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Under certain aging conditions, ferrihydrite evolves into hematite through intermediate products of enhanced magnetism. Although the magnetic properties of the end product, hematite, are to date satisfactorily known, those of ferrihydrite (and especially the products of its progressive aging) are not sufficiently explored. To this end, magnetic experiments, conducted mostly by using alternating magnetic fields, have been performed. The results reveal that two-line ferrihydrite exhibits conspicuous low-temperature spin glass behavior, a new finding that, on the other hand, could be expected given the previous microstructural observations that described this mineral as a very disordered gel. Upon aging, a progressive increase of the effective magnetic moment per iron ion is detected, in agreement with previous observations that ascribed ferrimagnetic character to an intermediate crystalline phase (sometime called ferriferrihydrite) that disappears just before the full transformation into hematite. Transmission electron microscopy observations suggest that this intermediate crystalline phase nucleates at the expense of the primordial gel, remaining physically attached to it until complete transformation into hematite. This microstructural picture appears well supported as, upon aging, the glassy magnetic dynamics found in ferrihydrite persists for the aged products even when dispersed in a nonmagnetic matrix, very likely because part of the so grown ferrimagnetic nanoparticles form aggregates within remains of the nontransformed gel. We propose a coherent model of ferrihydrite aging, based on integrated microstructural and magnetic observations, which is useful for the identification of aging products in geological and biological environments.

Methods for the quantitative determination of magnetic nanoparticles in biological matrices, in the frame of biomedical applications, are required to evaluate the particles biodistribution after systemic administration. AC magnetic susceptibility measurements are an alternative method to quantify magnetic nanoparticles in tissues, being able to provide also information on the particle transformations over time and allowing the distinction of the particles from other endogenous species such as the ferritin iron cores. The protocol for particle quantification using AC magnetic susceptibility measurements is described in detail in this article. A summary of synthesis routes towards magnetic nanoparticles is also provided. © 2014 the Owner Societies., LG is the beneficiary of a post-doctoral grant from the AXA Research Fund. This work was partially supported by grants from the Spanish Ministry of Economy and Competitiveness (MAT2011-23641), the Madrid regional government (S009/MAT-1726), Junta de Andalucía (P11-FQM-7074), Carlos III Health Institute (PI060549) and the EU-FP7 MULTIFUN project (NMPLarge Ref. 246479).

As radio- and chemotherapy-based cancer treatments affect both tumors and healthy tissue, cancer immunotherapy attempts to specifically enhance the natural immune response to tumor cells. In mouse models of cancer, we tested uniform dimercaptosuccinic acid (DMSA)-coated monodisperse magnetic nanoparticles as a delivery system for the anti-tumorigenic cytokine IFN-γ. IFN-γ-adsorbed DMSA-coated magnetic nanoparticles were targeted to the tumor site by application of an external magnetic field. We analyzed nanoparticle biodistribution before and after IFN-γ conjugation, as well as the efficiency of nanoparticle accumulation in tumors, IFN-γ release in the area of interest, and the effects of both on tumor development. At the tumor site, we observed a high degree of nanoparticle accumulation and of cytokine delivery, which led to increased T cell and macrophage infiltration and promoted an anti-angiogenic effect. The combined action led to a notable reduction in tumor size. Our findings indicate that IFN-γ-adsorbed DMSA-coated magnetite nanoparticles can be used as an efficient in vivo drug delivery system for tumor immunotherapy.

Atmospheric dust collected on filters at a coastal site in Gran Canaria has been analysed by a combination of chemical, magnetic and optical methods with the aim of determining the iron speciation. The fraction of total iron as particulate (oxyhydr)oxides, determined by the citrate–bicarbonate–dithionite method, was 0.39 ± 0.11 (mean ± s.d.); the fraction of (oxyhydr)oxide iron in ferrimagnetic form, through analysis of the saturation magnetisation, was 0.053 ± 0.038 (mean ± s.d.); and the fraction of haematite iron with respect to the iron in haematite + goethite form, by diffuse reflectance spectroscopy measurements, was 0.47 ± 0.12 (mean ± s.d.). Consistent with these findings, low temperature in-phase and out-of-phase AC susceptibility measurements reveal also the presence of paramagnetic iron, most likely in silicates with ionic substitution, and indicate that, while magnetite or haematite particles may be present in the dust, their particle size should be very small, as the typical magnetic transitions characteristic of large crystals of these oxides are practically impossible to detect. The comparison of the Fe/Al elemental ratios with typical crustal values indicates that the great majority of captured dust iron has a non-anthropogenic origin. Although no significant correlations have been found between the analysed dust properties and the dust provenance, the obtained magnetic data corresponding to the dust collected at this site may be useful, as a middle step, in future magnetic monitoring studies of the iron biogeochemical cycle.

