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The electron–electron, or zero-field interaction (ZFI) in the electron paramagnetic resonance (EPR) of high-spin transition ions in metalloproteins and coordination complexes, is commonly described by a simple spin Hamiltonian that is second-order in the spin S: H = D [ S z 2 − S S + 1 / 3 + E (S x 2 − S y 2) . Symmetry considerations, however, allow for fourth-order terms when S ≥ 2. In metalloprotein EPR studies, these terms have rarely been explored. Metal ions can cluster via non-metal bridges, as, for example, in iron-sulfur clusters, in which exchange interaction can result in higher system spin, and this would allow for sixth- and higher-order ZFI terms. For metalloproteins, these have thus far been completely ignored. Single-molecule magnets (SMMs) are multi-metal ion high spin complexes, in which the ZFI usually has a negative sign, thus affording a ground state level pair with maximal spin quantum number mS = ±S, giving rise to unusual magnetic properties at low temperatures. The description of EPR from SMMs is commonly cast in terms of the 'giant-spin model', which assumes a magnetically isolated system spin, and in which fourth-order, and recently, even sixth-order ZFI terms have been found to be required. A special version of the giant-spin model, adopted for scaling-up to system spins of order S ≈ 103–104, has been applied to the ubiquitous iron-storage protein ferritin, which has an internal core containing Fe3+ ions whose individual high spins couple in a way to create a superparamagnet at ambient temperature with very high system spin reminiscent to that of ferromagnetic nanoparticles. This scaled giant-spin model is critically evaluated; limitations and future possibilities are explicitly formulated. [ABSTRACT FROM AUTHOR]

Soy protein is one kind of environment‐friendly nonbioactive plant protein with excellent properties. Paramagnetic γ‐Fe2O3 nanoparticles with a mean particle size of 7 nm are prepared by submerged circulative impinging stream reactor (SCISR). And a series of soy protein isolate (SPI)/Fe2O3 nanocomposites with excellent magnetic performance have been successfully prepared with glycerol as plasticizer and γ‐Fe2O3 as fillers. The results show that the strong hydrogen bonds are formed between polar amino acids of SPI and the fillers. The Fe2O3 nanoparticles exhibit homogeneous dispersion in the protein matrix. The resulting SPI/Fe2O3 nanocomposites exhibit good paramagnetism and good microwave absorbing properties at 2–18 GHz. In addition, the mechanical properties and thermal stability of soy protein materials are improved with the addition of Fe2O3 nanocomposites. The interactions between the fillers and matrix as well as between the fillers would influence the reinforcing effect of Fe2O3. These natural resources based on green functional materials would exhibit wide potential applications in electromagnetic interference shielding and packaging areas. [ABSTRACT FROM AUTHOR]

Evidence for the suppression of collective magnetic behavior of dipolarly interacting Fe nanoparticles is found in Fe-Ag granular multilayers. Interaction of Fe particles separated by an Ag layer is studied as a function of the nominal thickness of the Ag layer. The surprisingly increasing interaction with increasing Ag-layer thickness, verified by memory-effect measurements, is explained by the formation of pinholes in the Ag layer at small Ag thicknesses, allowing direct ferromagnetic coupling between the Fe particles. This coupling may hinder the frustration of superspins favored by dipolar interactions. At larger Ag thicknesses, the Ag layer is continuous without pinholes and frustration leads to the appearance of the superspin-glass state. The effect of increasing interactions correlates well with the growing deviation at low temperatures of the measured field-cooled (FC) magnetization from the interaction-free FC curve calculated by a model based on the relaxation of two-level systems. Similar phenomenon is reported in a recently published paper (Sánchez et al., Small 2022 , 18, 2106762) where a dense nanoparticle system is studied. Here the collective magnetic behavior of the particles due to dipolar interactions is suppressed when the anisotropy energy of the individual particles exceeds a certain threshold. [ABSTRACT FROM AUTHOR]

The origin of magnetism in RF magnetron sputtered deposited carbon thin films is reported. Three different carbon thin films were deposited using RF magnetron sputtering of carbon target by Ar ions at RF power of 50 W, 100 W and 150 W, respectively. Microstructural characterization of films using Raman spectroscopy confirms the presence of graphitic crystallites in all three films and the crystallite edges are terminated with the sp3 bonding. However, increasing RF power results in an increase in graphitic crystallite size as well as the sp2/sp3 hybridization ratio, while the average density of the films decreases. The observed magnetization (measured using SQUID-VSM) in all the three-carbon films has a contribution from both superparamagnetic (SPM) particles (due to the interaction between unpaired spins) and the paramagnetic term due to isolated unpaired spins distributed throughout the film. The origin of experimentally observed magnetization in the carbon thin films and their correlation with the change in density and sp2/sp3 hybridization ratio due to variation in RF power is discussed. Image 1 [ABSTRACT FROM AUTHOR]

Cholera is a life-threatening disease caused by the cholera toxin (CT) as produced by some Vibrio cholerae serogroups. In this research we present a method which directly detects the toxin’s B subunit (CTB) in drinking water. For this purpose we performed a magnetic sandwich immunoassay inside a 3D immunofiltration column. We used two different commercially available antibodies to capture CTB and for binding to superparamagnetic beads. ELISA experiments were performed to select the antibody combination. The beads act as labels for the magnetic frequency mixing detection technique. We show that the limit of detection depends on the type of magnetic beads. A nonlinear Hill curve was fitted to the calibration measurements by means of a custom-written python software. We achieved a sensitive and rapid detection of CTB within a broad concentration range from 0.2 ng/ml to more than 700 ng/ml. [ABSTRACT FROM AUTHOR]

