Comparative study of magnetic ordering in bulk and nanoparticles of Sm0.65Ca0.35MnO3: Magnetization and electron magnetic resonance measurements

To explore the effect of size reduction to nanoscale on the hole doped Sm0.65Ca0.35MnO3 compound, dc magnetic measurements and electron magnetic resonance (EMR) were done on bulk and nanoparticle samples in the temperature range 10 <= T <= 300 K. Magnetization measurement showed that the bulk sample undergoes a charge ordering transition at 240K and shows a mixed magnetic phase at low temperature. However, the nanosample underwent a ferromagnetic transition at 75 K, and the charge ordered state was destabilized on size reduction down to nanoscale. The low-temperature ferromagnetic component is found to be enhanced in nanoparticles as compared to their bulk counterpart. Interestingly around room temperature, bulk particles show higher magnetization where as at low temperature nanoparticles show higher magnetization. Ferromagnetism in the bulk is due to super exchange where as ferromagnetism in nanoparticles is due to uncompensated spins of the surface layer. Temperature variation of EMR parameters correlates well with the results of magnetic measurements. The magnetic behaviour of the nanoparticles is understood in terms of the core shell scenario. (C) 2015 AIP Publishing LLC.

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.

Mono layer to Interdigitated Partial Bilayer Smectic C Transition in Thiophene-Based Spacer Mesogens: X-ray Diffraction and C-13 Nuclear Magnetic Resonance Studies

Mesophase organization of molecules built with thiophene at the center and linked via flexible spacers to rigid side arm core units and terminal alkoxy chains has been investigated. Thirty homologues realized by varying the span of the spacers as well as the length of the terminal chains have been studied. In addition to the enantiotropic nematic phase observed for all the mesogens, the increase of the spacer as well as the terminal chain lengths resulted in the smectic C phase. The molecular organization in the smectic phase as investigated by temperature dependent X-ray diffraction measurements revealed an interesting behavior that depended on the length of the spacer vis-a-vis the length of the terminal chain. Thus, a tilted interdigitated partial bilayer organization was observed for molecules with a shorter spacer length, while a tilted monolayer arrangement was observed for those with a longer spacer length. High-resolution solid state C-13 NMR studies carried out for representative mesogens indicated a U-shape for all the molecules, indicating that intermolecular interactions and molecular dynamics rather than molecular shape are responsible for the observed behavior. Models for the mesophase organization have been considered and the results understood in terms of segregation of incompatible parts of the mesogens combined with steric frustration leading to the observed lamellar order.

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.

Prior image based temporally constrained reconstruction algorithm for magnetic resonance guided high intensity focused ultrasound

Purpose: A prior image based temporally constrained reconstruction ( PITCR) algorithm was developed for obtaining accurate temperature maps having better volume coverage, and spatial, and temporal resolution than other algorithms for highly undersampled data in magnetic resonance (MR) thermometry. Methods: The proposed PITCR approach is an algorithm that gives weight to the prior image and performs accurate reconstruction in a dynamic imaging environment. The PITCR method is compared with the temporally constrained reconstruction (TCR) algorithm using pork muscle data. Results: The PITCR method provides superior performance compared to the TCR approach with highly undersampled data. The proposed approach is computationally expensive compared to the TCR approach, but this could be overcome by the advantage of reconstructing with fewer measurements. In the case of reconstruction of temperature maps from 16% of fully sampled data, the PITCR approach was 1.57x slower compared to the TCR approach, while the root mean square error using PITCR is 0.784 compared to 2.815 with the TCR scheme. Conclusions: The PITCR approach is able to perform more accurate reconstructions of temperature maps compared to the TCR approach with highly undersampled data in MR guided high intensity focused ultrasound. (C) 2015 American Association of Physicists in Medicine.

Ultrafine graphene oxide-CoFe2O4 nanoparticle composite as T-1 and T-2 contrast agent for magnetic resonance imaging

Graphene oxide-CoFe2O4 nanoparticle composites were synthesized using a two step synthesis method in which graphene oxide was initially synthesized followed by precipitation of CoFe2O4 nanoparticles in a reaction mixture containing graphene oxide. Samples were extracted from the reaction mixture at different times at 80 degrees C. All the extracted samples contained CoFe2O4 nanoparticles formed over the graphene oxide. It was observed that the increase in the reflux time significantly increased the saturation magnetization value for the superparamagnetic nanoparticles in the composite. It was also noticed that the size of the nanoparticles increased with increase in the reflux time. Transverse relaxivity of the water protons increased monotonically with increase in the reflux time. Whereas, the longitudinal relaxivity value initially increased and then decreased with the reflux time. Graphene oxide-CoFe2O4 nanoparticle composites also exhibit biocompatibility towards the MCF-7 cell line.

