稀土掺杂锰氧化物庞磁电阻效应

过去十多年来,具有庞磁电阻效应的稀土掺杂锰氧化物成为了凝聚态物理研究的重要领域。锰氧化物的载流子自旋极化率高,且在居里温度附近表现出很大的磁电阻效应,因此在自旋电子学中有潜在的应用前景。另一方面,锰氧化物是典型的强关联电子体系,它对目前有关强关联体系的认识提出了很大挑战。本文综述了锰氧化物的各种性质及其物理原因。全文首先概述了锰氧化物的庞磁电阻效应及其晶格和电子结构,简单介绍了其他一些庞磁电阻材料;随后综述了锰氧化物的电荷/轨道有序相及其输运性质;在第四部分简单介绍了锰氧化物中庞磁电阻效应的机制;最后讨论了锰氧化物的一些可能的应用,如低场磁电阻效应、磁隧道结、磁p＿n结以及全钙钛矿的场效应管和自旋极化电子注入装置等。     

稀土掺杂锰氧化物庞磁电阻效应

过去十多年来，具有庞磁电阻效应的稀土掺杂锰氧化物成为了凝聚态物理研究的重要领域。锰氧化物的载流子自旋极化率高，且在居里温度附近表现出很大的磁电阻效应，因此在自旋电子学中有潜在的应用前景。另一方面，锰氧化物是典型的强关联电子体系，它对目前有关强关联体系的认识提出了很大挑战。本文综述了锰氧化物的各种性质及其物理原因。全文首先概述了锰氧化物的庞磁电阻效应及其晶格和电子结构，简单介绍了其他一些庞磁电阻材料；随后综述了锰氧化物的电荷／轨道有序相及其输运性质；在第四部分简单介绍了锰氧化物中庞磁电阻效应的机制；最后讨论了锰氧化物的一些可能的应用，如低场磁电阻效应、磁隧道结、磁p-n结以及全钙钛矿的场效应管和自旋极化电子注入装置等。     

掺杂稀土锰氧化物的巨磁电阻效应

介绍了掺杂稀土锰氧化物的巨磁电阻效应研究概况和最新进展，在综合目前实验和理论研究结果的基础上，对在掺杂稀土锰氧化物材料中引起巨磁电阻效应的物理机制进行了探讨，对这一材料的应用前景和需要做的工作进行了讨论。     

稀土锰氧化物的低场磁电阻效应

具有庞磁电阻效应的掺杂稀土锰氧化物因为其高的自旋极化率和自旋极化输运行为而表现出显著的低场磁电阻效应。这一效应在氧化物自旋电子学中有着深远的潜在应用前景。本文综述了国内外近年来在锰氧化物低场磁电阻增强这一研究领域的进展和存在的一些问题。全文分三个部分，首先概述了基于自旋极化散射和自旋极化隧穿两种输运机制的磁电阻理论；然后重点介绍掺杂稀土锰氧化物低场磁电阻增强的主要研究进展，这些进展背后的基本物理图象是通过人为引入自旋无序介质形成自旋极化散射和自旋极化隧穿，从而增强其低场磁电阻；第三部分讨论了基于掺杂稀土锰氧化物的磁性隧道结制备和输运性质。本文最后提出了锰氧化物低场磁电阻增强研究应该关注的一些物理问题。     

磁电子学讲座 第二讲 掺杂稀土锰氧化物的巨磁电阻效应

介绍了掺杂稀土锰氧化物的巨磁电阻效应研究概况和最新进展．在综合目前实验和理论研究结果的基础上，对在掺杂稀土锰氧化物材料中引起巨磁电阻效应的物理机制进行了探讨．对这一材料的应用前景和需要做的工作进行了讨论     

混价锰氧化物超巨磁电阻材料研究进展

综述了钙钛矿结构锰氧化物超巨磁电阻材料的研究进展，主要包括Ｊａｈｎ－Ｔｅｌｅｒ畸变、磁性、电阻率、热电势、霍尔效应、比热、热膨胀、电荷有序、中子衍射和非弹性散射、电子能谱，以及同位素效应等方面     

多晶钙钛矿锰氧化物中的巨磁电阻与磁场关系

本论文研究了多晶锰氧化物磁电阻和磁场的关系,在低温和低场下,磁性纳米团簇的磁矩转动和晶粒边界的自旋二级隧穿对磁电阻起主要作用因高于Tc时,由弱化强度随温度关系的实验结果指出顺磁态中已出现铁磁团簇,因此它类似于磁性颗粒膜中的巨磁电阻（CMR）机制;在顺磁－铁磁相变区,既有颗粒贡献又有界面的隧道贡献,这一理论模型与多晶La0.825Sr0.175MnO3中的实验结果很好地吻合。     