Highlights • Discussion of the crystal and magnetic structure of ilmenite Mn 2 FeSbO 6. • Ferrimagnetic transition at T N = 270 K. • Identical zero field and field cooled (ZFC-FC) M vs T curves. • Significant changes in M vs T curves with H and T due to unsaturated moments. • Near-zero coercive field in magnetization M vs applied magnetic field H curves. Abstract In the work presented the magnetic properties of the high pressure ilmenite phase Mn 2 FeSbO 6 are investigated in detail. It is shown that Mn 2 FeSbO 6 ilmenite is ferrimagnetic (T N ≈ 270 K) and exhibits a near-zero coercive field H C. Furthermore, the Mn2+-Mn2+, Fe3+-Fe3+ and Mn2+-Fe3+ magnetic exchange interactions exhibit gradual and continuous variations with temperature and magnetic field. The ferrimagnetic moment is unsaturated even at 120 kOe below the theoretical value of 5 μ B per f.u. [ABSTRACT FROM AUTHOR]

In this work, we report the structural and magnetic properties of nanocrystalline Cu 1−x Zn x Fe 2 O 4 mixed ferrites synthesized using novel combustion method. The prepared samples are spinel structured and the secondary phase α-Fe 2 O 3 is also present in CuFe 2 O 4 . The size of the crystallites ranges between 9.6 nm and 31 nm and the lattice constant increases from 8.342 Å to 8.435 Å. The FTIR absorption bands of CuFe 2 O 4 are observed at 571 cm −1 and 409 cm −1 for tetrahedral and octahedral sites, respectively. These bands are shifting to 547 cm −1 and 397 cm −1 for Zn substitution. Raman spectra show the change in local environment of tetrahedral and octahedral interstitial sites with Zn. The FE-SEM images show that the prepared samples are nano sized and the substitution of Zn decreases the agglomeration. The magnetic study reveals that the saturation magnetizations initially increases from 21.45 emu/g (x = 0) to 44.16 emu/g (x = 0.2) and then decreases at 300 K. The superparamagnetic behavior is observed for samples of x ≥ 0.4 concentration. The temperature dependent zero field cooled and field cooled magnetic measurements confirm the superparamagnetic nature and the observed blocking temperature decreases from 120.4 K to 32.3 K. The hysteresis curves measured at 20 K show the coercivity and it varies from 663 Oe to 70 Oe for increasing Zn content. [ABSTRACT FROM AUTHOR]

We have studied the effect of replacing Mn 3+ by Cr 3+ on the structure, transport and magnetism in the bilayered manganite LaSr 2 Mn 2 O 7 . Although no structural transition was observed in LaSr 2 Mn 2−y Cr y O 7 (0.1 ≤ y ≤ 0.6), the electrical transport and the magnetic properties were found to be affected significantly by this substitution. Substitution of Cr 3+ reduces the conductivity by restricting the hopping of small polarons. Magnetization increases with increasing Cr 3+ concentration suggesting that Cr 3+ -ions induce ferromagnetic moments . The ferromagnetic and an antiferromagnetic phase observed above ∼60 K merge into an inhomogeneous phase below this temperature. Thermopower yields an essentially concentration independent charge density nearly equal to its value for chromium free composition inspite of its expected decrease with this substitution suggesting that the small charge density of the insulating AFM phase is supplemented by the free carriers in the FM phase. The inhomogeneous phase shows a relaxor type behavior which contrasts with the spin glass behavior seen in La 0.46 Sr 0.54 Mn 0.98 Cr 0.02 O 3 having an identical AFM magnetic state. The difference is attributed to the non-JT character of Cr-ions which reduce the distortion of the Mn O octahedra located within the FM domains. With a higher lattice strain in the surrounding AFM matrix the carriers remain confined within the FM domains leading to the relaxor type behavior. [ABSTRACT FROM AUTHOR]

This article reviews magnetic memory effects in spin-glasses (SG), superspin-glasses (SSG) and superparamagnetic (SPM) materials with detailed discussions about protocols and mechanisms. The origin of time (t) and temperature (T) dependent magnetic memory effects in different classes of materials have been discussed starting from the nanoscale towards the bulk magnetism. Magnetic memory effects (T and t dependent) in field cooled (FC) and zero field cooled (ZFC) protocols are focused to understand the physics of ageing and rejuvenation in SG, SSG and SPM materials. The article is enlightened with significant discussion to understand the diverse nature and classified properties of magnetic memory of the said materials. • M emory effects in SG (both in bulk and nano) and SPM materials. • Origin, footprint and diverse characteristics of memory effect. • Significantly covers the field of magnetism especially magnetic metastabilities. • Diverse nature and classified properties of magnetic memory. [ABSTRACT FROM AUTHOR]

The precipitation-hardenable nickel–chromium-based superalloy X-750 exhibits excellent mechanical properties at high-to-cryogenic temperatures. However, little is known about its other properties, even though it is used in a wide range of applications. To elucidate the additional properties of X-750, we studied the effects of its nanostructural evolution on both its mechanical and magnetic properties. We found that precipitation not only mechanically hardens this superalloy, but also induces soft magnetism as well as notable magnetic losses at ultralow temperatures. This information should aid the design of superconducting instruments based on the X-750 superalloy as a structural material. [ABSTRACT FROM AUTHOR]

