| In recent years, the colossal magneto-resistance effect in rare-earth manganite perovskites R1-xAxMnO3(where R is rear-earth irons La, Nd, Pr; A is alkaline earth iron Ca, Sr, Ba) has attracted considerable attention in scientific studies due to its potential application in fields of magnetic record and magnetic sensor and so on.However, the poor temperature dependence of MR, the low-temperature MR effect, and the ideal MR only in high field, severely limit their practical utilities. Many researches have thus been concentrated on making new material system that provides low field magnetoresistance (LFMR) around room temperature . On the other hand, recent efforts to broaden the CMR temperature range have been made by means of the Mn-site substitutionIn this paper, La0.67Sr0.33-x-yAx yMnO3(A=Cu, Ag) samples were prepared by sol-gel method and their structure and electromagnetic properties were researched in detail.There are several interesting results we have got:1. The phase identification of samples has been carried out by the X-ray diffraction (XRD) with a Rigaku max-RB diffractometer with Cu Ka radiation. The results indicate that the materials are a single phase with perovskite structure without any other secondary phase.2. The morphology was obtained by S-570 scanning electron microscopy (SEM). The results show that the particles in powder samples is closed to globular shape and the particle size is different with doped by different element, Cu-doped is about 150nm, while Ag-doped is about 100nm. On the other hand, we also measured the crystal grain size-in block sample by XRD and known that grain size is 145.0nm and 109.6nm, respectively. The results by XRD and SEM are similar, so we can conclusion that the particles in powder samples are grain of single-crystalline, and the crystal grains size have no difference.3. The Curie's temperature Tc and saturation magnetization decreased obviously by Sr substituted with Cu or Ag and Sr2+ vacancy in the samples, while the initial susceptibility is increased.4. For Cu-doped samples, under an applied filed of 1.8T, we obtained that the maximum MR is 28.5% at 328K for La0.67Sr0.23Cu0.1MnO3 sample, and 23% at 300K for La0.67Sr0.15Cu0.15d0.03MnO3 sample.5. For Ag-doped samples, under an applied filed of 1.8T, we obtained that the peak of MR is 40% at 333K for La0.67Sr0.13Ag0.15 0.05MnO3 sample and 26% at 300K forLa0.67Sr0.13Ag0.2MnO3 sample.6. It is interesting to see that there is a small MR change for La0.67Sr0.13Ag0.20MnO3 andLa0.67Sr0.13Ag0.20MnO3. The former MR changes from 23.6% to 25.7% and to 23.6% corresponding to temperature changes form 290K to 302K and to 315K over a wide temperature interval between 290-315K. Although the latter MR is only about 9%, there is little change of MR in the range between 275-31 OK. Which is important for application of this material.So we improved notably the magnetoresistance and the initial susceptibility of samples by Sr substitution with Cu or Ag, and Sr2+ vacancy, especially the MR at room temperature was raised. On the other hand, field susceptivity and temperature stability of magentoresistance at room temperature were enhanced. In our opinion, the enhancing of initial susceptibility may be probably one of factors induced increase of magnetoresistance. Up to day, temperature stability of magenetoresistance has not been reported about room temperature. |
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