Investigation of magnetic properties of interacting Fe2O3 nanoparticles

The magnetic properties of Fe₂O₃ nanoparticles (average diameter ∅≅3 nm) in alumina (68% Fe₂O₃ in weight) have been investigated by magnetization measurements. The results indicate a superparamagnetic behavior of interacting particles, which block with decreasing temperature (the zero-field-cooled susceptibility shows a maximum at T≅145 K) with a distribution of relaxation times. A change of magnetic regime is observed below ∼60 K, due to the increasing interparticle interactions and local surface anisotropy.     

Superparamagnetic properties of C60Co3 complexes

Preparation of fullerites containing cobalt and analyses of reactions based on semiempirical quantum calculations are described. The magnetic properties of thermally treated C₆₀Co₃ samples: Curie constant (C≈3500 emu K/mol Oe) temperature and field dependencies of magnetization and nonequilibrium effects of magnetization are interpreted in terms of superparamagnetic blocking model of the compound.     

Fabrication of cyclodextrin-functionalized superparamagnetic Fe3O4/amino-silane core–shell nanoparticles via layer-by-layer method

This paper presents a feasible protocol for the preparation of a novel versatile nanocomposite possessing superparamagnetism via a layer-by-layer method. We combined (3-aminopropyl)triethoxysilane-coated magnetic Fe₃O₄ nanoparticles (APTES-MNPs) with β-cyclodextrin (β-CD). The following unusual features were integrated in a single nano-system: (a) the silane coating outside the magnetic Fe₃O₄ cores derived from the hydrolysis of APTES acted as a coupling agent and provided amino group (–NH₂) for linking the CD molecule; (b) the outermost CD moieties can function as inclusion sites and specific containers for drugs and biomolecules; (c) the innermost magnetic cores were able to sense and respond to an externally applied magnetic field and their behaviors in vivo or in vitro can be artificially manipulated and navigated. The obtained nanocomposite turned out to be superparamagnetic with a relatively high saturation magnetization value of 69 emu g^?1, which implies potentially promising applications in magnetic drug delivery technology and bioseparation.     

Structural and Mössbauer investigation on barium titanate–cobalt ferrite composites

Perovskite and spinels oxides have received renewed attention due to the possibility of combining both structures in di-phase composites to obtain multifunctional materials. In this work, barium titanate (perovskite)-cobalt ferrite (spinel) composite powders with different microstructures were obtained from thermal treatment of amorphous precursors at 500–1100 °C. The precursors were prepared by combining coprecipitation and sol–gel routes. Lyophilization of ferrite prior to mixing was used as a strategy to control interphase reaction. Mössbauer spectroscopy showed that the dispersion of coprecipitated ferrite in a viscous BaTiO₃ precursor gel resulted in superparamagnetic behavior and reduction of the local magnetic field of site [B].     

Magnetic properties of rare-earth transition metal aluminides R6T4Al43 with Ho6Mo4Al43-type structure

The magnetic properties of 53 aluminium-rich intermetallic compounds R₆T₄Al₄₃ with R=rare-earth elements and T=Ti, V, Nb, Ta, Cr, Mo, W were investigated using polycrystalline samples and a SQUID magnetometer in the temperature range from 2 to 300 K with magnetic flux densities up to 5.5 T. The yttrium and lutetium compounds are Pauli paramagnetic, indicating that the transition metal atoms do not carry magnetic moments. The samarium compounds show van Vleck behavior and antiferromagnetic order with Néel temperatures of less than 12 K. Of these Sm₆Ti₄Al₄₃ becomes metamagnetic. The ytterbium compounds show a mixed or intermediate valent behavior and no magnetic order down to 2 K. All other compounds obey the Curie–Weiss law above 30 K. Their effective magnetic moments correspond to the theoretical moments of the rare-earth ions. They show ferromagnetic or metamagnetic behavior with ordering temperatures all below 20 K. The magnetization curves of most compounds (recorded up to 5.5 T) reach about 50% of the theoretical magnetization already at 0.5 T. The gadolinium compounds are exceptional in that they reach at 0.5 T only about 10% of their theoretical magnetization. The crystal structures of the isotypic compounds Yb₆V₄Al₄₃ and Yb₆Ta₄Al₄₃ were refined from single-crystal X-ray data.     

