Sher Singh Meena

Random site occupancy induced disordered Néel-type collinear spin alignment in heterovalent Zn2+–Ti4+ ion substituted CoFe2O4

CoFe2O4, cobalt ferrite (CFO) nano-particles with composition CoZnxTixFe2−2xO4 (0 ≤ x ≤ 0.4) were synthesized by sol–gel autocombustion method. The effect of Zn2+–Ti4+ substitution on the structural, magnetic and frequency dependent permeability properties of the CFO nano-particles were investigated by X-ray diffraction, 57Fe Mossbauer spectroscopy, vibrating sample magnetometry, transmission electron microscopy and permeability analysis. The Rietveld refinement of XRD patterns confirm the single spinel phase and the crystallite size is found in the range of 22–32 nm. Cation distribution was estimated by refining the XRD pattern by Rietveld method, and shows Zn2+ ions at the tetrahedral A-sites, and Co2+ and Ti4+ ions at octahedral B-sites. The saturation magnetization (Ms) increased from 58 to 75 emu g−1 for up to x = 0.2 and then decreased, while the coercivity decreased continuously with Zn2+–Ti4+ substitution. Two distinct composition ranges with Zn2+–Ti4+ substitution are identified for which Ms variation with x is explained by the Neel and Yafet–Kittel models. The room temperature Mossbauer spectra are analyzed in detail for probing the magnetic properties of Fe based Zn2+–Ti4+ substituted CFO. The effect of Zn2+–Ti4+ substitution on various Mossbauer parameters, viz. hyperfine field distribution, isomer shift, quadrupole splitting, and line width, has also been studied. The variation of nuclear magnetic fields at the A and B sites is explained on the basis of A–B and B–B supertransferred hyperfine interactions. The CFO nanoparticle is considered to possess a fully inverse spinel structure with a Neel-type collinear spin alignment, whereas the Zn2+–Ti4+ substitution in CFO is found to be structurally and magnetically disordered due to the nearly random distribution of cations and the canted spin arrangement. This study also demonstrates that one can tailor the magnetic properties of CFO particles by optimizing the Zn2+–Ti4+ substitution. The increase in the permeability, saturation magnetization and lower loss factor makes the synthesized materials suitable for applications in microwave devices and deflection yokes.

Crystal structure and magnetic properties of Bi0.8A0.2FeO3 (A = La, Ca, Sr, Ba) multiferroics using neutron diffraction and Mossbauer spectroscopy

Bi0.8A0.2FeO3 (A = La, Ca, Sr, Ba) multiferroics were studied using x-ray, neutron diffraction and magnetization techniques. All the samples crystallized in rhombohedral structure with space group R3c. The compounds exhibit antiferromagnetic (AFM) ordering at 300 K and no evidence of further structural or magnetic transition was observed on lowering of temperature below it. The magnetic structure of these substituted compounds are found to be collinear G-type AFM structure as against the non collinear incommensurate magnetic structure reported in the case of parent compound. The moments on Fe at 6 K are aligned along the a-axis in the case of Ca-doped sample. With increase in the ionic radii of dopant, the moments are found to be aligned in the ac plane and the angle of tilt away from the a-axis increases. The observed change in the magnetic structure with substitution is attributed to the intrinsic structural distortion as evidenced by the change in the bond angle (Fe-O-Fe) and bond distances (Bi-O, Fe-O...

Core–Shell Prussian Blue Analogue Molecular Magnet Mn1.5[Cr(CN)6]·mH2O@Ni1.5[Cr(CN)6]·nH2O for Hydrogen Storage

