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In the present study, polysulfone (PSF)-activated carbon nanocomposites were synthesized by a melt mixing technique. Here, 2 wt% activated carbon (CA, CA–Ni, and CA–Co) was used as filler, and effects on thermal, mechanical, magnetic, morphological, and carbon dioxide capture properties were studied. The pyrolysis of wood sawdust produced carbon materials activated by Co and/or Ni salt. The thermal degradation and the amount of metal in the carbon materials were investigated by thermogravimetric analysis. The maximum degradation temperature showed an improvement of up to 3 °C, while the initial degradation temperature decreased up to 4 °C with the addition of metal-activated carbons. The values of Tg estimated by differential scanning calorimetry appear to be practically identical for pure PSF and its nanocomposites. The elasticity modulus of the nanocomposite shows an enhancement of 17% concerning the neat PSF. The water contact angle showed a decrease with the incorporation of the fillers, indicating the hydrophilic nature of the composite. The carbon dioxide sorption capacity of the nanocomposite showed an enhancement of almost 10% in contrast to neat PSF. Ferromagnetic behavior of the thermoplastic nanocomposite was observed with the introduction of 2.0 wt% metal-carbonized filler. The exceptional magnetic properties, for a thermoplastic material such as polysulfone, make it promising for various industrial applications.

We report a peculiar magnetization reversal observed in magnetron-sputtered Co, Fe, and Ni films. We found that some recoil (minor) magnetization curves lie entirely and way outside the major loop, a phenomenon referred to here as a recoil-curve overshoot. The greatly enhanced recoil curve's remanence and coercivity result in an up to 2.5-fold increase of loop's area. The model of pairs of exchange-coupled grains with misaligned anisotropy axes reproduces, in a very good agreement with the experiment, all key features of the recoil-curve overshoot as well as the kink that some major loops present before saturation. The disclosed features of the ferromagnetic hysteresis provide further insights into this important classical phenomenon.

A study addressed to develop new recyclable and/or biodegradable magnetic polymeric materials is reported. The selected matrices were polypropylene (PP) and poly (lactic acid) (PLA). As known, PP corresponds to a non-polar homo-chain polymer and a commodity, while PLA is a biodegradable polar hetero-chain polymer. To obtain the magnetic nanocomposites, magnetite supported on thermally reduced graphene oxide (TrGO:Fe3O4 nanomaterial) to these polymer matrices was added. The TrGO:Fe3O4 nanomaterials were obtained by a co-precipitation method using two types of TrGO obtained by the reduction at 600 °C and 1000 °C of graphite oxide. Two ratios of 2.5:1 and 9.6:1 of the magnetite precursor (FeCl3) and TrGO were used to produce these nanomaterials. Consequently, four types of nanomaterials were obtained and characterized. Nanocomposites were obtained using these nanomaterials as filler by melt mixer method in polypropylene (PP) or polylactic acid (PLA) matrix, the filler contents were 3, 5, and 7 wt.%. Results showed that TrGO600-based nanomaterials presented higher coercivity (Hc = 8.5 Oe) at 9.6:1 ratio than TrGO1000-based nanomaterials (Hc = 4.2 Oe). PLA and PP nanocomposites containing 7 wt.% of filler presented coercivity of 3.7 and 5.3 Oe, respectively. Theoretical models were used to analyze some relevant experimental results of the nanocomposites such as mechanical and magnetic properties.

Cupric oxide (CuO) films were grown by radio frequency (RF) reactive sputtering technique. The relation between the deposition parameters andthe film structure was investigated aiming to obtain a CuO film with (111) preferential orientation at room temperature. High correlation between the CuO phase and pressure was found. Also,the preferential growthwas obtained controlling the RF power and O2 flow. Moreover, a high temperature growth of CuO was replaced by high RF power at RT. Finally, a Ru buffer layer enhanced the surface morphologyand the crystallinity of CuO along the [111] direction at room temperature.

The great majority of experimental studies on magnetic interactions employ a technique based on the Wohlfarth’s relation and require an initially demagnetized state. In the present work a relation analogous to that of Wohlfarth though between in-field magnetization curves instead of remanence curves is derived. This allowed to introduce interaction plots obtained via measurements of the major hysteresis loop and a recoil curve only. The method is effectively tested on three real magnetic systems which evidence different interaction effects. Although the plots introduced here seem to provide information about interactions alike the classical remanence plots, they present important advantages. These plots are acquired in an easier and faster manner and do not demand demagnetization, which significantly simplifies the measurement procedure and allows assessment of interaction effects in virtually impossible to demagnetize systems with rectangular major loops. Furthermore, this method can easily be adapted for estimation of effects leading to deviations from theoretical behavior of other hysteretic quantities.

