Oriented nanometric aggregates of partially inverted zinc ferrite: One-step processing and tunable high-frequency magnetic properties

In this work, it is demonstrated that the in situ growth of oriented nanometric aggregates of partially inverted zinc ferrite can potentially pave a way to alter and tune magnetocrystalline anisotropy that, in turn, dictates ferromagnetic resonance frequency (f(FMR)) by inducing strain due to aggregation. Furthermore, the influence of interparticle interaction on magnetic properties of the aggregates is investigated. Mono-dispersed zinc ferrite nanoparticles (<5 nm) with various degrees of aggregation were prepared through decomposition of metal-organic compounds of zinc (II) and iron (III) in an alcoholic solution under controlled microwave irradiation, below 200 degrees C. The nanocrystallites were found to possess high degree of inversion (>0.5). With increasing order of aggregation in the samples, saturation magnetization (at 5 K) is found to decrease from 38 emu/g to 24 emu/g, while coercivity is found to increase gradually by up to 100% (525 Oe to 1040 Oe). Anisotropy-mediated shift of f(FMR) has also been measured and discussed. In essence, the result exhibits an easy way to control the magnetic characteristics of nanocrystalline zinc ferrite, boosted with significant degree of inversion, at GHz frequencies. (C) 2015 AIP Publishing LLC.

Tailored mechanical behavior of magnetic particles loaded carbon nanotube foam in presence of magnetic field

The compressive behavior of carbon nanotube (CNT) foam with an entangled microstructure has become an important research area due to its excellent energy absorption capability. This report presents a tailored mechanical behavior of CNT foam under an applied magnetic field when all CNTs in the foam are coated with magnetic nanoparticles. The presence of nanoparticles not only enhanced the stiffness of the foam to four times but also revealed a nonlinear variation in both the stress and energy absorption capability with the gradual increase of the magnetic field. Magnetization of both CNT and attached nanoparticles along the magnetic field direction are shown to play a crucial role in determining the dominant deformation mechanism.

Spark Plasma Sintered HA-Fe3O4-Based Multifunctional Magnetic Biocomposites

Although HA is highly biocompatible, one of the major disadvantages of HA include the lack of antibacterial property. In an earlier study, we demonstrated the potential role of magnetic field stimulation on bactericidal property in vitro. Following this, it was hypothesized that antibacterial property can be realized if bacteria are grown on magnetic biocomposites in vitro. In addressing this issue, this study demonstrates the development of HA-Fe3O4-based magnetic substrate with multifunctional properties. For this purpose, HA-xFe(3)O(4) (x: 10, 20 and 40wt%) powder compositions were sintered using uniquely designed spark plasma sintering conditions (three stage sintering with final holding temperature of 1050 degrees C for 5min). A saturation magnetization of 24emu/g is measured with HA-40%Fe3O4. Importantly, all the HA-Fe3O4 composites demonstrated bactericidal property by rupturing the membrane of Escherichia coli bacteria, while supporting cell growth of metabolically active human fetal osteoblast cells over 8d culture. A systematic decrease in bacterial viability with Fe3O4 addition is consistent with a commensurate increase in reactive oxygen species (ROS).

Temperature-Dependent Magnetic and EPR Studies of Bulk and Nanoparticles of Bi0.1Ca0.9MnO3

We report temperature-dependent magnetic and electron paramagnetic resonance (EPR) properties of bulk and nanoparticle samples of Bi0.1Ca0.9MnO3 (BCMO). The nanoparticles of BCMO (dia similar to 50 nm) were prepared by the standard sol-gel technique and bulk samples by solid-state reaction method. We have investigated the magnetic ordering in the two samples by carrying out temperature-dependent magnetic and EPR studies and compared their properties. According to earlier reports, antiferromagnetic and ferromagnetic orders coexist in the bulk sample of Bi0.1Ca0.9MnO3. Our magnetization and EPR results show the existence of ferromagnetism in the bulk sample which is present in the nanosample as well but with somewhat weakened strength with the size reduction.

