Of Nd <sub>, 1-x </ Sub> Sr <sub> The X </ Sub> Mn <sub> 1-y </ Sub> Cu <sub> Y-the O <sub> 3 </ Sub> </ Sub> / Nife <sub> 2 </ Sub> O <sub> 4 </ Sub> Composite System Mag

In recent years , the colossal magnetoresistance(CMR) effect in rare-earth perovskite manganites R_1-xA_xMnO₃ (where R is a trivalent rare-earth element La³⁺, Nd³⁺, Pr³⁺ etc., A is a divalent alkaline-earth element Ca²⁺, Sr²⁺, Ba²⁺ etc.) has attracted considerable attention due to its potential application.On the other hand, the discovery that grain boundaries and surfaces can be important sources of IMR effect has stimulated renewed interest in polycrystalline samples.In this paper, we report an enhanced IMR effect in a structure by introducing soft magnetic material NiFe2O4 into polycrystalline Nd_1-xSr_xMn_1-yCu_yO₃.The Nd_1-xSr_xMn_1-yCu_yO₃ and NiFe₂O₄ composite samples are prepared by two steps. First, the Nd_1-xSr_xMn_1-yCu_yO₃ and NiFe₂O₄ powder are prepared by sol-gel process and chemical coprecipitation method respectively; Second, the Nd_1-xSr_xMn_1-yCu_yO₃ and NiFe2O4 powders are mixed. After fully mixed, the compounds are pressed into pellets and annealed at 1673K for 10 hours to obtain the Nd_1-xSr_xMn_1-yCu_yO₃ and NiFe₂O₄ composite samples.There are several interesting results we have got:1. The thermal analysis pattern of Nd_1-xSr_xMn_1-yCu_yO₃ pre-reaction material indicates that the pure perovskite phase has formed at 963 K, that is to say, the temperature we prepare Nd_1-xSr_xMn_1-yCu_yO₃ samples must higher than 963K;2. The influence of heat treatment temperature on the magnetization of our samples is remarkable. From the scanning electron microscope (SEM) images, the higher the heat treatment temperature is, the larger the grain size is. The magnetization of samples treated at 1273K is larger than that of samples treated at 1073K;3. With the increasing of NiFe₂O₄ contents, the resistivity of composite samples Nd_0.67Sr_0.33MnO₃/niFe₂O₄ is 1073K. increases dramatically. When the content of NiFe₂O₄ reaches to 30%, the maximum MR value gets to -6% at the temperature from 310K to 340K, and the resistivity ranges from 1354 to 646 cm, where applied magnetic field is 1.8T. It is interesting that the MR value is negative which is different from that of the usual material;4. With the decreasing of Sr contents, the resistivity of composite samples (Nd_1-xSr_xMnO₃)_0.7/(NiFe₂O₄)_0.3 changes apparently, at the same time, the MR gets worse;5. With the increasing of Cu contents, flie resistivity of composite samples Nd_0.67Sr_0.33Mn_1-yCu_yO₃)_0.7/(NiFe₂O₄)o.3 changes rapidly and the MR gets better;6. As for the composite samples composed of Nd_0.67Sr_0.33Mn_0.8Cu_0.2O₃ of different sizes, with the increasing of heat treatment temperature, the grain size becomes larger, so the resistivity decreases and the capability of MR gets worse;7. The advantage to enhance the MR in room temperature is that: the content of NiFe_<sub>2O₄ is 30%, the proportion of Sr to Nd is 1/2, and that of Cu to Mn is 1/4,the heat treatment temperature ofNd_0.67Sr_0.33Mn_0.8Cu_0.2O₃ is 1073K.