Exchange bias properties of MnFe$$_2$$ 2 O$$_4$$ 4 @$$\gamma$$ γ –Fe$$_2$$ 2 O$$_3$$ 3 core–shell nanoparticles are investigated. The measured field and temperature dependencies of the magnetization point out a well-ordered ferrimagnetic core surrounded by a layer with spin glass-like arrangement. Quasi-static SQUID magnetization measurements are presented along with high-amplitude pulse ones and are cross-analyzed by comparison against ferromagnetic resonance experiments at 9 GHz. These measurements allow one to discern three types of magnetic anisotropies affecting the dynamics of the magnetic moment of the well-ordered ferrimagnetic NP’s core viz. the easy-axis (uniaxial) anisotropy, the unidirectional exchange-bias anisotropy and the rotatable anisotropy. The uniaxial anisotropy originates from the structural core–shell interface. The unidirectional exchange-bias anisotropy is associated with the spin-coupling at the ferrimagnetic/spin glass-like interface; it is observable only at low temperatures after a field-cooling process. The rotatable anisotropy is caused by partially-pinned spins at the core/shell interface; it manifests itself as an intrinsic field always parallel to the external applied magnetic field. The whole set of experimental results is interpreted in the framework of superparamagnetic theory, i.e., essentially taking into account the effect of thermal fluctuations on the magnetic moment of the particle core. In particular, it is found that the rotatable anisotropy of our system is of a uniaxial type.

Made available in DSpace on 2021-06-25T10:22:09Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-03-01 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) This work illustrates an effective method for obtaining hybrid nanoparticles of Ni-Fe-Mo permalloy and reduced graphene oxide (rGO). The metallic nanoparticles were spread by the sputtering technique, which allowed a good dispersion of the metallic nanoparticles onto rGO substrate powder. TEM showed permalloy nanoparticles smaller than 8 nm uniformly distributed throughout rGO. Permalloy/rGO hybrid with 10.5 wt% loading of permalloy nanoparticles was calculated by TGA. RBS experiment reveals that permalloy target and the nanoparticles deposited have similar composition. The interaction between permalloy and rGO was studied by FT-IR. Ni-Fe-Mo/rGO presented an electrical conductivity of 122 Scm−1, significantly higher than the original rGO and a magnetization hysteresis-loop coercivity of 16 Oe at room temperature. To our knowledge this is the first work in which permalloy nanoparticles are deposited onto graphene powder substrate by a physical impregnation technique. Escola Politécnica Pontifícia Universidade Católica do Rio Grande do Sul, Av. Ipiranga, 6681 Universidade Estadual Paulista UNESP Faculdade de Engenharia, Campus de ilha solteira, Av. Brasil 56 Instituto de Física Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500 Instituto de Química Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500 Universidade Estadual Paulista UNESP Faculdade de Engenharia, Campus de ilha solteira, Av. Brasil 56 CAPES: 001

This work reports an unprecedented mechanochemistry synthesis of β-CoTe and its systematic characterization through X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and magnetometry techniques. The mechanical alloying produced the desired material within 6 h along with minor impurities, showing good stabilization for higher milling times (15 h) and long-term storage. XRPD characterization employed the Rietveld profile fitting analysis with fundamental parameters analysis in a direct convolution approach, giving the material's structure and microstructure information. For the spherical shape, the diameter mass average of the crystallites furnished values around 13 nm with 1.1% of microstrain. The double-Voigt procedure also modeled a triaxial ellipsoid shape for the crystallite size and obtained a surface-weighted average value for its volume around 150 nm3. TEM images confirmed the nanometric size visually and showed the crystallites to aggregate in large particles hundreds of nanometers in size. Measuring hundreds of supposed crystallite sizes, we could achieve a numerical distribution of their sizes with an average of 16 nm. The magnetization analysis performed both experimentally and via numerical simulations showed that β-CoTe is predominantly superparamagnetic with a magnetic domain size compatible with the double-Voigt one.

Morphological, structural and magnetic properties of Co nanowire/IrMn film heterostructures are studied by atomic force microscopy, non-coplanar X-ray diffraction and magnetization measurements, the latter performed with magnetic field parallel and perpendicular to the nanowire length direction and using remnant (as-grown) and field-cooling protocols. Two magnetic Co phases are established, namely as magnetically-hard hcp phase and soft fcc one (the amount of the latter increasing with the nanowires’ thickness). The magnetization hysteresis loops of these heterostructures present exchange bias, with field shift initially increasing up to a certain Co thickness, followed by a gradual decrease, an effect correlated with changes in the interface energy and shape of the nanowires. The experimental hysteresis loops are successfully reproduced using a modified polycrystalline model for exchange bias, taking into consideration dipolar interactions among the nanowires. Our results are shown that the competitions between anisotropies and dipolar interactions play important roles in the coherent magnetization reversal processes observed.

We present a systematic structural, microstructural and magnetic characterization of the hexagonal δ-FeSe nanophase produced by a simple one-step mechanochemical synthesis route, by using conventional X-ray powder diffraction (XRPD), Rietveld refinement, transmission electron microscopy (TEM) and magnetometry techniques. We observed the simultaneous formation of tetragonal β-FeSe and δ-FeSe after 3 h of milling (with minor amounts of unreacted iron), followed by complete β-FeSe → δ-FeSe phase transition as milling time increases to 6 h (no unreacted iron). The average crystallite size of the δ-FeSe phase of about 16 nm after 3 h milling time decreases by about 31% up to the final milling time (24 h). TEM images and electron diffraction patterns confirm the nanometric size of the crystalline domains in the irregularly-shaped agglomerated particles. Two ferromagnetic phases with distinct coercivity spectra were assumed here by considering an assembly of randomly-oriented weakly-anisotropic ferromagnetic particles, mixed at a 4 to 6 volume ratio with other randomly-oriented ferromagnetic grains. Four years after synthesis, the aged samples milled for less than 9 h revealed a certain amount of the β-FeSe phase that slightly affects the δ-FeSe (micro)structure but causes some variations (decreasing) in magnetic parameters. Milling times as long as 12 h were shown to be necessary to guarantee the δ-FeSe nanophase stability and to retain its magnetic properties over time.