Core-shell Prussian blue analogue molecular magnet Mn1.5[Cr(CN)6]·mH2O@Ni1.5[Cr(CN)6]·nH2O has been synthesized using a core of Mn1.5[Cr(CN)6]·7.5H2O, surrounded by a shell of Ni1.5[Cr(CN)6]·7.5H2O compound. A transmission electron microscopy (TEM) study confirms the core-shell nature of the nanoparticles with an average size of ∼25 nm. The core-shell nanoparticles are investigated by using x-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and elemental mapping, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and infrared (IR) spectroscopy. The Rietveld refinement of the XRD pattern reveals that the core-shell compound has a face-centered cubic crystal structure with space group Fm3m. The observation of characteristic absorption bands in the range of 2000-2300 cm(-1) in IR spectra corresponds to the CN stretching frequency of Mn(II)/Ni(II)-N≡C-Cr(III) sequence, confirming the formation of Prussian blue analogues. Hydrogen absorption isotherm measurements have been used to investigate the kinetics of molecular hydrogen adsorption into core-shell compounds of the Prussian blue analogue at low temperature conditions. Interestingly, the core-shell compound shows an enhancement in the hydrogen capacity (2.0 wt % at 123 K) as compared to bare-core and bare-shell compounds. The hydrogen adsorption capacity has been correlated with the specific surface area and TGA analysis of the core-shell compound. To the best of our knowledge, this is the first report on the hydrogen storage properties of core-shell Prussian blue analogue molecular magnet that could be useful for hydrogen storage applications.

Magnetic and dielectric behavior in YMn1−xFexO3 (x ≤ 0.5)

The role of doping Fe on the structural, magnetic, and dielectric properties of frustrated antiferromagnet YMn1−xFexO3 (x ≤ 0.5) has been investigated. The neutron diffraction analysis shows that the structure of these polycrystalline samples changes from hexagonal phase (space group P63cm) to orthorhombic phase (space group Pnma) for x > 0.2. The frustration parameter decreases with Fe substitution. All the compounds are antiferromagnetic, and the magnetic structure is described as a mixture of Γ3 and Γ4 irreducible representation (IR) in the hexagonal phase, and the ratio of these two IRs is found to vary with Fe doping (x ≤ 0.2). A continuous spin reorientation as a function of temperature is observed in these samples. The magnetic ground state in the orthorhombic phase of the higher doped samples (x ≥ 0.3) is explained by taking Γ1 (GxCyAz) representation of Pnma setting. In YMnO3 suppression of dielectric constant e′ is observed below TN indicative of magnetoelectric coupling. This anomalous behavior...

Quaternary ammonium bearing hyper-crosslinked polymer encapsulation on Fe3O4 nanoparticles

The Gibbs free energy involved in the transfer of ions from water to a hydrophobic medium with ion-exchange sites changes the selectivity pattern expected from the electrostatic interactions. Oxyanions are less hydrated and, therefore, their exchange with more hydrated anions at the ion-exchange sites of a hydrophobic matrix in contact with aqueous solution is energetically more favorable. This gives rise to a possibility of separating the highly toxic monovalent oxyanions such as HCrO4−, MnO4−, TcO4− and ClO4− etc. by the Gibbs free energy of the transfer controlled anion-exchange process. In the present work, hydrophobic anion-exchange polymer encapsulation was anchored on Fe3O4 particles by a simple and reproducible method for studying the selectivity pattern. The choice of Fe3O4 nanoparticles (NPs) as the host matrix was based on their higher dispersion, larger surface area, and easy retrieval using an external magnetic field that is best suited for treating a large volume of an aqueous stream and developing sustainable technology. The anion-exchange polymer encapsulation was formed by first coating Fe3O4 NPs with (3-aminopropyl)triethoxysilane, and subsequently reacting these precursor particles in a sequence with poly(vinylbenzyl chloride) (PVBCl), 1,4-diazabicyclo[2.2.2]octane (DABCO), and 1,8-dibromooctane (DBO). Each step involved in the chemical treatments was monitored by C, H and N analyses. Energy dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) were used to confirm the expected chemical structure of the encapsulation on the Fe3O4 NPs. This sequence of chemical treatments resulted in the formation of hyper-crosslinked, hydrophobic, and high fixed positive charge density polymer encapsulation on the Fe3O4 NPs. The analyses of the images obtained from field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) indicated that the sizes of the Fe3O4 NPs were increased from 13 ± 3 nm to 33 ± 3 nm due to the formation of polymer encapsulation and inter-particle crosslinking. Although a very dilute solution and ultrasonication were used, there was a possibility of crosslinking between the particles. However, the thus encapsulated Fe3O4 NPs retained their superparamagnetic properties having a reasonably good magnetic saturation (30 emu g−1). The anion-exchange capacity was found to be 0.65 ± 0.03 meq g−1. The Fe3O4@PVB–DABCO–DBO particles exhibited a selectivity pattern corresponding to the Hofmeister series, i.e. the least hydrated anions were adsorbed preferentially. For example, the least hydrated representative TcO4− anions adsorbed quantitatively in the Fe3O4@PVB–DABCO–DBO particles from aqueous solutions such as ground water, seawater, and 0.5 mol L−1 HNO3 with efficiencies of 98%, 80% and 75%, respectively.