Synthesis and investigation of magnetic properties of Gd-substituted Mn–Zn ferrite nanoparticles as a potential low-TC agent for magnetic fluid hyperthermia

Gd-substituted Mn–Zn ferrite nanoparticles of different compositions were synthesized by chemical co-precipitation method. To study the reduction of the Curie temperature (T_C) for different samples, their magnetic properties in dependence from the composition and cationic distribution were investigated. An attempt to lower the T_C of superparamagnetic particles to the optimal temperature required in magnetic fluid hyperthermia (44–47 °C) was made.     

Deposition of superconducting CeCoIn5 thin films by co-sputtering and evaporation

Thin superconducting films of CeCoIn₅ were prepared in situ by simultaneous thermal evaporation of indium and dc magnetic field assisted sputtering of planar metallic Ce and Co targets. To achieve an effective sputtering of the magnetic Co target a special geometry with two facing planar targets (Ce and Co) and magnetic field perpendicular to the targets was used. The stoichiometric (0 0 1)-oriented CeCoIn₅ films were grown on r-cut sapphire substrates with a high-rate of 100 nm/min. The temperature dependence of the electrical resistivity revealed the characteristic heavy-fermion behavior and a superconducting transition at about 2 K in agreement with the literature data for CeCoIn₅ bulk material and thin films.     

Preparation of monodisperse magnetite nanoparticles under mild conditions

In this paper, we reported a method to prepare monodisperse magnetite nanoparticles at mild temperature using cheap and non-toxic precursors. It overcomes the shortages of chemical co-precipitation method and thermal decomposition method and combines the advantages of facile, cheap, large-scale, monodisperse, nanosize, and low synthesis temperature and low toxic. In this method, FeCl₃ · 6H₂O, FeCl₂ · 4H₂O and sodium oleate were mixed in toluene/ethanol/water mixture solvent and refluxed at 74 °C to prepare magnetite nanoparticles directly. The nanoparticles were characterized by transmission electron microscopy, dynamic light scattering, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectrometer and thermogravimetric analysis. The magnetic properties of nanoparticles were measured by superconducting quantum interference device. The results showed that the nanoparticles are well-monodisperse with about 4–5 nm of average diameter. The nanoparticles were proved to be superparamagnetic with saturated magnetization 23.6 emu/g and blocking temperature 24.4 K. A possible formation mechanism of monodisperse magnetite nanoparticles was presented at the same time.     

Magnetic anisotropy and magnetostriction of Lu2Fe17 single crystal

The magnetic properties of Lu₂Fe₁₇ single crystal have been studied by means of magnetization, susceptibility and magnetostriction measurements. The unusual magnetic behavior with two magnetic phase transitions has been observed in magnetic fields up to 50 Oe. The magnetostriction of the Lu₂Fe₁₇ compound has the maximum at temperature T≈285 K at which the paraprocess makes the main contribution to the magnetization.     

Large magnetocaloric effect in HoFeO3 single crystal

Magnetocaloric properties of HoFeO₃ single crystal are investigated along the direction [100]. Magnetic field dependent magnetization isotherms at different temperatures undergo a metamagnetic transition, entropy change as large as 19.2 J/kg K and 15.8 J/kg K are obtained at 7 T in the vicinity of antiferromagnetic ordering temperature of Ho³⁺ and the metamagnetic transition, respectively. The coupling of Ho and Fe spins generates the compensation behavior at 6.5 K, separating the two large magnetic entropy change. Its refrigeration capacity (RC) value, as high as 220 J/kg, is appreciable and can be considered as a promising magnetic refrigerant. New evidence for spin reorientation of Fe³⁺ in HoFeO₃ is also provided by the change of magnetic entropy.     

Magnetic structure of the GdAl3 intermetallic compound as seen via Mössbauer spectroscopy data for 119Sn impurity atoms

The magnetic hyperfine interaction for ¹¹⁹Sn impurity atoms in GdAl₃ frustrated antiferromagnetic compound has been investigated by Mössbauer spectroscopy technique. Two magnetic subspectra with the ratio of the intensities 2:1 were observed. At 4.5 K, the values of the magnetic hyperfine field are 4.00(2) and 1.35(2) T. The peculiarities of the Mössbauer spectrum provide an opportunity to propose a plausible model of the spin arrangement in the GdAl₃ lattice. In each GdAl₃ basal layer, the magnetic moments of the Gd atoms form three-sublattice 120° spin structure that is peculiar to magnetically frustrated compounds. The appearance of two magnetically nonequivalent Sn sites is a result of vector summation of the transferred polarizations from Gd moments located in two adjacent GdAl₃ planes. The anomalous temperature behavior of the hyperfine field is characteristic of frustrated systems with competing exchange interactions.     