The exchange-bias properties of ferromagnetic, either Co or Ni, thin films deposited onto polycrystalline multiferroic CuO are investigated. After field cooling, the CuO/Co magnetization hysteresis loops show exchange bias at temperatures lower than 200 K, while the CuO/Ni system exhibits bias below about 5 K only. It is suggested that the exchange bias of CuO/Co is determined mainly by the magnetization reversal that takes place on the descending branch of the loop. Rather high values of both the interface coupling energy, 0.89 erg/cm 2 , and coercivity, 2.44 kOe, of the CuO/Co film are obtained at 5 K.

In this study we investigated the effect of carbon-based magnetic fillers on the thermal, mechanical, morphological and magnetic properties of polyethylene nanocomposites, with the filler range from 0 to 8.5 wt%, prepared by in situ polymerization. The nickel-carbonized material was prepared from biomass by the pyrolysis of wood sawdust. The thermal stability was investigated by thermogravimetric analysis (TGA) and differential calorimetric analysis (DSC); the results show that, with incorporation of the filler, the onset and maximum degradation temperatures increased by 24 and 7 °C, respectively. The DSC results indicate an enhancement of 4 °C in the melting and crystallization temperatures, as compared to neat polyethylene. The addition of the Ni filler resulted in a ferromagnetic behavior of the polyethylene nanocomposites. While the variation on the coercivity with the Ni filler content was very little (a rise of ∼30%), the effect on the normalized remnant magnetization was rather significant, presenting a threefold increase. The mechanical properties showed an improvement, mainly with the higher amount of the filler.

Magnetic and conducting polypropylene (PP) polymer nanocomposites with different loadings of synthetic carbon nanotubes (CNT-Fe) were fabricated by in-situ polymerization. Chemical vapor deposition was used as the synthetic route for carbon nanotube (CNT) synthesis, in which high-surface-area silica (SiO 2 ) acts as the support and ferrocene as the precursor and catalyst. Scanning and transmission electron microscopy analyses evidence the homogenous dispersion of the filler in the polymer matrix. It was found that, with the addition of 3.8 wt.% of the filler, the insulating PP matrix changes to a semiconductor. The magnetic properties of the nanocomposites were investigated using a vibrating sample magnetometer. The addition of 0.8 wt.% CNTs results in ferromagnetic behavior in the diamagnetic polymer matrix and high coercivities at room temperature. The thermal properties were investigated by thermogravimetric analysis and differential scanning calorimetry. Results show an increase in the maximum degradation, crystallization, and melting temperatures of the nanocomposites as compared with neat PP.

Magnetization and magnetoresistance properties of Py/Cu/Py/IrMn spin valve (SV) films are studied in the framework of the modified antiferromagnetic domain-wall model applied to a granular multidomain system. In the simulations, a misalignment between the in-plane easy magnetization axes of the Py and IrMn is considered. Magnetization and magnetoresistance data are simulated for a number of field orientations and a fairly good agreement with the experiment is found. The same holds for the respective distributions of the coercive and exchange-bias field values determined from magnetoresistance first-order reversal curves (MR-FORC) obtained for magnetic field parallel to the direction of that applied during the sample deposition. Both experimental and theoretical data of the angular variations of the magnetoresistance at constant fields are successfully used to obtain the misalignment angles. For some samples, mostly those with thinner Py layer coupled to IrMn one, our results indicate that the misalignment is due to interfacial magnetic frustration. Moreover, it is shown that MR-FORC diagrams are useful to extract information about the alignment of the grains along the field-cooling directions in SVs, as well as that these can be used to determine the threshold of the continuity of the pinned magnetic layer.

An in-field interaction plot, delta-M_R, has been recently introduced, presenting important advantages over the classical remanence plots. Here a general delta-M_R is proposed, allowing to assess interactions even in systems with shifted and asymmetric major loops. To construct such a plot, a recoil loop and the position of the center of the major loop are only needed. Applying the method on exchange-biased Co/IrMn bilayer gives two types of delta-M_R obtained for measuring field either parallel or antiparallel to the exchange-bias direction. This provides valuable information on the reversal mechanism and allows distinguishing between effects coming from magnetic coupling into the ferromagnet (Co) and those stemming from interactions in its interface with the antiferromagnet (IrMn). The essentially nonzero general delta-M_R plot obtained from the major loop revealed to be a precise measure of the reversal asymmetry. The technique can readily be adjusted for use in other scientific fields where hysteresis is observed. We provide free software which generates such delta-M_R plot(s)., Comment: 7 pages, 6 figures

Manipulating the exchange coupling at magnetic interfaces is crucial to spintronic devices. This study shows that inserting a very thin ferromagnetic spacer at the interface of a ferromagnet/antiferromagnet Co/Ir-Mn film substantially strengthens the coupling (if there are no important interfacial defects or interdiffusion in the antiferromagnet). Furthermore, although a Fe spacer film presents the same exchange bias after annealing or He-ion irradiation, its coercivity increases threefold after irradiation. This provides a way to fine-tune the characteristics of such structures.