The precise quantification of catalyst residues in purified carbon nanotubes is often a major issue in view of any fundamental and/or applicative studies. More importantly, since the best CNTs are successfully grown with magnetic catalysts, their quantification becomes strictly necessary to better understand intrinsic properties of CNT. For these reasons, we have deeply analyzed the catalyst content remained in nickel–yttrium arc-discharge single walled carbon nanotubes purified by both a chlorine-gas phase and a standard acid-based treatment. The study focuses on Ni analysis which has been investigated by transmission electron microscopy, X-ray diffraction, thermogravimetry analysis, and magnetic measurements. In the case of the acid-based treatment, all quantifications result in a decrease of the nanocrystallized Ni by a factor of two. In the case of the halogen gas treatment, analysis and quantification of Ni content is less straightforward: a huge difference appears between X-ray diffraction and thermogravimetry results. Thanks to magnetic measurements, this disagreement is explained by the presence of Ni 2+ ions, belonging to NiCl 2 formed during the Cl-based purification process. In particular, NiCl 2 compound appears under different magnetic/crystalline phases: paramagnetic or diamagnetic, or well intercalated in between carbon sheets with an ordered magnetic phase at low temperature. [ABSTRACT FROM AUTHOR]

Nanoporous palladium samples containing superparamagnetic Co-rich clusters were prepared by electrochemical dealloying and the magnetic properties were subsequently varied by thermal annealing at different temperatures. It is shown that in this way porous magnetic nanomaterials exhibiting various magnetic characteristics can be obtained. The as-dealloyed, unannealed sample is ferromagnetic at 4.2 K and superparamagnetic at 300 K, with a blocking temperature T B of 100 K. The observed general decrease of the magnetization with increased annealing temperature is explained by the redistribution of Co from the Co-rich superparamagnetic clusters into the Pd matrix, leading to a shrinkage of the clusters and the formation of a diluted Co–Pd alloy phase. As a consequence of the smaller magnetic cluster size a down-shift of T B occurs for annealing temperatures of 373 K and 573 K. After annealing at 773 K the stronger dissolution of Co from the clusters into the Pd-matrix reduces T B further down to 35 K. Additionally, a second blocked magnetic phase with much higher T B emerges, which leads to ferromagnetic behavior at room temperature. The occurrence of this phase is attributed to the formation of larger magnetic entities consisting of Co-rich clusters lying close together and a Co–Pd alloy phase with relatively high Co concentration in between these clusters. After high temperature annealing at 973 K a weakly ferromagnetic phase is obtained at 4.2 K and at 300 K, which can be explained by the nearly completed dissolution of the Co in the Pd matrix. • Co-rich clusters in nanoporous Pd matrix prepared by electrochemical dealloying. • Co-rich clusters show superparamagnetic behavior. • Annealing leads to diffusion of Co in matrix and shrinkage of magnetic cluster size. • Larger magnetic entities can form at increased annealing temperature of 773 K. • Formation of weakly ferromagnetic Co–Pd alloy phase upon annealing at 973 K. [ABSTRACT FROM AUTHOR]

One ‐ dimensional (1D) transition metal oxide (TMO) nanostructures are actively pursued in spintronic devices owing to their nontrivial d electron magnetism and confined electron transport pathways. However, for TMOs, the realization of 1D structures with long ‐ range magnetic order to achieve a sensitive magnetoelectric response near room temperature has been a longstanding challenge. Herein, we exploit a chemical hydric effect to regulate the spin structure of 1D V – V atomic chains in monoclinic VO2 nanowires. Hydrogen treatment introduced V3+ (3d2) ions into the 1D zigzag V – V chains, triggering the formation of ferromagnetically coupled V3+ – V4+ dimers to produce 1D superparamagnetic chains and achieve large room ‐ temperature negative magnetoresistance (−23.9 %, 300 K, 0.5 T). This approach offers new opportunities to regulate the spin structure of 1D nanostructures to control the intrinsic magnetoelectric properties of spintronic materials. Die Spinstruktur der eindimensionalen V ‐ V ‐ Ketten in monoklinen VO2 ‐ Nanodrähten wurde durch Wasserstoffbehandlung modifiziert, wodurch V3+ ‐ Ionen und ferromagnetisch gekoppelte V3+ ‐ V4+ ‐ Dimere in den Ketten erzeugt wurden. Die neue Struktur besteht aus superparamagnetischen Ketten und zeichnet sich durch einen großen negativen Magnetwiderstand bei Raumtemperatur aus. [ABSTRACT FROM AUTHOR]

The graphene system is actively pursued in spintronics for its nontrivial sp electron magnetism and its potential for the flexible surface chemical tuning of magnetoelectronic functionality. The magnetoresistance (MR) of graphene can be effectively tuned under high magnetic fields at cryogenic temperatures, but it remains a challenge to achieve sensitive magnetoelectric response under ambient conditions. We report the use of surface modulation to realize superparamagnetism in reduced graphene oxide (rGO) with sensitive magnetic field response. The superparamagnetic rGO was obtained by a mild oxidation process to partially remove the thiol groups covalently bound to the carbon framework, which brings about large low-field negative MR at room temperature (−8.6 %, 500 Oe, 300 K). This strategy provides a new approach for optimizing the intrinsic magnetoelectric properties of two-dimensional materials. [ABSTRACT FROM AUTHOR]