XRD and Mössbauer spectroscopy investigation of Fe2O3–Al2O3 nano-composite

Fe₂O₃–Al₂O₃ nano-composites were synthesized by sol–gel means. The properties of samples sintered at various thermal treatment temperatures were investigated by X-ray diffraction (XRD) and Mössbauer spectroscopy (MS). The experimental results show that the γ- to α-Al₂O₃ transformation occurs at lower temperature after iron oxide doping. The samples obtained at 1173 K contained poorly crystallized γ-Al₂O₃ phase and an amorphous iron oxide. When the temperature of heating was increased to 1373 K, the sample was composed of α-Fe₂O₃/α-Al₂O₃ nano-composite and some solid solution. A superparamagnetic phenomenon was observed until the thermal treatment temperature reached 1373 K.     

Electrostatic self-assembly of Fe3O4 nanoparticles on carbon nanotubes

Carbon nanotubes (CNTs)-based magnetic nanocomposites can find numerous applications in nanotechnology, integrated functional system, and in medicine owing to their great potentialities. Herein, densely distributed magnetic Fe₃O₄ nanoparticles were successfully attached onto the convex surfaces of carbon nanotubes (CNTs) by an in situ polyol-medium solvothermal method via non-covalent functionalization of CNTs with cationic surfactant, cetyltrimethylammonium bromide (CTAB), and anionic polyelectrolyte, poly(sodium 4-styrenesulfonate) (PSS), through the polymer-wrapping technique, in which the negatively charged PSS-grafted CNTs can be used as primer for efficiently adsorption of positively metal ions on the basis of electrostatic attraction. X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) analysis have been used to study the formation of Fe₃O₄/CNTs. The Fe₃O₄/CNTs nanocomposites were proved to be superparamagnetic with saturation magnetization of 43.5 emu g^?1. A mechanism scheme was proposed to illustrate the formation process of the magnetic nanocomposites.     

Neutron studies of the iron-based family of high TC magnetic superconductors

We review neutron scattering investigations of the crystal structures, magnetic structures, and spin dynamics of the iron-based RFe(As, P)(O, F) (R = La, Ce, Pr, Nd), (Ba,Sr,Ca)Fe₂As₂, and Fe_1+x(Te–Se) systems. On cooling from room temperature all the undoped materials exhibit universal behavior, where a tetragonal-to-orthorhombic/monoclinic structural transition occurs, below which the systems become antiferromagnets. For the first two classes of materials the magnetic structure within the a–b plane consists of chains of parallel Fe spins that are coupled antiferromagnetically in the orthogonal direction, with an ordered moment typically less than one Bohr magneton. Hence these are itinerant electron magnets, with a spin structure that is consistent with Fermi-surface nesting and a very energetic spin wave bandwidth ～0.2 eV. With doping, the structural and magnetic transitions are suppressed in favor of superconductivity, with superconducting transition temperatures up to ≈55 K. Magnetic correlations are observed in the superconducting regime, with a magnetic resonance that follows the superconducting order parameter just like the cuprates. The rare earth moments order antiferromagnetically at low T like ‘conventional’ magnetic superconductors, while the Ce crystal field linewidths are affected when superconductivity sets in. The application of pressure in CaFe₂As₂ transforms the system from a magnetically ordered orthorhombic material to a ‘collapsed’ non-magnetic tetragonal system. Tetragonal Fe_1+xTe transforms to a low T monoclinic structure at small x that changes to orthorhombic at larger x, which is accompanied by a crossover from commensurate to incommensurate magnetic order. Se doping suppresses the magnetic order, while incommensurate magnetic correlations are observed in the superconducting regime.     

Ferromagnetism in Zn1−xCrxTe (x = 0.05, 0.15) films grown on GaAs(1 0 0) substrate

Zn_1−xCr_xTe (x = 0.05, 0.15) films were grown on GaAs(1 0 0) substrate by thermal evaporation method. X-ray diffraction analysis showed the presence of ZnCrTe phase without any secondary phase. The surface was analyzed by high resolution magnetic force microscope and profile measurements showed orientation of magnetic domains in the range of 0.5–2 nm with increase of Cr content. Magnetic moment–magnetic field measurements showed a characteristic hysteresis loop even at room temperature. The Curie temperature was estimated to be greater than 300 K. From the electron spin resonance spectra, the valence state of Cr in ZnTe was found to be +2 with d² electronic configuration. Hall effect study was done at room temperature and the result showed the presence of p-type charge carriers and hole concentration was found to increase from 5.95 × 10¹² to 6.7 × 10¹² m⁻³ when Cr content increases. We deduce the origin of ferromagnetic behavior based on the observed experimental results.     