Magnetic superconductors: Model theories and experimental properties of rare-earth compounds

Superconducting and magnetically long-range ordered states were believed to be mutually exclusive phenomena. The discovery of rare-earth compounds in recent years, which exhibit both superconductivity and magnetic ordering (ferromagnetic, antiferromagnetic or sinusoidal), has led to considerable theoretical and experimental work on such systems. In the present article, we give a review of various theoretical models and important experimental results. In the theoretical sections, we start with the Abrikosov-Gorkov pair breaking theory for dilute alloys and discuss its improvement in the work of Müller-Hartmann and Zittartz. Then, in the context of magnetic superconductors, various microscopic theories that have been advanced are presented. These predict re-entrant behaviour in some systems (ferromagnetic superconductors) and coexistence regions in others (particularly antiferromagnetic superconductors). Following this, phenomenological generalized Ginzburg-Landau theories for two kinds of orders (superconducting and magnetic) are presented. A section dealing with renormalization group analysis of phase diagrams in magnetic superconductors is given. In experimental sections, the properties of each rare-earth compounds (ternary as well as some tetranery) are reviewed. These involve susceptibility, heat capacity, resistivity, upper critical field, neutron scattering and magnetic resonance measurements. The anomalous behaviour of the upper critical field of antiferromagnetic superconductors near the Néel temperature is discussed both in theory sections and experimental section for various systems.

Electron paramagnetic resonance study of vanadyl ion in K2Zn(SO4)2·6H2O and K2Mg(SO4)2·6H2O

EPR study of the vanadyl ion has been carried out in its paramagnetically dilute form in K2Zn(SO4)2·6H2O and K2Mg(SO4)2·6H2O at room temperature at X-band. The vanadyl ion enters the divalent metal site and preferentially orients itself in the direction of the water molecules forming the octahedron and forms the vanadyl sulfate pentahydrate complex. The g and A tensorscorresponding to the two populous V-O orientations have been analyzed to obtain the principal values and their direction cosines with respect to the crystallographics axes. It is found that the g and A tensor have the same principal frames of reference within the limits of eperimental error. A correlation between the metal-water distance and the populations of the different V-O orientations is observed.

Triple resonance, triple-quantum NMR studies of dipolar coupled spin systems

Multiple quantum-single quantum correlation experiments are employed for spectral simplification and determination of the relative signs of the couplings. In this study, we have demonstrated the excitation of three nuclei, triple quantum coherences and discussed the information obtainable from such experiments. The experiments have been carried out on doubly labeled acetonitrile and fluoroacetonitrile aligned in liquid crystalline media. The experiment is advantageous in providing many spectral parameters from a single experiment. The coherence pathways involved in the pulse sequence are described using product operators. (C) 2011 Elsevier Inc. All rights reserved.

Preparation, characterization and magnetic studies of Bi0.5X0.5(X=Ca,Sr)MnO3 nanoparticles

Nanoparticles (dia ~ 5 - 7 nm) of Bi0.5X0.5(X=Ca,Sr)MnO3 are prepared by polymer assisted sol-gel method and characterized by various physico-chemical techniques. X-ray diffraction gives evidence for single phasic nature of the materials as well as their structures. Mono dispersed to a large extent, isolated nanoparticles are seen in the transmission electron micrographs. High resolution electron microscopy shows the crystalline nature of the nanoparticles. Superconducting quantum interferometer based magnetic measurements from 10K to 300K show that these nanomanganites retain the charge ordering nature unlike Pr and Nd based nanomanganites. The CO in Bi based manganites is thus found to be very robust consistent with the observation that magnetic field of the order of 130 T are necessary to melt the CO in these compounds. These results are supported by electron magnetic resonance measurements.

Orthogonal biofunctionalization of magnetic nanoparticles via ``clickable'' poly(ethylene glycol) silanes: a ``universal ligand'' strategy to design stealth and target-specific nanocarriers

