Size effects on the magnetic and optical properties of CuO nanoparticles

Optical and magnetic studies on CuO nanoparticles prepared by a chemical route are reported and the effect of size variation on these properties is discussed. SEM images show that the nanoparticles are interlinked into microspheres with the cages containing visible nanoscale holes. Diffuse reflectance spectroscopy indicates a consistent red shift in the fundamental band gap (indirect band gap) from 1.23 to 1 eV as the size decreases from 29 to 11 nm. This observed red shift is attributed to the presence of defect states within the band gap. A clear blue shift is observed in the direct band gap of these nanoparticles presumably due to the quantum confinement effects. Air-annealed samples show a paramagnetic response whereas particles annealed in a reducing atmosphere show additionally a weak ferromagnetic component at room temperature. For both types of particles, the paramagnetic and ferromagnetic moments, respectively, increase with decreasing size. The role of oxygen vacancies is understood to relate to the generation of free carriers mediating ferromagnetism between Cu spins. AC susceptibility measurements show both the antiferromagnetic transitions of CuO including the one at 231 K which is associated with the onset of the spiral antiferromagnetic phase transition.     

Size and anisotropy effects on magnetic properties of antiferromagnetic nanoparticles

Based on the Heisenberg model taking into account single-ion anisotropy and using a Green's function technique we have studied the influence of size and anisotropy effects on magnetization M, Neel temperature T_N, coercive field H_c and spin excitation energy of antiferromagnetic nanoparticles. The properties are compared with those of ferromagnetic nanoparticles. We have shown that the enhanced magnetization M and coercive field H_c of antiferromagnetic nanoparticles is a surface effect, which is due to uncompensated surface spins. Moreover, the shape of the coercive field curve can be significantly influenced by surface magnetic anisotropy.     

Switching field distribution of magnetic fine particles

In the magnetic recording media the switching field distribution (SFD) is an important micromagnetic characteristic curve. However, in general case, methods of assessing SFD have not presented a reasonable relation between the macroscopic measurements and the microscopic properties of the system. The degree of alignment of particles easy axis in a texture and the angular dependence of switching field are two fundamental factors for the distribution function. A technique for determining the switching field distribution curve due to these factors is reported. This technique is performed for textured iron fine particles and samples of CrO₂ and γFe₂O₃ commercial tapes. The distribution differs from a gaussian function. However, the width and the mean switching field equal to twice of the width of dM_r/dH curve and the remanent coercivity, respectively. Furthermore, it is shown that the width of the switching field distribution decreases with increasing the degree of easy axis orientation.     

On preparation of fine particles of MnFe2O4 and their magnetic properties

Influence of the heat treatment, following a wet chemical process, has been examined on the sizes of the obtained particles of MnFe₂O₄. X-ray diffraction, magnetization and Mössbauer measurements have been used. The average sizes of the obtained particles are in the range of ～ 100 Å–300Å with the upper size limit being ～ 450 Å and more. The average size as well as size distribution is strongly influenced by the nature of the heat treatment followed for preparation of the sample. Prolonged hydrothermal heating resulted in larger average size than did dry heating at 400°C of the precipitate obtained from the initial wet process. Further dry heating following hydrothermal treatment did not lead to any major growth. Chemical nature of the starting materials also affects the sizes.     

Size effects on Curie temperature of ferroelectric particles

A model for the size dependence of the Curie temperature T_c of perovskite ferroelectric particles without any free adjustable parameters has been developed. The model predicts that T_c decreases with decreasing particle size. The predictions of the model are in agreement with experimental results for PbTiO₃ and BaTiO₃. PACS 61.46.+w; 77.80.Bh; 77.84.Dy     

Effect of Ni doping on the properties of fine magnetite particles

Vibrating sample magnetometer (VSM) and Mössbauer spectroscopy are used to characterize the magnetic behaviour of fine magnetite particles obtained from (i) pure goethite and (ii) Ni-doped goethite, in ammoniacal solution. The latter sample has 0.4 wt% Ni which has significantly changed the properties of the sample. The Ni-doped magnetite shows a much higher overoccupancy of tetrahedral sites by iron atoms as compared to the undoped sample. TEM study shows that presence of Ni ions leads to narrower size distribution of magnetite particles as compared to the magnetite obtained from undoped goethite. The coercive field is also affected by presence of Ni, being only 105 Oe for the Ni-doped magnetite as against 170 Oe in the undoped sample.     