In the present work, a new method is developed for the preparation of polyethylene magnetic nanocomposites by in situ polymerization. Carbon nanotubes (CNTs), synthesized by the chemical vapor deposition method using ferrocene as the precursor and catalyst and silica (SiO 2 ) as the support, were used as fillers and compared with a commercial CNT. The synthesized nanofillers had iron magnetic particles encapsulated in CNTs. The fillers were well dispersed into the polyethylene matrix, as evidenced by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs. Even for a very low filler concentration of 0.9 wt%, the presence of magnetic nanoparticles changed the diamagnetic nature of the polymer matrix to a ferromagnetic one. The thermal properties showed that the polymeric matrix did not change their properties significantly.

Proper estimation of magnetization curve parameters is vital in studying magnetic systems. In the present article, criteria for discrimination non-saturated (minor) from saturated (major) hysteresis loops are proposed. These employ the analysis of (i) derivatives of both ascending and descending branches of the loop, (ii) remanent magnetization curves, and (iii) thermomagnetic curves. Computational simulations are used in order to demonstrate their validity. Examples illustrating the applicability of these criteria to well-known real systems, namely Fe 3 O 4 and Ni fine particles, are provided. We demonstrate that the anisotropy-field value estimated from a visual examination of an only apparently major hysteresis loop could be more than two times lower than the real one.

This paper proposes the development of isotactic polypropylene (iPP) nanocomposites with magnetic carbon-based hybrid fillers, which contain magnetite, by melt mixing at 190 °C. The carbon-based fillers such as carbon nanotubes (CNTs) were synthesized via chemical vapor deposition (CVD), using ferrocene as catalyst and precursor synthesis, and using silica (SiO 2 ) or thermally reduced graphene oxide (TrGO) as support, obtaining SiO 2 /CNT Magnetite and TrGO/CNT Magnetite hybrid nanostructures, respectively. Mechanical, electrical and magnetic behaviors of the iPP nanocomposites with magnetic CNTs were evaluated; their performance against iPP composites with commercial CNTs was compared. The results show that the electrical conductivity of the iPP nanocomposites is not affected by the presence of magnetite, reaching a percolation threshold similar to that obtained in iPP nanocomposites with commercial CNTs. Likewise, the presence of CNTs with magnetic particles changes the diamagnetic nature of the polymeric matrix, transforming it into a ferromagnetic composite at low filler concentrations (2 wt.%).

Cu1−xFexO nanoparticles were prepared using a freeze-drying process followed by heat treatment. The particles were then characterized using Mossbauer spectroscopy and magnetization techniques. The results revealed complex magnetic behavior, which can be attributed to the presence of two different magnetic regions: the particle core, which has antiferromagnetic fluctuations, and the particle shell, where uncompensated spins are responsible for their superparamagnetic characteristics. At low temperatures, the moments freeze, revealing a ferromagnetic order for the shells and a dipolar magnetic interaction among the nanoparticles. In addition, an exchange-bias field revealed magnetic interactions between the core and the shell of the nanoparticles. The ferromagnetism observed in this system suggests that antiferromagnetic oxide matrices can be used for diluted magnetic semiconductor applications, if suitably doped.

Ultrasmall magnetic particles are notorious for exhibiting a magnetization increase even at quite intense applied fields, behavior which can be interpreted as a non-saturation of the magnetic disordered shell even if the nanoparticle magnetization is reversible. In this work we study two kinds of ultrasmall core@shell nanoparticles (3 nm) with contrasting core anisotropy, composed of MnFe2O4 and CoFe2O4 cores covered by a thin maghemite layer. In order to investigate the saturation criterion associated to the closure of major loops (in contrast to minor loops), we use several procedures to determine, at moderate fields, if the effective anisotropy energy barrier is overcome or not. Firstly, we carefully evaluate the closure field of the hysteresis loop, interpreted as the effective anisotropy field, correspondent to two different contributions. One, arising from the nanoparticle core, is related to the coercivity and the other one is associated to pinned spins of the nanoparticle shell. Secondly, the ZFC-FC magnetization measurements taken at different fields give us information about both the anisotropy energy barrier distribution and the thermal dependence of effective anisotropy field. Thirdly, forced minor loops are performed, measured after either ZFC or FC processes, to counter-check the effective magnetic anisotropy at low temperature of both samples. Finally, we perform major hysteresis loop calculations in order to better understand the magnetization processes involved. To go further, we propose in this study a procedure to extract information about the two contributions which composes the effective magnetic anisotropy field.