High coercivity and superparamagnetic behavior of nanocrystalline iron particles in alumina matrix

Single-domain nanoscale magnetic iron particles have been embedded uniformly in an amorphous matrix of alumina using a pulsed laser deposition technique. Structural characterization by transmission electron microscopy (TEM) reveals the presence of a crystalline iron and an amorphous alumina phase. Fine particle magnetism have been investigated by carrying out field and temperature dependence of magnetization measurements using superconducting quantum interference device magnetometer. The particle size of Fe in Al₂O₃ matrices prepared by changing the deposition time of Fe, have been found to be 9, 7 and 5 nm from TEM studies. At 10 K, the coercivities of these samples are found be 450, 350 and 150 Oe, respectively. At 300 K, the coercivity of Fe–Al₂O₃ sample decreases from 100 to 50 Oe as the particle size decreases from 9 to 7 nm and finally the sample turns superparamagnetic when the Fe particle size becomes around 5 nm. Based on the calculated value of blocking temperature, T_B, (481 K), magnetic anisotropy K (4.8×10⁵ erg/cm³) for Fe, and the Boltzmann constant k_B (1.38×10⁻¹⁶ erg/K) from T_B=KV/25k_B, the mean radius of Fe particles is found to be 9.3 nm. in one of the samples. This is in good agreement with the particle size measured using TEM studies.     

Uniaxial pressure effect on magnetic susceptibility of perovskite manganite La0.66Ba0.34MnO3

Magnetization of La_0.66Ba_0.34MnO₃ and its temperature behavior under a uniaxial pressure of 0.1 kbar are measured between 5 and 270 K in magnetic fields 0<H<120 Oe. The magnetization represents nearly linear dependence on an external magnetic field. Temperature dependence of the magnetic susceptibility found represents a plateau, that is considered as an evidence of the formation of a long period magnetic structure (probably a sort of helix) below the Curie point. Pressure derivative of magnetization displays a sharp minimum at 200 K, pointing to an instability of electronic structure of the compound near this temperature.     

Effect of Ti4+ ions doping on microstructure and dc resistivity of nickel ferrites

Nanostructured nickel ferrites (NiFe₂O₄) were prepared by doping with Ti⁴⁺ ions using solid-state reaction route. Lowest grain size of 55 nm was achieved in the specimens with 20 mole% TiO₂ doping. Magnetization in the specimens decreases with decreasing grain sizes. Lower volume fractions of ferrite phase due to dissociation of the magnetic phase into smaller particles by the disruption of super exchange interaction by the titanium substitution results a decrease in magnetizations. Coercivity showed an increasing trend. This was explained as arising due to multidomain/monodomain magnetic behavior of magnetic nanoparticles. Small polaron hopping conduction between Fe²⁺ and Fe³⁺ sites controls the dc electrical properties of the specimens. The presence of an interfacial amorphous phase between the sites is evident from Mott's analysis. Specimens containing 10 mole or more TiO₂ and sintered at 1350 °C contain NiTiO₃ as a secondary phase and show unusual dc conductivity.     

Structure,optical, and magnetic properties of rutile Sn1−xMnxO2 thin films

Sn_1?xMn_xO₂ (x ≤ 0.11) thin films were fabricated by sol–gel and spin-coated method on Si (1 1 1) substrate. X-ray diffraction revealed that single-phase rutile polycrystalline structure was obtained for x up to about 0.078. Evolution of the lattice parameters and X-ray photoelectron spectroscopy studies confirmed the incorporation of Mn³⁺ cations into rutile SnO₂ lattice. Optical transmission studies show that the band gap energy (E_g) broadens with the increasing of Mn content. Magnetic measurements revealed that all samples exhibit room temperature ferromagnetism (RTFM), which is identified as an intrinsic characteristic. Interestingly, the magnetic moment per Mn atom decreases with the increasing Mn content. The origin of RTFM can be interpreted in terms of the bound magnetic polaron model.