EMR searching of quantum behavior of magnetic γ-Fe<inf>2</inf>O<inf>3</inf> nanoparticles encapsulated into poly(Propylene imine) dendrimer

© Kazan Federal University (KFU).The superparamagnetic γ-Fe2O3 nanoparticles (average diameter of 2.5 nm) encapsulated in poly(propylene imine) dendrimer have been investigated by electron magnetic resonance (EMR). EMR measurements have been recorded in perpendicular and parallel configurations in the wide temperature range (4.2-300 K). It has been shown that the model based on the spin value S = 30, corresponding to the total magnetic moment of the nanoparticle, can be used to interpret the experimental results and the proof of the quantum behavior of γ-Fe2O3 nanoparticles

Blue shift in optical absorption, magnetism and light-induced superparamagnetism in γ-Fe2O3 nanoparticles formed in dendrimer

© 2015, Springer Science+Business Media Dordrecht. Abstract: We are presenting the investigation of the optical, magnetic, and photoinduced superparamagnetic properties of single-domain γ-Fe2O3 nanoparticles (NPs) with diameters of about 2.5 nm formed in second-generation poly(propylene imine) dendrimer. The optical absorption studies indicated direct allowed transition with the band gap (4.5 eV), which is blue shift with respect to the value of the bulk material. Low-temperature blocking of the NPs magnetic moments at 18 K is determined by SQUID measurements. The influence of pulsed laser irradiation on the superparamagnetic properties of γ-Fe2O3 NPs was studied by EPR spectroscopy. It has been shown that irradiation of the sample held in vacuo and cooled in zero magnetic field to 6.9 K leads to the appearance of a new EPR signal, which decays immediately after the irradiation is stopped. The appearance and disappearance of this new signal can be repeated many times at 6.9 K when we turn on/turn off the laser. We suppose that the generation of conduction band electrons by irradiation into the band gap of the γ-Fe2O3 changes the superparamagnetic properties of NPs. Graphical Abstract: [Figure not available: see fulltext.]Features of the behavior of single-domain γ-Fe2O3 nanoparticles formed in dendrimer were found by UV-Vis and EPR spectroscopy: “blue” shift in optical absorption, a significant increase in the band gap width and variation of superparamagnetic properties under light irradiation

Optical properties and photoinduced superparamagnetism of γ-Fe<inf>2</inf>O<inf>3</inf> nanoparticles formed in dendrimer

© 2014 Elsevier Ltd. All rights reserved. We are presenting the joint investigation of the optical and photoinduced superparamagnetic properties of a single-domain γ-Fe2O3 nanoparticles (NPs) formed in poly(propylene imine) (PPI)-dendrimer. The optical absorption studies indicated direct allowed transition with the band gap (4.5 eV), which is blue-shift with respect to the value of the bulk material. The influence of pulsed laser irradiation on the superparamagnetic properties of γ-Fe2O3 NPs was studied by Electron paramagnetic resonance (EPR) spectroscopy. It has been shown that irradiation of the sample in vacuo and cooled in zero magnetic field to 6.9 K leads to the appearance of a new EPR signal, which decays immediately after the irradiation is stopped. We suppose that the generation of conduction band electrons by irradiation into the band gap of the γ-Fe2O3 changes the superparamagnetic properties of NPs

High-spin Fe(III) Schiff based complexes with photoactive ligands. Synthesis, EPR study and magnetic properties