In situ X-ray photo-emission electron microscopy is used to investigate the magnetic properties of iron nanoparticles deposited on different single crystalline substrates, including Si(001), Cu(001), W(110), and NiO(001). We find that, in our room temperature experiments, Fe nanoparticles deposited on Si(001) and Cu(001) show both superparamagnetic and magnetically stable (blocked) ferromagnetic states, while Fe nanoparticles deposited on W(110) and NiO(001) show only superparamagnetic behaviour. The dependence of the magnetic behaviour of the Fe nanoparticles on the contact surface is ascribed to the different interfacial bonding energies, higher for W and NiO, and to a possible relaxation of point defects within the core of the nanoparticles on these substrates, that have been suggested to stabilise the ferromagnetic state at room temperature when deposited on more inert surfaces such as Si and Cu. [ABSTRACT FROM AUTHOR]

Magnetism been studied in two series of nanocrystalline SiC films obtained by the method of direct deposition of ions with an energy of ~100 eV at temperatures 1150 °С and 1200 °С. There were separated the contributions of diamagnetism, paramagnetism and superparamagnetism+ferromagnetism. Magnetization value of the films correlates with the deposition temperature. In the films deposited at higher temperatures the value of magnetization was by 1.5 times lower. It was concluded that induced magnetism in nanocrystalline SiC films is caused by interaction of magnetic moments of neutral V Si V C divacancies in separate nanocrystals. The estimated concentration of neutral V Si V C divacancies in nanocrystalline SiC films is ~10 20 сm −3 . [ABSTRACT FROM AUTHOR]

Abstract: A correlation between the DC magnetization and hyperthermia heating rate in high frequency magnetic fields was established for two distinctively different magnetic nanoparticle (MNP) systems: (1) polystyrene (PS)/Fe3O4 based composites, consisting of 10nm diameter Fe3O4 nanoparticles embedded in the matrix of polystyrene (PS) spheres (~100nm), and (2) similar Fe3O4 nanoparticles coated with polyacrylic acid (PAA) and dispersed in water. Due to physical confinement of Fe3O4 nanoparticles in the former, the PS/Fe3O4 composite exhibited much lower magnetic hyperthermia heating compared to the latter. The reduced magnetic hyperthermia heating in the polystyrene (PS)/Fe3O4 based composites was found to be associated with strong dipolar interactions. [Copyright &y& Elsevier]

Zinc-substituted nickel ferrite (Ni Zn Fe O with x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) nanoparticles were synthesized by solgel auto-combustion technique at low temperature and characterized by using X-ray diffraction, scanning electron microscopy, pulse field hysteresis loop technique, and Mossbauer spectroscopy. X-ray diffraction studies confirmed the formation of single-phase spinel structure of the prepared ferrite samples with average crystallite size of 30 nm, very close to that of the critical size for nanoparticles exhibiting superparamagnetism. Scanning electron micrographs of the ferrite samples showed uniform spherical morphology of nanograins with homogenous microstructure. Further investigations on magnetic properties by pulse field hysteresis loop technique and Mossbauer spectroscopy indicated the presence of superparamagnetic phases in the ferrite samples attributed to occupation of octahedral [B] sites by zinc ions in these Ni-Zn samples and also to the nanometer sizes of the ferrite particles. Magnetic behavior of the Ni-Zn ferrite system is in agreement, initially, with Neel's two-sublattice collinear model and then with the Yafet-Kittel model for samples with higher zinc content (x ≥ 0.4). Value of hyperfine splitting is found to decrease with increase in zinc content and is attributed to the reduction in particle size giving rise to superparamagnetism. Other Mossbauer parameters like quadrupole splitting and the isomer shift are within the reported range for those of ferrites with spinel structure. [ABSTRACT FROM AUTHOR]

Abstract: Conventional magnetic order in a material requires the partially filled d or f bands. The exchange interactions between the electrons in these partially filled bands give rise to a magnetic order. However, the discovery of unexpected magnetism observed in some nanomaterials, which have the d and f shells either completely empty or full, has challenged our understanding of magnetism in conventional materials. The magnetism in nanomaterials shows the effects of reduced dimensions, reduced coordination of atoms at the surface and some quantum effects which dominate at low dimensions. In this review paper we give a brief review and discuss the unexpected magnetism experimentally observed and/or theoretically predicted in nanomaterials of conventional magnetic and nonmagnetic bulk materials. [Copyright &y& Elsevier]

In this work, through a combination of photoluminescence spectroscopy, X-ray powder diffraction and magnetic measurements, it is determined that ZnO rods, made hydrothermally using a combination of magnetic field with respect to the force of gravity, exhibit superparamagnetic properties which emerge from Zn defects. These Zn defects result in a size-dependent superparamagnetic property of the rods. Red emissions, characteristic of Zn vacancies, and magnetic susceptibility both increased with decreasing rod size. The ZnO rods have significantly larger superparamagnetic cluster sizes (one order of magnitude) and lower fluctuation rates when compared to other superparamagnetic particles. [ABSTRACT FROM AUTHOR]