Origin of room temperature weak-ferromagnetism in antiferromagnetic Pb(Fe2/3W1/3)O-3 ceramic

We report the origin of room temperature weak ferromagnetic behavior of polycrystalline Pb(Fe2/3W1/3)O-3 (PFW) powder. The structure and magnetic properties of the ceramic powder prepared by a Columbite method were characterized by X-ray and neutron diffraction, Mossbauer spectroscopy and magnetization measurements. Rietveld analysis of diffraction data confirm the formation of single phase PFW, without traces of any parasitic pyrochlore phase. PFW was found to crystallize in the cubic structure at room temperature. The Rietveld refinement of neutron diffraction data measured at room temperature confirmed the G-type antiferromagnetic structure of PFW in our sample. However, along with the antiferromagnetic (AFM) ordering of the Fe spins, we have observed the existence of weak ferromagnetism at room temperature through: (i) a clear opening of hysteresis (M-H) loop, (ii) bifurcation of the field cooled and zero-field cooled susceptibility; supported by Mossbauer spectroscopy results. The P-E loop measurements showed a non-linear slim hysteresis loop at room temperature due to the electronic conduction through the local inhomogeneities in the PFW crystallites and the inter-particle regions. By corroborating all the magnetic measurements, especially the spin glass nature of the sample, with the conduction behavior of the sample, we report here that the observed ferromagnetism originates at these local inhomogeneous regions in the sample, where the Fe-spins are not perfectly aligned antiferromagnetically due to the compositional disordering. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Exchange spring behaviour in SrFe12O19-CoFe2O4 nanocomposites

Nanocomposites of hard (SrFe12O19) and soft ferrite (CoFe2O4) are prepared by mixing individual ferrite components at appropriate weight ratio and subsequent heat treatment. The magnetization of the composites showed hysteresis loop that is characteristic of the exchange spring system. The variation of J(r)/J(r)(infinity) vs. J(d)/J(r)(infinity) for these nanocomposites are investigated to understand the presence of both the interacting field and the disorder in the system. This is further corroborated with the First Order Reversal Curve analysis (FORC) on the nanocomposites of 1:4 (Cobalt Ferrite: Strontium Ferrite) and 1:16 (Cobalt Ferrite: Strontium Ferrite). The FORC distribution reveals that the pinning mechanism is stronger in the nanocomposite of 1:4 compared to 1:16. However, the nanocomposite of 1:16 exhibit superior exchange coupling strength in contrast to 1:4. The asymmetric nature of the FORC distribution at H-c = 0 Oe for both the nanocomposites validates the intercoupling between the reversible and irreversible magnetization. (C) 2015 Author(s).

Effect of Size Reduction on Magnetic Ordering in Sm1-x Ca (x) MnO3 (x=0.35, 0.65 and 0.92) Manganites: Magnetic and EMR Studies

We have employed the highly sensitive electron magnetic resonance technique complimented by magnetization measurements to study the impact of size reduction on the magnetic ordering in nanosized Sm1-x Ca (x) MnO3 (x = 0.35, 0.65 and 0.92). In the bulk form, x = 0.35 sample shows a charge ordering transition at 235 K followed by a mixed magnetic phase, the sample with x = 0.65 exhibits charge order below 275 K and shows an antiferromagnetic insulator phase below 135 K while that with x = 0.92 has a ferromagnetic-cluster glass ground state. Thus, a comparative study of magnetic ground states of bulk and nanoparticles (diameter similar to 25 nm) enables us to investigate size-induced effects on different types of magnetic ordering. It is seen that in the bulk samples the temperature dependences of the EPR parameters are quite different from each other. This difference diminishes for the nanosamples where all the three samples show qualitatively similar behavior. The magnetization measurements corroborate this conclusion.

