双钙钛矿氧化物的磁电阻效应及电磁性质
摘要
在标准双钙钛矿型氧化物Sr_2FeMoO_6中发现的室温低场下达10％的隧穿型磁
电阻现象（TMR），由于其在磁记录、磁传感器等方面具有的潜在应用前景，以及类
似结构的钙钛矿材料中相继发现的：磁记忆和热滞后以及电荷有序化转变等丰富的
物理现象，都使得一大类A_2B’B”O_6氧化物在磁电阻效应实用及理论研究的众多候
选对象中脱颖而出，吸引了物理学界相当大的关注。基于此，本论文着重于双钙钛
矿型氧化物的磁电阻效应，首先通过双钙钛矿型氧化物与钙钛矿型ABO_3氧化物材
料的类比，初步回顾了已有A_2B’B”O_6材料的研究成果，归纳了该系列材料共同遵循
的一些特征，然后以三种A_2B’B”O_6型氧化物为研究对象，首次制备出了具有大的室
温磁电阻效应的Ca_2FeMnO_(6-δ)以及Sr_2CrFeO_(6-δ)多晶样品，研究了其磁电阻效应及
电磁性能。主要工作包括：
1.双钙钛矿型氧化物La_2NiMnO_6多晶样品的电磁及输运性质
我们采用溶胶—凝胶法制备了优于传统固相烧结法的La_2NiMnO_6多晶样品。
其电阻显示出类似于半导体的温度依赖关系，在80K以下出现磁电阻效应。样品呈
亚铁磁性，Tc约为350K，随制备条件不同Tc有一定起伏。样品磁化强度与磁场
及热处理过程的关系观测到类似于自旋玻璃的时间依赖特征。
2.新型双钙钛矿氧化物Ca_2FeMnO_(6-δ)的磁电阻及独特磁行为
Ca_2FeMnO_(6-δ)样品利用溶胶—凝胶法制备，在不同温区烧结而成。XRD分析
表明均为单相，样品1100属正交结构（a_0=5.6505±0.0006A,b_0=7.6338±0.0008A,c_0=
5.3402±0.0005A）。在液氮致冷温区处于顺磁态，并由M-T曲线和穆斯堡尔谱测量结
果验证。室温下具有33％的磁电阻MR（定义为ρ=（ρ（H）-ρ（O）/ρ（H），μ_0H=1.25T）
效应。样品的MR随制备条件不同而有较大差异，其性能对制备条件及过程十分敏
感。样品的电阻随磁场的变化曲线显示了磁滞现象，这一铁磁体行为特征与实验中
观测到的顺磁性看似矛盾。本文给出由于电流诱导而在顺磁基底中出现纳米铁磁团
簇的物理图象，显示了电流诱导的磁电阻现象（在Cu/Co多层膜，颗粒合金，Ni纳
米线和Mn系隧道结中均有观测）具有一定的普遍性。样品的非线性I-V曲线也验证
了这一观点。经过液氦致冷温区处理，实验观测到样品1100发生顺磁到铁磁的不可
逆相变。Hc分别为3250Oe（80K）和4500Oe（300K），与相变前磁电阻测量中的ρ
峰值相对应。同时样品在M-T曲线中显示出磁性波动现象，与自旋玻璃相类似。穆
斯堡尔谱测量结果显示的B位多价态共存的特征，给了样品中多种亚稳态（B位不
南京大学硕士论文2001 二
一
同价态的组合）存在的可能，这种多亚稳态的存在正是自旋玻璃的本质特征．同时
实验观测到的不可逆相变显示出温度与磁场都对各个亚稳态间的相互转化起着相当
大的作用．
3．新型双钙钛矿氧化物Sr。CrFeO。－e的磁电阻及输运性质
不同温度烧结的Sr。CrFeO。＊样品制备方法同上．XRD分析表明样品为复相，
含有SrCrO。，SrFeO卜。晶相．该系列样品随烧结温度的升高，晶粒逐渐长大，相应
电阻率降低．尽管样品含有少量杂相，在300K温度和1．25T磁场下，具有磁滞的最
高磁电阻MR值可达19．4％．而静态磁测量显示出顺磁特征，这种矛盾和样品中由外
加电流而诱导出铁磁相的作用存在着密切关系．随着测量电流的升高MR显示出的
磁滞特征逐步显著，验证了外加电流诱导铁磁相的作用．低温对铁磁相的出现也起
着相当大的作用，从经低温处理后样品磁致回线的变化中可以看出．电阻测量中出
现的跳跃现象，除去接触电势的影响外，表明了样品内部与输运性质密切相关的B
位电子分布、自旋组态以及晶粒内部结构和晶界状况等具有的复杂性．这些因素的
联合作用形成了许多局部的能量极小值，部分导致了电阻的涨落．样品在80K－30（）
温区，P随着温度的降低而升高，具有典型的半导体导电特征，载流子的跃迁随温
度的下降而显著降低．热重测量的结果显示在300K－800K样品没有相变出现．
The discovery of intergrain magnetoresistance in Sr2FeMoO6 as high as 10% at room
temperature (300K), due to it’s potential teclrnological applications in magnetic recording,
sensors as well as it’s theoretical values, made a wide serial A2B払?6 double perovskite
materials attracted considerable interests from the physical fields. Concentrating on the
magnetoresistance (MR) phenomenon of double perovskite (DP), this thesis compared dou-
ble perovskite materials with the explicitly studied ABO3 materials at first. Some achieved
experimental and theoretical results are carefully reviewed and the common points underly-
ing them are also discussed. Then La2NiMnO6 was chosen to be studied about it’s electric,
magnetic and MR properties. hi the first time, new double perovskite Ca2FeMnO66 and