Recently, the in-field δ M R interaction-plot technique has been extended for systems with shifted and asymmetric major magnetization hysteresis loops. In the present work an alternative definition of δ M R , allowing a better graphic visualization of this plot, is given. The method is applied to a series of Co/Cu/IrMn films in order to examine the variations, with the thickness of the nonmagnetic Cu layer, of the interactions emerging from the Co/IrMn interface and those coming from magnetic coupling within the Co film. Model simulations indicate that, for the case of symmetric major loops, the effects of the ferromagnet/antiferromagnet exchange coupling might not be identified by interaction plots, being only evidenced by the shift along the magnetic-field axis and the coercivity enhancement.

© 2017 American Chemical Society. Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains. Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here, we demonstrate that δM plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the M plots of γ-Fe 2 O 3 nanoparticles isolated by thick SiO 2 shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. Our results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mössbauer measurements, that has a pronounced effect on the δM plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the δM plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions as well as offer new perspectives in the use of Henkel and δM plots to quantify the rather elusive inhomogeneous magnetization states in nanoparticles.

We investigate the broadband ferromagnetic resonance dispersion relation of NiFe/IrMn multilayers using the vector network analyzer ferromagnetic resonance (VNA-FMR) method. Multilayered films with structure [NiFe(20 nm)/IrMn ( t IrMn )/Ta(3 nm)]×10 (with t IrMn =4 and 15 nm) were produced onto Si(100) substrates using magnetron sputtering. The dispersion relations of the samples were extracted from the resonance spectra in the range of 0.1–7 GHz under magnetic fields of up to±300 Oe. Static magnetization curves were also obtained in the same field range. The experimental data were compared with numerical calculations performed using the granular exchange-bias model which takes into account both, the unidirectional and rotatable anisotropy present in the samples. A good agreement between numerical and experimental results was achieved without the need of any frequency or field-dependent parameter.

The exchange bias properties of Co/CoO coaxial core/shell nanowires were investigated with cooling and applied fields perpendicular to the wire axis. This configuration leads to unexpected exchange-bias effects. First, the magnetization value at high fields is found to depend on the field-cooling conditions. This effect arises from the competition between the magnetic anisotropy and the Zeeman energies for cooling fields perpendicular to the wire axis. This allows imprinting predefined magnetization states to the antiferromagnetic (AFM) shell, as corroborated by micromagnetic simulations. Second, the system exhibits a high-field magnetic irreversibility, leading to open hysteresis loops attributed to the AFM easy axis reorientation during the reversal (effect similar to athermal training). A distinct way to manipulate the high-field magnetization in exchange-biased systems, beyond the archetypical effects, was thus experimentally and theoretically demonstrated., The work was supported by the 2014-SGR-1015 and 2014-SGR-150 projects of the Generalitat de Catalunya, the Spanish MAT2012-35370 project, European Community’s Seventh Framework Programme (FET-Open/FP7/2007-2013) under grant agreement 296679 (MANAQA), and the Brazilian agency CAPES (BEX 0298/2015-08). G.S.A. thanks the Knut and Alice Wallenberg Foundation (Project 3DEM-NATUR) for the partial financial support. A.S. acknowledges funding from an ICREA Academia award. The authors thank Maria Esther Mata-Zamora and Raul Valenzuela for providing the nanowires. ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295).

The present study discusses on modification of the exchange-bias parameters due to magnetic annealing during 15 min of sputtered IrMn/Co and IrMn/Cu(0.25 nm)/Co films. The annealing temperature Tann has been systematically varied in the presence of sufficiently strong magnetic fields to ensure saturation of the ferromagnetic layers. Broad maxima at T ann ≈ 200 ° C have been observed in the in-plane exchange-bias field Heb versus Tann variations for both series of samples. Numerical simulations have been employed in order to fit the magnetization curves in both easy and hard axis configurations from which the magnetic anisotropy parameters were extracted. This work indicates that, in the films studied, an improvement in the bulk IrMn crystallinity and/or grain growth, which may lessen the number of uncompensated spins at the last-grown IrMn sublattice, seems to be a plausible candidate to account, in part, for the observed decrease of Heb after rapid anneal for Tann between 200 and 300 °C. This mechanism might be considered as an alternative and/or adding to the commonly observed IrMn and Co interdiffusion.

The perpendicular exchange bias and magnetic anisotropy were investigated in IrMn/Pt/[Co/Pt]3 multilayers through the analysis of in-plane and out-of-plane magnetization hysteresis loops. A phenomenological model was used to simulate the in-plane curves and the effective perpendicular anisotropies were obtained employing the area method. The canted state anisotropy was introduced by taking into account the first and second uniaxial anisotropy terms of the ferromagnet with the corresponding uniaxial anisotropy direction allowed to make a nonzero angle with the film's normal. This angle, obtained from the fittings, was of approximately 15° for IrMn/[Co/Pt]3 film and decreases with the introduction of Pt in the IrMn/Pt/ [Co/Pt]3 system, indicating that the Pt interlayer leads to a predominant perpendicular anisotropy. A maximum of the out-of-plane anisotropy was found between 0.5 and 0.6 nm of Pt, whereas a maximum of the perpendicular exchange bias was found at 0.3 nm. These results are very similar to those obtained for IrMn/Cu/[Co/Pt]3 system; however, the decrease of the exchange bias with the spacer thickness is more abrupt and the enhacement of the perpendicular anisotropy is higher for the case of Cu spacer as compared with that of Pt spacer. The existence of a maximum in the perpendicular exchange bias as a function of the Pt layer thickness was attributed to the predominance of the enhancement of exchange bias due to more perpendicular Co moment orientation over the exponential decrease of the ferromagnetic/antiferromagnetic exchange coupling and, consequently, of the exchange-bias field.

