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The complex dielectric permittivities ([variant_greek_epsilon]′,[variant_greek_epsilon]″) of RMn2O5 (R=Tb, Dy, and Er) are measured in the temperature range of 78–328 K with frequencies ranging from 100 Hz to 1 MHz. A large real part of the dielectric constant [variant_greek_epsilon]′ (40–140) is observed at room temperature in all three samples studied. These values are higher than the peak value found at the ferroelectric transition of RMn2O5 single crystals. Our results reveal that high dielectric permittivities are related to thermally activated dipolar motions with activation energies of ∼300 meV and a relaxation frequency of ∼3×1012 Hz. The possible relation between the high dielectric permittivity and the size effect of R ions based on the spin-phonon interaction is suggested. [ABSTRACT FROM AUTHOR]

Mn2.5Co0.5O4 films are grown on MgO(001) and (011) substrate by molecular beam epitaxy. Detailed structural and magnetic analyses are carried out by x-ray diffraction and magnetization measurements. The reduction of the lattice parameter c/a* ratio and the enhancement of the Curie temperature (from 43 to 66 K), due to the weakening of the Jahn-Teller effect, are observed. In addition, thermomagnetic reversibility is observed at lower temperature and low external filed indicating the existence of local canting states. The randomness of Co cation and uniaxial anisotropy further enhance the local canting effect. An H-T phase diagram is mapped out, which indicates a short rang order state in the temperature 66-85 K separating the ferrimagentic and paramagnetic phases. [ABSTRACT FROM AUTHOR]

In the present study, iron/iron oxide (with a ratio of 4/1, 1/1, and 1/4) composite particles were prepared by precipitation. The morphology and phase composition were examined by scanning electron microscopy and X-ray diffraction techniques. The as-prepared composite particles were mixed with epoxy resin to prepare microwave absorbers and the corresponding electromagnetic properties were evaluated. The experimental results showed that the composite powder exhibited a mixture of α-Fe (∼4 μm) and Fe3O4(∼30 nm) phases. The amount of iron oxide plays an important role in the electromagnetic properties of microwave absorbers. In general, complex permittivity (ϵ’ and ϵ”) and permeability (μ’ and μ”) increased with increasing amount of iron oxide additions. The minimum reflection loss can reach −16.7 dB at 9.0 GHz for 50 wt% of Fe/Fe3O4= 1/4 composite absorber with 3 mm in thickness. [ABSTRACT FROM PUBLISHER]

We propose an energy diagram for the isolated Mn^2+ free electron interacted with external field and crystal field in tetragonal spinel Mn3 - xCoxO4 films and provide a simple and effective way to analyze the distribution of cations via the measurement of electron spin resonance (ESR). Based on our ESR results, we suggest that Co ions prefer to enter the tetrahedral sites (A-sites) of spinel structure. However, when the amount of Co ions at A-site reach a critical value, the partial octahedral site (B-site) of Mn ions start to be replaced. With increasing Co content up to 1, the Co ions substitute both A- and B-sites simultaneously and the structure transforms into two phase at x=1.5. [ABSTRACT FROM AUTHOR]