing Mn composition.

Model for temperature-dependent magnetization of nanocrystalline materials

A magnetization model of nanocrystalline materials incorporating intragrain anisotropies, intergrain interactions, and texture effects has been extended to include the thermal fluctuations. The method relies on the stochastic Landau–Lifshitz–Gilbert theory of magnetization dynamics and permits to study the magnetic properties of nanocrystalline materials at arbitrary temperature below the Currie temperature. The model has been used to determine the intergrain exchange constant and grain boundary anisotropy constant of nanocrystalline Ni at 100 K and 298 K. It is found that the thermal fluctuations suppress the strength of the intergrain exchange coupling and also reduce the grain boundary anisotropy. In comparison with its value at 2 K, the interparticle exchange constant decreases by 16% and 42% and the grain boundary anisotropy constant decreases by 28% and 40% at 100 K and 298 K, respectively. An application of the model to study the grain size-dependent magnetization indicates that when the thermal activation energy is comparable to the free energy of grains, the decrease in the grain size leads to the decrease in the magnetic permeability and saturation magnetization. The mechanism by which the grain size influences the magnetic properties of nc–Ni is discussed.

Size-dependent multiple magnetic phases and exchange bias effect in hole-doped double perovskite La1.6Sr0.4NiMnO6

La1.6Sr0.4NiMnO6 nanoparticles of different sizes (18–150 nm) have been prepared by a sol–gel method, and the size effects on their magnetic properties are investigated. It is found that there is a ferromagnetic (FM) transition at TC ∼ 245 K for the sample with particle size D ∼ 150 nm. As the particle size decreases, a spin glass (SG) transition appears at TG ∼ 60 K and becomes more obvious. Meanwhile, the TC shifts to a lower temperature and the FM transition becomes indistinct. The size dependent saturation magnetization MS, which is in the range 0.78 ∼ 1.27 μB/f.u., shows a non-monotonic variation with a maximum of 1.27 μB/f.u. at the particle size DC ∼ 42 nm. The exchange bias (EB) effect is clearly observed for D > 32 nm and becomes indistinct as D ⩽ 32 nm. As the particle size decreasing, the EB field HE decreases and becomes near nil at DC. Meanwhile, the coercive field HC first increases to a maximum at DC and then decreases. All the results indicate the coexistence of AFM APBs related to the anti-site disorders, FM domains, and SG phase in La1.6Sr0.4NiMnO6 nanoparticles. The magnetic properties of the nanoparticles are understood by a complex core-shell model, i.e. both AFM APBs and FM domains coexist in the core, and the SG phase mainly resides on the surface (shell) of each particle, which reveals that the size effects on the FM phase, AFM APBs and surface SG phase in grain are different, even are opposite. The nanometer size effect is a crucial factor to influence the magnetic properties of La1.6Sr0.4NiMnO6 nanoparticles.

Grain size dependent magnetization, electrical resistivity and magnetoresistance in mechanically milled La 0.67Sr 0.33MnO 3

We have studied magnetization, ac susceptibility, resistivity and magnetoresistance in mechanically milled La 0.67Sr 0.33MnO 3. The material with grain size micron to nanometer scale has stabilized in rhombohedral crystal structure with space group R3 C. We have found various grain size effects, e.g., decrease of ferromagnetic moment, increase of surface spin disorder, and appearance of insulator/semiconductor type resistivity. In addition to these conventional features, we have identified a magnetic anomaly at 45 K in bulk sample. Ferromagnetic to paramagnetic transition temperature ( T C) is above room temperature for all samples. The samples are typical soft ferromagnet that transformed from multi-domain state to single domain state in nanocrystalline samples. The remarkable observation is that low temperature freezing of ferromagnetic domains/clusters does not follow the conventional spin glass features. Experimental results clearly showed the enhancement of high field magnetoresistance in nanocrystalline samples below 200 K, whereas low field magnetoresistance gradually decreases above 200 K and almost absent at 300 K. We have discussed few more magnetic and electrical changes, highly relevant to the progress of nanomaterial research in ferromagnetic manganites.

Memory and aging effects in antiferromagnetic nanoparticles

We investigate slow dynamics of collection of a few noninteracting antiferromagnetic NiO nanoparticles. Our purpose is to enquire the role of size-dependent magnetization fluctuations in temperature and time dependent properties of antiferromagnetic nanoparticles. The zero-field cooled magnetization exhibits size dependent fluctuations. We find memory effects in field cooled magnetization, as well as aging effects in thermoremenant magnetization of antiferromagnetic nanoparticles. The antiferromagnetic nanoparticles show a stronger memory effect than the corresponding effect in the ferromagnetic particles, when the distribution of particles include very small sizes. The situation reverses for bigger sizes. The relaxation of the magnetization after a sudden cooling, heating and removal of fields reiterate the memory effects. We also see a weak signature of size-dependent magnetization fluctuations in aging effect of antiferromagnetic nanoparticles. We find a two-step relaxation of thermoremenant magnetization in antiferromagnetic case, which differs qualitatively from relaxation of ferromagnetic nanoparticles.

