陈 鹏

论文题目：磁性氧化物的磁电阻和磁热效应

作者简介：陈 鹏，男，1969年12月出生，1998年09月师从于南京大学都有为教授，于2001年06月获博士学位。

摘 要

1988年M.N.Baibich在反铁磁场耦合的Fe/Cr多层膜中观察到巨磁电阻效应,随后在多种组合的磁性多层膜以及颗粒膜中观察到巨磁电阻效应, 在掺杂钙钛矿型锰氧化物中观察到庞磁电阻效应。由于巨磁电阻效应巨大的应用前景和内在丰富多彩的物理现象引起了物理学界和材料学界的广泛关注，成为了现代科学研究的一个热点。人们一方面对已有的磁电阻材料进行广泛而深入的研究，以弄清巨磁电阻效应的机理和影响它的因素，研制出具有更高磁场灵敏度的材料。另一方面，人们还在努力地寻找具有巨磁电阻效应的新材料。

其中1989年在掺杂钙钛矿型锰氧化物R_1-xA_xMnO₃（其中A为二价碱土金属离子，如Ca²⁺、Sr²⁺、Ba²⁺等，R为三价稀土金属离子，如La³⁺、Pr³⁺、Tb³⁺、Sm³⁺等）中发现巨磁电阻（GMR），由于其在磁记录、磁传感器等方面潜在的应用前景，以及金属—绝缘体相变等所涉及的强关联效应，使该类化合物吸引了物理学界的广泛注意。最近几年里，在该类材料中还相继发现了许多新的物理现象，例如：大的磁致伸缩、磁致结构相变以及异常的热膨胀等。

数年来，围绕着钙钛矿材料中巨磁电阻的机理问题，人们先后设想了多种理论方案或物理图象。五十年代初期，由Zener创立的，其后由Anderson及de Gennes等人发展起来的双交换作用理论，为今天解释巨磁电阻效应提供了一条途径。到九十年代，Millis 等人和邢定钰等人分别对巨磁电阻效应的机理作了重要的探索。

七十年代末至八十年代初，人们在半导体材料以及顺磁材料中发现了由量子相干效应（由于无序而加强的载流子库仑相互作用）导致的正磁电阻，并建立了一套基于无序的理论来解释所观察到的实验现象。去年, Manyala在Fe_1-XCo_XSi中首次观察到铁磁材料中的由量子相干效应导致的正磁电阻。

另一方面，随着郭载兵等在1997年首次发现钙钛矿型锰氧化物La_1-xCa_xMnO₃具有较大的磁热效应后^[40,41]，钙钛矿型锰氧化物的磁热效应引起了人们的注意。

本论文主要对La_0.7Pb_0.3MnO₃单晶样品的由量子相干效应导致的正磁电阻效应、A_0.5Sr_0.5MnO₃ (A= Pr, Nd) 的巨磁热效应、多晶锌铁氧体和多晶Ni_XFe_1-XS的巨磁电阻效应进行了系统的研究。

我们的工作主要包括：

1．掺杂钙钛矿型锰氧化物中由量子相干效应导致的正磁电阻。

我们通过旋转坩锅熔融法制备了La_0.7Pb_0.3MnO₃单晶样品，在国际上首次报道了掺杂钙钛矿型锰氧化物中由量子相干效应导致的正磁电阻, 这样的量子相干效应在4.2K< T < 50K的温区出现, 远远超出了以往半导体材料以及顺磁材料中发现量子相干效应的极低温区( ~mK )。测量单晶样品在温度4.2K-400K的磁电阻，显示出奇特的特性。在高温区T > 70K，单晶样品显示出由双交换作用导致的正常的负磁电阻，在居里温度340K在8T磁场下达70%。但在低温区（4.2K< T < 50K）却显示出正磁电阻，在最大外场为8T时样品在4.2K磁电阻高达20%。并且发现样品电阻率在低温（T < 50K）满足这样两个关系式: ，（其中T为温度，H为外加磁场），也即电阻率与温度及磁场的平方根成正比，这正是量子相干效应的特征表现。。