Size dependence of the magnetic moments of exposed nanoscale iron particles

The magnetic moments in exposed, mass-selected, nanoscale Fe clusters in the size range 1.89–2.20 nm (300–475 atoms), deposited onto graphic in situ have been measured by X-ray magnetic circular dichroism. The smallest clusters possess moments that are enhanced by around 4% for m_spin and 80% for m_orb and decrease towards the bulk value with increasing size. The larger clusters show an in-plane anisotropy that is consistent with the anisotropy in the orbital moment. The smallest clusters are, within experimental error, magnetically isotropic. The anisotropy constant in the 475-atom clusters is significantly higher than the bulk value.     

Contrasting magnetic behavior of fine particles of some Kondo lattices

We have investigated the magnetic behavior of ball-milled fine particles of well-known Kondo lattices, CeAu₂Si₂, CePd₂Si₂ and CeAl₂, by magnetization and heat-capacity studies in order to understand the magnetic behavior when the particle size is reduced. These compounds have been known to order antiferromagnetically in the bulk form near ($T_N=$) 10, 10 and 3.8 K respectively. We find that the features due to magnetic ordering get suppressed to temperatures below 1.8 K in the case of fine particles of ternary alloys, though trivalence of Ce as inferred from the effective moment remains unchanged. In contrast to this, in CeAl₂, there appears to be a marginal enhancement of $T_N$, when the particle size is reduced to less than a micron. These results can be consistently understood by proposing that these compounds move toward left in the Doniach magnetic phase-diagram, for instance, due to relatively more 4f-localization, as the particle size is reduced.     

Fluctuations of magnetic moments in fine particles: SEDM studies

The spectra of γ-quanta resonantly scattered on the nuclei⁵⁷Fe contained in the goethite FeOOH microparticles reveal the presence of energetically shifted (satellite) component that arises due to the repopulation of hf sublevels during the lifetime of excited nuclear state. This observation confirms the hypothesis of relaxation behaviour of magnetic moments in microcrystalline goethite.     

Size effects in electrical and magnetic properties of quasi-one-dimensional tin wires in asbestos

Bulk composites have been prepared based on one-dimensional fibers of natural chrisothil-asbestos with various internal diameters (d = 6–2.5 nm) filled with tin. The electrical and magnetic properties of quasi-one-dimensional Sn wires have been studied at low temperatures. The electrical properties have been measured at T = 300 K at a pressure P = 10 kbar. It has been found that the superconducting (SC) characteristics of the nanocomposites (critical temperature T_c and critical magnetic field H_c) increase as the Sn filament diameter decreases. The temperature spreading of the resistive SC transition also increases as the Sn filament diameter decreases, which is explained by the SC order parameter fluctuations. The size effects (the increase in critical temperature T_c and transition width ΔT_c) in Sn nanofilaments are well described by the independent Aslamazov–Larkin and Langer–Ambegaokara fluctuation theories, which makes it possible to find the dependence of T_c of the diffuse SC transition on the nanowire diameter. Using the temperature and magnetic-field dependences of the magnetic moment M(T, H), it has been found that the superconductor–normal metal phase diagram of the Sn–asbestos nanocomposite has a wider region of the SC state in T and H as compared to the data for bulk Sn. The magnetic properties of chrisotil-asbestos fibers unfilled with Sn have been studied. It has been found that the Curie law is fulfilled and that the superparamagnetism is absent in such samples. The obtained results indicate the absence of magnetically ordered impurities (magnetite) in the chrisotil-asbestos matrix, which allowed one to not consider the problem of the interaction of the magnetic subsystem of the asbestos matrix and the superconducting subsystem of Sn nanowires.     

Size effects in the optical absorption of ultrafine aluminium particles

Parallel band absorption at 1.55 eV, which is clearly seen in bulk aluminium, becomes much less visible in transmittance spectra on films consisting of gas evaporated particles with diameters of 3 to 4 nm. This can be understood, at least partially, from an enhancement of the Drude absorption caused by the lowered electron lifetime.     