Abstract: We developed a simple synthesis method for production of CoFe2O4 nanoparticles. The method is based on the solid state ball-milling and calcinations of acetate precursors and citric acid. The samples were characterized using X-ray diffraction, transmission electron microscope, Fourier transform infrared and energy-dispersive X-ray spectroscopy. Magnetic properties of the samples were studied using vibrating sample magnetometer and ac susceptibility measurements. The results show that the products mainly consist of CoFe2O4 nanoparticles. Magnetization measurements demonstrate the effect of calcination temperature on particle size and hence, on the magnetic properties of the CoFe2O4 nanoparticles. By increasing the particle size from 3.5 to 7.2nm, the room temperature coercivity and magnetization increase from 0 to 152Oe and from 8 to 30emu/g respectively. Also, the low temperature calcinated samples, C250 and C30 are superparamagnetic in nature due to it’s near-zero coercivity and remanence. We estimated the mean value of the effective anisotropy constant, K eff =9.2×106 erg/cm3. This value is much larger than the bulk value, due to the surface effects. [Copyright &y& Elsevier]

Abstract: We demonstrate through experiment and simulation that when mono-domain Fe nanoparticles are formed into chains by the application of a magnetic field, the susceptibility of the resulting structure is greatly enhanced (11.4-fold) parallel to the particle chains and is much larger than transverse to the chains. Simulations show that this significant enhancement is expected when the susceptibility of the individual particles approaches 5 in MKS units, and is due to the spontaneous magnetization of individual particle chains, which occurs because of the strong dipolar interactions. This large enhancement is only possible with nanoparticles, because demagnetization fields limit the susceptibility of a spherical multi-domain particle to 3 (MKS). Experimental confirmation of the large susceptibility enhancement is presented, and both the enhancement and the susceptibility anisotropy are found to agree with simulation. The specific susceptibility of the nanocomposite is 54 (MKS), which exceeds the highest value we have obtained for field-structured composites of multi-domain particles by a factor of four. [Copyright &y& Elsevier]

Abstract: The response of multi-walled carbon nanotubes encapsulating Co nanorods to ion irradiation was studied. The irradiation experiments with medium ion energies (40–500keV) were carried out at high temperatures and combined with transmission electron microscopy and Raman characterization of the irradiated samples. Contrary to electron irradiation and high-energy (100MeV) ion irradiation, we did not see accumulation of pressure inside irradiated nanotubes. We found that nanotubes with Co nanorods inside were transformed to amorphous carbon rods encapsulating Co clusters with typical diameters of 3–6nm. As Co is magnetic, such one-dimensional composite systems could be used for various applications such as magnetic data storage or magnetic resonance imaging. [Copyright &y& Elsevier]

Mössbauer study of La1 − x Sr x 57Fe0.025Co0.975O3 (0.15 ≤ x ≤ 0.25) perovskites below their magnetic ordering temperature shows cooperative electronic and magnetic phase separation for all compositions investigated. It is shown that the increase of the Sr concentration results in a transition from superparamagnetism to long range ferromagnetism of Co clusters between x = 0.18 and 0.2. In-field Mössbauer measurements revealed the existence of frustrated magnetic interactions at x = 0.22. [ABSTRACT FROM AUTHOR]

Abstract: Temperature and field dependencies of the magnetization of melt-spun ribbons of Ni45.7Mn37.2Ga17.1 (T CT A), where T A and T C are the reverse martensitic transformation and Curie temperatures, respectively, have been studied in broad ranges of magnetic fields (±30kOe) and temperatures (10–350K). It was found that the magnetic and thermomagnetic properties have a number of peculiarities and anomalies. Particularly, the magnetization values measured at both low and high magnetic fields increase significantly after annealing. Low-temperature anomalies of magnetization suggest the presence of the superparamagnetic behavior and/or phenomena related to the concurrent exchange interactions in the as-spun state. The obtained results can be explained by the influence of concentration inhomogeneities and atomic disordering in the rapidly quenched ribbons, which can be reduced by annealing. [Copyright &y& Elsevier]

Abstract: Magnetite particles of 2–4 nm were synthesized by an economic, biocompatible chemical coprecipitation route, with their size tuned by the reaction temperature. The microstructure and morphology of the nanoparticles were characterized by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM), whereas the magnetic properties were investigated by vibrating sample magnetometry (VSM) and superconducting quantum interference device (SQUID). It is found that the nanoparticles demonstrate well-defined superparamagnetic behavior as prepared and after annealing. Distinct lattices were observed which manifest the high crystallinity of such ultrasmall particles and the finite-size effect was revealed by analyzing the corresponding microstructure and magnetism. [Copyright &y& Elsevier]

Abstract: Nanoparticles of Ni0.25Co0.25Zn0.5Fe2O4 with different particle sizes were synthesized by chemical coprecipitation technique. The prepared nanoparticles were characterized and studied by X-ray diffraction, room temperature 57Fe Mössbauer spectroscopy and DC magnetization. Increase in annealing temperature leads to inversion of cations from their normal configurations which results in an increase in the hyperfine fields at both tetrahedral A and octahedral B sites. The decrease in saturation magnetization values with an increase in annealing temperature can be attributed to the variation in proportions of superparamagnetic phase and also the redistribution of cations in these ferrite nanoparticles. Variation of coercivity with annealing temperatures can be explained in terms of the size effect. [Copyright &y& Elsevier]

Nanoscale maghemite samples were obtained by a solid-state displacement reaction activated by mechanochemical treatment for two different milling times, 30min (sample S1) and 4h (sample S2). Samples were characterized by X-ray powder diffraction, transmission electron microscopy and SQUID magnetometry. The results show that prolongation of the milling time increases particle size while reducing the crystallinity, with a strong impact on the magnetic properties. Coercivity of 3.3kOe at 5K temperature for S2 is among the highest values reported so far for the maghemite nanoparticles. [Copyright &y& Elsevier]