2．A_0.5Sr_0.5MnO₃ (A= Pr, Nd)大块多晶样品的巨磁热效应

我们通过固相烧结法制备了系列的A_1-xSr_xMnO₃(A= Pr, Nd) 大块多晶样品（x = 0.3, 0.4, 0.5），对样品的晶体结构，磁特性以及磁卡效应作了详细的研究。在实验表明，A_0.5Sr_0.5MnO₃ (A= Pr, Nd)都具有一个从低温反铁磁电荷有序态到高温铁磁电荷无序态的一级相变。低温反铁磁电荷有序态在磁场作用下会崩塌，转变为铁磁电荷无序态。在一级相变温度161K（A= Pr）和183K（A= Nd）附　近，观察到在1T外磁场下7.1J/kg.K（A= Pr）和7.3J/kg.K（A= Nd）的巨大的正磁熵变，这一数值比金属Gd在居里温度T_C = 293K附近的磁熵变（3.1J/kg . K）的两倍还多, 给引人注目的掺杂钙钛矿型锰氧化物电荷有序态注入了新的物理含义, 同时也显示该材料是150K-180K温区理想的磁制冷工质材料。

3. 多晶锌铁氧体的巨磁电阻效应

我们利用溶胶-凝胶方法制作了在晶粒表面包裹着一层a-Fe₂O₃薄层的多晶锌铁氧体,其中锌铁氧体晶粒尺寸约为150nm, 而a-Fe₂O₃薄层厚约6nm。在国际上首次报道了氧化物系统中巨大的隧穿磁电阻效应。(Zn_0.41Fe_2.59O₄)_0.89 (a-Fe₂O₃)_0.11的隧穿磁电阻在4.2K达1280%，在室温下仍有158%的值,这是目前国际报道的最大隧穿磁电阻效应。这样一个氧化物系统的电阻率显示出rµexp(1/T)的温度依赖关系, 这不同于传统颗粒系统中rµexp(1/)的电阻率-温度依赖关系。这样一种不同的电阻率-温度关系起源于系统中较大的锌铁氧体晶粒尺寸(约150nm)和均匀厚薄的a- Fe₂O₃层(6nm)。同时我们的实验结果显示Zn_0.41Fe_2.59O₄具有很高的自旋极化率,室温自旋极化率大于66%,是一种理想的半金属材料。

4. NiS及Ni_XFe_1-XS的巨磁电阻效应

我们在封闭的真空石英管中高温退火,用固相烧结法制得多晶NiS及Ni_XFe_1-XS系列材料. 在国际上首次报道了此系列材料中磁场诱导的非金属到金属的相变以及巨磁电阻效应.在4T磁场作用下, NiS在温度268K显示出1530%的巨磁电阻效应, Ni_XFe_1-XS在室温显示出730%的巨磁电阻效应,这是目前国际上报道的最大室温巨磁电阻效应。

ABSTRACT

Giant magnetoresistance (GMR) has attracted much attention since 1988 M. N. Baibich et al observed 50% magnetoresistance(MR) in Fe/Cr magnetic multilayers due to its intrinsic rich variety of physical phenomena and great potential applications.

Hole- doped perovskite manganites R_1-xA_xMnO₃ (R is a trivalent rare-earth ion and A is a divalent alkali earth ion) has attracted much attention since 1989 due to not only its technological applications in magnetic recording and sensor, but also the effect of the strong correlation concerning metal-insulator transition, etc. In recent years, a lot of new physical phenomena have also been observed in these compounds, for example, large magnetostriction, structural phase transition driven by an external field, and anomalous thermal expansion, etc.

Since then, several models or physical pictures have been suggested to explain the mechanism of GMR. The double exchange interaction founded by Zener in early 1950’s, and developed by Anderson and de Gennes later is a good theory in describing the conductive behavior in the compounds. It provides us a correct route to make clear the origin of GMR. Since 1995, Millis et al. and Xing et al. have made a lot of work, respectively, in order to explain the mechanism of GMR.