Effect of nanocrystalline particles on the magnetic properties of Z-type hexaferrites

In order to improve the magnetic properties of Z-type hexaferrites such as high initial permeability and high-quality factor, the nanocrystalline hexaferrite particles (NHPs) with the same chemical composition were introduced. The influence of NHPs on the densification, microstructures and magnetic properties of the ceramics prepared by a combined method was investigated. The results show that these NHPs, which spread around the micron-sized hexaferrite particles (MHPs), enhance the densification by increasing the inter-diffusion of the particles due to the increase of contact area; simultaneously, the grain growth in the direction of c-plane is dominant. Due to the special microstructure, high sintering density, and no addition sintering aids with different chemical composition, relatively high initial permeability and Q-factor than those of the samples with 1.0 wt% Bi₂O₃ were obtained in the samples with proper nanocrystalline particles sintered at 900 °C.     

Spinel oxides growing on Fe-Cr alloy particles during plasma spraying

Oxidation reactions during plasma spraying of metallic powders give rise to oxide crusts on powder particle surfaces. The first oxidation stage occurs in flight of molten particles. It is usually followed by the second stage after hitting a substrate. To investigate the oxidation products immediately after the first stage, abrupt stopping of in-flight oxidation is possible by trapping and quenching the flying particles in liquid nitrogen. In oxide crusts on plasma sprayed and liquid nitrogen quenched particles of a Fe-12%Cr alloy, two spinel oxides were indicated by Mössbauer spectroscopy and X-ray diffraction. Both are solid solutions of the type Fe₃O₄ - Cr₃O₄ (i.e., Fe_3?x Cr _x O₄, 0 ≤x ≤ 3). One of the oxides, tetragonally distorted spinel, is characterized by the mean value ofx ≈ 2.3. It is only stable at very high temperatures. The other spinel oxide is cubic withx slightly lower than 2, i.e. almost stoichiometric chromite FeCr₂O₄. From thermodynamic considerations it follows that in the Fe₃O₄ - Cr₃O₄ system there is no miscibility gap at high temperatures. The simultaneous existence of both oxides is probably due to non-equilibrium conditions during liquid nitrogen quenching of trapped particles.     

Magnetic cluster expansion simulation and experimental study of high temperature magnetic properties of Fe-Cr alloys

We present a combined experimental and computational study of high temperature magnetic properties of Fe-Cr alloys with chromium content up to about 20?at.%. The magnetic cluster expansion method is applied to model the magnetic properties of random Fe-Cr alloys, and in particular the Curie transition temperature, as a function of alloy composition. We find that at low (3-6?at.%) Cr content the Curie temperature increases with the increase of Cr concentration. It is maximum at approximately 6?at.% Cr and then decreases for higher Cr content. The same feature is found in thermo-magnetic measurements performed on model Fe-Cr alloys, where a 5?at.% Cr alloy has a higher Curie temperature than pure Fe. The Curie temperatures of 10 and 15?at.% Cr alloys are found to be lower than the Curie temperature of pure Fe.     

Dimension effects on the dielectric properties of fine BaTi03 ceramics

It is found that the core-shell structured grains are easy to produce for fine grain doped BaTiO3 ceramics in the sintering process. We study the influence of the core-shell structure on the Curie-Weiss temperature and dielectric properties of BaTiO3 ceramics by using effective medium approximation （EMA）. Considering the second approximation, the dielectric properties of fine grain doped BaTiO3 ceramics are consistent with experimental data.     

Structure and magnetic properties of the oxide layers on iron ultrafine particles

5 . The γ-Fe particles, because of their paramagnetic nature, are very convenient for investigation on the attributes of iron oxide layers formed on the particle surfaces. Structures, morphologies and magnetic properties of the oxide layers covering the iron ultrafine particles have been studied using transmission electron microscopy observation, magnetic property measurement, X-ray diffraction and annealing treatment. Convincing evidences established that the iron oxide layers are not continuous and consist of very fine crystallites, and that these layers are non-ferromagnetic and have no contribution to the saturation magnetization of the iron particles. The iron oxide layers formed at room temperature was determined to be Fe₃O₄. Additionally, a brief annealing of the iron particles in air were performed to examine magnetic properties of the formed iron oxide layers and ultrafine oxide particles. Received: 30 April 1996/Accepted: 5 November 1996     

Ordering and magnetic properties of nanostructured CoPt particles

The kinetics of ordering in nanostructured CoPt particles is investigated. The low activation energy (58 ± 2 kJ/mol) of the ordering process indicates that the kinetics of ordering is not limited by bulk diffusion. Coercivity grows along with the degree of ordering and reaches its maximum value of 14.2 kOe upon the complete ordering of CoPt alloy with a crystallite size of no more than that of a single domain.