The effects of the deposition of ultrathin 57Fe layers on both sides of the NiFe layers in NiFe/Cu multilayers were investigated by focusing on their structural, magnetic and magnetoresistance properties. Conversion electron Mossbauer spectroscopy measurements showed an out-of-plane magnetic anisotropy of the Fe layers. The magnetoresistance curves showed an unusual shape, where up to three peaks were observed. Eight variables computer simulations, based on a phenomenological model that considers bilinear and biquadratic couplings between layers with cubic and in-plane uniaxial anisotropies, were used in order to calculate the best-fitting magnetization curves for the NiFe/Cu and Fe/NiFe/Fe/Cu multilayers. Both model and Mossbauer spectroscopy results showed that it is the rotation of the Fe magnetic moment from out-of-plane to in-plane orientation that provokes the unusual magnetoresistance curve shape. The observed reduction of the magnetoresistance amplitude with the addition of one monolayer of Fe in the NiFe/Cu multilayer was attributed to a less-effective spin-dependent scattering that occurs at Fe/Cu and Fe/NiFe interfaces than at the NiFe/Cu interfaces.

The exchange-bias, H EB and coercivity, H C of IrMn / Cu ( t Cu ) / Co films have been investigated as a function of the Cu spacer thickness, t Cu before and after magnetic annealing. A large increase of H EB and a significant decrease of H C with the treatment were observed for the samples with t Cu ⩽ 1.0 nm . H C ( t Cu ) and H EB ( t Cu ) showed a general trend of rapid decrease for all samples. The phenomenological models used to derive the anisotropy and exchange coupling parameters from the magnetization data indicated that the antiferromagnetic part of the interface is fully spin-compensated for the as-made films and nearly uncompensated for the treated ones. The annealing, despite the H EB and H C changes, did not alter the Co anisotropy but only improved the interfacial IrMn spins alignment.

A method for the estimation of reversible and irreversible susceptibilities of initial magnetization curves has been developed. It deals with the energy necessary for magnetizing and demagnetizing the sample, but neither with the nature of the magnetization processes nor with a specific type of anisotropy, so it could be applied for a wide variety of real materials. A set of minor hysteresis loops of an initially demagnetized sample, plotted with progressively increasing maximum magnetic field, has been used. The obtained results showed excellent agreement with those calculated by the remanence curve method for a Stoner–Wohlfarth model system.

Colloidal suspensions of cubic fcc Ni(0) nanoparticles of 5.0–6.0 nm in sizes were obtained in ionic liquids derived of the 1- n -butyl-3-methylimidazolium cation associated with different anions (tetrafluoroborate (BF 4 ), trifluoromethane sulphonate (CF 3 SO 3 ), and bis-trifluoromethane-sulfonimidate (N(Tf) 2 )). These ionic liquids act as stabilizing agents of the nanoparticles. X-ray diffraction shows clearly Ni(0) nanoparticles embedded in these ionic liquids avoiding their agglomeration. X-ray absorption spectroscopy evidenced that the Ni(0) nanoparticles are surrounded by a cap layer due to the steric and electrostatic interaction with the ionic liquids. Transmission electron microscopy and Small angle X-ray scattering also show a strong evidence of this interaction. The magnetization curves exhibit typical superparamagnetic behavior with no hysteresis. The thermomagnetic curves show superparamagnetic response with blocking temperatures around 200 K.

We have studied the effects of ion irradiation on the structural and magnetic properties of FeCo/Cu multilayers. The films were irradiated at room temperature with 600 keV Kr ions, with fluences ranging from 1 to 5 · 10 15 ions/cm 2 . X-ray absorption spectroscopy (XAS) was used to characterize the structural changes around Fe and Co atoms. The XAS data have shown a phase transformation of FeCo alloy from bcc for the as-deposited sample to fcc after irradiation. The structural effects induced by irradiation changed the saturation magnetization (MS), the coercive field (HC) and the magnetic particle size. The interplay between structural and magnetic effects indicates that some degree of mixing took place even being the elements immiscible. � 2007 Elsevier B.V. All rights reserved.

We report on the dependence of the ferromagnetic resonance and magnetization on the NiFe thickness, t, of NiFe/Cu(9 A) magnetic monolayers and symmetrical NiFe/Cu(9 A)/NiFe/Cu(9 A) exchange-coupled magnetic bilayer structures, deposited onto Si(1 0 0) substrate. The out-of-plane resonance field variations showed the presence of both the acoustic and optic precession modes in most of the bilayer structures. These variations and the magnetization curves were fitted using a phenomenological model, which indicated different effective magnetizations for the first and second NiFe layers as well as dependence of the magnetization and the coupling on t. Up to t = 43 A , the bilayers are discontinuous and direct interaction between the adjacent NiFe layers occurs. For higher t, they could be considered continuous and the exchange coupling changes from ferromagnetic to antiferromagnetic at 64 A t 128 A . Coexistence of ferromagnetic and antiferromagnetic coupling was detected for the bilayer with t = 1280 A , for which a biquadratic interlayer exchange coupling was also considered in the fittings, indicating the presence of pinholes. The results show that the techniques used are very sensitive for the evaluation of the continuity of the layers, the exchange coupling, and the possible presence of pinholes in such structures.