From 1970s to 1980s, Positive magnetoresistance from quantum interference effects (QIE--Coulomb interaction between electrons) was observed in semiconductor and many paramagnetic disordered materials. Theory was developed to explain the experimental facts. Recently, Manyala et al. found positive MR from QIE in ferromagnet Fe_1-xCo_xSi for the first time .

On the other hand, the magnetothermal effect in hole- doped perovskite manganites has attracted much attention since Guo et al. Observed large magnetic entropy change in perovskite manganites in1997.

In this thesis we concentrated on the studies of the positive MR from QIE in La_0.7Pb_0.3MnO₃ single crystals, the large magnetothermal effect of bulk polycrystalline A_0.5Sr_0.5MnO₃ ( A = Pr, Nd ), the giant magnetoresistance effect in polycrystalline Zn_0.41Fe_2.59O₄ and Ni_XFe_1-XS.

Our main work includes:

1 The positive MR from QIE in perovskite manganites.

High-quality single crystal of La_0.7Pb_0.3MnO₃ was grown by the flux growth using the accelerated crucible rotation technique. We report QIE-induced positive MR in perovskite manganites for the first time. The MR in single crystal of La_0.7Pb_0.3MnO₃ shows strange character in 4.2K~400K. Negative CMR due to double-exchange interaction was observed at temperature >70K, and maximal CMR (over 70%) occurs around T_C=340K under a magnetic field of H=8T. The CMR decreases with temperature lowered and almost vanishes at T70K. As the sample is further cooled to 50K, an unexpected positive MR appears. The magnitude of positive MR increases with temperature lowered, a positive MR of 20% was found at T= 4.2K and H= 8T. It is found that is well fit for T ^1/2 dependence at low temperatures below 50K, while for H>3kOe, approach asymptotes proportional to H^1/2. The behavior of the type has been regarded as a characteristic signature of the QIE-induced transport anomalies

2. The large magnetothermal effect of bulk polycrystalline A_0.5Sr_0.5MnO₃ ( A = Pr, Nd ).

A_0.5Sr_0.5MnO₃ ( A = Pr, Nd ) perovskite–type manganites have been prepared , and found that the samples show first-order magnetic, electron and structural phase transition from antiferromagnetic charge-ordered state at low temperature to ferromagnetic charge-disordered state at high temperature. Collapse of the antiferromagnetic charge-ordered state take place under a magnetic field. Giant magnetic entropy change (7~8J/kg. K) has been discovered in these samples under a low magnetic field of 10kOe near the first-order transition temperature (150K~180K). It exceeds the magnetic entropy change in Gd by a factor of 2.3 at the same condition. The manganese perovskites compound A_0.5Sr_0.5MnO₃ ( A = Pr, Nd ) is suitable candidate for magnetic refrigerant at temperature 150K~180K.

3. Giant tunneling magnetoresistance in polycrystalline Zn_0.41Fe_2.59O₄ with grain boundaries coated by a-Fe₂O₃ Layer

we report a new class of nanostructured material, polycrystalline Zn _0.41Fe _2.59 O₄, exhibiting an intrinsic tunneling-type MR as large as 158% at T ~ 300 K and H ~ 5 kOe. The polycrystalline material reported here is a two-phase system in which the Zn_0.41Fe_2.59O₄ grains (~150nm) are separated by uniform a-Fe₂O₃ grain boundaries (6nm). Such a huge room temperature tunneling MR indicates that Zn_0.41Fe_2.59O₄ is a highly spin-polarized FM. As the temperature is lowered from room temperature, the tunneling MR first increases slowly and then has a big enhancement below 100 K, reaching 1280% at 4.2 K.

4. Giant magnetoresistance in polycrystalline NiS and Ni_1-xFe_xS

Large magnetoresistance (1530%) in NiS has been observed in an applied

magnetic field of 4 T at a temperature of 268 K. On he other hand, we report another material Ni_1-xFe_xS with a large MR (MR~730%) near room temperature at 299 K in a magnetic field of 4 T for x=0.12. Such a large MR near room temperature has never been reported before. The large magnetoresistance is due to a magnetic field induced transition from the AFM nonmetal phase to the PM metal phase.

　回主页