A Mossbauer study of the spin-flop phase transition in some antiferromagnetic materials

The spin-flop phase transition has long been of interest to both theoretical and experimental physicists, ever since it was first predicted by Louis Neel in 1936. In this work spin-flop transitions in the antiferromagnetic materials K 2 FeF 5 , Rb 2 FeF 5 and a-Fe 2 0 3 have been 57 studied by means of Fe MBssbauer spectroscopy. Spectra of K2FeF5 and Rb2FeF5 single crystals were recorded at 4.2K with external magnetic fields of up to 14T applied in different modes, providing information on the effect of misalignment on the sharpness or 'order' of the observed transition. Directing the applied field parallel to the easy anisotropy axis resulted in a sharp 'first-order' transition, while misaligning the field by ~30o produced a broadened 'second-order' transition. Field-induced spin-flop transitions in a-Fe203 single crystal samples were studied at low temperatures by applying fields of up to lOT either parallel to or perpendicular to the easy anisotropy axis. The observed transitions were found to be of first-order in the 'parallel' case, and second-order in the 'perpendicular' case. The Morin transition, a temperature driven spin-flop which occurs naturally in a-Fe 2 0 3 at ~260K, was also studied and was found to be of first-order. Comparison of the character of the observed transitions with the predictions of several theoretical models of the spin-flop led to the conclusion that the conventional mean-field theory of the transition provides a good qualitative description of the phenomenon

Chemomagnetic Measurements of Electric Signals in Combustion Reactions of “Metal-Oxide”

The affect of an external magnetic field of 0.2 T on the Self-propagating High-temperature Syntheses (SHS) of a mixture of first row transition metals (Fe, Co, Ni, Mn and Ti) and their oxides (Fe3O4, Co3O4, NiO, MnO and TiO2) with solid oxidizer (NaClO4) was studied for the first time with respect to the chemomagnetic signals generated of each system during the transformations in the combustion wave under different external conditions. Reactions were carried out in zero and an applied magnetic field of 0.2 T and effect of magnetic fields on the combustion wave propagation were also studied

Dynamic phase transition properties and hysteretic behavior of a ferrimagnetic core-shell nanoparticle in the presence of a time dependent magnetic field

We have presented dynamic phase transition features and stationary-state behavior of a ferrimagnetic small nanoparticle system with a core-shell structure. By means of detailed Monte Carlo simulations, a complete picture of the phase diagrams and magnetization profiles have been presented and the conditions for the occurrence of a compensation point $T_{comp}$ in the system have been investigated. According to N\'{e}el nomenclature, the magnetization curves of the particle have been found to obey P-type, N-type and Q-type classification schemes under certain conditions. Much effort has been devoted to investigation of hysteretic response of the particle and we observed the existence of triple hysteresis loop behavior which originates from the existence of a weak ferromagnetic core coupling $J_{c}/J_{sh}$, as well as a strong antiferromagnetic interface exchange interaction $J_{int}/J_{sh}$. Most of the calculations have been performed for a particle in the presence of oscillating fields of very high frequencies and high amplitudes in comparison with exchange interactions which resembles a magnetic system under the influence of ultrafast switching fields. Particular attention has also been paid on the influence of the particle size on the thermal and magnetic properties, as well as magnetic features such as coercivity, remanence and compensation temperature of the particle. We have found that in the presence of ultrafast switching fields, the particle may exhibit a dynamic phase transition from paramagnetic to a dynamically ordered phase with increasing ferromagnetic shell thickness.Comment: 12 pages, 12 figure

Magnetically assisted delivery of cells using a magnetic resonance imaging system

A simple analytical model is presented which enables rapid interactive prediction and control of magnetically labelled cells in an arterial bifurcation using magnetic field gradients produced by a magnetic resonance imaging (MRI) system. This model is compared against experimental results for human mononuclear cells labelled with micrometre sized superparamagnetic iron oxide particles. Experimental and theoretical results highlight the importance of cell aggregation for magnetic targeting in a strong magnetic field. These predicted aggregates are confirmed via confocal endoscopy which allows the visualization of cell aggregates and their movement inside a vascular flow model in a 9.4 T preclinical MRI scanner. © 2011 IOP Publishing Ltd

Size effects in the magnetic behaviour of TbAl_2 milled alloys

The study of the magnetic properties depending upon mechanical milling of the ferromagnetic polycrystalline TbAl_2 material is reported. The Rietveld analysis of the X-ray diffraction data reveals a decrease of the grain size down to 14 nm and -0.15 % of variation of the lattice parameter, after 300 hours of milling time. Irreversibility in the zero field cooled - field cooled (ZFC-FC) DC-susceptibility and clear peaks in the AC susceptibility between 5 and 300 K show that the long-range ferromagnetic structure is inhibited in favour of a disordered spin arrangement below 45 K. This glassy behaviour is also deduced from the variation of the irreversibility transition with the field (H^{2/3}) and frequency. The magnetization process of the bulk TbAl_2 is governed by domain wall thermal activation processes. By contrast, in the milled samples, cluster-glass properties arise as a result of cooperative interactions due to the substitutional disorder. The interactions are also influenced by the nanograin structure of the milled alloys, showing a variation of coercivity with the grain size, below the crossover between the multi- and single-domain behaviours.Comment: 23 pages, 11 figures, to appear in J. Phys.: Condens. Ma

