Ling-Fang Xu

Voltage dependent capacitances in CaCu3Ti4O12

The capacitance–dc voltage relation has been investigated in the giant dielectric constant material CaCu3Ti4O12. A capacitance drop, rise and even maxima, minima have been found with increasing dc voltage, which could not simply be explained by the back-to-back Schottky barrier theory. A trap charges repositioning between field perpendicular boundaries and field parallel boundaries has been put forward to explain the strange C-V relations. In addition, a first order calculation of C-V has been done based on the charge repositioning model and has led to a successful fitting of the experimental curves, extracting parameters such as a polarisability of about 1.5×10−39 for the induced dipoles.

Effect of thermal cycle on the interfacial antiferromagnetic spin configuration and exchange bias in Ni-Mn-Sb alloy

Effect of thermal cycle on the interfacial antiferromagnetic (AFM) spin configuration and exchange bias in Ni50Mn36Sb14 alloy has been investigated. The results indicate thermal cycle can induce further martensitic transition from part of arrested FM phase to AFM phase, leading to the reconstruction of interfacial antiferromagnetic spin configuration. The shape of hysteresis loops at 5 K after cooling back can be tuned from a single-shifted loop to a nearly symmetric double-shifted loop gradually accompanied with exchange bias field increasing to peak value and then decreasing. The evolutions can be illustrated intuitively by a simple AFM bidomain model.

Positive to negative zero-field cooled exchange bias in La0.5Sr0.5Mn0.8Co0.2O3 ceramics.

Exchange bias effect obtained after zero-field cooling from unmagnetized state usually exhibits a shift of hysteresis loop negative to the direction of the initial magnetic field, known as negative zero-field cooled exchange bias. Here, positive zero-field cooled exchange bias is reported in La0.5Sr0.5Mn0.8Co0.2O3 ceramics. In addition, a transition from positive to negative exchange bias has been observed with increasing initial magnetization field and measurement temperature. Based on a simple spin bidomain model with variable interface, two type of interfacial spin configuration formed during the initial magnetization process are proposed to interpret the observed phenomenon.

Premartensitic transition and relevant magnetic effects in Ni50Mn34In15.5Al0.5 alloy

Resistance measurement, in situ optical microscopic observation, thermal and magnetic measurements have been carried out on Ni50Mn34In15.5Al0.5 alloy. The existence of a pronounced premartensitic transition prior to martensitic transition can be characterized by microstructure evolution as well as exothermic peak and smooth decrease of resistance and magnetization with obvious hysteresis over a wide temperature range upon cooling. Consequently, the alloy undergoes two successive magneto-structural transitions consisting of premartensitic and martensitic transitions. Magnetoelastic coupling between magnetic and structural degrees of freedom would be responsible for the appearance of premartensitic transition, as evinced by the distinct shift of transitions temperatures to lower temperature with external applied field of 50 kOe. The inverse premartensitic transition induced by magnetic field results in large magnetoresistance, and contributes to the enhanced inverse magnetocaloric effect through enlarging the peak value and temperature interval of magnetic entropy change ΔSm.

Effect of post-annealing on the phase transitions and magnetocaloric properties in bulk Ni44Mn45Sn11 alloy

The effect of low temperature post-annealing on the phase transitions and magnetocaloric properties in bulk Ni44Mn45Sn11 alloy has been investigated. The results indicate that with the increase of annealing temperature, the martensite start temperature decreases rapidly, while the Curie temperature of austenitic phase shows a small decrease. The variations of phase transition temperatures can be attributed to the decrease of atomic order degree characterized by the increases of cell volume. Although the peak value of magnetic entropy change decreases rapidly, the refrigerant capacity shows a slight increase at the annealing temperature ≤300 °C due to the wider peaks.

Effect of Ge and Al substitutions on exchange bias in Ni-Mn-Sb alloy

Magnetic properties, martensitic transition, and exchange bias in Ni50Mn36Sb14, Ni50Mn36Sb12Ge2, and Ni50Mn36Sb12Al2 alloys have been investigated. The results reveal that the martensitic transition temperatures and blocking temperature shift to low temperature for Ge substitution and shift to high temperature for Al substitution. The exchange bias field decreases/increases in case of Ge/Al substitution below blocking temperature. All the results can be ascribed to the increase/decrease of ferromagnetic interaction.

Ball Milling Synthesis and Property of Nd0.7Sr0.3MnO3 Manganites

Strontium doped perovskite-type Nd0.7Sr0.3MnO3 ceramics were synthesized completely by high-energy ball milling raw oxides of Nd2O3, SrCO3 and MnO2. The optimal ball milling time and mass ratio of milling balls to raw materials are 4 h and 10:1, respectively. The grain size of as-milled Nd0.7Sr0.3MnO3 ceramics ranges from 51 to 93 nm, and the fine particles contain two phases of crystalline phase and amorphous phase. For the Nd0.7Sr0.3MnO3 synthesized by ball milling and sequent heat treatment, a remarkable colossal electroresistance (CER) effect is observed and the CER ratio reaches 900% at Curie temperature TC when the load voltage increases from 0.1 to 0.8 V.