Anisotropy and domain state dependent enhancement of single domain ferrimagnetism in cobalt substituted Ni–Zn ferrites

Nanocrystalline Co substituted Ni–Zn ferrites with the general formula CoxNi0.6−xZn0.4Fe2O4 (x = 0.0 to 0.6) were prepared by a precursor combustion method. Average crystallite size, as estimated by using a Scherrer method and the particle size obtained from transmission electron microscopy (TEM) techniques, was found to be in the range of 10–30 nm. EDX and XRD analysis confirms the presence of Co, Ni, Zn, Fe and oxygen and the desired phases in the prepared nanoparticles. Selective area electron diffraction (SAED) analysis confirms the crystalline nature of the prepared nanoparticles. It is observed that Co substitution has a pronounced effect on the magnetic properties such as MR, MS and HC and also on the Curie temperature (TC), which is found to decrease from 420 °C for non-substituted Ni–Zn ferrites to 325 °C for the highest substitution of x = 0.6. These effects are assigned to the higher magnetocrystalline anisotropy of Co than Ni and to the size dependent existence of single domain–superparamagnetic particles distribution. Comparatively, the dominant existence of single domain particles with Co substitution over superparamagnetic particles in non-substituted nickel zinc ferrites is extensively investigated in this article.

Magnetic and electric properties of nanoparticles of Ni-substituted ferrites synthesized using a microwave refluxing process

Microwave refluxing was found to be a suitable method to produce single phase magnetic nanoparticles of NixFe3−xO4. Ethylene glycol employed in the synthesis plays a crucial role in restricting the particle size to nano dimensions. X-ray diffraction studies, Mössbauer spectroscopy and magnetic measurements suggest that as-prepared samples were small in size (∼15 nm) and hence display low saturation magnetization values (44–60 emu g−1). NixFe3−xO4 transforms into γ-NiyFe2−yO3 and α-NiyFe2−yO3 after sintering at 720 and 1020 K respectively as indicated by X-ray diffraction patterns. This was also confirmed by resistivity measurements. The electrical properties of the two transformed phases were comparable to their bulk values. It has also been found that a large substitution of Ni stabilizes γ-phase at higher temperature.

Structural and Mossbauer spectroscopic studies of heat-treated NixZn1−xFe2O4 ferrite nanoparticles

NixZn1−xFe2O4(x = 0.5,0.6,0.7) nanoparticles were prepared using coprecipitation method and were heat treated at 200, 500 and 800 °C. Structure and hyperfine interactions were studied by X-ray diffraction and Mossbauer spectroscopic techniques, respectively. The particle size increases with increasing the heat treatment (HT) temperature and Ni ion concentration. Only a quadrupole doublet was observed for Ni0.5Zn0.5Fe2O4 ferrite, heat treated at 200 °C. For higher heat treatment temperatures, hyperfine sextets appear and become predominant in nanoparticles with 800 °C HT. However, the quadrupole doublet remains with reduced intensity. The results interpreted in terms of an existence of size distribution of nanoparticles.

Hydrothermally synthesized oxalate and phenanthroline based ferrimagnetic one-dimensional spin chain molecular magnets [{Fe(Δ)Fe(Λ)}1−x{Cr(Δ)Cr(Λ)}x(ox)2(phen)2]n (x = 0, 0.1 and 0.5) with giant coercivity of 3.2 Tesla