Sr2CrFeO66 are synthesized successfully. They both show the room temperature MR as
high as 33% and 19.4%. What distinguished them with the former CMR materials is their
PM character at the temperature when the MR shows. Main results of this thesis are shown
as following:
1. The electronic and magnetic characters of double perovskite La2NiMnO6
Using the Sol-gel method, we prepared fine La2NiMnO6 samples superior than the
traditional solid-state method making sample. The resistance of the sample dramatically
increases when the temperature decrease which is similar to the semiconductors. Below
80K the MR effect is observed. The sample behaves as ferrimagnet under Tc~350K.The
Curie temperature shows a little fluctuation with different samples made in various sintering
processes. Their magnetization show time dependent characters somewhat similar to the
spin glasses.
2. The MR effect of new DP Ca2FeMnO66 and its novel magnetic behavior
Ca2FeMnO6 samples are made by the Sol-gel methods following with different
temperature sintering. XRD analysis indicate a single phase of orthorhombic structure,
where a0 5.6505 + 0.0006A,b0 7.6338 + 0.0008A,c0 5.3402 ?0.000SAfor the 1373K
sintered sample. In the temperature range cooling by liquid Nitrogen, it is at the param-
agnetic state, which is confirmed by the M-T curve and the Msbauer spectra. At 300K,
the samples have the MR effects as high as 33% ,12% and 9.5%,where certain hysteresis
as FM appear. Seemingly it is conflicted with the PM character observed under statistic
magnetic measurements. The physical picture of nanoferromagnetic clusters induced by
the electric current in PM matrix is given to interpret this confliction. It is supported by
the non-linear I-V curve. After disposition under 70K, irreversible phase transition from
PM to FM is observed in the 1373K sintered sample. The FM phase has coercive force
32500e and 45000e at 80K and 300K respectively, this coercive force is corespondent to
the maxim of resistivity in the MR data obtained before the irreversible phase transition.
Some fluctuations of magnetization are also observed in the M-T curve, they are similar to
the spin glasses. Msbauer measurement shows the coexistence of different valence state of
B site ions, which give the possibility of various valence B and B?ions combination states.
Tins leads to many metastable states in Ca2FeMnO66 sample, it is the existence of many
metastable states that make a spin glass. Experiments also show the temperature and the
magnetic field have strong influences on the transition between these different metastable
states.
