Nd_（1-x）Sr_xMn_（1-y）Cu_yO_3/NiFe_2O_4复合体系磁电阻效应研究
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摘要
近些年来，人们发现在钙钛矿锰氧化物R_(1-x)A_xMnO_3(R为三价稀土金属元素La~(3+)，Nd~(3+)，Pr~(3+)等；A为二价碱土金属元素Ca~(2+)，Sr~(2+)，Ba~(2+)等)中具有庞磁电阻(colossal magnetoresistance)效应，由于它在磁记录、磁传感器方面具有广泛的应用前景，同时也向传统的磁记录材料提出了挑战，因而引起了物理学界的广泛关注。
另一方面，研究表明纳米微粒复合体系由于表面和界面效应，主要呈现粒间隧穿磁电阻(IMR)效应，其本身就具有低场灵敏磁电阻效应，具有很好的潜在应用价值。
本文中，我们通过在多晶Nd_(1-x)Sr_xMn_(1-y)Cu_yO_3粉体中复合软磁材料NiFe_2O_4来得到增强的IMR效应。样品制备分为两步：首先，采用溶胶—凝胶法制备Nd_(1-x)Sr_xMn_(1-y)Cu_yO_3粉体，采用化学共沉淀法制备NiFe_2O_4软磁材料；然后将二者按不同比例混合，经充分研磨后压成片体，1673K下烧结10小时即得到所需复合样品。
我们得到如下主要结论：
1．热重分析表明，Nd_(1-x)Sr_xMn_(1-y)Cu_yO_3前驱物粉体在963K形成单相多晶，即在此温度成相，所以制备此类样品的最低成相温度应该不低于963K；
2．热处理温度和热处理时间对样品的磁性质有较大影响。经扫描电镜分析，高温下烧结的样品粒径较大，而且热处理时间越长，粒径越大，热处理温度为1273K的Nd_(1-x)Sr_xMn_(1-y)Cu_yO_3样品，其磁化强度明显大于1073K下热处理的样品，这是由于前者晶粒较大，表面原子占总原子数比例较小，磁结构的有序性相对较强造成的；
摘要
3.随着NIFeZO;含量的增大，Ndo67Sr033MnO3/N iFeZO;系列复合样
品的电阻率迅速增大，当NIFeZO4的含量达到30%时，在3 10K一340K
温度范围内，样品电阻率在1354一6460一cm范围内变化，磁电阻在这一
温区出现一个平台，其值在一6%左右。值得注意的是，在这一温区中，
这种复合样品的磁电阻是负的，即给样品加磁场后，样品的电阻率反而
增大，与纯Nd067sro33MnO3样品的磁电阻性能恰好相反，其机制有待进
一步研究;
4.随着S:含量的减小，(Ndl一SrxMnO3)。:/困iFeZO4)。3系列复合样品
的电阻率显著变化，同时磁电阻性能逐渐变差;
5.随着Cu含量的增大，伽嘶67Sr033Mnl一uyO3)07/困iFeZO4)03系列
复合样品的电阻率显著变化，同时磁电阻性能得到明显改善，当Cu含量
增大到y=0.2时，样品磁电阻值在293K到3 1 7K较宽的温度范围内基本
保持在一13.6%，随温度变化很小，由于该温区恰好在室温范围且温区宽，
因而具有很好的潜在应用价值。
6.经不同温度热处理后的N断67Sr033Mn0.sCu0203粉体与NIFeZO;复
合制得的困衡67Sr033Mno.sCu0203)0袱困iFeZO4)03复合样品，随着粉体热处
理温度的升高，Nd067Sr033MnosCu02O3颗粒粒径逐渐增大，从而导致复
合体系电阻率减小，同时磁电阻性能逐渐变差。
7.对室温附近磁电阻效应有利的条件是:
与Nd的比例为
体热处理温度为
1/2，Cu与Mn的比例为1/4，
1073K。
NIFeZO4含量为30%，Sr
Ndo石7Sr0.33Mn0名Cuo.ZO3粉
In recent years , the colossal magnetoresistance(CMR) effect in rare-earth perovskite manganites R_(1-x)A_(x)MnO_(3) (where R is a trivalent rare-earth element La~(3+), Nd~(3+), Pr~(3+) etc., A is a divalent alkaline-earth element Ca~(2+), Sr~(2+), Ba~(2+) 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-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3).The Nd_(1-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3) and NiFe_(2)O_(4) composite samples are prepared by two steps. First, the Nd_(1-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3) and NiFe_(2)O_(4) powder are prepared by sol-gel process and chemical coprecipitation method respectively; Second, the Nd_(1-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3) 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-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3) and NiFe_(2)O_(4) composite samples.
