Wei Zhong

Influences of La3+ substitution on the structure and magnetic properties of M-type strontium ferrites

Abstract M-type strontium ferrites with substitution of Sr 2+ by rare-earth La 3+ , according to the formula Sr 1− x La x Fe 12 O 19 , are prepared by the ceramic process. Influences of the substituted amount of La 3+ on structure and magnetic properties of Sr 1− x La x Fe 12 O 19 compounds have systematically been investigated by XRD, VSM and Mossbauer spectrum. When the substituted amount x is below 0.30, X-ray diffraction shows that the samples are single M-type hexagonal ferrites. It is found that the suitable amount of La 3+ substitution may remarkably increase saturation magnetization σ s and intrinsic coercivity H cJ . With the La 3+ addition for the same sintering temperature, σ s and H cJ increase at first, then decrease gradually. However, the substituted amount x at the maximum value of H cJ is bigger than that of σ s . Mossbauer spectroscopy of 57 Fe in Sr 1− x La x Fe 12 O 19 has shown that the substitution of Sr 2+ by La 3+ is associated with a valence change of Fe 3+ to Fe 2+ at 2a or 4 f 2 site. The magnetic properties are reflected in the Mossbauer spectra which indicate that the magnetic hyperfine field ( H hf ) is detected with the highest value at x =0.20. The different exchange paths between the iron sublattices are discussed according to the increased hyperfine fields of the 12k- and 2b-site. The Curie temperature of Sr 1− x La x Fe 12 O 19 decreases linearly with increasing La 3+ substitution.

Key step in synthesis of ultrafine BaFe12O19 by sol-gel technique

Abstract Barium hexaferrite powders have been prepared by sol-gel technique. Different ways in heat treatment of the gel are adopted to study the factors that influence the formation mechanism, morphology and magnetic properties of BaFe12O19. Results show that preheating the gel between 400 and 500°C for several hours is a key step, it can prevent the formation of α-Fe2O3 intermediate and obtain ultrafine BaFe12O19 single phase with improved magnetic properties and narrow size distribution at a low temperature. The barium ferrite powder had a coercive force of 5950 Oe and a magnetization of 70 emu/g, both are in good agreement with the expected values theoretically.

Magnetocaloric properties of Na-substituted perovskite-type manganese oxides

Abstract The dependence of magnetization on the applied magnetic field and temperature was measured carefully near their Curie temperature for polycrystalline perovskite-type manganese oxides La 1− x Na x MnO 3 (0.05 X ≤ 0.20) prepared by sol-gel technique. The large magnetic entropy changes upon an application of a low magnetic field and the convenient adjustment of the Curie temperature make the perovskite-type manganese oxides useful for magnetic refrigerants in an extended high temperature range even at room temperature.

Preparation and magnetic properties of SrFe12O19 particles prepared by the salt-melt method

Abstract The salt-melt method was adopted to prepare high-performance Sr ferrite without further pulverizing. The reaction path, particles size and magnetic properties have been investigated by using high-purity starting raw materials to synthesize Sr ferrite by replacing SrSO 4 with celestite. The magnetic properties are compared with those of materials prepared by the ceramic process. Due to the lower reaction temperature, no further pulverization is needed and due to low-cost minerals, the process should be more economical than the conventional ceramic process.

Spin-dependent interfacial tunnelling and tunnel-type GMR in granular perovskite family

The granular perovskites , with the concentration x from 0.05 to 0.5, have been fabricated by the sol-gel method. The concentration-dependent transport properties in the granular perovskites have been investigated. Spin-dependent interfacial tunnelling and the corresponding tunnel-type magnetoresistance have been observed in the samples sintered at lower temperatures in the doping range from x = 0.05 to 0.45 for the temperatures below the Curie point. The magnetization, the heat capacity and the oxygen stoichiometry for the sample family have been measured to show that the interfacial tunnelling stems from the magnetic difference between the surfaces and the cores of the grains, and that the tunnel-type GMR originates from the field-induced change of the surface magnetic configuration.

Giant uniaxial piezoresistance in GMR perovskite La0.85Sr0.15MnO3

Abstract The uniaxial-pressure dependences of electric and magnetic properties have been investigated in perovskite-type Laue5f8Srue5f8Mrue5f8O. A giant piezoresistance (GPR) effect and a remarkable pressure-induced increase of T p , the onset of the metal-semiconductor-like (M-S) transition, have been observed. The piezoresistance value (PR) and the pressure coefficient of T p are all larger by two orders of magnitude than that in the case of hydrostatic pressure. Piezoresistance and magnetoresistance peaks were found to locate at almost the same temperature. This fact suggests that GMR and GPR may originate from the same mechanism(s) and may be relative to lattice transformation(s).

Enhanced photoluminescence of C60 incorporated into interlayers of hydrotalcite

Abstract Strong photoluminescence of sodium-reduced C 60 incorporated into interlayers of hydrotalcite is observed. This phenomenon is correlated to the fact that the reduced C 60 is positioned between positively charged layers of the anion clay. The interaction between the layers and reduced C 60 alters the photophysical properties of C 60 and relaxes the electron transition inhibition, thus enhancing photoluminescence.

A structure-dependent change of magnetism in the magnetic oxide

The grain size dependences of magnetic properties in the granular perovskite have been investigated. A structural phase transition from cubic perovskite to orthorhombic form and lattice parameter enlargements with grain growth have been observed. Simultaneously, a series of magnetic changes, including magnetization and Curie temperature, has been obtained with the change of grain size. These phenomena suggest a structure-coupled change of magnetism in the hole-doped manganese perovskites.

Magnetoelectric transport properties of quenched polycrystalline magnetite

Abstract Ultrafine Fe3O4 particles were prepared through chemical co-precipitation. Powder compact was sintered and quenched in a controlled CO2/CO atmosphere to obtain bulk specimen. X-ray diffraction and Mossbauer spectroscopy showed homogeneous polycrystalline Fe3O4 phase. The electrical resistance was measured in applied magnetic fields up to 5xa0T over the temperature range of 55–155xa0K. The temperature dependence of resistivity showed a semiconductor behavior without discontinuity at Verwey point, while the magnetoresistance (MR) exhibited an extremum in its temperature dependence and correspondingly the magnetization manifested a discontinuous decrease, with temperature decreasing through Verwey point. Meanwhile X-ray diffraction showed no observable structural transition in the range from 77 to 300xa0K. The results different from those of well-annealed samples are suggested to be attributed to quench related stress and defects. It is proposed that near Verwey point the MR behavior in Fe3O4 is not completely determined by the `structural–electronic’ mechanism. Below Verwey temperature Tv, MR value increased sharply when temperature decreased. The transport behavior is modeled as an activated mechanism above Tv and a variable range hopping mechanism below Tv.

Structure and Magnetocaloric Properties in Perovskitelike La1-xNaxMnOz Powders

Perovskitelike manganese oxides La1-xNaxMnOz (xxa0=xa00.075, 0.100, 0.165, 0.200) have been fabricated using sol-gel technique. Structure analysis, temperature programmed reduction (TPR) technique and magnetic measurement have been used to study the effect of Na ion substituted amount on Mn-O distance, the oxygen content, and the magnetocaloric properties of these materials. Results show there is no clear correlation between the substituted amount and mean Mn valence, but a close relationship is observed between Tc and the bond distance of Mn-O. These materials have quite large magnetic entropy changes upon an application of a low magnetic field and can be utilized as more suitable candidates for magnetic refrigerants with wide temperature span.