High-pressure structural analysis of (Nd,Sm)1/2Sr1/2MnO3

a ridge of high pressure~

a ridge of high pressure是（气象学术语）高压脊
天气预报广播中，常常出现“低压槽”和“高压脊”两个名词，实际上就是气旋和反气旋。

在北半球的西风带里，大气是呈波浪起伏式运动的。波浪的低谷区就是低压槽，气流作反时针方向旋转，气压分布是中间低、四周高，空气自外界向槽内流动，槽内空气辐合上升，形成阴雨天气。波浪的高峰区就是高压脊，气流作顺时针方向旋转，气压分布是中间高、四周低，空气自中心向外辐散，脊内盛行下沉气流，一般天气晴好。一对槽脊，一低一高组成一个波动。西风带里的高空槽脊系统就叫西风波。

高空的槽脊系统与地面的天气变化有密切的关系。如在北半球的西风带里，高空槽前一般吹西南风，这种风能把孟加拉湾和印度洋上空的暖湿空气输送到我国中纬度地区，为形成云雨创造了条件。而高空槽后（即高压脊前）一般吹西北风，地面是一个高气压区，天气由阴转晴。

高空槽脊形成后，不停地移动和变化，有时加强，有时减弱。随着高空槽脊的移动变化和加强减弱，地面的天气也会随之发生相应的变化。因此，做好高空槽脊系统活动变化的预报，是天气预报中的重要内容。


who had the presence of mind ：那些在危难中镇定自若的人。

这里的 teacher's 是名词所有格，相当于汉语的“的字结构”。在指代清楚的前提下，后面的名词完全可以省略，如：我的书是关于科技方面的，你的呢？这个“你的”就是省略了上文中的“书”。
英语也是如此。本句中，上文提到了 student's physical fitness，下文为避免重复，就不在提 physical fitness，直接用了 teacher's，其实就是 teacher's physical fitness

Title:
High-pressure structural analysis of (Nd,Sm)1/2Sr1/2MnO3: Origin for pressure-induced charge ordering
Authors:
Kuriki, A.; Moritomo, Y.; Machida, A.; Nishibori, E.; Takata, M.; Sakata, M.; Ohishi, Y.; Shimomura, O.; Nakamura, A.
Affiliation:
AA(Department of Crystalline Materials Science, Nagoya University, Nagoya 464-8603, Japan) AB(Center for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464-8601, Japan) AC(Department of Crystalline Materials Science, Nagoya University, Nagoya 464-8603, Japan) AD(Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan) AE(Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan) AF(Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan) AG(SPring-8/JASRI, 1-1-1 Koto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan) AH(SPring-8/JAERI, 1-1-1 Koto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan) AI(Center for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464-8601, Japan)
Publication:
Physical Review B, vol. 65, Issue 11, id. 113105 (PhRvB Homepage)
Publication Date:
03/2002
Origin:
APS
Abstract Copyright:
(c) 2002: The American Physical Society
DOI:
10.1103/PhysRevB.65.113105
Bibliographic Code:
2002PhRvB..65k3105K

Abstract
Pressure effects on the structural parameters are investigated in a half-doped manganites (Nd0.125Sm0.875)1/2Sr1/2MnO3, by means of the synchrotron-radiation x-ray powder diffraction measurement. The compound shows a pressure-induced metal-insulator transition due to the subtle competition between the double-exchange interaction and the charge-ordering instability [Y. Tokura et al., Phys. Rev. Lett. 76, 3184 (1996)]. We have found that application of pressure induces the four-long and two-short deformation of the MnO6 octahedra. We present a possible scenario for the pressure-induced charge-ordering.

参考：http://adsabs.harvard.edu/abs/2002PhRvB..65k3105K

中、英对照

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R1-xAxMnO3（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Fe1-XCoXSi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La1-xCaxMnO3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La0.7Pb0.3MnO3单晶样品的由量子相干效应导致的正磁电阻效应、A0.5Sr0.5MnO3 (A= Pr, Nd) 的巨磁热效应、多晶锌铁氧体和多晶NiXFe1-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La0.7Pb0.3MnO3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．A0.5Sr0.5MnO3 (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A1-xSrxMnO3(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，A0.5Sr0.5MnO3 (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TC = 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。(Zn0.41Fe2.59O4)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Zn0.41Fe2.59O4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NiXFe1-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NiXFe1-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, NiXFe1-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 R1-xAxMnO3 (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 Fe1-xCoxSi 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 La0.7Pb0.3MnO3 single crystals, the large magnetothermal effect of bulk polycrystalline A0.5Sr0.5MnO3 ( A = Pr, Nd ), the giant magnetoresistance effect in polycrystalline Zn0.41Fe2.59O4 and NiXFe1-XS.

Our main work includes:

1 The positive MR from QIE in perovskite manganites.

High-quality single crystal of La0.7Pb0.3MnO3 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 La0.7Pb0.3MnO3 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 TC=340K under a magnetic field of H=8T. The CMR decreases with temperature lowered and almost vanishes at T 70K. 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 H1/2. 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 A0.5Sr0.5MnO3 ( A = Pr, Nd ).

A0.5Sr0.5MnO3 ( 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 A0.5Sr0.5MnO3 ( A = Pr, Nd ) is suitable candidate for magnetic refrigerant at temperature 150K~180K.

3. Giant tunneling magnetoresistance in polycrystalline Zn0.41Fe2.59O4 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 Zn0.41Fe2.59O4 grains (~150nm) are separated by uniform a-Fe2O3 grain boundaries (6nm). Such a huge room temperature tunneling MR indicates that Zn0.41Fe2.59O4 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 Ni1-xFexS

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 Ni1-xFexS 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.

http://scholar.ilib.cn/abstract.aspx?A=zgkx-eg200305003

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