Q. Q. Cao

Magnetocaloric properties of Ag-substituted perovskite-type manganites

Abstract Magnetic entropy change larger than that of gadolinium has been observed near room temperature in polycrystalline of La 0.8 Ag 0.2 MnO 3 perovskite-type manganite. The large magnetic entropy change produced by the abrupt reduction of magnetization is associated with a first-order phase transition of the sample near the Curie temperature. This phenomenon indicates that perovskite manganites have some potential applications for magnetic refrigerants in an extended high-temperature range.

Excellent microwave absorption property of Graphene-coated Fe nanocomposites

Graphene has evoked extensive interests for its abundant physical properties and potential applications. It is reported that the interfacial electronic interaction between metal and graphene would give rise to charge transfer and change the electronic properties of graphene, leading to some novel electrical and magnetic properties in metal-graphene heterostructure. In addition, large specific surface area, low density and high chemical stability make graphene act as an ideal coating material. Taking full advantage of the aforementioned features of graphene, we synthesized graphene-coated Fe nanocomposites for the first time and investigated their microwave absorption properties. Due to the charge transfer at Fe-graphene interface in Fe/G, the nanocomposites show distinct dielectric properties, which result in excellent microwave absorption performance in a wide frequency range. This work provides a novel approach for exploring high-performance microwave absorption material as well as expands the application field of graphene-based materials.

Giant magnetoresistance of the La1−xAgxMnO3 polycrystalline inhomogeneous granular system

A series of bulk polycrystalline La1−xAgxMnO3 samples with x ranging nominally from 0 to 0.5 were prepared by conventional solid-state reaction processing in air. X-ray diffraction patterns show that the samples contain a single perovskite phase when x⩽0.25, and are composed of two phases (a magnetic perovskite phase and a nonmagnetic Ag-rich phase) for x>0.25. It is found that, in this series of polycrystalline samples, maximum magnetoresistance occurs for x=0.30, i.e., for a composite of the two phases whose magnetoresistance ratio is about 25.5% at room temperature. The enhancement of the magnetoresistance effect in such an inhomogeneous granular system can be attributed to the spin-dependent scattering of electrons at the interface of the two phases.

Large exchange bias field in the Ni–Mn–Sn Heusler alloys with high content of Mn

The exchange bias properties have been investigated in bulk Mn50Ni40−xSn10+x (x=0, 0.5, and 1) Heusler alloys with high content of Mn, in which the largest exchange bias field is up to 910 Oe for Mn50Ni40Sn10 alloy. In these alloys, the excess Mn atoms would occupy not only the Sn sites but also the Ni sites, and the moments of Mn on Sn or Ni sites are coupled antiferromagnetically to those on the regular Mn sites, respectively. The origin of this considerably large exchange bias field has been discussed.

Magnetostructural phase transition and magnetocaloric effect in off-stoichiometric Mn1.9−xNixGe alloys

Phase transitions and magnetic entropy changes are studied in Mn1.9−xNixGe (x=0.85, 0.855) alloys. In these off-stoichiometric alloys, the crystallographic transition temperature decreases remarkably due to the deficiency of transition-metal atoms, and, consequently, a magnetostructural transition from the antiferromagnetic TiNiSi-type structure to the ferromagnetic Ni2In-type structure is observed. Owing to the abrupt change in magnetization, large magnetic entropy changes are obtained. The effect of transition-metal vacancies on the phase transition temperature is discussed.

Electric control of magnetism at room temperature.

In the single-phase multiferroics, the coupling between electric polarization (P) and magnetization (M) would enable the magnetoelectric (ME) effect, namely M induced and modulated by E, and conversely P by H. Especially, the manipulation of magnetization by an electric field at room-temperature is of great importance in technological applications, such as new information storage technology, four-state logic device, magnetoelectric sensors, low-power magnetoelectric device and so on. Furthermore, it can reduce power consumption and realize device miniaturization, which is very useful for the practical applications. In an M-type hexaferrite SrCo2Ti2Fe8O19, large magnetization and electric polarization were observed simultaneously at room-temperature. Moreover, large effect of electric field-controlled magnetization was observed even without magnetic bias field. These results illuminate a promising potential to apply in magnetoelectric devices at room temperature and imply plentiful physics behind them.

Large polarization and enhanced magnetic properties in BiFeO3 ceramic prepared by high-pressure synthesis

The BiFeO3 ceramics were prepared by sol-gel method (BFO-1) and high-pressure synthesis (BFO-2). X-ray diffraction showed that these ceramics are almost of single phase. It is difficult to observe a ferroelectric loop of BFO-1 even at an electric field of 6kV∕cm. Compared to BFO-1, the high-pressure synthesized one has higher resistivity, higher density, and better crystallization. Under an applied electric field of 120kV∕cm, the values of remanent polarization and the coercive field are 46μC∕cm2 and 73kV∕cm, respectively. At room temperature, a magnetic hysteresis loop with enhanced magnetization can be observed in BFO-2.

The magnetostructural transformation and magnetocaloric effect in Co-doped MnNiGe1.05 alloys

A series of MnNi1−xCoxGe1.05 (x = 0, 0.03, 0.05, 0.07, 0.09, and 0.11) alloys were prepared by the arc-melting method. With increasing content of Co, a first-order magnetostructural transformation between the antiferromagnetic TiNiSi-type phase and the ferromagnetic Ni2In-type phase was observed. A magnetic and crystallographic phase diagram for MnNi1−xCoxGe1.05 alloys was proposed in this paper. Owing to the abrupt and large jump of magnetization around the magnetostructural transformation, MnNi1−xCoxGe1.05 (x = 0.07, 0.09, 0.11) alloys show large and positive magnetic entropy changes at relatively low field.

Study of uniaxial magnetism and enhanced magnetostriction in magnetic-annealed polycrystalline CoFe2O4

Magnetic and magnetostrictive properties of magnetic-annealed polycrystalline CoFe2O4 were investigated. The magnetic hysteresis loops showed obvious uniaxiality with an induced easy direction parallel to the annealing field. Magnetic force microscopy study revealed that the domains were fixed by magnetic annealing. The uniaxial behavior was also observed in the magnetostrictive measurement, which showed a significantly enhanced magnetostriction of − 273 PPM when the external field was applied perpendicular to the annealing field direction. A physical mechanism for the effect of magnetic annealing on polycrystalline CoFe2O4 is developed, in which the induced uniaxiality is ascribed to the realignment of easy axes in polycrystals. The uniaxial behavior of magnetism and enhanced magnetostriction could be well explained by this model.

The magnetocaloric effect and magnetic phase transitions in Dy(Co1−xAlx)2 compounds

Abstract Polycrystalline samples of Laves-phase compounds Dy(Co 1− x Al x ) 2 ( x =0.0, 0.02, 0.04, 0.06, 0.08, 0.1) have been prepared and their magnetic properties were investigated by means of magnetization and ac susceptibility measurements. Results on the magnetocaloric effect (MCE) of these samples have been obtained from the magnetization data, which suggest their potential as working substance of magnetic refrigerators at cryogenic temperature from 142 to 200 K. Both the ac susceptibility results and Arrott plots were used to characterize the magnetic phase transitions in these compounds. By discussing the influence of partial replacement of Co by Al on the magnetic phase transitions and magnetocaloric properties of them, we conclude that the size of the MCE and the order of the magnetic phase transition are closely related.