3 The magneteresistan
电阻现象（TMR），由于其在磁记录、磁传感器等方面具有的潜在应用前景，以及类
似结构的钙钛矿材料中相继发现的：磁记忆和热滞后以及电荷有序化转变等丰富的
物理现象，都使得一大类A_2B’B”O_6氧化物在磁电阻效应实用及理论研究的众多候
选对象中脱颖而出，吸引了物理学界相当大的关注。基于此，本论文着重于双钙钛
矿型氧化物的磁电阻效应，首先通过双钙钛矿型氧化物与钙钛矿型ABO_3氧化物材
料的类比，初步回顾了已有A_2B’B”O_6材料的研究成果，归纳了该系列材料共同遵循
的一些特征，然后以三种A_2B’B”O_6型氧化物为研究对象，首次制备出了具有大的室
温磁电阻效应的Ca_2FeMnO_(6-δ)以及Sr_2CrFeO_(6-δ)多晶样品，研究了其磁电阻效应及
电磁性能。主要工作包括：
1.双钙钛矿型氧化物La_2NiMnO_6多晶样品的电磁及输运性质
我们采用溶胶—凝胶法制备了优于传统固相烧结法的La_2NiMnO_6多晶样品。
其电阻显示出类似于半导体的温度依赖关系，在80K以下出现磁电阻效应。样品呈
亚铁磁性，Tc约为350K，随制备条件不同Tc有一定起伏。样品磁化强度与磁场
及热处理过程的关系观测到类似于自旋玻璃的时间依赖特征。
2.新型双钙钛矿氧化物Ca_2FeMnO_(6-δ)的磁电阻及独特磁行为
Ca_2FeMnO_(6-δ)样品利用溶胶—凝胶法制备，在不同温区烧结而成。XRD分析
表明均为单相，样品1100属正交结构（a_0=5.6505±0.0006A,b_0=7.6338±0.0008A,c_0=
5.3402±0.0005A）。在液氮致冷温区处于顺磁态，并由M-T曲线和穆斯堡尔谱测量结
果验证。室温下具有33％的磁电阻MR（定义为ρ=（ρ（H）-ρ（O）/ρ（H），μ_0H=1.25T）
效应。样品的MR随制备条件不同而有较大差异，其性能对制备条件及过程十分敏
感。样品的电阻随磁场的变化曲线显示了磁滞现象，这一铁磁体行为特征与实验中
观测到的顺磁性看似矛盾。本文给出由于电流诱导而在顺磁基底中出现纳米铁磁团
簇的物理图象，显示了电流诱导的磁电阻现象（在Cu/Co多层膜，颗粒合金，Ni纳
米线和Mn系隧道结中均有观测）具有一定的普遍性。样品的非线性I-V曲线也验证
了这一观点。经过液氦致冷温区处理，实验观测到样品1100发生顺磁到铁磁的不可
逆相变。Hc分别为3250Oe（80K）和4500Oe（300K），与相变前磁电阻测量中的ρ
峰值相对应。同时样品在M-T曲线中显示出磁性波动现象，与自旋玻璃相类似。穆
斯堡尔谱测量结果显示的B位多价态共存的特征，给了样品中多种亚稳态（B位不
南京大学硕士论文2001 二
一
同价态的组合）存在的可能，这种多亚稳态的存在正是自旋玻璃的本质特征．同时
实验观测到的不可逆相变显示出温度与磁场都对各个亚稳态间的相互转化起着相当
大的作用．
3．新型双钙钛矿氧化物Sr。CrFeO。－e的磁电阻及输运性质
不同温度烧结的Sr。CrFeO。＊样品制备方法同上．XRD分析表明样品为复相，
含有SrCrO。，SrFeO卜。晶相．该系列样品随烧结温度的升高，晶粒逐渐长大，相应
电阻率降低．尽管样品含有少量杂相，在300K温度和1．25T磁场下，具有磁滞的最
高磁电阻MR值可达19．4％．而静态磁测量显示出顺磁特征，这种矛盾和样品中由外
加电流而诱导出铁磁相的作用存在着密切关系．随着测量电流的升高MR显示出的
磁滞特征逐步显著，验证了外加电流诱导铁磁相的作用．低温对铁磁相的出现也起
着相当大的作用，从经低温处理后样品磁致回线的变化中可以看出．电阻测量中出
现的跳跃现象，除去接触电势的影响外，表明了样品内部与输运性质密切相关的B
位电子分布、自旋组态以及晶粒内部结构和晶界状况等具有的复杂性．这些因素的
联合作用形成了许多局部的能量极小值，部分导致了电阻的涨落．样品在80K－30（）
温区，P随着温度的降低而升高，具有典型的半导体导电特征，载流子的跃迁随温
度的下降而显著降低．热重测量的结果显示在300K－800K样品没有相变出现．
The discovery of intergrain magnetoresistance in Sr2FeMoO6 as high as 10% at room
temperature (300K), due to it’s potential teclrnological applications in magnetic recording,
sensors as well as it’s theoretical values, made a wide serial A2B払?6 double perovskite
materials attracted considerable interests from the physical fields. Concentrating on the
magnetoresistance (MR) phenomenon of double perovskite (DP), this thesis compared dou-
ble perovskite materials with the explicitly studied ABO3 materials at first. Some achieved
experimental and theoretical results are carefully reviewed and the common points underly-
ing them are also discussed. Then La2NiMnO6 was chosen to be studied about it’s electric,
magnetic and MR properties. hi the first time, new double perovskite Ca2FeMnO66 and
Sr2CrFeO66 are synthesized successfully. They both show the room temperature MR as
high as 33% and 19.4%. What distinguished them with the former CMR materials is their