The dynamic magnetic susceptibility of several haematinics in which iron is present as antiferromagnetic salts or iron oxyhydroxide nanoparticles has been measured. Among other parameters, the temperature dependence of the AC susceptibility of each compound acts as a fingerprint that informs about microstructural aspects of the presence of iron. The physicochemical characterisation of these compounds is of great relevance with respect to their bioavailability in the treatment of iron deficiency anaemia.

The conspicuous magnetic properties of iron, paradoxically, rarely participate in the methods routinely employed in the clinical environment to detect iron containing species in tissues. In the organism iron is just a trace metal and it mostly occurs as part of haemoproteins or ferritin, which show paramagnetic, diamagnetic or antiferromagnetic behaviour, hence resulting in a very low contribution to the tissue susceptibility. Detailed magnetic measurements make it nowadays possible to identify such species in tissues that correspond to individuals with iron overload pathologies. Since, as alternatives to the conventional biopsy, magnetism-based noninvasive techniques to diagnose and manage such diseases are recently under development, the deep knowledge of the magnetic properties of the different forms of iron in tissues is of high applied interest.

The use of magnetic ac susceptibility measurements of biological tissues in the quantitative determination of their particulate magnetic carrier content has been investigated. In a first step, an ad hoc series of agar dilutions of the superparamagnetic contrast agent Endorem, used as an example of magnetic carrier, has been characterized to determine the influence of the dipolar interaction. With this result in hand, the quantitative determination of the content of a magnetic carrier in the ex vivo liver and spleen tissues of rats, to which the same compound was previously administered, has been accomplished. It is shown that, by careful interpretation of the temperature dependent out-of-phase susceptibility profiles in the cryogenic range, it is possible to discern between the magnetic contribution of the carrier and that of biomineral iron, being able to detect magnetic carrier iron concentrations of the order of 1 microg Fe g(-1) dry tissue. At the usual dosages in humans, necessarily small to avoid toxicity, the amount of magnetic carrier in terms of elemental iron is small compared to physiological iron. The choice of their most salient property, that is, the magnetic moment, therefore makes the quantification possible even in such a minority proportion. By analysing the magnetic dynamics, through a method that just considers the in-phase and the out-of phase components of the susceptibility at only one frequency, it has been possible to decouple the carrier concentration from eventual local aggregations, opening the possibility of investigating the degree of particle clustering at a larger observation scale compared with transmission electron microscopy, and independently of physiological iron.

A magnetic AC susceptibility characterisation of rat tissues after intravenous administration of superparamagnetic iron oxide (Endorem ® ), at the same dose as established for Magnetic Resonance Imaging (MRI) contrast enhancement in humans, has been carried out. The measurements reveal the presence of the contrast agent as well as that of physiological ferritin in liver and spleen while no traces have been magnetically detected in heart and kidney. This preliminary work opens suggestive possibilities for future biodistribution studies of any type of magnetic carriers.

From magnetic and structural analyses performed on ferrimannitol ovalbumin, iron in this drug is observed to be present as two-line ferrihydrite nanoparticles, of around 4 nm in diameter, which are superparamagnetic above approximately 20 K. Among other parameters, the temperature dependence of the magnetic out-of-phase susceptibility of this compound has been fully characterised and yields results clearly different from that of ferritin. This characterisation, performed for the first time for an oral haematinic, is of interest in the context of future pharmacological studies of this compound.

Magnetic and structural analyses have been performed on an iron sucrose complex used as a haematinic agent. The system contains two-line ferrihydrite particles of about 5 nm that are superparamagnetic above approximately 50 K. The observed low-temperature magnetic dynamics of this compound is closer to simple models than in the case of other iron-containing drugs for intravenous use like iron dextran.