Ferromagnetic glass–crystalline materials were produced by sintering of glasses from the systems Bi 2O3–B2O3–MnO and crystalline phase La0.6Sr0.4MnO3. The appropriate preparation procedure was applied as follows: synthesis of the starting components, mixing, milling, pressing and sintering at selected temperatures. The steps of the technological scheme were controlled by X-ray diffraction and infrared spectroscopy. Small crystals with size in the order of 1–5 µm were detected by transmission electron microscopy observations [TEM] after heath treatment. The magnetic properties were studied by a vibrating sample magnetometer [VSM] and the Curie temperature was detected at 353 K. The presence of a narrow hysteresis loop proves the ferromagnetic properties of the materials.  2003 Elsevier SAS. All rights reserved.

Semi-conducting polyethylene (PE) nanocomposites with outstanding magnetic properties at room temperature were synthesized. These exceptional properties, for a diamagnetic and insulating matrix as PE, were obtained by polymerizing ethylene in the presence of a catalytic system formed by a metallocene catalyst supported on a mixture of reduced graphene oxide (rGO) and carbon nanotubes with encapsulated iron (CNT-Fe). It was used a constant and very low amount of CNT-Fe, obtained by vapor chemical deposition using ferrocene. The percolation threshold, to achieve conductivity, was obtained using a variable amount of rGO. The nanocomposites were semiconductors with the addition of 2.8 wt % and 6.0 wt % of the filler, with electrical conductivities of 4.99 × 10−6 S cm−1 and 7.29 × 10−4 S cm−1, respectively. Very high coercivity values of 890–980 Oe at room temperature were achieved by the presence of only 0.04–0.06 wt % of iron in the nanocomposites. The novelty of this work is the production of a thermoplastic with both, magnetic and electric properties at room temperature, by the use of two fillers, that is rGO and CNT-Fe. The use of a small amount of CNT-Fe to produce the magnetic properties and variable amount of rGO to introduce the electrical conductivity in PE matrix let to balance both properties. The encapsulation strategy used to obtain Fe in CNT, protect Fe from easy oxidation and aggregation. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45382.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al., The interdependence between training and magnetization reversal in granular Co-CoO exchange bias (EB) systems prepared by O ion implantation in Co thin films is demonstrated by polarized neutron reflectometry. While high-fluence O-implanted thin films show reduced relative training values and no asymmetry in magnetization reversal (all reversals take place by domain wall nucleation and motion), low-fluence O ion implantation results in an increased relative training and a magnetization reversal asymmetry between the first descending and the first ascending branches. Whereas the untrained decreasing field reversal occurs mainly by domain wall nucleation and motion, traces of a domain rotation contribution are evidenced in the increasing field reversal. This is explained by the evolution of the CoO structure and the contribution of the out-of-plane magnetization with ion implantation. The amount of incorporated O, which determines the threshold between both behaviors, is around 20 at.%. This reveals that the interdependence between training and magnetization reversal is insensitive to the morphology of the constituents (i.e., granular or layered), indicating that this is an intrinsic EB effect, which can be conveniently tailored by the interplay between the intrinsic properties of the investigated materials and ion implantation., This work was financed by the Research Foundation - Flanders (FWO), the KU Leuven Concerted Action (GOA/09/006 and GOA/14/007) programs, the 2009-SGR-1292 project of the Generalitat de Catalunya, the MAT2010-20616-C02 project of the Spanish Ministerio de Economía y Competitividad, and the European Commission under the 7th Framework Programme through the “Research Infrastructure” action of the “Capacities” Programme, NMI3-II Grant No. 283883. We thank HZB (Proposal No. PHY-04-2130) and ESRF (Proposal No. HC-1012, BM20 beamline) for the allocation of neutron and synchrotron radiation beamtime, respectively, and C. Bähtz for the assistance during the synchrotron measurements. E.M. and L.M.C.P. also thank the FWO for financial support. T.D. thanks the CNPq agency (Project No. 245897/2012-7) for financial support.

Magnetization and remanent magnetization curves for noninteracting single-domain particles with cubic magnetocrystalline anisotropy have been calculated, taking into account the first two anisotropy constants. The dependencies of the saturation remanence, coercivity, remanence coercivity, and δM plots on the anisotropy constants ratio have been discussed. It has been found that the calculated saturation remanence shows maxima in the vicinity of the spin-reorientation transition points for both negative and positive first cubic anisotropy constants. The remanent magnetization for the case when the face diagonals ‹110› are the easiest magnetization orientations has been analytically determined as well. It has been shown that when more than one type of easiest directions coexist, the remanence of the system can only be obtained numerically because of the history dependence of the remanent magnetization for some particles’ configurations. The predicted remanence behavior for systems representing spin-reorientation has been compared with that of a Tb0.6Ho0.4Fe2 powder sample, and an excellent agreement between numerical and experimental data has been found.

