Solveig Felton

Evaluation of La1−xSrxMnO3 (0 ≤ x < 0.4) synthesised via a modified sol–gel method as mediators for magnetic fluid hyperthermia

A range of lanthanum strontium manganates (La1−xSrxMnO3–LSMO) where 0 ≤ x < 0.4 were prepared using a modified peroxide sol–gel synthesis method. The magnetic nanoparticle (MNP) clusters obtained for each of the materials were characterised using scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and infra-red (IR) spectroscopy in order to confirm the crystalline phases, crystallite size and cluster morphology. The magnetic properties of the materials were assessed using the Superconducting quantum interference device (SQUID) to evaluate the magnetic susceptibility, Curie temperature (Tc) and static hysteretic losses. Induction heating experiments also provided an insight into the magnetocaloric effect for each material. The specific absorption rate (SAR) of the materials was evaluated experimentally and via numerical simulations. The magnetic properties and heating data were linked with the crystalline structure to make predictions with respect to the best LSMO composition for mild hyperthermia (41 °C ≤ T ≤ 46 °C). La0.65Sr0.35MnO3, with crystallite diameter of 82.4 nm, (agglomerate size of ∼10 μm), Tc of 89 °C and SAR of 56 W gMn−1 at a concentration 10 mg mL−1 gave the optimal induction heating results (Tmax of 46.7 °C) and was therefore deemed as most suitable for the purposes of mild hyperthermia, vide infra.

Spin-Based Diagnostic of Nanostructure in Copper Phthalocyanine–C60 Solar Cell Blends

Nanostructure and molecular orientation play a crucial role in determining the functionality of organic thin films. In practical devices, such as organic solar cells consisting of donor-acceptor mixtures, crystallinity is poor and these qualities cannot be readily determined by conventional diffraction techniques, while common microscopy only reveals surface morphology. Using a simple nondestructive technique, namely, continuous-wave electron paramagnetic resonance spectroscopy, which exploits the well-understood angular dependence of the g-factor and hyperfine tensors, we show that in the solar cell blend of C(60) and copper phthalocyanine (CuPc)-for which X-ray diffraction gives no information-the CuPc, and by implication the C(60), molecules form nanoclusters, with the planes of the CuPc molecules oriented perpendicular to the film surface. This information demonstrates that the current nanostructure in CuPc:C(60) solar cells is far from optimal and suggests that their efficiency could be considerably increased by alternative film growth algorithms.

Fragility of the spin-glass-like collective state to a magnetic field in an interacting Fe-C nanoparticle system

Magnetic properties of nanoparticle systems and spin glasses have been investigated theoretically, and experimentally by squid magnetometry.Two model three-dimensional spin glasses have been studied: a long-range Ag(11 at% Mn) Heisenberg spin glass and a short-range Fe0.5Mn0.5TiO3 Ising spin glass. Experimental protocols revealing ageing, memory and rejuvenation phenomena are used. Quantitative analyses of the glassy dynamics within the droplet model give evidences of significantly different exponents describing the nonequilibrium dynamics of the two samples. In particular, non-accumulative ageing related to temperature-chaos is much stronger in Ag(11 at% Mn) than in Fe0.5Mn0.5TiO3.The physical properties of magnetic nanoparticles have been investigated with focus on the influence of dipolar interparticle interaction. For weakly coupled nanoparticles, thermodynamic perturbation theory is employed to derive analytical expressions for the linear equilibrium susceptibility, the zero-field specific heat and averages of the local dipolar fields. By introducing the averages of the dipolar fields in an expression for the relaxation rate of a single particle, a non trivial dependence of the superparamagnetic blocking on the damping coefficient is evidenced. This damping dependence is interpreted in terms of the nonaxially symmetric potential created by the transverse component of the dipolar field.Strongly interacting nanoparticle systems are investigated experimentally in terms of spin-glass behaviour. Disorder and frustration arise in samples consisting of frozen ferrofluids from the randomness in particle position and anisotropy axes orientation. A strongly interacting system is shown to exhibit critical dynamics characteristic of a spin-glass phase transition. Ageing, memory and rejuvenation phenomena similar to those of conventional spin glasses are observed, albeit with weak temperature-chaos effects.

Magnetic properties of copper hexadecaphthalocyanine (F16CuPc) thin films and powders

The structural and magnetic properties of F16CuPc thin films and powder, including x-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometry, and theoretical modelling of exchange interactions are reported. Analysis of XRD from films, with thickness ranging between 100 and 160 nm, deposited onto Kapton and a perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) interlayer shows that the stacking angle (defined in the text) of the film is independent of the thickness, but that the texture is modified by both film thickness and substrate chemistry. The SQUID measurements suggest that all samples are paramagnetic, a result that is confirmed by our theoretical modelling including density functional theory calculations of one-dimensional molecular chains and Greens function perturbation theory calculations for a molecular dimer. By investigating theoretically a range of different geometries, we predict that the maximum possible exchange interaction between F16CuPc molecules is twice as large as that in unfluorinated copper-phthalocyanine (CuPc). This difference arises from the smaller intermolecular spacing in F16CuPc. Our density functional theory calculation for isolated F16CuPc molecule also shows that the energy levels of Kohn-Sham orbitals are rigidly shifted ∼1 eV lower in F16CuPc compared to CuPc without a significant modification of the intra-molecular spin physics, and that therefore the two molecules provide a suitable platform for independently varying magnetism and charge transport.