Investigations on Controlled-Size-Precipitated Cobalt Ferrite Nanoparticles

Size-dependent nanocrystalline cobalt ferrite (CoFe2O4) particles have been synthesized by coprecipitation and investigated in detail. The effects of reducing the particle size on physical properties like crystalline phase, crystallite size, lattice strain, IR absorption, thermomagnetic measurements, and magnetization parameter are studied. Structural parameters in relevance to the shape and size of nanoparticles are discussed. The crystalline phase of cobalt ferrite remains stable down to 5 nm. Size-dependent magnetization and the effect of surface area are presented. Characterization of samples has been performed using X-ray diffractometer, transmission electron microscope, Fourier transform infrared spectrophotometer, vibrating sample magnetometer, and thermogravimetric analysis techniques.

Unusual size-dependent magnetization in near hemispherical Co nanomagnets on SiO2 from fast pulsed laser processing

Nanosecond pulsed laser melting of ultrathin metal films can lead to self-organized arrays of spherical nanoparticles. We have applied this technique to assemble arrays of nanoparticles of the soft elemental ferromagnet Co on SiO2. Surface morphology studies by using scanning electron microscopy and atomic force microscopy established that the nanoparticles were nearly hemispherical with an average contact angle of ∼104±22°. Magnetic properties of these nanoparticles in the size range of 30–250nm diameter were investigated by magnetic force microscopy under zero applied field in conjunction with simulations of the magnetic tip-particle interaction. Particles up to 180nm diameter were found to be single domain with the magnetization direction oriented predominantly in-plane for the smaller particles (⩽75nm) and out-of-plane for the larger particles (⩽180nm). Multidomain behavior was observed for particles larger than 180nm. Magnetic hysteresis measurements at room temperature confirmed that the arrays consisted of a mixture of in-plane and out-of-plane orientations. Microstructural analysis by transmission electron microscopy revealed that the nanoparticles had a granular microstructure with the average grain size increasing with particle size. This size-dependent magnetic orientation is inconsistent with the expected in-plane orientation due to shape anisotropy. We suggest that a size-dependent residual strain and the microstructure formed by rapid laser processing determine the orientation of nanomagnets. This idea was supported by the significant increase in in-plane orientation of larger particles following thermal annealing. These studies suggest that anisotropic nanomagnets of near hemispherical polycrystalline particles with desired magnetic orientation can be prepared by fast laser thermal processing.

Size dependence of ferromagnetism in gold nanoparticles: Mean field results

In this paper, a simple spin-spin Ising interaction model for the surface ferromagnetism is combined with the bulk Au diamagnetic response to model the size dependence of the magnetization of a Au nanoparticle. Using the maximum entropy formalism, we obtain the average temperature dependent magnetization within a mean field model. Our results qualitatively reproduce recent experimental observations of size-dependent magnetization of Au nanoparticles in which the ferromagnetic moment of thiol-capped nanoparticles is seen to increase for diameters larger than $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, peaking at approximately $3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, and subsequently decreasing as the particle diameter increases further.

Size dependence in the magnetization reversal of Fe∕Gd multilayers on self-assembled arrays of nanospheres

The size dependent magnetization reversal of Fe∕Gd multilayers on self-assembled silica nanospheres has been studied by scanning transmission x-ray microscopy. Caps on 800nm spherules show S-shaped hysteresis loops while the caps on 330 and 160nm spherules exhibit rectangular loops. This drastic change is attributed to a transition from a radial to a uniaxial magnetization distribution with decreasing diameter and results from the increasing exchange interaction between adjacent moments. Furthermore, element selective studies reveal a transition from an aligned to a twisted magnetic state with reducing size of the nanocaps.

Magnetization enhancement in nanosized MgFe 2O 4 prepared by mechanosynthesis

The magnetization enhancement in nanoscale mechanosynthesized MgFe 2O 4 is reported and is discussed in terms of a modified core–shell model, in which a competition between the effects of spin canting and site exchange of cations plays a decisive role. Because of the nearly random distribution of cations, the surface shell of nanoparticles exhibits an effective magnetic moment that is about 2 times larger than that of the particle core. The thickness of the “magnetically active” surface shell is calculated from the size-dependent magnetization measurements.

Saturation magnetization of ferromagnetic and ferrimagnetic nanocrystals at room temperature

The size-dependent saturation magnetization Ms(D) of ferromagnetic and ferrimagnetic nanocrystals at room temperature, without free parameters, has been predicted in terms of a size-dependent cohesive energy model, where D denotes the diameter of nanoparticles or the thickness of thin films. The given Ms(D) functions, which are also a function of interface conditions for substrate supported nanocrystals, drop as D decreases, which correspond to the available experimental and theoretical results for ferromagnetic Ni films, Fe, Co, Ni nanoparticles, and ferrimagnetic γ-Fe2O3, Fe3O4, MnFe2O4 and CoFe2O4 nanoparticles.

XMCD Study of Dilutely Fe Doped Pd Fine Particles

X-ray magnetic circular dichroism (XMCD) of Pd 97.1 Fe 2.9 fine particle samples is studied at the Pd L 2,3 edges. The magnetic moment of a Pd atom, which is estimated using the magneto-optical sum rule, is ∼0.06µ B /atom independent of particle size, while the total magnetization of the particle samples decreases monotonically with decreasing particle size. We attribute the size dependent magnetization to a reduction in the Fe magnetic moment at the surface region with decreasing particle size. Analysis of the XMCD data is reported in detail.