A method for the preparation of Cr-doped Y2Sn2O7 powders based on the pyrolysis of aerosols generated from aqueous salt solutions of the corresponding metal cations is reported. These powders yielded Cr-doped pyrochlore red-shade pigments on calcination in the absence of fluxes at lower temperature (900°–1100°C) than that involved in the traditional ceramic method (1400°C). This process resulted in pigment particles with spherical shape and broad size distribution (

The oxidation state and the localization of the chromium ions in the ceramic matrix of Cr-doped cassiterite (SnO2) and Cr-doped malayaite (CaSnSiO5) pigments have been investigated through the use of X-ray absorption near-edge (XANES), extended X-ray absorption fine structure (EXAFS), optical absorption and electron spin resonance (ESR) spectroscopies as well as by measurements of the unit cell parameters and the magnetic susceptibility of the pigments. We have found that three types of chromium species are present in the Cr-doped cassiterite pigments; a majority phase consisting of Cr(III) oxide clusters, a small amount of CrO2 nanoparticles and Cr(IV) ions dissolved in the cassiterite lattice (∼5% of the total Cr amount), which must be responsible for the violet colour of this pigment. In the case of Cr-doped malayaite, most chromium cations are tetravalent forming a solid solution with the malayaite lattice by mainly substituting for Sn(IV) cations in isolated octahedral positions, although a very small amount of Cr(IV) also substitutes for tetrahedral Si(IV).

Dynamic AC magnetic susceptibility, magnetisation and electron microscopy analyses have been performed on several samples derived from a commercial iron–dextran complex. The system contains nanometric spindle shape nanoparticles whose electron diffraction pattern corresponds to that of the iron oxyhydroxide akaganeite (β-FeOOH) and whose magnetic behaviour is superparamagnetic above approximately 100 K. Below such temperature the system exhibits outstanding magnetic relaxation, associated to the blocking of the magnetic moments which result from the incomplete compensation of the intraparticle antiferromagnetic spins. This phenomenon, which is clearly manifested in the temperature dependent out-of-phase susceptibility χ ″( T ) profile, does not fit the Arrhenius relaxation model and seems not to depend on the type of sample processing, in particular on the degree of particle dilution.

This study investigated the ability for magnetic nanoparticles to influence cellular migration in the presence of an external magnetic field. We found that the direction of migrating keratinocytes can be controlled and the migration speed of fibroblasts can be increased with the internalisation of these nanoparticles in the presence of a magnetic field. The possibility of shepherding cells towards a region of interest through the use of internalized nanoparticles is an attractive prospect for cell tracking, cell therapies, and tissue engineering applications., This work was funded by the Australian Research Council (ARC), and the National Health & Medical Research Council (NHMRC) of Australia. The authors acknowledge the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, the University of Western Australia; funded by the University, State and Commonwealth Governments. LG is the beneficiary of a post-doctoral grant from the AXA Research Fund.

A magnetic study of iron sorbitol, an iron-containing drug to treat the iron deficiency anemia is presented. Transmission electron microscopy reveals that the system contains nanometric particles with an average diameter of 3 nm whose composition is close to two-line ferrihydrite. The characterisation by magnetisation and AC susceptibility measurements indicates superparamagnetic behaviour with progressive magnetic blocking starting at 8 K. The quantitative analysis of the magnetic results indicates that the system consists of an assembly of very small magnetic moments, presumably originated by spin uncompensation of the antiferromagnetic nanoparticles, with Arrhenius type magnetic dynamics.

Metal containing hydrotalcites, where metal oxides present redox properties and hydrotalcite shows a basic character, appear to be new important environmental catalysts for the removal of SOx and NOx. Redox and basic properties of a mixed Co/Mg/Al oxide derived from hydrotalcites are tuned in order to achieve the optimal catalytic behavior required. This sample has been characterized showing that cobalt is present in two forms, as isolated and well dispersed paramagnetic ions, and as very small Co-containing particles (in the nanometric range), with an internal antiferromagnetic ordering at low temperature. The redox properties of cobalt allow the reduction of NO with propane at high temperatures and in presence of oxygen. The reduced cobalt species are proposed as the active sites. Nevertheless, for the removal of SO2 and contrary to the case of Cu/Mg/Al samples, the addition of an oxidant as cerium oxide on Co/Mg/Al is necessary in order to oxidize SO2 to SO3. In this case, similar results than those obtained with previously reported catalyst, i.e. cerium or copper–cerium hydrotalcite, are obtained. These results indicate that this catalyst could be an adequate material for the simultaneous removal of SO2 and NOx in a FCC unit.