Electron Magnetic Resonance Studies of Nanosized Nd0.65Ca0.35 Mn1-xCrxO3 (x=0, 0.06, 0.1) Manganite

We report a systematic investigation of the temperature dependence of electron magnetic resonance (EMR) line width, intensity and resonance field for similar to 25 nm Nd0.65Ca0.35MnO3 (NCMO1), Nd0.65Ca0.35Mn0.94Cr0.06O3 (NCMO2) and Nd0.65Ca0.35Mn0.9Cr0.1O3 (NCMO3) nanoparticles prepared by sol-gel method. The EMR line widths for the three nano-samples differ significantly from one another below a temperature T (min) where the line width has a minimum. T (min) was found to be 130, 100 and 120 K for NCMO1, NCMO2 and NCMO3, respectively. Well above T (min) the line width values for the three samples are close to one another. The sharp upturn of EMR line width below T (min) is attributed to the formation of short range, ferromagnetically ordered clusters. Temperature dependence of EMR intensity shows a residual CO transition in NCMO1 and NCMO2 and a complete disappearance of it in NCMO3. The intensity undergoes significant increase below 120, 80 and 100 K for NCMO1, NCMO2 and NCMO3, respectively, indicating the onset of ferromagnetic transitions. The occurrence of ferromagnetic transition is further confirmed by magnetization hysteresis measurements. The decrease in T (C) in NCMO2 and NCMO3 compared to NCMO1 nanoparticles is understood to be due to the destruction of the double-exchange interaction by chromium doping. The resonance field decreases below the ferromagnetic onset temperatures for all the samples as expected. The combined effects of the reduction in size and of chromium doping in Mn site are discussed.

New physical insights into the electromagnetic shielding efficiency in PVDF nanocomposites containing multiwall carbon nanotubes and magnetic nanoparticles

This work attempts to bring critical insights into the electromagnetic shielding efficiency in polymeric nanocomposites with respect to the particle size of magnetic nanoparticles added along with or without a conductive inclusion. To gain insight, various Ni-Fe (NixFe1-x; x = 10, 20, 40; Ni: nickel, Fe: iron) alloys were prepared by a vacuum arc melting process and different particle sizes were then achieved by a controlled grinding process for different time scales. Poly(vinylidene fluoride), PVDF based composites involving different particle sizes of the Ni-Fe alloy were prepared with or without multiwall carbon nanotubes (MWNTs) by a wet grinding approach. The Ni-Fe particles were thoroughly characterized with respect to their microstructure and magnetization; and the electromagnetic (EM) shielding efficiency (SE) of the resulting composites was obtained from the scattering parameters using a vector network analyzer in a broad range of frequencies. The saturation magnetization of Ni-Fe nanoparticles and the bulk electrical conductivity of PVDF/Ni-Fe composites scaled with increasing particle size of NiFe. Interestingly, the PVDF/Ni-Fe/MWNT composites showed a different trend where the bulk electrical conductivity and SE scaled with decreasing particle size of the Ni-Fe alloy. A total SE of similar to 35 dB was achieved with 50 wt% of Ni60Fe40 and 3 wt% MWNTs. More interestingly, the PVDF/Ni-Fe composites shielded the EM waves mostly by reflection whereas, the PVDF/Ni-Fe/MWNT shielded mostly by absorption. A minimum reflection loss of similar to 58 dB was achieved in the PVDF/Ni-Fe/MWNT composites in the X-band (8-12 GHz) for a particular size of Ni-Fe alloy nanoparticles. This study brings new insights into the EM shielding efficiency in PVDF/magnetic nanoparticle based composites in the presence and absence of conducting inclusion.