Abstract: A new magnetic transition in the ruthenocuprate parent compound RuSr2EuCeCu2O10 has been observed below 10K. It shows up only as a kink in the imaginary part of the ac susceptibility and exhibits a pronounced frequency dependence. At the same time, the real part of the ac susceptibility and the dc magnetization study show very little change in the same temperature window suggesting only a minor fraction of the material to be involved in the transition. Frequency dependence shows excellent agreement with the predictions of the Arrhenius law known to describe well the dynamics of the superparamagnetic particles. The same type of the investigation on the RuSr2Eu1.1Ce0.9Cu2O10 composition showed no evidence of the similar transition, which points to a possible intrinsic behavior. [Copyright &y& Elsevier]

Abstract: Static and dynamic magnetic properties are examined for granular systems Co x (TiO2)1− x with Co volumetric contents of 31.6, 41.4, 49.4 and 57.6vol%. The ferromagnetic resonance (FMR) technique in the 38–45GHz band and the vibrating sample magnetometer technique were applied. The results from comparative analysis of saturation magnetization defined by both methods are discussed and the estimation of the most probable sizes and shapes of magnetic particles forming the nanocomposite is carried out. [Copyright &y& Elsevier]

Abstract: Superparamagnetic γ-Fe2O3 nanocrystallites have been prepared by γ-irradiating ferrocene in the presence of isopropyl alcohol to get Fe nanoparticles in nitrogen atmosphere and at room temperature, followed by oxidization in air to obtain γ-Fe2O3. The final black powder was characterized with X-ray powder diffraction (XRD), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). From XRD pattern and XPS spectrum, we can confirm to get γ-Fe2O3. The particle size is several nanometers as shown in TEM image. Magnetic hysteresis loop measurements exhibited that the γ-Fe2O3 nanoparticles display superparamagnetism. However, a trace black powder was obtained in kerosene oil using the same method. A possible formation mechanism of the γ-Fe2O3 nanoparticles was suggested. [Copyright &y& Elsevier]

Abstract: Yttrium iron garnet (YIG) powders with particle sizes of 9, 14, 25 and 60nm have been synthesised and their magnetic properties have been studied as a function of particle size and temperature. The particles with sizes of 9 and 14nm exhibited superparamagnetism. The blocking temperature () decreased with decreasing particle size, whereas the anisotropy constant (K) increased. The saturation magnetisation () decreased with decreasing particle size. These variations are attributed to large surface-to-volume ratios in nanoparticles where the surface moments are predominant. [Copyright &y& Elsevier]

Abstract: The magnetic and magnetoresistive properties of nanostructured Fe/Cr multilayers with ultrathin Fe layers (the nominal Fe thickness is down to ) were investigated at low temperatures (down to 4.2K) in magnetic fields up to 90kOe. Nanostructures were MBE grown in ultrahigh vacuum on single crystal MgO substrates. It is shown that Fe layers in nanostructures with are not continuous but consist of separate ferromagnetic clusters. Such cluster-layered nanostructures exhibit the superparamagnetic properties and the Kondo-like behavior of the resistance. [Copyright &y& Elsevier]

Abstract: Temperature-dependent zero-, longitudinal- and transverse-field spectra have been recorded in nanometric obtained by mechanical alloying. The alloy comprises Fe(Cu) nanoparticles (5nm) in a Ag matrix formed by 16nm particles, as deduced from the Rietveld refinement of the X-ray diffraction pattern. The muon depolarization can be described by a Dynamical Lorentzian Kubo–Toyabe function. This is in agreement with a high degree of interparticle dipolar interactions, giving rise to a cluster spin glass-like behavior below 100K. The intra particle interactions are also probed by the , and influence the thermal variation of the static width. [Copyright &y& Elsevier]

Abstract: We propose a short overview of a few selected issues of magnetism in reduced dimensions, which are the most relevant to set the background for more specialized contributions to the present Special Issue. Magnetic anisotropy in reduced dimensions is discussed first, on a theoretical basis, then with experimental reports and views from surface to single-atom anisotropy. Then, conventional magnetization states are reviewed, including macrospins, single domains, multidomains, and domain walls in stripes. Dipolar coupling is examined for lateral interactions in arrays, and for interlayer interactions in films and dots. Finally thermally-assisted magnetization reversal and superparamagnetism are presented. For each topic we have sought a balance between well established knowledge and recent developments. To cite this article: O. Fruchart, A. Thiaville, C. R. Physique 6 (2005). [Copyright &y& Elsevier]

Magnetic grain-size and coercivity distributions of a superparamagnetic (SP) particle ensemble together determine its frequency dependence of susceptibility (FDS). Investigating the mathematical theory of this dependence leads to a general dispersion relation between real and imaginary parts of the complex susceptibility for SP particle ensembles, which extends the previous treatment by Néel. Using the new theory, it is demonstrated that the inverse problem of determining the combined grain-size and coercivity distribution from FDS measurements is not uniquely solvable. The inversion of the FDS at one temperature can be described by a deconvolution integral, the kernel of which is analytically calculated. The deconvolved FDS corresponds to an energy barrier distribution. Only using a priori assumptions about the relation between particle volume and coercivity it can be interpreted in terms of a volume or grain-size distribution. In order to deconvolve natural rock measurements, a semi-analytical parametric deconvolution method has been developed, which allows to reconstruct the SP grain-size distribution even from relatively noisy data. Dense measurements of FDS at several temperatures can be used to check for the applicability of the above theory. Observed deviations can be interpreted in terms of magnetostatic particle interaction. A quantitative estimate is presented, which allows to determine the average interaction field together with the volume distribution. [ABSTRACT FROM AUTHOR]