巨磁电阻锰氧化物材料的低温电阻率行为的研究

对巨磁电阻锰氧化物材料La2/3Ca1/3MnO3/YSZ(钇稳定氧化锆)(LCMO/YSZ)系列样品低温下电阻率行为进行研究,低温下电阻率行为一般是ρ=ρ0 ATa BTb的形式,其中ρ0为剩余电阻率,a=2或3/2,b=5或9/2.T2代表电子-电子散射项,T3/2代表被无序自旋玻璃散射的电子项,T6代表电子.声子散射项,Tθ/2们代表电子-双磁子散射项.在此指出b=5是合理的,代表电子-声子散射项.     

混合价锰氧化物的金属—绝缘体转变和特大磁电阻理论

提出了一个包括双交换自旋无序和非磁无序的局域化模型，解释了锰氧化物的异常磁性和输运性质．     

Pressure Dependence of the Transport and Magnetic Properties of Colossal Magnetoresistance Tl 2 Mn 2 O 7 Pyrochlore System

The Tl 2 Mn 2 O 7 pyrochlore system, pure or doped at the Tl position (Cd, Bi, Sc) or at Mn position (Sb, Te, Cr, Pb), present interesting magnetotransport properties characterized by the Colossal Magnetoresistance (CMR) effect. Our data for Tl 2 Mn 2 O 7 and Tl 1.9 Bi 0.1 Mn 2 O 7 indicate a decrease of the ordering temperature, measured from resistivity and magnetic susceptibility up to 70-90 kbars, with a clear increase of T C for higher pressures (up to 120 kbars). By contrast, in the case of Tl 2 Mn 1.8 Sb 0.2 O 7 , T C increases with pressure in the explored pressure range. A recent theory from Nuñez-Regueiro and Lacroix explains this dependence in terms of the existence of a second coupling mechanism between Mn ions, mediated by the conduction band (Tl (6s)). The doping at the Mn position (Sb) strongly increases the charge carrier density, so that this second coupling mediated by the conduction band, is strongly enhanced, in such a way that T C increases with pressure.     

Chemical Substitutions in Tl 2 Mn 2 O 7 Pyrochlore,Synthesized Under High Pressure: Effects on the Colossal Magnetoresistance

Tl 2 Mn 2 O 7 pyrochlore has recently been described as a half-metallic ferromagnet showing colossal magnetoresistance (CMR) properties. New series of Tl 2 Mn 2 O 7 derivatives have been prepared under high pressure conditions. We have replaced Tl 3+ cations by Bi 3+ and Cd 2+ , leading to different modifications of the physical properties, with dramatic improvements of CMR. The Mn 4+ cations have also been partially replaced by Sb 5+ and Te 6+ . In particular, moderate Sb substitution leads to significant increments of T C . In this work we discuss on the effects of the different chemical substitutions on the structural, magnetic and transport properties.     

Colossal magnetoresistive manganites

Magnetoelectronic features of the perovskite-type manganites are overviewed in the light of the mechanism of the colossal magnetoresistance (CMR). The essential ingredient of the CMR physics is not only the double-exchange interaction but also other competing interactions, such as ferromagnetic/antiferromagnetic superexchange interactions and charge/orbital ordering instabilities as well as their strong coupling with the lattice deformation. In particular, the orbital degree of freedom of the conduction electrons in the near-degenerate 3d e_g state plays an essential role in producing the unconventional metal–insulator phenomena in the manganites via strong coupling with spin, charge, and lattice degrees of freedom. Insulating or poorly conducting states arise from the long or short-range correlations of charge and orbital, but can be mostly melted or turned into the orbital-disordered conducting state by application of a magnetic field, producing the CMR or the insulator–metal transition.     

Description of the Colossal Magnetoresistance of La 1.2 Sr 1.8 Mn 2 O 7 Based on the Spin-Polaron Conduction Mechanism in the Ferromagnetic Temperature Range

We have analyzed the resistance of La1.2Sr1.8Mn2(1 – z)O7 single crystal in magnetic fields from 0 to 90 kOe in the ferromagnetic temperature range. The observed magnetoresistance of La1.2Sr1.8Mn2O7 is described based on the spin-polaron conduction mechanism. The magnetoresistance is determined by the change in the sizes and magnetic moment directions of magnetic inhomogeneities (polarons). It is shown that the colossal magnetoresistance is ensured by an increase (along the magnetic field) of the polaron linear size. It is found using the method for separating the contributions of different conduction mechanisms to the magnetoresistance that the contribution to the magnetoresistance from the orientation mechanism at 80 K in low magnetic fields is close to 50%. With increasing magnetic field, this contribution decreases and becomes small in fields exceeding 30 kOe. The comparable contributions to the conductivity from the orientational and spin-polaron mechanisms unambiguously necessitate the inclusion of both conduction mechanisms in the magnetoresistance calculations. We have calculated the temperature variation of the polaron size (in relative units) in zero magnetic field and in a magnetic field of 90 kOe.     