Structural modification and enhanced magnetic properties with two phonon modes in Ca-Co codoped BiFeO3 nanoparticles

Bi1-xCaxFe1-xCoxO3 nanoparticles with x=0.0, 0.05, 0.10 and 0.15 were successfully synthesized by cost effective tartaric acid based sol gel route. The alkali earth metal Ca2+ ions and transition metal Co3+ ions codoping at A and B-sites of BiFeO3 results in structural distortion and phase transformation. Rietveld refinement of XRD patterns suggested the coexistence of rhombohedral and orthorhombic phases in codoped BiFeO3 samples. Both XRD and Raman scattering studies showed the compressive lattice distortion in the samples induced by codoping of Ca2+ and Co3+ ions. Two-phonon Raman spectra exhibited the improvement of magnetization in these samples. X-ray photoelectron spectroscopy (XPS) showed the dominancy of Fe3+ and Co3+ oxidation states along with the shifting of the binding energy of Bi 4f orbital which confirms the substitution Ca2+ at Bi-site. The magnetic study showed the enhancement in room temperature ferromagnetic behavior with co-substitution consistent with Rama analysis. The gradual change in line shape of electron spin resonance spectra indicated the local distortion induced by codoping. (C) 2015 Published by Elsevier Ltd and Techna Group S.r.l.

Organic fluorine involved intramolecular hydrogen bonds in the derivatives of imides: NMR evidence corroborated by DFT based theoretical calculations

The rare occurrence of intramolecular hydrogen bonds (HBs) of the type N-H center dot center dot center dot F-C is detected in the derivatives of imides in a low polarity solvent by using multi-dimensional and multinuclear NMR experiments. The observation of (1h)J(FH), (2h)J(FN), and (2h)J(FF), where the spin magnetization is transmitted through space among the interacting NMR active nuclei, provided strong and unambiguous evidence for the existence of intra-molecular HBs. The variation in the chemical shifts of labile protons depending on physical conditions, such as the solvent dilution and the systematic alteration of temperature confirmed the presence of weak interactions through intramolecular HBs in all the investigated fluorine substituted molecules. The self or cross dimerization of molecules is unequivocally discarded by the analysis of the rates of diffusion obtained using pseudo-two dimensional DOSY experiments. The Density Function Theory (DFT) calculations based on the Quantum Theory of Atoms In Molecules (QTAIM) and Non Covalent Interaction (NCI), are in close agreement with the NMR experimental findings.

Effect of Sm3+-Gd3+ on structural, electrical and magnetic properties of Mn-Zn ferrites synthesized via combustion route

Nanocrystalline Mn0.4Zn0.6SmxGdyFe2-(x+y)O4 (x = y = 0.01, 0.02, 0.03, 0.04 and 0.05) were synthesized by combustion route. The detailed structural studies were carried out through X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM). The results confirms the formation of mixed spine phase with cubic structure due to the distortion created with co-dopants substitution at Fe site in Mn-Zn ferrite lattice. Further, the crystallite size increases with an increase of Sm3+-Gd3+ ions concentration while lattice parameter and lattice strain decreases. Furthermore, the effect of Sm-Gd co-doping in Mn-Zn ferrite on the room temperature electrical (dielectric studies) studies were carried out in the wide frequency range 1 GHz-5 GHz. The magnetic studies were carried out using vibrating sample magnetometer (VSM) under applied magnetic field of 1.5T and also room temperature electron paramagnetic resonance (EPR) spectra's were recorded. From the results of dielectric studies, it shows that the real and imaginary part of permittivities are increasing with variation of Gd3+ and Sm3+ concentration. The magnetic studies reveal the decrease of remnant, saturation magnetization and coercivity with increasing of Sm3+-Gd3+ ion concentration. The g-value, peak-to-peak line width and spin concentration evaluated from EPR spectra correlated with cations occupancy. The electromagnetic properties clearly indicate that these materials are the good candidates which are useful at L and C band frequency. (C) 2015 Elsevier B.V. All rights reserved.