Combustion Reactions of Some “Metal-Oxide” Systems under Conditions of Zero and Applied Magnetic Fields: Thermal Imaging Experiments

The effect of an external magnetic field of 0.2 T on the self-propagating high temperature syntheses (SHS) of a mixture of first row transition metals and their oxides was studied by using a very sensitive thermal imaging method involving an IR-camera and software developed by MIKRON Instrument Co., Inc. (M9100 Pyrovision Series – Imaging Pyrometer). For the basic conversion of first row transition metals to their corresponding oxide, there was no observable difference in propagation behavior between applied and zero field reactions. However the average wave velocity for the iron system showed a significantly greater value when the SHS was conducted under conditions of an applied field. This enabled accurate monitoring of the combustion process in particular propagation velocity, maximum temperature, cooling rates, synthesis wave width and pathway. Several interesting phenomena, such as hole formation in the pellet and combustion wave segmentation were detected in some systems

Thermosensitive polymer-grafted iron oxide nanoparticles studied by in situ dynamic light backscattering under magnetic hyperthermia

© 2015 IOP Publishing Ltd. Thermometry at the nanoscale is an emerging area fostered by intensive research on nanoparticles (NPs) that are capable of converting electromagnetic waves into heat. Recent results suggest that stationary gradients can be maintained between the surface of NPs and the bulk solvent, a phenomenon sometimes referred to as \u27cold hyperthermia\u27. However, the measurement of such highly localized temperatures is particularly challenging. We describe here a new approach to probing the temperature at the surface of iron oxide NPs and enhancing the understanding of this phenomenon. This approach involves the grafting of thermosensitive polymer chains to the NP surface followed by the measurement of macroscopic properties of the resulting NP suspension and comparison to a calibration curve built up by macroscopic heating. Superparamagnetic iron oxide NPs were prepared by the coprecipitation of ferrous and ferric salts and functionalized with amines, then azides using a sol-gel route followed by a dehydrative coupling reaction. Thermosensitive poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) with an alkyne end-group was synthesized by controlled radical polymerization and was grafted using a copper assisted azide-alkyne cycloaddition reaction. Measurement of the colloidal properties by dynamic light scattering (DLS) indicated that the thermosensitive NPs exhibited changes in their Zeta potential and hydrodynamic diameter as a function of pH and temperature due to the grafted PDMAEMA chains. These changes were accompanied by changes in the relaxivities of the NPs, suggesting application as thermosensitive contrast agents for magnetic resonance imaging (MRI). In addition, a new fibre-based backscattering setup enabled positioning of the DLS remote-head as close as possible to the coil of a magnetic heating inductor to afford in situ probing of the backscattered light intensity, hydrodynamic diameter, and temperature. This approach provides a promising platform for estimating the response of magnetic NPs to application of a radiofrequency magnetic field or for understanding the behaviour of other thermogenic NPs

Equilibrium susceptibilities of superparamagnets: longitudinal & transverse, quantum & classical

The equilibrium susceptibility of uniaxial paramagnets is studied in a unified framework which permits to connect traditional results of the theory of quantum paramagnets, \Sm=1/2, 1, 3/2, ..., with molecular magnetic clusters, \Sm\sim5, 10, 20, all the way up, \Sm=30, 50, 100,... to the theory of classical superparamagnets. This is done using standard tools of quantum statistical mechanics and linear response theory (the Kubo correlator formalism). Several features of the temperature dependence of the susceptibility curves (crossovers, peaks, deviations from Curie law) are studied and their scalings with \Sm identified and characterized. Both the longitudinal and transverse susceptibilities are discussed, as well as the response of the ensemble with anisotropy axes oriented at random. For the latter case a simple approximate formula is derived too, and its range of validity assessed, so it could be used in modelization of experiments.Comment: 32 pages, 5 figures. Submitted to J.Phys.Condens.Matte

European Research on Magnetic Nanoparticles for Biomedical Applications: Standardisation Aspects

Magnetic nanoparticles have many applications in biomedicine and other technical areas. Despite their huge economic impact, there are no standardised procedures available to measure their basic magnetic properties. The International Organization for Standardization is working on a series of documents on the definition of characteristics of magnetic nanomaterials. We review previous and ongoing European research projects on characteristics of magnetic nanoparticles and present results of an online survey among European researchers

Real-time tracking of delayed-onset cellular apoptosis induced by intracellular magnetic hyperthermia

Aim: To assess cell death pathways in response to magnetic hyperthermia. Materials & methods: Human melanoma cells were loaded with citric acid-coated iron-oxide nanoparticles, and subjected to a time-varying magnetic field. Pathways were monitored in vitro in suspensions and in situ in monolayers using fluorophores to report on early-stage apoptosis and late-stage apoptosis and/ or necrosis. Results: Delayed-onset effects were observed, with a rate and extent proportional to the thermal-load-per-cell. At moderate loads, membranal internal-to-external lipid exchange preceded rupture and death by a few hours (the timeline varying cell-to-cell), without any measurable change in the local environment temperature. Conclusion: Our observations support the proposition that intracellular heating may be a viable, controllable and nonaggressive in vivo treatment for human pathological conditions