Xiao-Bo Yuan

Large room-temperature magnetoresistance in Ag–Ti-added La0.67Ba0.33MnO3

The effects of Ag addition in Ti-doped La0.67Ba0.33MnO3 (LBT) (abbreviated as LBT/Agx, where x is the nominal molar ratio of Ag) on the electric and magnetoresistance (MR) properties have been studied systematically. The results show that the resistivity of the samples decreases dramatically with increasing Ag addition and reaches its minimum for x = 0.27. The room-temperature MR ratio of LBT/Agx increases with increasing x and reaches its maximum at x = 0.27. The MR ratio at 280 K for the x = 0.27 sample at H = 10 kOe is as large as 41%, which is about 3.4 times larger than that for LBT. The Ag addition makes the LBT crystallites more perfect and uniform, and suppresses the magnetic scattering at the grain boundaries. The Ag conduction channels penetrating into the insulating regions can also lower the resistivity. The near room-temperature TC and low resistivity of the Ag-added LBT are responsible for the large intrinsic room-temperature MR.

Large room-temperature magnetoresistance in Pd-added manganites

A series of La0.67(Ca0.65Ba0.35)0.33MnO3∕Pdx composites were synthesized using a sol-gel method followed by a conventional solid-state reaction route. The results show that the resistivity of the composites decreases dramatically with Pd addition, while the room-temperature magnetoresistance (MR) increases remarkably. A large MR of about 170% is obtained at room temperature and 1T applied magnetic field for x=0.27 sample. The large enhancement of the MR can be attributed to the decrease in resistivity caused by the good conductive metal Pd, which improves the disordered atomic structure and magnetic property on the grain surfaces/boundaries. In addition, the polarization of Pd atoms near the Mn ions on the grain surfaces/boundaries also plays a very important role on the enhancement of the MR, which induces a large number of spin clusters.

Electronic transport and extra large magnetoresistance in LCBMO/Pdx composites

Two-phase composites LCBMO/Pdx were synthesized using the sol?gel technique followed by the solid-state reaction. Pd addition induces a remarkable decrease in resistivity, which is mainly related to the improvement of grain boundaries/surfaces caused by the segregation of Pd. The resistivity data for all samples follow the adiabatic small-polaron-hopping model at high temperature above the Curie temperature TC, while in the low temperature region, they are proportional to T2, reflecting that the electron?electron scattering mechanism is dominant in the ferromagnetic metallic state. In addition, Pd addition induces a large enhancement of room temperature magnetoresistance (MR). Especially for the x = 0.27 sample, an extra large magnetoresistance over 170% is obtained at 10?kOe and 289?K. This is the largest MR obtained at room temperature in all kinds of colossal magnetoresistance (CMR) materials. The good conductivity and polarizing effect of Pd are responsible for the enhancement of MR. The former leads to the decrease in resistivity and the latter induces a large number of spin clusters.

Electrical properties and room temperature magnetoresistance of La0.67(Ca0.65Ba0.35)0.33MnO3/Agx composites

The electrical properties of the La0.67(Ca0.65Ba0.35)0.33MnO3/Agx composite system are systematically investigated as a function of Ag-added content. Ag addition induces a decrease in resistivity (ρ) and an increase in temperature TP where the ρ peak is located. This is due to the improvement of grain boundaries caused by the segregation of Ag on the grain surfaces. The T2 dependence of the resistivity data in the low temperature region of T < TP reflects that the conductive mechanism in the ferromagnetic (FM) metallic state mainly arises from electron–electron scattering. At high temperatures, above the Curie temperature TC, the resistivity data for all samples follow the adiabatic spin polaron hopping model. Moreover, the ρ –T curves for x = 0.27 and 0.30 samples fit well to the phenomenological percolation model based on phase segregation. This result confirms the coexistence of FM clusters and paramagnetic regions near TC, and indicates the prominent intrinsic behaviour of the La0.67(Ca0.65Ba0.35)0.33MnO3 matrix. Note that a 27% molar ratio of added Ag induces a large room temperature magnetoresistance (MR) ratio of up to 41%. This value is attractive for the study of applications. The good agreement of experimental data with the Brillouin function indicates that the MR behaviour in this composite system accounts for the spin-dependent hopping of the electrons among the spin clusters.

LARGE ROOM TEMPERATURE MAGNETORESISTANCE IN (La0.7Sm0.3)0.7Sr0.3MnO3/Agx COMPOSITES

The effects of Ag addition in the composite (La0.7Sm0.3)0.7Sr0.3MnO3/Agx (abbreviated as LSSMO/Agx) has been studied. The results showed that Ag addition induces the decrease in resistivity (ρ) due to the improvement of grain boundaries resulted from the segregation of Ag on the grain surfaces. In addition, 27% molar ratio of Ag addition induces a large room temperature magnetoresistance (MR) ratio of 35%. The good agreement of experimental data with Brillouin function indicates that the MR behavior in this composite system accounts for the spin-dependent hopping of the electrons between the spin clusters.