Benxi Gu

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.

Structure and Magnetic Properties of La3+-Substituted Strontium Hexaferrite Particles Prepared by Sol–Gel Method

Influences of the substituted amount of La 3 + on structure and magnetic properties of Sr 1 - x La x Fe 1 2 O 1 9 compounds prepared by the sol-gel method have systematically been investigated by XRD, VSM, and TEM. Optimum magnetic properties of σ s = 74.1 Am 2 /kg and H c J = 498.4 kA/m are achieved with material of the composition Sr 0 . 8 5 La 0 . 1 5 Fe 1 1 . 8 5 O 1 9 heated at 850 °C in air. It is found that the suitable amount of La 3 + substitution may remarkably increase the saturation magnetization σ s and the intrinsic coercivity H c J . The Curie temperatures of Sr 1 - x La x Fe 1 2 O 1 9 decrease linearly with increasing La 3 + substitution.

Ferromagnetism in Mn-doped CuO

Ferromagnetic properties have been observed in CuO doped with 3.5–15 at.u200a% of Mn. The transition from ferromagnetic to paramagnetic phase at TC=80u2002K is associated with the metal–insulator transition. Magnetoresistance is weakly negative in the vicinity of the transition, but positive in a wide range of temperatures below TC. The experimental results suggest a possibility of interpretation in terms of the Zener double-exchange mechanism and strong electron–phonon interactions.

Controlled growth of well-faceted zigzag tin oxide mesostructures

Reproducibly high-yield growth of zigzag fibers and well-faceted nanobelts of SnO2 was achieved via tuning the reactant vapor. The investigation of the morphological evolution via scanning electron microscopy and transmission electron microscopy hints that the formation of the zigzag SnO2 fiber is based on the pregrowing SnO2 nanobelt. The elucidation of the growth mechanism should provide a fully controlled route for reproducibly high-yield growth of zigzag fibers of SnO2 and give some valuable hints to synthesis other zigzag mesostructures. Optical characterizations of these structures show very weak defect-related emissions.

Magnetic properties of X‐type Ba2Me2Fe28O46 (Me=Fe, Co, and Mn) hexagonal ferrites

Polycrystalline samples of X‐type Ba2Me2Fe28O46 (Me2‐X, Me=Fe, Co, and Mn) hexagonal ferrite have been prepared by the classic ceramic method. Low‐temperature magnetic properties, Curie temperatures, and distribution of divalent cations were studied. The substitution of Fe2+ ions with Co2+ and Mn2+ ions causes a decrease of the spontaneous moment μ and the Curie temperature Tc of X‐type hexaferrites. The lower μ and Tc of Co2‐X hexaferrite originate from the smaller magnetic moment of Co2+ ions. In the case of Mn2‐X, the lower μ may be due to the valence state of Mn and the lower Tc may be caused by the fact that the space distribution of 3d electrons of Mn is different from that of Fe. In the Co2‐X and Fe2‐X compounds the Co2+ and Fe2+ ions enter the octahedral sites of spinel S blocks.

Exchange-coupling interaction in nanocomposite SrFe12O19/γ-Fe2O3 permanent ferrites

Nanocomposite SrFe12O19/γ-Fe2O3 permanent ferrites have been prepared by the sol-gel method and their structure and magnetic properties have been analyzed by x-ray diffraction, transmission electron microscopy, and vibrating sample magnetometer. When thermal treatment temperature is below 800u2009°C, the samples pressed into a flake exhibit exchange-coupling interaction between the two nanostructured phases of γ-Fe2O3 and SrFe12O19. The flake sample treated at 800u2009°C produces an isotropic nanocomposite with a intrinsic coercivity of 6015 Oe, a maximum magnetization of 75.6 emu/g and a maximum energy product of 1.87 MGOe as compared with the powder sample of 6400 Oe, 75.9 emu/g, and 1.52 MGOe, respectively. The exchange-coupling interaction and the enhancement of isotropic (BH)max in nanocomposite SrFe12O19/γ-Fe2O3 permanent ferrites have been discussed.

Metal nanotubes prepared by a sol?gel method followed by a hydrogen reduction procedure

A sol?gel method followed by a hydrogen reduction procedure was proposed in this work for the preparation of metal nanotubes. The tube length, diameter, wall thickness and, most importantly, the chemical components can be adjusted conveniently. Several examples of metal nanotubes (Fe, Ni, Pb and CoFe) were prepared by using anodic aluminium oxide (AAO) templates. The fabricated nanotubes are uniform in diameter and wall thickness through the entire tubes. The length of the nanotubes is about 100??m, with an aspect ratio of about 1000.

Role of amorphous grain boundaries in nanocomposite NdFeB permanent magnets

Two typical Nd–Fe–B permanent magnetic alloys of B-enriched Nd4.5Fe77B18.5 and Fe-enriched Nd9Fe85B6Zr0.5 have been employed to study the effect of annealing on the microstructure and the magnetic properties, respectively. It has been found that the maximum remanence, remanence ratio, intrinsic coercivity, and energy product are 1.32 T, 0.86, 212 kA/m, and 130 kJ/m3, respectively, for the B-enriched alloy, while they are 1.15 T, 0.82, 496 kA/m, and 162 kJ/m3, respectively, for the Fe-enriched alloy. In these samples, x-ray diffraction, Mossbauer spectroscopy, and high-resolution transmission electron microscopy revealed the existence of an amorphous phase between grains. It is believed that enhancement of the remanence and remanence ratio in the magnets originates from the amorphous grain boundaries, which increase the exchange coupling between neighboring grains.

Magnetic properties and magneto-optical effect of Co0.5Fe2.5O4 nanostructured films

Co0.5Fe2.5O4 ferrite films with nanostructure have been prepared by rf sputtering on a quartz substrate and annealed at temperatures from 400 to 600u200a°C in air. The magnetic properties and magneto-optical effect have been studied. It has been found that nanocrystalline Co0.5Fe2.5O4 films exhibit good magnetic properties and a large magneto-optical effect. The magnetization, coercivity, remanence ratio, and maximum energy product are 455 emu/cm3, 2.8 kOe, 0.72 MGOe, and 2.4 MGOe, respectively. The magneto-optical Faraday rotation is <−3.4°/μm at short wavelength and 4°/μm at the wavelength of 740 nm. These characteristics make them very promising to be used for magneto-optical recording and for permanent magnets. The influence of the distribution of Co2+ ions on magnetic properties and magneto-optical effect has been discussed.

Grain-size dependence of complex permeability in polycrystalline perovskite La0.7Sr0.3MnO3

Abstract The frequency-response complex permeability of polycrystalline perovskite La 0.7 Sr 0.3 MnO 3 with mean grain size range from 1200 to 45xa0nm has been investigated. The pronounced dispersions with relaxation character have been observed in the samples with mean grain size larger than 105xa0nm. The dispersions are due to domain wall displacements. The damping of wall displacements mainly originates from the eddy currents associated with wall motions. The resistivity and mean grain size may be responsible for the change of relaxation frequency.