Oxalate (ox) and phenanthroline (phen) ligands based one dimensional spin chain molecular magnets, [{Fe(Δ)Fe(Λ)}1−x{Cr(Δ)Cr(Λ)}x(ox)2(phen)2]n (x = 0, 0.1, and 0.5) have been designed, and synthesized using a hydrothermal synthesis method. The Rietveld refinement of the powder X-ray and neutron diffraction patterns at room temperature confirms the single-phase formation of the compounds in the monoclinic structure with a space group P21. The compounds consist of two ligands, the oxalate (C2O42−) as a coordination acceptor building block and the neutral phen (C12H8N2) as a coordination donor building block. Both ligands are connected to Fe ions of different symmetry {Fe(Δ) and Fe(Λ)}, thus forming an alternating zigzag chain like crystal structure having the repeating unit of [phen-Fe(Δ)-C2O4-Fe(Λ)-phen]n. The chain is infinite in length and lies in the crystallographic ac plane. The interchain is well separated with an intermetallic distance of ∼8.8 A and the absence of an interchain π–π overlap between the organic ligands, resulting in a magnetic isolation between the interchains. The Mossbauer spectroscopy reveals the presence of high spin states of the Fe2+ ions of the compound for x = 0 whereas, both high-spin Fe2+ (t2g4eg2, S = 2) as well as low spin Fe2+ (t2g6eg0, S = 0) states are present for the compounds x = 0.1 and 0.5. The dc magnetization measurements show that the compounds exhibit spontaneous magnetization below ∼9 K. The transition temperature is found to be ∼8.7, 8.2 and 4.0 K for x = 0, 0.1 and 0.5 compounds, respectively. Moreover, a short range antiferromagnetic spin–spin correlation around 18–45 K has been observed for the compounds x = 0 and 0.1. An application of the Ising chain model to the dc magnetization data reveals the presence of a one-dimensional magnetic nature of all compounds with alternately spaced magnetic Fe sites. It is observed that the different Lande g factors (3.4 and 2.8) and exchange coupling constant values (−86 and −54 K) for x = 0 at two alternating Fe sites give rise to a ferrimagnetic-like behavior of the chains. The ferrimagnetic chain like structure transforms toward antiferromagnetic with Cr doping i.e. for x = 0.1 and 0.5. A hysteresis loop with a giant coercivity (3.2 T for x = 0) has been observed at 1.6 K, indicating a hard magnet-type behavior. The frequency dependence of the peak temperature in ac susceptibility vs. temperature curves for the x = 0 compound has been fitted and analyzed using the Arrhenius law as well as the power law, which exclude the possibility of a spin glass behavior. The fitted parameters (Δ/kB = 208 K and τ0 = 2.9 × 10−14 s obtained from the Arrhenius law, and τ0 = 6.1 × 10−8 s, and zν = 2.6 from the power law) show that the compound obeys the Glauber dynamics and is a real ferrimagnetic one-dimensional single chain magnet. In addition, the high pressure magnetization measurements for the x = 0 compound show an enhancement in the transition temperature from ∼8.7 to 10.7 K with increasing pressure. The observation of both a one-dimensional spin chain nature and giant coercivity (3.2 Tesla) in such compounds opens up new opportunities to design and develop low dimensional molecular chain magnets through the appropriate choice of ligands using the hydrothermal synthesis method, because the observation of magnetic hysteresis of molecular origin in single-molecule magnets is considered one of the most relevant achievements in molecular magnetism.

Enabling the Electrochemical Activity in Sodium Iron Metaphosphate [NaFe(PO3)3] Sodium Battery Insertion Material: Structural and Electrochemical Insights

Sodium-ion batteries are widely pursued as an economic alternative to lithium-ion battery technology, where Fe- and Mn-based compounds are particularly attractive owing to their elemental abundance. Pursuing phosphate-based polyanionic chemistry, recently solid-state prepared NaFe(PO3)3 metaphosphate was unveiled as a novel potential sodium insertion material, although it was found to be electrochemically inactive. In the current work, employing energy-savvy solution combustion synthesis, NaFe2+(PO3)3 was produced from low-cost Fe3+ precursors. Owing to the formation of nanoscale carbon-coated product, electrochemical activity was enabled in NaFe(PO3)3 for the first time. In congruence with the first principles density functional theory (DFT) calculations, an Fe3+/Fe2+ redox activity centered at 2.8 V (vs Na/Na+) was observed. Further, the solid-solution metaphosphate family Na(Fe1-xMnx)(PO3)3 (x = 0-1) was prepared for the first time. Their structure and distribution of transition metals (TM = Fe/Mn) was analyzed with synchrotron diffraction, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy. Synergizing experimental and computational tools, NaFe(PO3)3 metaphosphate is presented as an electrochemically active sodium insertion host material.