The stochastic Landau-Lifshitz-Gilbert equation of motion for a classical magnetic moment is numerically solved (properly observing the customary interpretation of it as a Stratonovich stochastic differential equation), in order to study the dynamics of magnetic nanoparticles. The corresponding Langevin-dynamics approach allows for the study of the fluctuating trajectories of individual magnetic moments, where we have encountered remarkable phenomena in the overbarrier rotation process, such as crossing-back or multiple crossing of the potential barrier, rooted in the gyromagnetic nature of the system. Concerning averaged quantities, we study the linear dynamic response of the archetypal ensemble of noninteracting classical magnetic moments with axially symmetric magnetic anisotropy. The results are compared with different analytical expressions used to model the relaxation of nanoparticle ensembles, assessing their accuracy. It has been found that, among a number of heuristic expressions for the linear dynamic susceptibility, only the simple formula proposed by Shliomis and Stepanov matches the coarse features of the susceptibility reasonably. By comparing the numerical results with the asymptotic formula of Storonkin {Sov. Phys. Crystallogr. 30, 489 (1985) [Kristallografiya 30, 841 (1985)]}, the effects of the intra-potential-well relaxation modes on the low-temperature longitudinal dynamic response have been assessed, showing their relatively small reflection in the susceptibility curves but their dramatic influence on the phase shifts. Comparison of the numerical results with the exact zero-damping expression for the transverse susceptibility by Garanin, Ishchenko, and Panina {Theor. Math. Phys. (USSR) 82, 169 (1990) [Teor. Mat. Fiz. 82, 242 (1990)]}, reveals a sizable contribution of the spread of the precession frequencies of the magnetic moment in the anisotropy field to the dynamic response at intermediate-to-high temperatures.

Temperature and frequency-dependent AC susceptibility has been used to characterize Cu 90 Co 10 melt-spun ribbons, about 15 μm thick, in order to see to what extent this technique yields information about the segregation of cobalt in this alloy. The interpretation of the results includes, as a prerequisite, a transmission electron microscopy (TEM) characterization and makes use of previous field-dependent magnetization data on the same samples. Due to their different dynamical magnetic properties, the large intergrain precipitates, the small intragrain aggregates and the remaining Cu–Co solid solution, previously detected in these alloys, are independently observed by AC susceptibility as ferromagnetic, superparamagnetic and spin-glass species. Contrary to other, mostly local, microstructural characterization techniques of use with nanostructured materials, the AC susceptibility yields information about the whole sample. Furthermore, unlike the measurement of the temperature-dependent magnetization which is the magnetic technique mostly used until now, the results are basically independent of the thermal history. The correlation between microstructure and magnetic properties is illustrated by a scheme which includes magnetization, AC susceptibility and TEM data.

Equilibrium nonlinear susceptibilities of an overdamped three-dimensional rotator in a uniaxial anisotropy potential }cos u ~u is the angle between the rotator axis and the potential symmetry axis!, which apply to independent magnetic particles and other rotationally bistable entities, are derived. In the crossover range from free-rotator to either two-state or plane-rotator regime induced by that potential, the dependences of the nonlinear susceptibilities on T can be steeper than those of the limit inverse-temperature power laws. The nonlinear susceptibilities can then resemble the high-temperature ranges of quantities diverging at low temperature, misleadingly suggesting interparticle interactions. @S0163-1829~97!04002-2#

Metals have vital functions as prosthetic groups in enzymes, but in labile form they can propagate oxidative stress. The primary function of ferritin is to store bioavailable iron in the form of ferrihydrite. In animals, ferritin is also used to traffic and recycle iron, and to modulate intestinal iron absorption. However, the effect of ferritin accumulation on cellular iron bioavailability remains poorly understood. Moreover, putative in vivo interactions of ferritin with other metal ions have been proposed, but their physiological relevance remains unclear. Here, heterozygous mutant and overexpression ferritin strains of Drosophila melanogaster were subjected to dietary iron manipulations to study the dynamics of iron partition between ferritin and other proteins. Quantitative magnetic analysis of whole fly samples indicated that iron loading of the ferritin core varied in the different genotypes. Total paramagnetic iron content, a likely correlate of bioavailable iron, was reduced in flies overexpressing ferritin when compared with control white flies. Further, three-dimensional maps of the ferritin protein shell and iron core were obtained from single particle transmission electron microscopy imaging and confirmed the similarity between Drosophila and Trichoplusia ferritin structures. Purified Drosophila ferritin also contained small amounts of zinc and manganese. Flies that overexpressed ferritin accumulated in their bodies half the amount of manganese compared to their respective controls. Our results indicate that ferritin may be involved in the homeostasis of other divalent metals, besides iron, and that overexpression of ferritin, sometimes employed to rescue neurodegenerative models of disease, serves to limit divalent metal bio-availability in cells. © 2013 The Royal Society of Chemistry., This work was supported by the CONACYT project 179835 to F.M.

