Nb5+的掺杂对La0.67Sr0.33MnO3磁性和传输性的影响

将用溶胶-凝胶法制备的La0.67Sr0.33MnO3(LSMO)微粉与(Nb2O5)x/2粉在1100℃下混合烧结,形成了具有钙钛矿结构的锰氧化物与LaNbO4的复合体系,x是掺入的Nb5 离子与母体材料的摩尔比.在x=0.07的样品中得到最大电阻率为23.74 Ω·cm,比LSMO 高三个数量级.Nb5 离子的掺杂使样品的低场磁电阻(LFMR)和高场磁电阻(HFMR)效应都有所增强.77 K下,0.1 T和1 T磁场下在x=0.07 样品中分别得到24 % 和33.8 %的磁电阻效应,是LSMO样品的2倍和1.7倍.室温下x=0.05样品的磁电阻最大,为9 %.其中,LFMR来源于颗粒晶界处电子的自旋相关隧穿及散射作用,而HFMR来源于表面层的自旋非共线结构.     

La0.67Sr0.33MnO3中Ag-Ti的掺杂效应

将La0.67Sr0.33MnO3（LSMO）、Ag2O及TiO2粉混合经高温烧结后制备了钙钛矿相/xAg两相复合体系（x是Ag与钙钛矿材料的物质的量比）,系统地研究了Ag-Ti的共掺杂对LSMO电性和磁电阻效应的影响.0.07摩尔比Ti4＋离子的B位掺杂使LSMO的居里温度降至室温.Ag的掺入对Tc影响不大,Tp逐渐升高.由于钙钛矿颗粒属性的改善和金属导电通道的出现,材料的电阻率明显下降.Ag掺杂使室温磁电阻得到显著增强,室温下从x=0.30样品中得到最大的磁电阻,约为32%,是La0.67Sr0.33MnO3样品的8倍,La0.67Sr0.33Mn0.93Ti0.07O3样品的1.6倍.     

La0.67Ba0.33MnO3中Ag的掺杂效应

在溶胶-凝胶法制备的La0.67 Ba0.33 MnO3(LBMO)微粉中掺入Ag2O粉,制成一系列(LBMO)／(Ag2O)x／2(x=0～0．35,为摩尔比)掺杂材料,实验结果发现Ag掺杂可明显降低材料的电阻率。当掺Ag量为x=0．25时,样品的电阻率达到最低值。同时在居里点附近,样品的峰值磁电阻得到显著增强。微量的Ag掺杂有助于提高样品的自旋相关隧穿磁电阻,使低场磁电阻得到显著增强。     

La0.67-xNdxSr0.33MnO3体系的磁性和电性

通过M-T曲线,ESR曲线,红外光谱,拉曼光谱,P-T曲线和MR-T曲线的测量,研究了La0.67-xNdxSr0.33MnO3(x=0.10,0.20,0.30,0.40,0.50)样品的磁电性质.实验结果表明:随着Nd掺杂的增加,样品的磁结构从长程铁磁有序向自旋团簇玻璃态、反铁磁态转变;当x=0.30、0.40时发生相分离,且x=0.40样品的ZFC曲线随温度升高在160K左右突然跌落,出现少见的"肩膀";样品的电行为随着Nd掺杂的增加发生变化.样品的磁电行为和CMR效应来源于Nd掺杂引起的次晶格不同耦合和与自旋相关的界面隧穿效应.     

La0.67Sr0.33MnO3中Bi的掺杂效应

将Bi2O3掺杂到用溶胶—凝胶法制备的La0.6Sr0．33MnO3(LSMO)微粉中,XRD测量结果证实有过量的Bi析出。随着Bi掺杂量的增加,LSMO／(Bi2O3)x/2材料电阻率发生明显变化,在x=(0—0．10)摩尔比的掺杂范围内,电阻率先上升后突然下降。当X=0．1时,电阻率比未掺杂样品下降了一个数量级。Bi掺杂对低温和室温磁电阻有着完全不同的影响。低温下,随掺杂量增加,磁电阻下降;室温下Bi的微量掺杂可以使磁电阻增大,掺入x=0.03Bi使室温磁电阻由-4.4％提高到-5．6％。     

