Brant Cage

Ripening during magnetite nanoparticle synthesis: Resulting interfacial defects and magnetic properties

The structure and magnetic properties of magnetite (Fe3O4) nanoparticles synthesized by a solvothermal processing route are investigated. The nanoparticles are grown from the single organometallic precursor Fe(III) acetylacetonate in trioctylamine (TOA) solvent at 260°C, with and without the addition of heptanoic acid (HA) as a stabilizing agent. From the temporal particle size distributions, x-ray-diffraction patterns, high-resolution transmission electron microscope tilt series experiments, and superconducting quantum interference device magnetometry, we demonstrate that HA, a strong Lewis acid stabilizing agent, slows growth processes during ripening thus reducing the formation of interfacial defects, which we observe in the TOA-only synthesis. Nanoparticles grown with HA remain single crystalline for long growth times (up to 24h), show a focused particle size distribution for intermediate growth times (3h), and possess a higher magnetic anisotropy (15.8×104J∕m3) than particles grown without the additi...

Design for a multifrequency high magnetic field superconducting quantum interference device-detected quantitative electron paramagnetic resonance probe: Spin-lattice relaxation of cupric sulfate pentahydrate (CuSO4⋅5H2O)

We have designed a spectrometer for the quantitative determination of electron paramagnetic resonance (EPR) at high magnetic fields and frequencies. It uses a superconducting quantum interference device (SQUID) for measuring the magnetic moment as a function of the applied magnetic field and microwave frequency. We used powdered 2,2-diphenyl-1-picrylhydrazyl to demonstrate resolution of g-tensor anisotropy to 1 mT in a magnetic field of 3 T with a sensitivity of 1014 spins per 0.1 mT. We demonstrate multifrequency operation at 95 and 141 GHz. By use of an aligned single crystal of cupric sulfate pentahydrate (chalcanthite) CuSO4⋅5H2O, we show that the spectrometer is capable of EPR line shape analysis from 4 to 200 K with a satisfactory fit to a Lorentzian line shape at 100 K. Below 100 K, we observed line-broadening, g shifts, and spectral splittings, all consistent with a known low-dimensional phase transition. Using SQUID magnetometry and a superconducting magnet, we improve by an order of magnitude th...

Advantages of superconducting quantum interference device-detected magnetic resonance over conventional high-frequency electron paramagnetic resonance for characterization of nanomagnetic materials

A dc-detected high-frequency electron paramagnetic resonance (HF-EPR) technique, based on a standard superconducting quantum interference device (SQUID) magnetometer, has significant advantages over traditional HF-EPR based on microwave absorption measurements. The SQUID-based technique provides quantitative determination of the dc magnetic moment as a function of microwave power, magnetic field and temperature. The EPR spectra obtained do not contain variability in the line shape and splittings that are commonly observed in the standard single-pass transmission mode HF-EPR. We demonstrate the improved performance by comparing EPR spectra for Fe8 molecular nanomagnets using both SQUID-based and conventional microwave-absorption EPR systems.

Resonant microwave power absorption and relaxation of the energy levels of the molecular nanomagnet Fe8 using superconducting quantum interference device-based magnetometry

Energy levels and saturation of molecular nanomagnet Fe8 crystals were investigated using a 95 and 141 GHz electron paramagnetic resonance (EPR) technique based on a standard superconducting quantum interference device (SQUID) magnetometer. The technique provides quantitative determination of the dc magnetic moment as a function of microwave power, magnetic field, and temperature.

EPR probing of bonding and spin localization of the doublet-quartet states in a spin-frustrated equilateral triangular lattice: Cu3(O2C16H23)6·1.2 C6H12

We present high-frequency (34 and 95 GHz) EPR spectroscopic measurements of the magnetic parameters of both the ground state (spin-doublet) and the excited state (spin-quartet) of the model frustrated-spin triangular lattice of Cu3(O2C16H23)6·1.2 C6H12, containing the Cu36+ core. From 295 down to about 100 K, the EPR spectra from single crystals consist of a well-resolved triplet, but with the central component being overlapped by a single peak. At 4 K, the triplet is replaced by a singlet. The triplet is shown to arise from the quartet state, located at 324 K above the ground state. Its magnetic parameters are: D = –535 G, E = 0, g// = 2.209, g⊥ = 2.057 with the parallel direction being the three-fold axis of the Cu36+ core. The singlet is assigned to the S = 1/2 ground state, with gxx = 2.005, gyy = 2.050, and gzz = 2.282. Its hyperfine structure was that from a single Cu nucleus, with Azz = 157 G, and Axx = Ayy < 60 G, demonstrating that in the doublet state the unpaired electron is localized on only one of the three Cu2+ ions. We ascribe this localization to an antiferromagnetic exchange interaction between Cu36+ cores, with zJ′ = –0.15 K. These results serve as a basis for detailed theoretical calculations of spin dynamics and electronic bonding in a frustrated triangular magnetic lattice. To cite this article: B. Cage et al., C. R. Chimie 6 (2003) 000–000.