The role of anesthesia on magnetic nanoparticle biodistribution among different organs after intravenous injection has been studied in a murine model. Animals were anesthetized by inhalation with isoflurane (0.5% in oxygen) or by intraperitoneal injection with a mixture of ketamine and xylazine. Then, monodisperse dimercaptosuccinic acid coated magnetic nanoparticles (diameter of 9.2 nm 1.2 nm) were administered intravenously to the animals. Lung and liver tissues were collected after the particle administration and the amount of particles in each tissue was determined by alternating current magnetic susceptibility measurements. Whereas the amount of particles that reaches the liver seems not to be affected by the anesthesia used, the amount of particles that reaches the lungs for inhaled isoflurane is three times less than for the intraperitoneally injected anesthetic. © 1965-2012 IEEE.

Although iron oxide magnetic nanoparticles (MNP) have been proposed for numerous biomedical applications, little is known about their biotransformation and long-term toxicity in the body. Dimercaptosuccinic acid (DMSA)-coated magnetic nanoparticles have been proven efficient for in vivo drug delivery, but these results must nonetheless be sustained by comprehensive studies of long-term distribution, degradation and toxicity. We studied DMSA-coated magnetic nanoparticle effects in vitro on NCTC 1469 non-parenchymal hepatocytes, and analyzed their biodistribution and biotransformation in vivo in C57BL/6 mice. Our results indicate that DMSA-coated magnetic nanoparticles have little effect on cell viability, oxidative stress, cell cycle or apoptosis on NCTC 1469 cells in vitro. In vivo distribution and transformation were studied by alternating current magnetic susceptibility measurements, a technique that permits distinction of MNP from other iron species. Our results show that DMSA-coated MNP accumulate in spleen, liver and lung tissues for extended periods of time, in which nanoparticles undergo a process of conversion from superparamagnetic iron oxide nanoparticles to other non-superparamagnetic iron forms, with no significant signs of toxicity. This work provides the first evidence of DMSA-coated magnetite nanoparticle biotransformation in vivo., RM holds a post-doctoral contract supported by EU-FP7 MULTIFUN project (no. 262943), LG holds a Sara Borrell post-doctoral contract (CD09/00030) from the Carlos III Health Institute, Spanish Ministry for Health, Social Services and Equality (MSSSI), and TMZ received a FPU pre-doctoral fellowship from the Spanish Ministry of Economy and Competitiveness (MINECO). This work was partially supported by grants from the MINECO (SAF-2011-23639 to DFB and MAT2011-23641 and CSD2007-00010 to MPM), the Research Network in Inflammation and Rheumatic Diseases (RIER) of the ISCIII-MSSSI Cooperative Research Thematic Network program (RD08/0075/0015 to DFB), the Madrid regional government (S009/MAT-1726 to MPM), and EU-FP7 MULTIFUN project (no. 262943 to DFB and MPM)., S2009/MAT-1726/Nanobiomagnet

There is no effective treatment for the cardiomyopathy of the most common autosomal recessive ataxia, Friedreich ataxia (FA). This disease is due to decreased expression of the mitochondrial protein, frataxin, which leads to alterations in mitochondrial iron (Fe) metabolism. The identification of potentially toxic mitochondrial Fe deposits in FA suggests Fe plays a role in its pathogenesis. Studies using the muscle creatine kinase (MCK) conditional frataxin knockout mouse that mirrors the disease have demonstrated frataxin deletion alters cardiac Fe metabolism. Indeed, there are pronounced changes in Fe trafficking away from the cytosol to the mitochondrion, leading to a cytosolic Fe deficiency. Considering Fe deficiency can induce apoptosis and cell death, we examined the effect of dietary Fe supplementation, which led to body Fe loading and limited the cardiac hypertrophy in MCK mutants. Furthermore, this study indicates a unique effect of heart and skeletal muscle-specific frataxin deletion on systemic Fe metabolism. Namely, frataxin deletion induces a signaling mechanism to increase systemic Fe levels and Fe loading in tissues where frataxin expression is intact (i.e., liver, kidney, and spleen). Examining the mutant heart, native size-exclusion chromatography, transmission electron microscopy, Mössbauer spectroscopy, and magnetic susceptibility measurements demonstrated that in the absence of frataxin, mitochondria contained biomineral Fe aggregates, which were distinctly different from isolated mammalian ferritin molecules. These mitochondrial aggregates of Fe, phosphorus, and sulfur, probably contribute to the oxidative stress and pathology observed in the absence of frataxin., This work was supported by the National Health and Medical Research Council, the Muscular Dystrophy Association, and the Canadian Institutes of Health Research.