© 2018 Elsevier Ltd A series of three novel Fe(III) compounds of the formula [FeL2]X (where X = Cl− (1), PF6− (2), NO3– (3), and L is a photoactive ligand, (4)-4-(((2-(ethylamino)ethyl)imino)methyl)-3-hydroxyphenyl 4-bromobenzoate) was synthesized and studied by means of electron paramagnetic resonance (EPR) and pulsed laser irradiation. The Fe3+ ions in these compounds are in a high-spin state. A thorough analysis of the EPR data suggests that compounds 1 and 2 undergo an order–disorder ferroelectric phase transition, and below the phase transition temperature (Tc = 100 and 200 K for compounds 1 and 2, respectively) a nonzero average electric dipole moment appears. To get an insight into molecular structure of Fe3+ ions and their supramolecular organization in low-temperature (LT) and high-temperature (HT) phases of compounds 1 and 2, a series of density functional theory calculations was performed. On the basis of our findings, the LT- and HT-phase structures were proposed for these compounds. It was also shown that, whereas the chloride and hexafluorophosphate anions are able to form a network of hydrogen bonds between the [FeL2]X units (ionic pairs), which enable an electric dipole ordering in the sample, the nitrate anions, in contrast, tend to form hydrogen bonds inside the ionic pair. This conclusion is evidenced by the observed EPR spectra, which are different for compound 3 and are not indicative of the existence of an order–disorder ferroelectric phase transition. The EPR data obtained upon irradiation of compound 1 show that photoexcitation in the UV region at 5 K destroys hydrogen bonds and converts cationic complexes into ligand-to-metal charge transfer (LMCT) states, in which the iron is ferrous, and the unpaired electron is located on the salicylidene moieties. The LMCT states decay back to the ferric one, and ferric complexes further form the most stable (LT) phase structure

EPR detection of presumable quantum behavior of iron oxide nanoparticles in dendrimeric nanocomposite

© 2017 Elsevier B.V.The superparamagnetic γ-Fe2O3 nanoparticles (average diameter of 2.5 nm) encapsulated in poly(propylene imine) dendrimer have been investigated by Electron Magnetic Resonance (EMR). EMR measurements have been recorded in perpendicular and parallel configurations in the wide temperature range (4.2–300 K). It has been shown that the model based on the spin value S = 30, corresponding to the total magnetic moment of the nanoparticle, can be used to interpret the experimental results and the proof of the quantum behavior of γ-Fe2O3 nanoparticles

Coexistence of spin crossover and magnetic ordering in a dendrimeric Fe(III) complex

© 2015 AIP Publishing LLC. The magnetic properties of a new dendrimeric spin crossover Fe(III) complex, [Fe(L)2]+PF6, where L = 3,5-di[3,4,5-tris(tetradecyloxy) benzoyloxy]benzoyl-4-salicylidene-N-ethyl-N-ethylene-diamine, are reported for the first time. EPR studies show that this compound undergoes a gradual spin transition in the temperature range 70-300K and has antiferromagnetic ordering below 10K. Mössbauer spectroscopy at 5K confirms the presence of magnetic ordering in the dendrimeric iron complex

Counterion effect on the spin-transition properties of the second generation iron(III) dendrimeric complexes

© 2017 Elsevier B.V.The magnetic properties and the influence of counterions on the spin crossover properties of two novel Fe(III) dendrimeric complexes of the second generation, namely [Fe(L)2]+X−, where L = 3,5-di(3,4,5-tris(tetradecyloxy)benzoyloxy)benzoyl-4-oxy-salicylidene-N’-ethyl-N-ethylenediamine X = Cl− (1), ClO4− (2), have been studied for the first time by magnetic susceptibility measurements and electron paramagnetic resonance (EPR) method in a wide (4.2–300 K) temperature range. EPR results showed that compound 1 contains about 98% of high-spin (HS, S = 5/2) and ∼2% of low-spin (LS, S = 1/2) Fe(III) centers, and undergoes an antiferromagnetic ordering below 7 K. The EPR integrated intensity of a broad line (g ≈ 2), corresponding to the HS iron(III) centers, passes through a broad maximum at Tmax ≈ 100 K, which is indicative of short-range correlation effects. The anomalous broadening of this EPR line at low temperatures with the critical exponent β = 1.5 upon approaching the long-range ordering transition (TNEPR = 7 K) from above indicates the quasi-two-dimensional antiferromagnetic nature of magnetism in complex 1. The spin-crossover effect is completely suppressed in compound 1. The complex with ClO4− counterion demonstrates a different magnetic behavior. EPR data showed that compound 2 contains about 77% of LS and ∼23% of HS Fe(III) centers at TNEPR = 10.2 K. It displays a partial spin crossover (S = 5/2 ↔ 1/2) above 150 K and undergoes the antiferromagnetic ordering below 10.2 K. The obtained results and the results of DFT calculations allowed us to conclude that a bilayered packing with a chain structure of Fe(III) centers in ionic bilayers is formed in compound 1, whereas a dimeric structure of Fe(III) centers is formed in compound 2. Thus, the ability of the counterion to form an effective network of hydrogen bonds and its size define the packing motif of the [Fe(L)2]+ complexes. Therefore, the replacing of the counterion has a significant impact on the magnetic properties of the compound