PM character at the temperature when the MR shows. Main results of this thesis are shown
as following:
1. The electronic and magnetic characters of double perovskite La2NiMnO6
Using the Sol-gel method, we prepared fine La2NiMnO6 samples superior than the
traditional solid-state method making sample. The resistance of the sample dramatically
increases when the temperature decrease which is similar to the semiconductors. Below
80K the MR effect is observed. The sample behaves as ferrimagnet under Tc~350K.The
Curie temperature shows a little fluctuation with different samples made in various sintering
processes. Their magnetization show time dependent characters somewhat similar to the
spin glasses.
2. The MR effect of new DP Ca2FeMnO66 and its novel magnetic behavior
Ca2FeMnO6 samples are made by the Sol-gel methods following with different
temperature sintering. XRD analysis indicate a single phase of orthorhombic structure,
where a0 5.6505 + 0.0006A,b0 7.6338 + 0.0008A,c0 5.3402 ?0.000SAfor the 1373K
sintered sample. In the temperature range cooling by liquid Nitrogen, it is at the param-
agnetic state, which is confirmed by the M-T curve and the Msbauer spectra. At 300K,
the samples have the MR effects as high as 33% ,12% and 9.5%,where certain hysteresis
as FM appear. Seemingly it is conflicted with the PM character observed under statistic
magnetic measurements. The physical picture of nanoferromagnetic clusters induced by
the electric current in PM matrix is given to interpret this confliction. It is supported by
the non-linear I-V curve. After disposition under 70K, irreversible phase transition from
PM to FM is observed in the 1373K sintered sample. The FM phase has coercive force
32500e and 45000e at 80K and 300K respectively, this coercive force is corespondent to
the maxim of resistivity in the MR data obtained before the irreversible phase transition.
Some fluctuations of magnetization are also observed in the M-T curve, they are similar to
the spin glasses. Msbauer measurement shows the coexistence of different valence state of
B site ions, which give the possibility of various valence B and B?ions combination states.
Tins leads to many metastable states in Ca2FeMnO66 sample, it is the existence of many
metastable states that make a spin glass. Experiments also show the temperature and the
magnetic field have strong influences on the transition between these different metastable
states.
3 The magneteresistan
引文
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[2] P.W.Anderson, Phys. Rev. 115 (1959) 2．
[3] G.H.Jonker, Physica 22 (1956) 707．
[4] G.Blasse, Philips Res.Rep. 20 (1965) 327．
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