The present work demonstrates a novel strategy to synthesize orthogonally bio-engineered magnetonanohybrids (MNPs) through the design of versatile, biocompatible linkers whose structure includes: (i) a robust anchor to bind with metal-oxide surfaces; (ii) tailored surface groups to act as spacers and (iii) a general method to implement orthogonal functionalizations of the substrate via ``click chemistry''. Ligands that possess the synthetic generality of features (i)-(iii) are categorized as ``universal ligands''. Herein, we report the synthesis of a novel, azido-terminated poly(ethylene glycol) (PEG) silane that can easily self-assemble on MNPs through hetero-condensation between surface hydroxyl groups and the silane end of the ligand, and simultaneously provide multiple clickable sites for high density, chemoselective bio-conjugation. To establish the universal-ligand-strategy, we clicked alkyl-functionalized folate onto the surface of PEGylated MNPs. By further integrating a near-infrared fluorescent (NIRF) marker (Alexa-Fluor 647) with MNPs, we demonstrated their folate-receptor mediated internalization inside cancer cells and subsequent translocation into lysosomes and mitochondria. Ex vivo NIRF imaging established that the azido-PEG-silane developed in course of the study can effectively reduce the sequestration of MNPs by macrophage organs (viz. liver and spleen). These folate-PEG-MNPs were not only stealth and noncytotoxic but their dual optical and magnetic properties aided in tracking their whereabouts through combined magnetic resonance and optical imaging. Together, these results provided a strong motivation for the future use of the ``universal ligand'' strategy towards development of ``smart'' nanohybrids for theragnostic applications.

Size Control and Magnetic Property Trends in Cobalt Ferrite Nanoparticles Synthesized Using an Aqueous Chemical Route

Cobalt ferrite (CoFe2O4) is an engineering material which is used for applications such as magnetic cores, magnetic switches, hyperthermia based tumor treatment, and as contrast agents for magnetic resonance imaging. Utility of ferrites nanoparticles hinges on its size, dispersibility in solutions, and synthetic control over its coercivity. In this work, we establish correlations between room temperature co-precipitation conditions, and these crucial materials parameters. Furthermore, post-synthesis annealing conditions are correlated with morphology, changes in crystal structure and magnetic properties. We disclose the synthesis and process conditions helpful in obtaining easily sinterable CoFe2O4 nanoparticles with coercive magnetic flux density (H-c) in the range 5.5-31.9 kA/m and M-s in the range 47.9-84.9 A.m(2)Kg(-1). At a grain size of similar to 54 +/- 2 nm (corresponding to 1073 K sintering temperature), multi-domain behavior sets in, which is indicated by a decrease in H-c. In addition, we observe an increase in lattice constant with respect to grain size, which is the inverse of what is expected of in ferrites. Our results suggest that oxygen deficiency plays a crucial role in explaining this inverse trend. We expect the method disclosed here to be a viable and scalable alternative to thermal decomposition based CoFe2O4 synthesis. The magnetic trends reported will aid in the optimization of functional CoFe2O4 nanoparticles

Enhanced electric field tunable magnetic properties of lead-free Na0.5Bi0.5TiO3-MnFe2O4 multiferroic composites

Strong magnetoelectric (ME) interaction was exhibited at both dc and microwave frequencies in a lead-free multiferroic particulate composites of Na0.5Bi0.5TiO3 (NBT) and MnFe2O4 (MFO) multiferroic, which were prepared by sol-gel route. The room temperature permeability measurements were carried out in the frequency range of 1 MHz-1 GHz. A systematic study of structural, magnetic and ME properties were undertaken. The room temperature ferromagnetic resonance (FMR) was studied. Strong ME coupling is demonstrated in 70NBT-30MFO composite by an electrostatically tunable FMR field shift up to 428 Oe (at E = 4 kV/cm), which increases to a large value of 640 Oe at E = 8 kV/cm. Furthermore, these lead-free multiferroic composites exhibiting electrostatically induced magnetic resonance field at microwave frequencies provide great opportunities for electric field tunable microwave devices.

Structural and magnetic properties of Dy3+ doped Y3Fe5O12 for microwave devices

The Dy3+ doped Y3-xDyxFe5O12 (x=0-3) nanopowders were prepared using microwave hydrothermal route. The structural and morphological studies were analyzed using transmission electron microscope, X-ray diffractometer and field emission scanning electron microscope. The nanopowders were sintered at 900 degrees C/90 min using microwave furnace. Dense ceramics with theoretical density of around 95% was obtained. Ferro magnetic resonance (FMR) spectrum and microwave absorption spectrum of Dy3+ doped YIG were studied, the signal exhibits a resonance character for all Dy3+ variations. It was observed that the location of the FMR signal peak at the field axes monotonically shifts to higher field with increasing Dy3+ content. The dielectric and magnetic properties (epsilon', epsilon `', mu' and mu `') of Dy3+ doped YIG were studied over a wide range of frequency (1-50 GHz). With increase of Dy3+ both epsilon' and mu' decreased. The low values of dielectric, magnetic properties and broad distribution of FMR line width of these ceramics are opening the real opportunity to use them for microwave devices above K- band frequency. (C) 2015 Elsevier Ltd. All rights reserved.