Restoring interlayer Josephson coupling in La1.885Ba0.115CuO4 by charge transfer melting of stripe order

We show that disruption of charge-density-wave (stripe) order by charge transfer excitation, enhances the superconducting phase rigidity in La1.885Ba0.115CuO4. Time-resolved resonant soft x-ray diffraction demonstrates that charge order melting is prompt following near-infrared photoexcitation whereas the crystal structure remains intact for moderate fluences. THz time-domain spectroscopy reveals that, for the first 2 ps following photoexcitation, a new Josephson plasma resonance edge, at higher frequency with respect to the equilibrium edge, is induced indicating enhanced superconducting interlayer coupling. The fluence dependence of the charge-order melting and the enhanced superconducting interlayer coupling are correlated with a saturation limit of ∼0.5mJ/cm2. Using a combination of x-ray and optical spectroscopies we establish a hierarchy of timescales between enhanced superconductivity, melting of charge order, and rearrangement of the crystal structure

Magnetic properties of novel dendrimeric spin crossover iron(III) complex

© 2015 Elsevier B.V. All rights reserved. The synthesis and magnetic properties of novel dendrimeric spin crossover Fe(III) complex of formula [Fe(L)2]+PF6 -, where L = 3,5-di(3,4,5-tris(tetradecyloxy)benzoyloxy)benzoyl-4-oxy-salicylidene-N′-ethyl-N-ethylenediamine have been studied for the first time by magnetic susceptibility, Electron Paramagnetic Resonance (EPR) and Mössbauer spectroscopy in the wide (2-300 K) temperature range. EPR showed that the compound is magnetically inhomogeneous, consists of two magnetic sub-lattices, displays a partial spin crossover (S=5/2 1/2) of ∼25% of the Fe(III) molecules above 160 K and undergoes the antiferromagnetic (AF) ordering below 10 K. High-spin (HS, S = 5/2) Fe(III) centers with weakly distorted octahedral environment most probably form chains in layers. The dimeric molecules, formed from low-spin (LS, S = 1/2) centers and HS centers with strongly distorted octahedral environment are likely located between the layers and are involved in the spin crossover. EPR has shown the presence of AF dynamical spin clusters in the high temperature (70-300 K) range, which are visible in the short time scale (10-10 s) and could not be registered in the static magnetic measurements. Mössbauer spectra demonstrated in a paramagnetic state of the compound a quadrupole doublet with average isomer shift of 0.35 mm/s and splitting 0.72 mm/s corresponding to HS Fe(III) centers. Below 60 K, the spectra displayed the appearance of magnetic hyperfine structure, whose relaxation nature testifies the collective spin flips of small clusters in the material. Mössbauer spectroscopy confirmed the existence of AF ordering in the Fe(III) dendrimeric complex at 5 K

Femtosecond dynamics of the collinear-to-spiral antiferromagnetic phase transition in CuO

We report on the ultrafast dynamics of magnetic order in a single crystal of CuO at a temperature of 207 K in response to strong optical excitation using femtosecond resonant x-ray diffraction. In the experiment, a femtosecond laser pulse induces a sudden, nonequilibrium increase in magnetic disorder. After a short delay ranging from 400 fs to 2 ps, we observe changes in the relative intensity of the magnetic ordering diffraction peaks that indicate a shift from a collinear commensurate phase to a spiral incommensurate phase. These results indicate that the ultimate speed for this antiferromagnetic re-orientation transition in CuO is limited by the long-wavelength magnetic excitation connecting the two phases.Comment: Accepted by Physical Review Letters (Dec. 2, 2011