IFN-γ-adsorbed DMSA-coated magnetite nanoparticles can be used as an efficient in vivo drug delivery system for tumor immunotherapy. Magnetic nanoparticles, with adsorbed interferon-γ, were targeted to the tumor site by application of an external magnetic field. A relevant therapeutic dosage of interferon in the tumor was detected and led to a notable reduction in tumor size. In general, only 10% of the total injected nanoparticles after multiple exposures were found in tissues by AC susceptibility measurements of the corresponding resected tissues. Magnetic nanoparticle biodistribution is affected by the application of an external magnetic field.

Quantitative analysis of the temperature dependent AC magnetic susceptibility of freeze-dried mouse tissues from an Hfe hereditary haemochromatosis disease model indicates that iron predominantly appears biomineralised, like in the ferritin cores, in the liver, the spleen and duodenum. The distribution of the amount of ferritin-like iron between genders and genotypes coincides with that of elemental iron and nonheme iron. Importantly, the so-called paramagnetic iron, a quantity also determined from the magnetic data and indicative of nonmineralised iron forms, appears only marginally increased when iron overload takes place.

International audience; We analysed magnetic nanoparticle biodistribution, before and after cytokine conjugation, in a mouse model by AC susceptibility measurements of the corresponding resected tissues. Mice received repeated intravenous injections of nanoparticle suspension for two weeks and they were euthanized one hour after the last injection. In general, only 10% of the total injected nanoparticles after multiple exposures were found in tissues. The rest of the particles may probably be metabolised or excreted by the organism. Our findings indicate that the adsorption of interferon to DMSA-coated magnetic nanoparticles changes their biodistribution, reducing the presence of nanoparticles in lungs and therefore their possible toxicity. The specific targeting of the particles to tumour tissues by the use of an external magnetic field has also been studied. Magnetic nanoparticles were observed by transmission electron microscopy (TEM) in the targeted tissue and quantified by AC magnetic susceptibility.

The influence of hydrogen absorption on the magnetic properties of the compounds Sm 2 Fe 14 C, Er 2 Fe 14 C and Tm 2 Fe 14 C was studied using X-ray diffraction and measurements of the temperature dependences of the magnetization, the a.c.-susceptibility and the resistivity. The latter two compounds have a first-order spin reorientation transition close to room temperature, as confirmed by DSC measurements. It is shown that this property is largely retained upon hydrogen absorption although the corresponding Curie temperatures increase strongly with hydrogen concentration. The maximum hydrogen concentration decreases with increasing atomic number of the R component in the R 2 Fe 14 C compounds investigated.

Results of x ac ( T ) in powder oriented samples of R 2 Fe 14 BH x (R = rare earth) very often show peaks, approximately between 100 and 300 K, when the exciting ac field is parallel to the easy axis. This work presents the existence of time effects as well as thermal hysteresis on these anomalies. An explanation of this phenomenon is given in terms of the extrinsic contributions to x ac ( T ).

The influence of hydrogen absorption on the magnetic properties of the compounds Sm 2 Fe 14 C, Er 2 Fe 14 C and Tm 2 Fe 14 C was studied using X-ray diffraction and measurements of the temperature dependences of the magnetization, the a.c.-susceptibility and the resistivity. The latter two compounds have a first-order spin reorientation transition close to room temperature, as confirmed by DSC measurements. It is shown that this property is largely retained upon hydrogen absorption although the corresponding Curie temperatures increase strongly with hydrogen concentration. The maximum hydrogen concentration decreases with increasing atomic number of the R component in the R 2 Fe 14 C compounds investigated.

Magnetisation and AC susceptibility measurements have been performed to characterize the melt-spun Cu97.5Co2.5 alloy. The magnetisation curve, at room temperature, shows two contributions, one superparamagnetic and the other nearly paramagnetic. The temperature dependence of the in-phase AC susceptibility shows two maxima with different dynamical character. These two anomalies are ascribed to be superparamagnetic blocking and spin-glass freezing and agree with previous magnetic and structural studies of the same alloy system.