磁场退火对La_(0.7)Sr_(0.3)MnO_3低场磁电阻增强效应的研究

为了提高La0.7Sr0.3MnO3的低场磁电阻效应,将样品进行了磁场退火处理。实验结果表明,磁场退火并没有改变样品的磁矩,但对其磁各向异性产生了较大的影响。与零磁场下退火的样品相比,磁场退火样品的磁电阻明显提高。这种增强效应在低磁场下更加明显,在0.1 T的外磁场下,磁退火样品的磁电阻比零场退火的提高了140%。利用自旋极化隧穿机制解释了这种增强的磁电阻效应。     

Sr2Fe1-xAlxMoO6(x=0,0.3)陶瓷的低场磁电阻效应研究

用传统的固相反应法合成了Fe位Al掺杂的双钙钛矿型氧化物Sr2Fe1-xAlxMoO6(x=0,0.3),并研究了它们的低场磁电阻效应.无磁性Al3+离子的Fe位掺杂打破了晶粒内部Fe离子和Mo离子的交替有序排列,将晶粒内部的亚磁性区域分割成很多尺寸更小的区域,提高了磁场灵敏度,从而使得Al掺杂样品在低温时的磁电阻比未掺杂时的Sr2FeMoO6提高一倍以上;但是Al掺杂后样品的磁电阻随着温度的升高而迅速下降,表现出更强烈的温度依赖性.     

磁性隧道结中自旋相关的隧穿输运

全面回顾和总结了磁性隧道结中自旋相关的隧穿这一研究领域的理论和实验方面的最新研究进展。讨论了影响磁性隧道结的自旋极化和隧穿磁电阻的各种因素及反映铁磁层和铁磁／绝缘层界面电子结构在隧穿中重要作用的理论模型和近期实验，同时也讨论了绝缘势垒和铁磁／绝缘层界面中的无序性在隧穿过程中对自旋极化与磁电阻效应的影响。     

界面自旋翻转对有限尺寸的铁磁体/非磁性半导体/铁磁体体系自旋注入的影响

本文考虑了界面自旋翻转效应后对有限尺寸的铁磁体/非磁性半导体/铁磁体异质结中的自旋注入问题进行了系统的理论探讨.由于自旋在两种介质界面上发生的翻转散射,自旋极化流的每一个分量在界面上都不可能连续.计算结果表明,当自旋注入效率从0增加到100%的过程中,铁磁体/非磁性半导体/铁磁体异质结的隧穿磁阻增大了两个数量级.这一事实证明界面的自旋翻转效应直接影响着铁磁体/非磁性半导体/铁磁体异质结的隧穿磁阻.     

Na0．75Co1-xErxO2的磁电阻

NaxCoO2具有奇异热电性质和电磁性质的强关联体系。为进一步探索材料中的电磁性质,制备出Na0．75Co1-xErxO2（0≤x≤0．10）多晶样品。对系统的电阻率、磁电阻和磁化率进行测量,结果显示,加磁场后系统的电阻减小,表明磁性离子对传导电子的散射作用;有效磁矩会随着Er的掺杂浓度增加迅速增大,3％的Er掺杂就能使有效磁矩几乎增大一倍;当系统的磁电阻达到15％,意味着Er的掺杂导致系统出现异常的电磁性质。     

Fundamentals of MRAM Technology

Developments in portable communication and computing systems are creating a growing demand for nonvolatile random access memory that is dense and fast. None of the existing solid-state memories can provide all the needed attributes in a single memory solution. Therefore, to achieve the required multiple functionality requirements, a number of different memories are being used while compromising performance and adding cost to the system. Magnetoresistive Random Access Memory (MRAM) has the potential to replace these memories in various systems with a single, universal memory solution. Key attributes of MRAM technology are nonvolatility and unlimited read and write endurance. In addition, MRAM can operate at high-speed and is expected to have competitive densities. In this paper we describe several fundamental technical and scientific aspects of MRAM with emphasis on recent accomplishments that enabled our successful demonstration of a 256 kbit memory chip.     

Variations in BOLD response latency estimated from event‐related fMRI at 3T: Comparisons between gradient‐echo and Spin‐echo

