J.H. Miao

Giant magnetoresistance and unusual hysteresis behavior in La0.67Ca0.33MnO3∕xCuO (x=20%) composite

La0.67Ca0.33MnO3(LCMO)∕xCuO (x=0 and 20mole%) composites were fabricated and investigated in detail for their electrical and magnetic transport properties. Compared with pure LCMO, the magnetoresistance (MR) effect for x=20% sample is substantially enhanced especially at the vicinity of TIM, where TIM is the transition temperature from insulator to metallic state. Application of even a low magnetic field of H=0.5T leads to a MR value as large as ∼98%. It is interesting to note that at the same temperature region where substantial MR effect is obtained, considerable thermal and magnetic hystereses in resistivity (ρ) appear. The measurement of magnetization (M) versus magnetic field (H) also exhibits an unusual magnetic hysteresis. Due to the absence of these unusual hysteresis behaviors in pure LCMO, it is discussed that the abnormal experimental observations are attributed to the spin disorder especially at grain boundaries induced by the interaction product layer between LCMO and CuO.

Enhancement of room temperature magnetoresistance in (1 − x)La0.67Sr0.33 MnO3/x Sb2O5 composites

The electrical transport and magnetoresistance (MR) properties of the composites (1 − x)La0.67Sr0.33MnO3 (LSMO)/x Sb2O5 with different Sb2O5 molar percentages x were investigated. Experimental results show that the Sb2 O5 addition plays a key role in the electrical transport behaviour of the composites. Different transport and MR behaviours are observed for x ≤ 2% and x≥2%. Compared with pure LSMO, an enhanced MR effect for the composites is found over a wide temperature range under applied magnetic fields of both 0.3 and 3 T. Specially, a large and constant MR value of ~18% is observed at room temperature, which is attractive for the study of applications. We argue that such an enhancement in MR is attributed to the magnetic disorder at the LSMO grain surfaces or boundaries caused by the Sb2O5 addition. Moreover, the resistivity (ρ) versus T curves for x = 1% and 2% samples fit well to the phenomenological model derived from spin-polarized tunnelling at grain boundaries and thermal activation. This result confirms that the grain boundaries have been modified in the composites with low Sb2O5 addition and provides information about the microscopic transport mechanism in the system.

Enhancement of magnetoresistance in La2/3Ca1/3MnO3/xCuMn2O4 nanocomposites

Nanocomposites of La2/3Ca1/3MnO3(LCMO)/xCuMn2O4 (0 ≤ x ≤ 40%) have been prepared by a citrate gel route and characterized for microstructural and magnetotransport properties. Results show that fabrication with CuMn2O4 has an important effect on the electrical transport behaviour of the composites. With the increment of CuMn2O4 content x, the metal–insulator transition temperature TMI for the composites shifts downwards and the resistivity increases. The susceptibility analysis indicates that the composites with x = 4, 20, 30 and 40% have a similar paramagnetic–ferromagnetic transition temperature TC ~ 240 K, which is lower than TC of pure LCMO (~262 K). The high temperature (T > TMI) semi-conducting part of the resistivity (ρ) data follows a small polaron hopping conduction mechanism, and the metallic behaviour of the samples (T < TMI) fits the model in terms of electron–magnon scattering of the carriers. Furthermore, a significant enhancement both in low-field magnetoresistance (LFMR) and in high-field magnetoresistance (MR) is observed for the composites at a wide temperature range below TMI. The LFMR measured at 0.3 T reaches the maximum for the x = 40% sample when T = 10 K with the value of ~23%, which is much larger than that of the pure LCMO (~6.9%). We argue that such an enhancement in MR is attributed to the enhanced spin-polarized tunnelling, which is manipulated by the spin disorder at the LCMO surfaces caused by CuMn2O4 addition.

Electrical and Magnetic Properties of Bilayer Manganites La1.4Sr1.6Mn1.96TE0.04O7 (TE = Mn, Fe, Ti, Nb)

Abstract The electrical and magnetic properties of bilayer manganites La 1.4 Sr 1.6 Mn 1.96 TE 0.04 O 7 (TE = Mn, Fe, Ti, Nb) were investigated. Doping caused obvious changes in electrical and magnetic behaviors such as decrease of insulator-metal transition and magnetic transition temperatures, increase of peak resistivity, and different magnetoresistance effect. These changes had a significant degree of correlation with the valence of doped ions. From Fe, Ti to Nb doping, the effect was doubly stronger. The results could be well understood by considering the different destructions on double-exchange interaction and different influences on lattice distortion caused by Fe, Ti and Nb doping. The temperature dependence of magnetization measured at high field showed that the influence of doping was greatest near three-dimensional magnetic transition temperature of parent phase.

Effect of sintering temperature on electrical transport in La0.67 Ca0.33 MnO3/BaTiO3composites

It is shown that the electrical transport in ferromagnet–ferroelectric–type La0.67Ca0.33MnO3/BaTiO3 composites fabricated by the sol–gel method strongly depends on sintering temperature (Ts). For the Ts > 1000 °C samples, the insulator–metal transition temperature (TIM) decreases with increasing BaTiO3 content and the magnetoresistance (MR) is enhanced at low temperature (T < TIM). For the Ts = 1000 °C samples, the TIM value initially increases with an increase of BaTiO3 content and thereafter it decreases as BaTiO3 content further increases. For the Ts = 900 °C samples, the TIM value increases throughout the increase in BaTiO3 content and MR is enhanced over the whole temperature range studied. The experimental results have been discussed in terms of the magnetoelectric coupling effect and the interfacial diffusion reaction, especially at grain boundaries.