Abstract: Self-organization of magnetic materials is an emerging and active field. An overview of the use of self-organization for magnetic purposes is given, with a view to illustrate aspects that cannot be covered by lithography. A first set of issues concerns the quantitative study of low-dimensional magnetic phenomena (1D and 0D). Such effects also occur in microstructured and lithographically-patterned materials but cannot be studied in these because of the complexity of such materials. This includes magnetic ordering, magnetic anisotropy and superparamagnetism. A second set of issues concerns the possibility to directly use self-organization in devices. Two sets of examples are given: first, how superparamagnetism can be fought by fabricating thick self-organized structures, and second, what new or improved functionalities can be expected from self-organized magnetic systems, like the tailoring of magnetic anisotropy or controlled dispersion of properties. To cite this article: O. Fruchart, C. R. Physique 6 (2005). [Copyright &y& Elsevier]

Nano-particles of Ni0.8Cu0.2Fe2O4 have been synthesized by the co-precipitation method. Langevin function fitting of the superparamagnetic M– N curve at 290 K provides a log-normal distribution with median diameter of 30 Å and standard deviation of 0.4. Outside a core of ordered spins, moments in the surface layer are disordered. Magnetization evolves over a long period of time t going linearly with log t. Magnetic anisotropy which was estimated by fitting the M–log t curve shows many fold increase over that of bulk particle sample. Major contribution to this enhancement comes form the disordered surface spins. [ABSTRACT FROM AUTHOR]

Magnetic nanoparticles are fabricated in silica glass (SiO2) using high-flux implantation of 60 keV negative nickel ions Ni-. Photo-absorption measurements and cross-sectional transmission electron microscopy (XTEM) observation confirm the formation of metallic Ni nanoparticles in SiO2, and exclude the possible formation of Ni silicides (Ni3Si, Ni2Si, NiSi) and oxides (NiO) as major products. The mean diameter of the nanoparticles was ∼2.9 nm, and the observed depth distribution was similar to a prediction from the TRIDYN code when sputtering was accounted for. Temperature- and field-dependences of magnetization show that the nanoparticles are in the superparamagnetic state at T>∼50 K. From the peak temperature of the zero-field cooling (ZFC) magnetization, which is ∼27 K, and the mean diameter determined from XTEM, it is suggested that the magnetic anisotropy constant K of the nanoparticles is enhanced by ∼8 times as compared to the bulk value. [Copyright &y& Elsevier]

Nanocrystalline nickel–zinc, zinc–manganese, and nickel–manganese ferrites synthesized by reverse micelle synthesis technique were characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction, and the magnetic behavior studied by Superconducting Quantum Interference Device. The mixed (Ni–Zn, Zn–Mn, and Ni–Mn) ferrites exhibit blocking temperatures of 11, 20, and 15 K, respectively. The small differences in the blocking temperatures are attributed to the magnetocrystalline anisotropy and L–S coupling. The saturation magnetization of nanocrystalline Ni–Zn, Zn–Mn, and Ni–Mn at 300 K was 4.5, 9, and 7 emu/g, respectively, and at 2 K was 15, 19, and 14 emu/g, respectively. The lower saturation magnetization of the nanocrystalline ferrites compared to their bulk counterparts is attributed to the core–shell morphology of the particles consisting of ferromagnetically aligned core spins and a spin-glass-like surface layer. [Copyright &y& Elsevier]

We have investigated the magnetic state of Fe thin films grown on Al2O3(0001) inclined substrates at the Fe thickness range of 5–25 ML (monolayer) (1.0–5.0 nm). For 5 ML Fe films, the magnetic state is superparamagnetic state and the effect of inclination of substrate is negligible. The magnetic state of 10 ML Fe films is in the coexistence of superparamagnetic state and ferromagnetic state, and the effect of inclination of substrate appears but is still a little. In the ferromagnetic region, above Fe thickness of 15 ML, the magnetic easy axis is influenced by the inclination of substrate. Fe thin films grown on the flat substrate have no preferred direction of magnetization in the film plane because of the three equivalent variants of the epitaxial Fe(110) on Al2O3(0001). On the other hand, Fe thin films grown on the inclined substrate have a uniaxial magnetic anisotropy, in which the magnetic easy axis is parallel to the step edge. The strength of uniaxial magnetic anisotropy decreases with increasing Fe thickness. This uniaxial magnetic anisotropy might be derived from the effective demagnetization field due to the magnetic charge distribution at the corrugated surface caused by the steps. [Copyright &y& Elsevier]

X-ray diffraction, Mo¨ssbauer effect and magnetisation investigations have been performed on sputtered Fe40(Al2O3)60 thin films. Ultrafine Fe particles of nanometer size in an amorphous Al2O3 matrix have been formed by annealing in the temperature range from 100°C to 500°C. Their particle sizes, however, show a rather wide distribution. Mo¨ssbauer spectra are constituted of both paramagnetic and ferromagnetic contributions. The paramagnetic contribution is associated with small grains of Fe, whereas the magnetic component is contributed by large iron grains. This assumption is supported by the ZFC- and FC-measurements, in which the “blocking” temperatures of 60 and 90 K were evidenced for as-deposited and 200°C-annealed films, respectively. [Copyright &y& Elsevier]