MALTS: A Tool to Simulate Lorentz Transmission Electron Microscopy From Micromagnetic Simulations

Here we describe the development of the MALTS software which is a generalized tool that simulates Lorentz Transmission Electron Microscopy (LTEM) contrast of magnetic nanostructures. Complex magnetic nanostructures typically have multiple stable domain structures. MALTS works in conjunction with the open access micromagnetic software Object Oriented Micromagnetic Framework or MuMax. Magnetically stable trial magnetization states of the object of interest are input into MALTS and simulated LTEM images are output. MALTS computes the magnetic and electric phases accrued by the transmitted electrons via the Aharonov-Bohm expressions. Transfer and envelope functions are used to simulate the progression of the electron wave through the microscope lenses. The final contrast image due to these effects is determined by Fourier Optics. Similar approaches have been used previously for simulations of specific cases of LTEM contrast. The novelty here is the integration with micromagnetic codes via a simple user interface enabling the computation of the contrast from any structure. The output from MALTS is in good agreement with both experimental data and published LTEM simulations. A widely-available generalized code for the analysis of Lorentz contrast is a much needed step towards the use of LTEM as a standardized laboratory technique.

Electron paramagnetic resonance studies of nitrogen interstitial defects in diamond

We report on electron paramagnetic resonance (EPR) studies of nitrogen doped diamond that has been (15)N enriched, electron irradiated and annealed. EPR spectra from two new nitrogen containing [Formula: see text] defects are detected and labelled WAR9 and WAR10. We show that the properties of these defects are consistent with them being the ⟨001⟩-nitrogen split interstitial and the ⟨001⟩-nitrogen split interstitial-⟨001⟩-carbon split interstitial pair, respectively. We also provide an explanation for why these defects have previously eluded discovery.

Easily accessible rare-earth-containing phosphonium room-temperature ionic liquids: EXAFS, luminescence, and magnetic properties

A range of liquid rare earth chlorometallate complexes with the alkyl-phosphonium cation, [P666 14](+), has been synthesized and characterized. EXAFS confirmed the predominant liquid-state speciation of the [LnCl6](3-) ion in the series with Ln = Nd, Eu, Dy. The crystal structure of the shorter-alkyl-chain cation analogue [P4444](+) has been determined and exhibits a very large unit cell. The luminescence properties, with visible-light emissions of the liquid Tb, Eu, Pr, and Sm and the NIR emissions for the Nd and Er compounds, were determined. The effective magnetic moments were measured and fitted for the Nd, Tb, Ho, Dy, Gd, and Er samples.

Improving the crystallinity and magnetocaloric effect of the perovskite La0.65Sr0.35MnO3 using microwave irradiation

The use of microwave (MW) irradiation to assist the modified peroxide sol–gel synthesis of La1−xSrxMnO3 was shown to 1) improve crystallinity and 2) enhance the magnetic susceptibility and magnetocaloric effect (MCE) of samples compared to those synthesised using the conventional method. The improvement in crystallinity of the samples synthesised using MW irradiation was evident in the structural characterisation conducted using SEM and XRD. Magnetic heating experiments on La1−xSrxMnO3 (x = 0.25, 0.35 and 0.4) showed that for the MW-assisted method, the most suitable material for magnetic fluid hyperthermia applications was La0.65Sr0.35MnO3. MW settings (time and power) were investigated in an attempt to further optimise the MCE. Increasing MW time settings used (10 min, 30 min and 2 hours) led to increased saturation temperatures (45.9 °C, 57.0 °C and 58.3 °C, respectively), with a linear relationship between the MW power settings used and the MCE also observed during the magnetic heating experiments. Specific absorption rates as high as 175 W gMn−1 were achieved compared to the conventional maximum of 56 W gMn−1 in previous work. The enhancement of MCE was ascribed to a greater core ferromagnetic contribution as a result of the improvement in the degree of crystallinity and magnetic susceptibility of the samples synthesised with MW irradiation.

TlCu1.73Fe0.27Se2 studied by means of Mössbauer spectroscopy and SQUID magnetometry

TlCu2?xFexSe2 is a p-type metal for x<0.5 which crystallizes in a body-centred tetragonal structure. The metal atoms are situated in ab-planes, ~7?? apart, while the metal?metal distance within the plane is ~2.75??. Due to the large difference in cation distances, the solid solutions show magnetic properties of mainly two-dimensional character. The SQUID measurements performed for x = 0.27 give the c-axis as the easy axis of magnetization, but also show clear hysteresis effects at 10?K, indicating a partly ferromagnetic coupling. The magnetic ordering temperature Tc is 55(5)?K as found from both SQUID and M?ssbauer spectra. At the magnetic hyperfine fields are distributed with a maximum at about 30?T, which are compared to the measured magnetic moment per iron atom, which is 0.97??B/Fe as found from SQUID measurements. The experimental results are compared to results using other methods on isostructural Tl selenides.

Magnetic properties of TlCu2-xFexSe2 crystals

The magnetic properties of two compositions of random solutions in the TlCu2–xFexSe2 system with x=0.2 and 0.45 have been investigated by superconducting quantum interference device magnetometry. T ...