A method for evaluating the temperature-dependent magnetic anisotropy field from thermomagnetic curves for Stoner-Wohlfarth-like systems is proposed. It allows the anisotropy parameters of a sample with unknown spontaneous magnetization to be obtained by using magnetic fields considerably lower than the anisotropy field.

The influence of the Cu layer thickness on the magnetic and magnetotransport properties has been investigated in Ta/NiFe/Cu/NiFe/FeMn spin valves. The magnetization and magnetoresistance measurements were carried out for magnetic field applied along the easy-axis direction. A phenomenological model, which assumes formation of a planar domain wall at the anti-ferromagnetic side of the interfaces as well as bilinear coupling between the ferromagnetic layers, was used to derive the anisotropy characteristics and orientation of each NiFe layer magnetization. The anisotropy and spin valve magnetoresistance were simulated numerically and compared with the experiment. It was found that the anisotropy magnetoresistance is negligible and that there is a poor agreement for the spin-valve one, which was attributed to the model (valid for ferromagnetic layers in single-domain state only) used for its calculation. It was found that the increase of the Cu layer thickness provokes a decrease of the interdiffusion between the NiFe and FeMn layers, and, as consequence, changes of the uniaxial anisotropy of the pinned NiFe layer, of the exchange interaction between the pinned NiFe layer and the FeMn ones, as well as of the exchange-bias field of the pinned NiFe layer.

In recent paper Mumtaz et al. [J. Magn. Magn. Mater. 313 (2007) 266] have reported magnetic characterization of CoFe 2 O 4 nanoparticles prepared by co-precipitation. The authors have interpreted the horizontal shift of the hysteresis loops, traced after magnetic field-cooling, as a result of magnetic exchange between surface spins (spin-glass-like) and core (ferrimagnetic) spins. The observed vertical shift of these loops has been attributed to strongly pinned uncompensated spins at the core–shell interface. The present comment demonstrates that the work of Mumtaz et al. is deficient in proofs supporting their statements. A very simple and natural reason for the appearance of both shifts is the fact that the samples measured at low temperatures are unsaturated, i.e., the authors have actually measured minor hysteresis loops which are, unsurprisingly, shifted along both field and magnetization axes.

The magnetic properties of noninteracting single-domain particles whose anisotropy is made up of a cubic magnetocrystalline and a uniaxial components were investigated. Various directions of the uniaxial anisotropy were considered and the dependencies of the reduced remanence as a function of the ratio between the two anisotropies were obtained. It was found that for sufficiently strong uniaxial anisotropy and random arrangement of the particle orientations, reduced remanence lower than 0.5 is, in general, an intrinsic property of the system due to the negative remanent magnetization of some of the particles.

The exchange-bias effect was investigated through the analyses of the magnetic hysteresis loops and anisotropic magnetoresistance measurements of NiFe/FeMn bilayers. The angular variations of the exchange-bias field and coercivity were fitted in the framework of a domain-wall formation model which assumes formation of a planar domain wall at the antiferromagnetic side of the interface. The anisotropy parameters of both layers were extracted from the fits and were also used to obtain the orientation of the NiFe magnetization for different magnetic field in-plane directions. The angular dependence of the anisotropic magnetoresistance was then calculated and a comparison with the experimental measurements was made. The agreement between these curves was good for magnetic field much higher than the exchange-bias field and only reasonable for magnetic field of the order of or lower than the exchange-bias field.

In the present work, we studied the magnetic and structural properties of Co ultrathin films grown by electrodeposition onto Au substrates. The influence of the solution pH is particularly investigated using in situ and ex situ magnetization measurements. The results indicated that the magnetization is out of plane for both basic and acid solutions. Deposits performed at pH 8.5 keep their perpendicular component up to thickness of 20 monolayers (ML) either in or out of the solution, whereas the magnetization becomes in plane for a much lower thickness for deposits performed at pH 3.5 (above 2 ML in solution and 6−7 ML after capping with Cu).

In this study we present the in-plane angular dependences of the exchange-bias and coercive fields of magnetron-sputtered IrMn/Co exchange-coupled system before and after magnetic field annealing. Two experimental techniques were employed for the magnetic characterization of the samples, i.e., alternate gradient field magnetometry and anisotropic magnetoresistance measurements, which gave practically the same value for the exchange-bias field shift. An increase of the exchange-bias field and a substantial decrease of the coercivity are observed after the annealing. A phenomenological model was also used to adjust the data and to derive the anisotropy characteristics.

The magnetic switching mechanism in Fe and Co thin films deposited on vicinal Si(1 1 1) substrates was investigated experimentally and by model calculations. Although only one-step switching that depends on the applied field and on the uniaxial magnetic anisotropy orientations was observed, it was found that the reversal could be rather complex as compared to (1 0 0)-oriented films. The model that involves combined coherent rotation and domain wall unpinning allows predicting the switching filed even when the reversal magnetic field is not along the easiest direction and when the domain-wall angle and unpinning energy are not constant.