Functional magnetic resonance imaging (fMRI) commonly uses gradient‐recalled echo (GRE) signals to detect regional hemodynamic variations originating from neural activities. While the spatial localization of activation shows promising applications, indexing temporal response remains a poor mechanism for detecting the timing of neural activity. Particularly, the hemodynamic response may fail to resolve sub‐second temporal differences between brain regions because of its signal origin or noise in data, or both. This study aimed at evaluating the performance of latency estimation using different fMRI techniques, with two event‐related experiments at 3T. Experiment I evaluated latency variations within the visual cortex and their relationship with contrast‐to‐noise ratios (CNRs) for GRE, spin echo (SE), and diffusion‐weighted SE (DWSE). Experiment II used delayed visual stimuli between two hemifields (delay time = 0, 250, and 500 ms, respectively) to assess the temporal resolving power of three protocols: GRE_TR1000, GRE_TR500, and SE_TR1000. The results of experiment I showed the earliest latency with DWSE, followed by SE, and then GRE. Latency variations decreased as CNR increased. However, similar variations were found between GRE and SE, when the latter had lower CNR. In experiment II, measured stimulus delays from all conditions were significantly correlated with preset stimulus delays. Inter‐subject variation in the measured delay was found to be greatest with GRE_TR1000, followed by GRE_TR500, and the least with SE_TR1000. Conclusively, blood oxygenation level‐dependent responses obtained from GRE exhibit greater CNR but no compromised latency variations in the visual cortex. SE is potentially capable of improving the performance of latency estimation, especially for group analysis. © 2013 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 23, 215–221, 2013     

Spin Injection and Spin Accumulation in Permalloy–Copper Mesoscopic Spin Valves

We study the electrical injection and detection of spin currents in a lateral spin valve device, using permalloy (Py) as ferromagnetic injecting and detecting electrodes and copper (Cu) as nonmagnetic metal. Our multiterminal geometry allows us to experimentally distinguish different magnetoresistance signals, being (1) the spin valve effect, (2) the anomalous magnetoresistance (AMR) effect, and (3) Hall effects. We find that the AMR contribution of the Py contacts can be much larger than the amplitude of the spin valve effect, making it impossible to observe the spin valve effect in a conventional measurement geometry. However, these contact magnetoresistance signals can be used to monitor the magnetization reversal process, of the spin injecting and detecting Py contacts. In a nonlocal spin valve measurement we are able to completely isolate the spin valve signal and observe clear spin accumulation signals at T = 4.2 K as well as at room temperature. We obtain spin diffusion lengths in Cu of 1 m and 350 nm at T = 4.2 K and room temperature respectively.     

Spin current,spin accumulation and spin Hall effect

Abstract

Nonlocal spin transport in nanostructured devices with ferromagnetic injector (F1) and detector (F2) electrodes connected to a normal conductor (N) is studied. We reveal how the spin transport depends on interface resistance, electrode resistance, spin polarization and spin diffusion length, and obtain the conditions for efficient spin injection, spin accumulation and spin current in the device. It is demonstrated that the spin Hall effect is caused by spin–orbit scattering in nonmagnetic conductors and gives rise to the conversion between spin and charge currents in a nonlocal device. A method of evaluating spin–orbit coupling in nonmagnetic metals is proposed.     

Spin Injection: A Survey and Review

The study of the transport and relaxation of spin-polarized carriers in the solid state began about 30 years ago. Tunneling spectroscopy was applied to ferromagnet–insulator–superconductor junctions to demonstrate the polarization of interfacial currents. The use of a ferromagnetic material as an injector and/or detector of polarized carriers has since become a valuable tool, and spin injection has been applied to nonmagnetic metals, superconductors, and semiconductors. The spin injection phenomenology is reviewed in the context of two topics of continuing importance for basic and applied research: (i) the transmission of polarized carriers across ferromagnet/nonmagnetic material interfaces and (ii) carrier spin relaxation inside the nonmagnetic material.     

Spin Dynamics and Spin Transport

Spin-orbit (SO) interaction critically influences electron spin dynamics and spin transport in bulk semiconductors and semiconductor microstructures. This interaction couples electron spin to dc and ac electric fields. Spin coupling to ac electric fields allows efficient spin manipulating by the electric component of electromagnetic field through the electric dipole spin resonance (EDSR) mechanism. Usually, it is much more efficient than the magnetic manipulation due to a larger coupling constant and the easier access to spins at a nanometer scale. The dependence of the EDSR intensity on the magnetic field direction allows measuring the relative strengths of the competing SO coupling mechanisms in quantum wells. Spin coupling to an in-plane electric field is much stronger than to a perpendicular field. Because electron bands in microstructures are spin split by SO interaction, electron spin is not conserved and spin transport in them is controlled by a number of competing parameters, hence, it is rather nontrivial. The relation between spin transport, spin currents, and spin populations is critically discussed. Importance of transients and sharp gradients for generating spin magnetization by electric fields and for ballistic spin transport is clarified.