Colossal dielectric constants in transition-metal oxides

Many transition-metal oxides show very large (“colossal”) magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development of new capacitance-based energy-storage devices. In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation. In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu₃Ti₄O₁₂ and related systems, which is today’s most investigated material with colossal dielectric constant. Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La_2−xSr_xNiO₄ where electronic phase separation may play a role in the generation of a colossal dielectric constant.     

Description of the Colossal Magnetoresistance of La1.2Sr1.8Mn2O7 Based on Spin-Polaron and Orientation Conductivity Mechanisms in the Paramagnetic Temperature Region

Physics of the Solid State - The resistance of single-crystal La1.2Sr1.8Mn2(1 – z)O7 is studied experimentally and theoretically in the 75–300 K temperature range in...     

巨磁电阻效应——2007年诺贝尔物理学奖简介

瑞典皇家科学院宣布，法国科学家阿尔伯特·费尔（Albert Vert）和德国科学家彼得·格鲁伯格（Peter Grunberg），共同获得2007年诺贝尔物理学奖．获奖的原因是这两位科学家先后独立发现了“巨磁电阻”（giant magnetoresistance，GMR）效应．这个发现引发的技术进步极大地提高了计算机硬盘磁头的数据读取能力，使硬盘无论从容量还是体积上都产生了质的飞越．这个发现还导致了新一代磁传感器的出现，而且巨磁电阻被认为是纳米技术最重要的应用之一．     

Magnetoresistance in amorphous germanium

Magnetoresistance of sputtered films of a-Ge has been studied. The experiments were performed in the temperature range 78–450 K and in magnetic fields up to 7 kG. The magnetoresistance has been found to be negative up to 450 K and changes its sign at 80 K. The experiments are confronted with a model based on the Zeeman splitting of localized electronic levels in the applied magnetic field. It is shown that such model can lead to relatively good agreement between the theory and the experiment.We are grateful to B.Velický for discussions about the subject matter of this paper and to J.Zemek for preparation of germanium samples.     

Colossal dielectric constant in high entropy oxides

Entropic contributions to the stability of solids are very well understood and the mixing entropy has been used for forming various solids, for instance such as inverse spinels, see Nawrotsky et al., J. Inorg. Nucl. Chem. 29 , 2701 (1967) [1]. A particular development was related to high entropy alloys by Yeh et al., Adv. Eng. Mater. 6 , 299 (2004) [2] and Cantor et al., Mater. Sci. Eng. A 375–377 , 213 (2004) [3] (for recent reviews see Zhang et al., Prog. Mater. Sci. 61 , 1 (2014) [4] and Tsai et al., Mater. Res. Lett. 2 , 107 (2014) [5]) in which the configurational disorder is responsible for forming simple solid solutions and which are thoroughly studied for various applications especially due to their mechanical properties, e.g. Gludovatz et al., Science 345 , 1153 (2014) [6] and Lu et al., Sci. Rep. 4 , 6200 (2014) [7], but also electrical properties, Kozelj et al., Phys. Rev. Lett. 113 , 107001 (2014) [8], hydrogen storage, Kao et al., Int. J. Hydrogen Energy 35 , 9046 (2010) [9], magnetic properties, Zhang et al., Sci. Rep. 3 , 1455 (2013) [10]. Many unexplored compositions and properties still remain for this class of materials due to their large phase space. In a recent report it has been shown that the configurational disorder can be used for stabilizing simple solid solutions of oxides, which should normally not form solid solutions, see Rost et al., Nature Commun. 6 , 8485 (2015) [11] these new materials were called ”entropy‐stabilized oxides”. In this pioneering report, it was shown that mixing five equimolar binary oxides yielded, after heating at high temperature and quenching, an unexpected rock salt structure compound with statistical distribution of the cations in a face centered cubic lattice. Following this seminal study, we show here that these high entropy oxides (named HEOx hereafter) can be substituted by aliovalent elements with a charge compensation mechanism. This possibility largely increases the potential development of new materials by widening their (already complex) phase space. As a first example, we report here that at least one HEOx composition exhibits colossal dielectric constants, which could make it very promising for applications as large‐k dielectric materials. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)