There are several interesting results we have got:
1. The thermal analysis pattern of Nd_(1-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3) pre-reaction material indicates that the pure perovskite phase has formed at 963 K, that is to say, the temperature we prepare Nd_(1-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3) 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 NiFe_(2)O_(4) contents, the resistivity of composite samples Nd_(0.67)Sr_(0.33)MnO_(3)/niFe_(2)O_(4) is 1073K. increases dramatically. When the content of NiFe_(2)O_(4) 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-x)Sr_(x)MnO_(3))_(0.7)/(NiFe_(2)O_(4))_(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_(0.67)Sr_(0.33)Mn_(1-y)Cu_(y)O_(3))_(0.7)/(NiFe_(2)O_(4))o.3 changes rapidly and the MR gets better;
6. As for the composite samples composed of Nd_(0.67)Sr_(0.33)Mn_(0.8)Cu_(0.2)O_(3) 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__(2)O_(4) is 30%, the proportion of Sr to Nd is 1/2, and that of Cu to Mn is 1/4,the heat treatment temperature of
Nd_(0.67)Sr_(0.33)Mn_(0.8)Cu_(0.2)O_(3) is 1073K.
另一方面，研究表明纳米微粒复合体系由于表面和界面效应，主要呈现粒间隧穿磁电阻(IMR)效应，其本身就具有低场灵敏磁电阻效应，具有很好的潜在应用价值。
本文中，我们通过在多晶Nd_(1-x)Sr_xMn_(1-y)Cu_yO_3粉体中复合软磁材料NiFe_2O_4来得到增强的IMR效应。样品制备分为两步：首先，采用溶胶—凝胶法制备Nd_(1-x)Sr_xMn_(1-y)Cu_yO_3粉体，采用化学共沉淀法制备NiFe_2O_4软磁材料；然后将二者按不同比例混合，经充分研磨后压成片体，1673K下烧结10小时即得到所需复合样品。
我们得到如下主要结论：
1．热重分析表明，Nd_(1-x)Sr_xMn_(1-y)Cu_yO_3前驱物粉体在963K形成单相多晶，即在此温度成相，所以制备此类样品的最低成相温度应该不低于963K；
2．热处理温度和热处理时间对样品的磁性质有较大影响。经扫描电镜分析，高温下烧结的样品粒径较大，而且热处理时间越长，粒径越大，热处理温度为1273K的Nd_(1-x)Sr_xMn_(1-y)Cu_yO_3样品，其磁化强度明显大于1073K下热处理的样品，这是由于前者晶粒较大，表面原子占总原子数比例较小，磁结构的有序性相对较强造成的；
摘要
3.随着NIFeZO;含量的增大，Ndo67Sr033MnO3/N iFeZO;系列复合样
品的电阻率迅速增大，当NIFeZO4的含量达到30%时，在3 10K一340K
温度范围内，样品电阻率在1354一6460一cm范围内变化，磁电阻在这一
温区出现一个平台，其值在一6%左右。值得注意的是，在这一温区中，
这种复合样品的磁电阻是负的，即给样品加磁场后，样品的电阻率反而
增大，与纯Nd067sro33MnO3样品的磁电阻性能恰好相反，其机制有待进
一步研究;
4.随着S:含量的减小，(Ndl一SrxMnO3)。:/困iFeZO4)。3系列复合样品
的电阻率显著变化，同时磁电阻性能逐渐变差;
5.随着Cu含量的增大，伽嘶67Sr033Mnl一uyO3)07/困iFeZO4)03系列
复合样品的电阻率显著变化，同时磁电阻性能得到明显改善，当Cu含量
增大到y=0.2时，样品磁电阻值在293K到3 1 7K较宽的温度范围内基本
保持在一13.6%，随温度变化很小，由于该温区恰好在室温范围且温区宽，
因而具有很好的潜在应用价值。
6.经不同温度热处理后的N断67Sr033Mn0.sCu0203粉体与NIFeZO;复
合制得的困衡67Sr033Mno.sCu0203)0袱困iFeZO4)03复合样品，随着粉体热处
理温度的升高，Nd067Sr033MnosCu02O3颗粒粒径逐渐增大，从而导致复
合体系电阻率减小，同时磁电阻性能逐渐变差。
7.对室温附近磁电阻效应有利的条件是:
与Nd的比例为
体热处理温度为
1/2，Cu与Mn的比例为1/4，
1073K。
NIFeZO4含量为30%，Sr
Ndo石7Sr0.33Mn0名Cuo.ZO3粉