A section adjacent to a building structure at She¨n De¨li, Butrint, Southern Albania exposes what is believed to be the remains of a former Roman bath-house. This section shows a transition from the underlying natural marsh clay into an archaeological deposit, which is both the basal remains of a building structure, and what appears to be an ash and charcoal enriched midden. The micro and macrocharcoal, magnetic and physical properties indicate a number of phases in the archaeological deposits, culminating in the micro and macrocharcoal rich and magnetically enhanced midden. The magnetic properties of these deposits are strongly related to the charcoal content that is probably a reflection of the ash content. The midden deposit is strongly enhanced in both superparamagnetic and low coercivity magnetite. The charcoal content and magnetic properties of the midden indicate a high content of wood-ash produced at the bath-house site, which is consistent with the anticipated fuel source for the region in Roman times. [Copyright &y& Elsevier]

• Mn-doped nanosized titanium dioxide powders were synthesized by a sol–gel method. • Mn doping leads to the predominance of the paramagnetic contribution to the magnetization over the diamagnetic contribution and a small increase in the number of the ferromagnetic clusters. • The linear part along with the diamagnetic contribution typical for metal oxides is due to the contribution of two types of paramagnetic centers. • These regions do not interact magnetically, and the powder as a whole exhibits superparamagnetic behavior. Studies of magnetization of undoped and Mn-doped nanosized titanium dioxide powders showed that its dependence on the magnetic field can be approximated by the function that is a linear combination of two Langevin functions with different parameters. The linear part along with the diamagnetic contribution typical for metal oxides is due to the contribution of two types of paramagnetic centers. The first one is present in both undoped and doped powders. As follows from the published models, the centers are complex lattice defects Ti+3 ions + O− vacancies. The second is the vast majority of the manganese ions (and related oxide vacancies) in the doped powders that are uniformly distributed over all nanoparticles. The nonlinear part, even present in the undoped samples, is caused by the contribution of a small number of regions with a high magnetic moment (ferromagnetic clusters). These regions do not interact magnetically, and the powder as a whole exhibits superparamagnetic behavior. According to our estimates, the introduction of Mn into the TiO 2 lattice mainly leads to the predominance of the paramagnetic contribution to the magnetization over the diamagnetic contribution characteristic of the TiO 2 lattice and a small increase in the number of the ferromagnetic clusters. [ABSTRACT FROM AUTHOR]

Heterostructured Co-Au nanoparticles are direct synthesized from gas phase. Magnetic properties of Co-Au nanoparticles are characterized. Low-temperature and room-temperature hysteresis loops show that the magnetic properties of Co are kept in Co-Au nanoparticles. A blocking temperature of 290 K is measured for Co-Au particles with an average size of 12 nm. A field-cooled low-temperature hysteresis loop of Co-Au nanoparticles indicates the existence of oxide, even the particles have a core-shell-like structure. [ABSTRACT FROM AUTHOR]

SQUID magnetometry and electron magnetic resonance (EMR) spectroscopy at 9.28 GHz were employed to probe magnetism and surface spin disorder in Fe3 O4 nanoparticles. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed magnetite particles with size ≃8 nm. Magnetization (M) versus temperature studies yielded blocking temperature TB ≃ 170 K in H = 50 Oe. For T > TB, M scales approximately as H/T with μP ≃ 5400 μB as the magnetic moment/particle. At 5 K, the magnitudes of hysteresis loop parameters viz. coercivity Hc ≃ 37 Oe and saturation magnetization M8 = 62 emu/g are considerably smaller than the reported values for larger 150 nm Fe3 O4 particles. In EMR studies, a single symmetric line at g ≃ 2.11 with linewidth ΔH = 850 Oe is observed at 300 K with the g-value and ΔH increasing with decreasing T. These results are interpreted in terms of a disordered surface spin layer of 0.5 nm thickness. [ABSTRACT FROM AUTHOR]

Well-defined CoFe2 O4 nanoparticles embedded in an amorphous SiO2 matrix have been synthesized by a sol-gel method, characterized and investigated by Mössbauer spectroscopy and magnetic properties measurements. The mean particle size increases from 3 to 15 nm consistently with the temperature of the subsequent annealing, which varied from 800°C to 1100°C, respectively. The composites exhibit superparamagnetic behavior with the blocking temperature TB increasing with the mean particle size from 80 to 350 K, respectively. Sufficiently high value of Hc ∼ 2.5 T (at 2 K) has been observed for the sample annealed at 1100°C. [ABSTRACT FROM AUTHOR]

We report on the magnetic properties of CuO nanoparticles prepared by the sol-gel method. M (T) curves show a blocking temperature at TB = 135 K and 160 K for 13 nm and 17 nm size particles, respectively, in measuring field H = 100 Oe. The M-H data for both the samples show a reversal magnetization at 300 K, whereas below TB it exhibit symmetrical hysteresis loops with a coercive field of 200 Oe (155 Oe) and a remanence 0.306 emu/mole (0.171 emu/mole) for 13 nm (17 nm) at 5 K. Presence of superparamagnetism and net magnetic moments of CuO nanoparticles are attributed to the uncompensated Cu2+ ions at the surface of the particles. [ABSTRACT FROM AUTHOR]