Molecular sieves of the type CoAPO-5 have been studied by TEM and ac susceptibility measurements. In the as-synthesised state the material is paramagnetic due to the isolated cobalt ions and microstructurally it looks very homogeneous. After calcination the system develops very small cobalt-containing particles of A size between 1 and 3 nm, is paramagnetic at high temperature, superparamagnetic below about 20 K and starts to experience particle blocking below about 5 K. The tendency of cobalt to produce oxides and hydroxides of antiferromagnetic nature, together with the observed reduction of the effective moment below 20 K suggests that this is indeed a paramagnetic-superparamagnetic transition of antiferromagnetic particles. The susceptibility data have been used to estimate the degree of intraparticle spin uncompensation.

The magnetic properties and the ultrastructure, with special emphasis on the nanometric range, of liver tissues in an iron overload rat model have been investigated. The tissues of the animals, sacrificed at different times after a single iron dextran injection, have been characterised by magnetic AC susceptibility measurements together with transmission electron microscopy (TEM) and selected area electron diffraction (SAED) as helping techniques. It has been observed that few days after the iron administration the liver contains at least two iron species: (i) akaganeite nanoparticles, coming from iron dextran and (ii) ferrihydrite nanoparticles corresponding to ferritin. The magnetic susceptibility of the tissues depends not only on the elemental iron content but also on its distribution among chemical species, and varies in a remarkable regular manner as a function of the elapsed time since the iron administration. The results are of relevance with respect to non-invasive techniques for liver iron determination, directly or indirectly based on the magnetic susceptibility of the tissues, as biomagnetic liver susceptometry (BLS) and magnetic resonance (MRI) image treatment.

The anomaly detected in the magnetic AC susceptibility of many intermetallic compounds between 100 and 300 K, and in particular in Nd/sub 2/Fe/sub 14/B at 220 K, has been induced in a controlled manner by thermal annealing. The anomaly has been interpreted in terms of thermal activated processes of defects imposing their dynamical behavior on the domain walls coupled to them, thus solving the controversy on its origin. >

The magnetic behavior of a unique sample of ${\mathrm{Cu}}_{97.5}{\mathrm{Co}}_{2.5}$ is shown to vary from spin-glass-like, in the as-spun state, towards that of a noninteracting nanoparticle ensemble after sequential annealing treatments. This result is explained by specifically considering the role of the remaining copper-cobalt solid solution regions. In particular a progressive cobalt depletion of the nanoparticle surroundings seems to affect the effectiveness of the Ruderman-Kittel-Kasuya-Yosida interaction mechanism between the Co-rich particles. The presented explanation may also be of help in understanding the magnetic properties of other metallic alloys much beyond the copper-cobalt case.

The effect of hydrogenation in the magnetic properties of ( RE ) 2 Fe 14 X , X = B , C compounds has been studied by ac magnetic susceptibility technique. These experiments have results very sensitive to the magnetic ordering detection of secondary phases and we report the variations induced by hydrogen uptake in intrinsic properties like anisotropy field or spin reorientation phase transition.

Metallic iron clusters dilutely supported in zeolite NaX have been studied by magnetization, ac susceptibility, and transmission electron microscopy. Their behavior is superparamagnetic at high temperatures with negligible intercluster interactions and the cluster magnetic moments become blocked at low temperatures. From the ac susceptibility data, at frequencies between 1 Hz and 1 kHz in the temperature range 1.8--300 K, the distribution of activation energies has been determined. The results have been used to check a previously proposed scaling of the ac susceptibility and justify the use of the Arrhenius law for these systems. Although the obtained information is limited by the temperature and frequency window of the experiment, the quantitative analysis of the ac susceptibility provides, with respect to the magnetization data, additional information about the actual distribution of cluster sizes and it is a valuable tool to avoid misinterpretations about the interaction effects. \textcopyright{} 1996 The American Physical Society.

The characterization of novel hard magnetic materials produces a huge amount of χ ac (T) curves, some of them exhibiting peaks and shoulders whose origin is at this moment under discussion. Some of these anomalies have been studied in the case of R 2 Fe 14 B (R=rare earth) compounds. From the strong time effects observed, an origin due to the presence of soft magnetic particulate spurious phases is suggested.