In recent years , the colossal magnetoresistance(CMR) effect in rare-earth perovskite manganites R_(1-x)A_(x)MnO_(3) (where R is a trivalent rare-earth element La~(3+), Nd~(3+), Pr~(3+) etc., A is a divalent alkaline-earth element Ca~(2+), Sr~(2+), Ba~(2+) 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-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3).The Nd_(1-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3) and NiFe_(2)O_(4) composite samples are prepared by two steps. First, the Nd_(1-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3) and NiFe_(2)O_(4) powder are prepared by sol-gel process and chemical coprecipitation method respectively; Second, the Nd_(1-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3) 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-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3) and NiFe_(2)O_(4) composite samples.
There are several interesting results we have got:
1. The thermal analysis pattern of Nd_(1-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3) pre-reaction material indicates that the pure perovskite phase has formed at 963 K, that is to say, the temperature we prepare Nd_(1-x)Sr_(x)Mn_(1-y)Cu_(y)O_(3) 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 NiFe_(2)O_(4) contents, the resistivity of composite samples Nd_(0.67)Sr_(0.33)MnO_(3)/niFe_(2)O_(4) is 1073K. increases dramatically. When the content of NiFe_(2)O_(4) 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-x)Sr_(x)MnO_(3))_(0.7)/(NiFe_(2)O_(4))_(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_(0.67)Sr_(0.33)Mn_(1-y)Cu_(y)O_(3))_(0.7)/(NiFe_(2)O_(4))o.3 changes rapidly and the MR gets better;
6. As for the composite samples composed of Nd_(0.67)Sr_(0.33)Mn_(0.8)Cu_(0.2)O_(3) 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__(2)O_(4) is 30%, the proportion of Sr to Nd is 1/2, and that of Cu to Mn is 1/4,the heat treatment temperature of
Nd_(0.67)Sr_(0.33)Mn_(0.8)Cu_(0.2)O_(3) is 1073K.
引文
[1] M.N.Baibich,A.Fert,etal.Phys.Rev.Lett.61(1988)
[2] R.M.Kusters etal. Phys.B 155(1989)
[3] 方光旦，巨磁电阻效应在信息存储等领域中的应用
[4] 卢正启，自旋阀巨磁电阻效应及其应用，物理，1998，27(6)：373
[5] R.Von Helmolt al,Gaint negative magnetoresistance in perovskite La_(2/3)Ba_(1/3)MnO ferromagnetic films Phys.Rev.Lett, 1993,71:2331
[6] 严纯华等，稀土锰酸盐纳米颗粒复合体系的软化学制备及磁电阻增强效应，物理，(2001)30卷，2期，100-105
[2] R.M.Kusters etal. Phys.B 155(1989)
[3] 方光旦，巨磁电阻效应在信息存储等领域中的应用
[4] 卢正启，自旋阀巨磁电阻效应及其应用，物理，1998，27(6)：373
[5] R.Von Helmolt al,Gaint negative magnetoresistance in perovskite La_(2/3)Ba_(1/3)MnO ferromagnetic films Phys.Rev.Lett, 1993,71:2331
[6] 严纯华等，稀土锰酸盐纳米颗粒复合体系的软化学制备及磁电阻增强效应，物理，(2001)30卷，2期，100-105