New class of magnetic refrigerants based on the Cr(V) tetraperoxides

We have recently investigated a series of compounds based on the Cr(V) tetraperoxides (general formulas M3CrO8 where M=Li, Na, K, Rb, Cs) that exhibit novel magnetic and heat capacity properties. Chief among them is the promise of superior performance for magnetic refrigeration. Currently, adiabatic demagnetization refrigeration (ADR) compounds are generally limited in their effective cooling temperature range. These limitations are a function of the magnetic spin ordering temperature, which limits the lowest attainable temperature, and the spin concentration, which limits the refrigeration power. Our investigations indicate that the peroxychromates can be tuned to possess maximal refrigeration power at a desired operating temperature. Evidence for this claim is suggested by the heat capacity coefficient (b). The lower the magnitude of b, the greater the ADR potential. Experimental evidence shows introducing larger cations significantly lowers b, while basically maintaining the spin concentration. This pa...

Novel magnetic and heat capacity properties of the Cr(IV) diperoxides

This study presents, to our knowledge, the first observation of long range magnetic order in a simple Cr(IV) organo-metallic complex. Magnetic susceptibility and heat capacity results are given for an Cr(IV) ethylenediamine diperoxo complex, [Cr(En)(O2)2(H2O)]⋅H2O. For the investigated compound, the spin state of S=1 was determined from the analysis of susceptibility data above 20 K. Below this temperature the susceptibility was observed to deviate from Curie–Weiss behavior. We find evidence for long range ordering similar to ferromagnetism around 4.5 K, and the appearance of short range spin interactions at higher temperatures. The mechanism for the ferromagnetic like peak has tentatively been attributed to canted anti-ferromagnetism. Analysis of the heat capacity determined that the majority of the spin entropy is engaged at temperatures above 10 K.

Preparation of spin-labeled styrene-divinylbenzene copolymers and a new approach for quantitative determination of the resin loading using ESR spectroscopy

Abstract Several spin-labeled car☐yl-containing resins were prepared, containing different concentrations of spin labels (nitroxyl or verdazyl stable free radicals). ESR studies of the prepared resin specimens demonstrated that the dependence between their ESR signal area and estimated and/or calculated amount of attached spin labels is linear in the case of verdazyl radicals, which makes this non-destructive approach a promising fast and accurate method for a resin loading determination.

Tailored Nanoscale Contrast Agents for Magnetic Resonance Imaging

Two potential molecular imaging vectors are investigated for material properties and magnetic resonance imaging (MRI) contrast improvement. Monodisperse magnetite (Fe3 O4 ) nanocrystals ranging in size from 7 to 22 nm are solvothermally synthesized by thermolysis of Fe(III) acetylacetonate (Fe(AcAc)3 ) both with and without the use of heptanoic acid (HA) as a capping ligand. For the resulting Fe3 O4 nanocrystals, X-Ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and superconducting quantum interference device magnetometry (SQUID) is used to identify the average particle size, monodispersity, crystal symmetry, and magnetic properties of the ensembles as a function of time. The characterization study indicates that the HA synthesis route at 3 hours produced nanoparticles with the greatest magnetic anisotropy (15.8 × 104 J/m3 ). The feasibility of Fe8 single molecule magnets (SMMs) as a potential MRI contrast agent is also examined. SQUID magnetization measurements are used to determine anisotropy and saturation of the potential agents. The effectiveness of the Fe3 O4 nanocrystals and Fe8 as potential MRI molecular probes is evaluated by MRI contrast improvement using 1.5 mL phantoms dispersed in de-ionized water. Results indicate that the magnetically optimized Fe3 O4 nanocrystals and Fe8 SMMs hold promise for use as contrast agents based on the reported MRI images and solution phase T1 /T2 shortening.Copyright