The metal-organic polymer [{Fe(C13H17N3)2}SO4·6H2O]n is synthetized from the reaction of FeSO4·7H2O with the polymeric Schiff base (C13H17N3)n. On the basis of its anomalous magnetic properties, this material has been recently characterized as a superparamagnet with a blocking temperature above room temperature. In this communication we show by means of electron microscopy techniques that the amorphous polymeric material contains small particles of iron oxides which are, indeed, responsible for the magnetic properties. The average size of the particles is in the range of 100 to 300A. The chemical process through which the particles are formed is also investigated. A more complete account of the magnetic properties of this material is also given. The presence of α-Fe2O3 in these preparations is confirmed; the data also suggest the presence of α-FeO(OH) rather than the γ-Fe2O3 previously reported.

Within the search for magnetic ordering in organic and metal-organic compounds, the polymeric material of formula {[Fe(C13H17N3)2]SO4·6H2O}n has been extensively studied by magnetic measurements. Our results show a basic agreement with the previous ones. Nevertheless, our study, including more data, leads to an interpretation in terms of superparamagnetism rather than metamagnetism as was previously reported., The research has been supported by grant CICYT MAT88-0174.

Magnetization and frequency dependence a.c. magnetic susceptibility measurements of the metal-organic polymers of general formula {[M(C13H17N3)2]SO4 6H2O}n, where M = Fe(II) or Co(II), are indicative of superparamagnetic behavior as a consequence of the existence of strong ferromagnetic interactions in the materials. Scanning electron microscopy and X-ray dispersive spectrometry have been used to study the homogeneity in the composition of the iron polymer.

The magnetic properties of iron-containing ETS-10 zeolite and its calcined variety have been studied by magnetic measurements. The results are consistent with the presence of paramagnetic ions and superparamagnetic clusters. Calcination results in a shift of the blocking temperatures, although their frequency dependence cannot be ascribed to non-interacting clusters. The hypothesis of cluster-glass like behaviour is discussed.

In this work we demonstrate the existence of a clear correlation between the type of anomaly present in both ac susceptibility and magnetic after-effect (MAE) measurements performed on the R2Fe14BHx compounds. These anomalies are explained as caused by disaccommodation of the domain walls coupled to interstitial hydrogen atoms. These atoms provoke orientational after-effect because of the thermal activation of their motion. The anomalies detected in the MAE measurements represent the first experimental detection of diffusion magnetic after-effect in R2Fe14B compounds.

NdNi 1− x CO x O 3 (0 ≤ x ≤ 1) samples have been synthesized using a sol-gel method. The samples have been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The magnetic properties have been studied by means of ac magnetic susceptibility and dc magnetization. NdNiO 3 and NdCoO 3 show a Curie-Weiss-like behaviour while samples with 0.3 ≤ x ≤ 0.8 show a magnetic anomaly around 28 K. The experimental results suggest a spin-glass phase transition.

Small particles of γ-Fe2O3 synthesized by spray pyrolysis have been studied by ac susceptibility. They are made out of nanometric crystallites resulting in dense and hollow spherical aggregates. The results show the transition from the superparamagnetic to blocked state. The anisotropy and magnetic moment per aggregate are less than for single particles of the same dimensions.

Zeolites offer an interesting type of matrix for confinement of magnetic materials at the nanoscopic level. We report on magnetic measurements performed on iron containing ZSM-5 zeolite. The ac susceptibility is sensitive to the extraction of iron ions from the zeolite framework by controlled thermal and hydrothermal treatments, forming small extra-framework iron oxide clusters.

AC susceptibility measurements show that in Fe 89 Zr 7 B 4 amorphous material the ferromagnetic-reentrant spin-glass transition follows the paramagnetic-ferromagnetic transition as the temperature is lowered, whereas the behavior of nanocrystalline sample is characteristic of ferromagnetic material. The results are interpreted in terms of a non-collinear magnetic structure in amorphous alloy which changes considerably after crystallization.

We report here on the magnetic properties in the temperature range of 4.2 to 300 K and as a function of the frequency of the excitation current, of a series of Co-containing polymers of general formula X = NO 3 , ClO 4 and ( SO 4 ) t 2 , and [Co(C13H17N3)Cl2·3H2O]n. At high temperature, these substances follow a Curie-Weiss law. However, in the low-temperature region the ac susceptibility presents a peak which is accompanied by an out-of-phase component. The frequency dependence of the magnetic susceptibility at zero dc magnetic field is characteristic of spin-glass behavior.

A systematic study of the spin reorientation transitions in RE 2 Fe 14 B, RE = Nd, Ho, Er, Tm, Yb has been performed using magnetic ac susceptibility, heat-capacity and ac resistivity.