MnFe2O4-Fe3O4 core-shell nanoparticles as a potential contrast agent for magnetic resonance imaging

In recent years, magnetic core-shell nanoparticles have received widespread attention due to their unique properties that can be used for various applications. We introduce here a magnetic core-shell nanoparticle system for potential application as a contrast agent in magnetic resonance imaging (MRI). MnFe2O4-Fe3O4 core-shell nanoparticles were synthesized by the wet-chemical synthesis method. Detailed structural and compositional charaterization confirmed the formation of a core-shell microstructure for the nanoparticles. Magnetic charaterization revealed the superparamagnetic nature of the as-synthesized core-shell nanoparticles. Average size and saturation magnetization values obtained for the as-synthesized core-shell nanoparticle were 12.5 nm and 69.34 emu g(-1) respectively. The transverse relaxivity value of the water protons obtained in the presence of the core-shell nanoparticles was 184.1 mM(-1) s(-1). To investigate the effect of the core-shell geometry towards enhancing the relaxivity value, transverse relaxivity values were also obtained in the presence of separately synthesized single phase Fe3O4 and MnFe2O4 nanoparticles. Average size and saturation magnetization values for the as-synthesized Fe3O4 nanoparticles were 12 nm and 65.8 emu g(-1) respectively. Average size and saturation magnetization values for the MnFe2O4 nanoparticles were 9 nm and 61.5 emu g(-1) respectively. The transverse relaxivity value obtained in the presence of single phase Fe3O4 and MnFe2O4 nanoparticles was 96.6 and 83.2 mM(-1) s(-1) respectively. All the nanoparticles (core-shell and single phase) were coated with chitosan by a surfactant exchange reaction before determining the relaxivity values. For similar nanoparticle sizes and saturation magnetization values, the highest value of the transverse relaxivity in the case of core-shell nanoparticles clearly illustrated that the difference in the magnetic nature of the core and shell phases in the core-shell nanoparticles creates greater magnetic inhomogeneity in the surrounding medium yielding a high value for proton relaxivity. The MnFe2O4-Fe3O4 core-shell nanoparticles exhibited extremely low toxicity towards the MCF-7 cell line. Taken together, this opens up new avenues for the use of core-shell nanoparticles in MRI.

Ferromagnetism and the effect of free charge carriers on electric polarization in the double perovskite Y2NiMnO6

The double perovskite Y2NiMnO6 displays ferromagnetic transition at T-c approximate to 81 K. The ferromagnetic order at low temperature is confirmed by the saturation value of magnetization (Ms) and also validated by the refined ordered magnetic moment values extracted from neutron powder diffraction data at 10 K. This way, the dominant Mn4+ and Ni2+ cationic ordering is confirmed. The cation-ordered P2(1)/n nuclear structure is revealed by neutron powder diffraction studies at 300 and 10 K. Analysis of the frequency-dependent dielectric constant and equivalent circuit analysis of impedance data take into account the bulk contribution to the total dielectric constant. This reveals an anomaly which coincides with the ferromagnetic transition temperature (T-c). Pyrocurrent measurements register a current flow with onset near T-c and a peak at 57 K that shifts with temperature ramp rate. The extrinsic nature of the observed pyrocurrent is established by employing a special protocol measurement. It is realized that the origin is due to reorientation of electric dipoles created by the free charge carriers and not by spontaneous electric polarization at variance with recently reported magnetism-driven ferroelectricity in this material.

Magnetic properties of nanocrystalline Mn1-xZnxFe2O4

Nanocrystalline Mn1-xZnxFe2O4 (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) were prepared via solution combustion method. Structural and morphology of Mn-Zn ferrites were characterized by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). Magnetic properties were carried out using vibrating sample magnetometer (VSM) at room temperature (RT) up to maximum field of 1.5 T. The room temperature real and imaginary part of permeability(mu' and mu'') has been measured in the frequency range of 1MHz to 1GHz. The room temperature XRD patterns exhibits the spinel cubic (Fm-3m) structure and broad XRD patterns shows the presence of nanoparticles. The imaginary part of the permeability (mu'') gradually increased with the frequency and took a broad maximum at a certain frequency, where the real permeability (mu') rapidly decreases, which is known as natural resonance. The coercive filed values are low, hence probability of domain rotation is also lower and the magnetization decreased with zinc substitution. The values of mu' and mu'' increases sharply, attained a maximum and then decreases with zinc content.