L. G. Wang

Negative magnetization and zero-field cooled exchange bias effect in Co0.8Cu0.2Cr2O4 ceramics

The negative magnetization and zero-field cooled exchange bias (ZFC EB) effect are observed in Co0.8Cu0.2Cr2O4 polycrystalline ceramics. 20% Cu substitution for Co in CoCr2O4 leads to the evident magnetization reversal at the compensation temperature (Tcomp ∼ 50 K) with applied magnetic field of 500 Oe. Besides, Tcomp decreases monotonously with increasing applied field, and the negative magnetization finally disappears when the field increases to 9000 Oe. Different temperature dependence of sublattice magnetization at different crystallographic sites is proved to induce the magnetization reversal. In addition, ZFC EB effect can be tuned by measuring temperature and presents the maximum of exchange bias field (HEB) with ∼2300 Oe at 50 K. This unconventional EB effect can be attributed to the coupling interaction between the two sublattices.

Synthesis and excellent microwave absorption properties of reduced graphene oxide/FeNi3/Fe3O4 composite

A reduced graphene oxide/FeNi3/Fe3O4 composite was successfully synthesized by a one-pot solvothermal reduction method. The microstructure and morphology were confirmed through X-ray diffraction and scanning electron microscopy. The loss tangent parameters showed that the electromagnetic attenuation mechanism of RGO/FeNi3/Fe3O4 is mainly dependent on dielectric loss. The microwave absorption properties showed a maximum absorption of −46.6 dB at 12.5 GHz with a thickness of 1.9 mm. The absorption bandwidth with reflection loss below −20 dB is increased by 9.26 GHz (from 6.1 to 15.36 GHz) with a thickness in the range of 1.6–3.5 mm. The excellent microwave absorption properties of RGO/FeNi3/Fe3O4 make it a candidate for a microwave absorptive material.

Unusual magnetic transition near metal-insulator transition and paramagnetic anomaly in VO2

Experiments for vanadium dioxide show a magnetic transition characterized by a sharp variation in susceptibility associated with the metal-insulator transition at the temperature Tc. The sample is shown to be of Pauli paramagnetism above Tc but of anomalous paramagnetism below Tc. Considering co-contributions of Pauli paramagnetism of residual V4+ ions and Curie-like paramagnetism of V-V dimers, we propose a phenomenological expression for the temperature-dependent susceptibility below Tc, which yields an excellent agreement with the experimental data. The unusual temperature dependence of Curie-like susceptibility below Tc is explained to be due to the fact that every dimer is formed by spin paring at an angle θ close to 180° and hence has an effective spin expressed as S e f f = cos ( θ / 2 ) which decreases with lowering temperature. The observed metal-insulator and magnetic transitions are argued to be due to a transition from the high-temperature Pauli paramagnetic state of V4+ ions to the low-temper...

Room-temperature magnetoelectric coupling study of multiferroic (1-x)(0.7BiFeO3-0.3Bi0.5Na0.5TiO3)-xCoFe2O4 ceramics

A series of (1−x)(0.7BiFeO3-0.3Bi0.5Na0.5TiO3)-xCoFe2O4 ceramics have been synthesized using the sol–gel method. Structural, microstructural, and multiferroic properties of the samples have been investigated. Magnetoelectric coupling has been also measured at room temperature. The structural and microstructural results show that perovskite 0.7BiFeO3-0.3Bi0.5Na0.5TiO3 and spinel CoFe2O4 can coexist in the ceramics without any observable impurities. The ceramics exhibit distinct ferromagnetic characteristics at room temperature. The dielectric constant and dielectric loss have been also studied as the function of frequency and temperature. The temperature dependence of dielectric properties displays a typical relaxor-like behavior at the temperature range of 150–300 °C. Due to magnetoelectric coupling, the dielectric anomaly at about 340 °C and the peak near 510 °C are respectively related to the ferroelectric transition of Bi0.5Na0.5TiO3 and magnetic transition of CoFe2O4. Impedance properties show the segment of arc at room temperature and the nearly perfect semicircle at higher temperature. Both Z′ and Z″ present regular change with the content of CoFe2O4. The room-temperature multiferroic properties of these new ceramics may be suitable for storage device applications.Graphical AbstractVariation of magnetoelectric coupling coefficient αE with the applied magnetic field.