金属离子掺杂对CuO基纳米复合材料的交换偏置调控

为了研究反铁磁基体中掺杂的金属离子对交换偏置效应的影响, 本文采用非均相沉淀法制备了纳米复合材料. X射线衍射图(XRD)和场发射扫描电子显微镜(SEM) 照片清晰表明CuO纳米复合样品具有统一的颗粒尺寸, 约为80 nm. 通过体系中掺杂磁性金属离子Ni和Fe, 实现了亚铁磁MFe₂O₄ (M=Cu, Ni)晶粒镶嵌在反铁磁(AFM) CuO 基体中. 在CuO基体中加入少量的Ni能改变两相交界面的磁无序从而生成类自旋玻璃相, 相应提高对铁磁相磁矩的钉扎作用. 同时, 场冷过程中反铁磁相内形成磁畴, 冻结在原始状态或磁场方向上, 畴壁也起到钉扎铁磁自旋的作用, 进而提高交换偏置效应. 随后加入的Ni 会生成各向异性能较大的NiO, 也能够提高交换偏置场. 在带场冷却下, 所有样品均发生垂直交换偏置, 也证明了样品在场冷过程中形成了自旋玻璃相, 正是由于亚铁磁与自旋玻璃相界面上的磁交换耦合, 才导致回线在整个测量范围内发生了向上偏移. 零场冷却和场冷却(ZFC/FC)情况下磁化强度与温度变化曲线(M-T)说明在这些复合材料中的交换偏置效应是由于存在亚铁磁颗粒和类自旋玻璃相界面处的交换耦合作用. 研究发现随着持续掺杂Ni离子, 交换偏置场先缓慢增加后又急剧增加, 生成各向异性能高的反铁磁相NiO 和反铁磁相内的畴态组织是这一结果的原因.     

Heusler合金NiCoMnSn中的磁场驱动马氏体相变、超自旋玻璃和交换偏置

合成了一系列Ni_50-xCo_xMn₃₉Sn₁₁ (8≤x≤10) 样品, 并对它们的结构和磁性进行了研究. 发现随Co含量的增加, 样品的饱和磁化强度逐渐增强, 并在Ni₄₂Co₈Mn₃₉Sn₁₁中实现了磁场诱发马氏体相变. 另外, 在Co大于8.0的成分中探测到了超自旋玻璃, 并且观察到交换偏置现象. 证实了超自旋玻璃的马氏体相和铁磁奥氏体母相共存, 这也是产生交换偏置的原因.我们猜测超自旋玻璃的形成可能是来源于Mn-Mn团簇的存在, 这和之前报道的Mn₂Ni_1.6Sn_0.4 的结果相一致^[1]. 关键词： NiCoMnSn Heusler合金 超自旋玻璃 交换偏置     

Bi0.8Ba0.2FeO3/La0.7Sr0.3MnO3异质结制备及其交换偏置效应研究

采用脉冲激光沉积方法, 通过调节激光能量、激光频率、衬底温度、氧压、靶基距等工艺参数, 在(100)取向的铝酸镧单晶衬底上制备出Bi_0.8Ba_0.2FeO₃/La_0.7Sr_0.3MnO₃多铁性异质结. X射线衍射图谱表明薄膜呈钙钛矿结构, 高分辨透射电镜图谱和能量色散X射线图谱表明两相界面清晰且具有良好的匹配度, 异质结呈(00l)取向性生长. 加场冷却条件下不同温度的磁滞回线(M-H)测量结果表明样品具有明显的交换偏置效应, 交换偏置场(H_EB)随温度的线性变化可能与异质结界面处电子轨道的重构和界面处自旋、轨道自由度之间的复杂的相互作用有关.     

Heusler合金Mn50–xCrxNi42Sn8的相变、磁性与交换偏置效应

研究了Mn50–xCrxNi42Sn8 (x=0, 0.4, 0.6, 0.8)多晶样品的相变、磁性和交换偏置效应.结果表明,该系列合金在室温下都具有非调制的四方马氏体结构.马氏体逆相变温度随Cr含量增加而逐渐降低. 20 k Oe磁场下的M-T曲线表明,该系列合金的磁性比较弱.两相之间的磁性差最大为△M=7.61 emu/g.磁性的变化主要与Mn-Mn间距的变化以及Ni(A位)-Mn(D位)间杂化作用的强弱有关.在低温下,马氏体相的磁性随着Cr含量增加而增强.在500 Oe的外加磁场作用下,从室温冷却到5 K,在Mn50Ni42Sn8合金中观察到高达2624 Oe的交换偏置场.随着Cr含量的增加,交换偏置场逐渐减小.当Cr含量x=0.8时,随着冷却场的增加, 5 K时的交换偏置场先迅速增加然后逐渐减小.当冷却场为500 Oe时,交换偏置场最大.这主要归因于自旋玻璃态与反铁磁性区域的界面交换耦合作用的变化.     

含铁磁颗粒的颗粒膜自旋激发弛豫研究

应用s-d交换作用模型，在对自旋格林函数s-d交换相互作用单环近似下，求出了含铁磁纳米颗粒的颗粒膜中自旋激发弛豫.并以(a-C：H)_1-xCo_x颗粒膜 为例，探讨了自旋激发弛豫随温度的变化规律.结果表明：经自旋极化激发实现的自旋激化弛豫过程与温度无关，而经热激活的自旋激发弛豫过程与温度有关. 关键词： 自旋激发弛豫 颗粒膜 铁磁颗粒     

V替代Mn对La_(0.45)Ca_(0.55)MnO_3电荷有序相及自旋玻璃态的影响

用固相反应法制备了La0.45Ca0.55Mn1-xVxO3(x=0.00,0.10)多晶样品.通过X射线衍射谱、质量磁化强度-温度曲线、电子自旋共振谱,研究了V5+替代Mn3+/Mn4+对La0.45Ca0.55MnO3电荷有序相和自旋玻璃态的影响.实验结果表明,当x=0.10时,不仅母体的电荷有序相基本破坏,而且母体在40K左右出现的自旋玻璃态也被融化.电荷有序相被破坏的主要原因是用V5+替代Mn3+/Mn4+后,增加了Mn3+与Mn4+的比例,使铁磁双交换作用优于反铁磁超交换作用;自旋玻璃态的融化是由于V替代Mn后破坏了反铁磁背景下有少量铁磁成分的自旋玻璃态的形成条件.     

La0.67Ca0.33MnO3纳米多晶环的时效现象

利用脉冲激光沉积(PLD)方法在多孔Al2O3模板上制备了纳米结构La0.67Ca0.33MnO3(LCMO)多晶膜.温度为80K时,LCMO纳米多晶环样品的电阻随时间连续下降,表现出明显的时效特征.时效后,样品具有保持低电阻状态的性质.该时效行为可用扩展的指数函数拟合.通过拟合可以得到相应条件下的样品平衡态电阻、样品的特征驰豫时间等信息.利用Core-Shell模型以及自旋玻璃相的存在对观察到的时效行为及拟合结果进行了解释.     

(Ni0.81Fe0.19)1－xCrx作为种子层对NiFe/PtMn双层膜交换偏置的影响

将NiFe/PtMn双层膜生长在(Ni_0.81Fe_0.19)_1-xCr_x种子层材料上，通过改变种子层中Cr的原子含量，系统的研究了NiFe/PtMn双层膜中PtMn晶粒尺寸和织构对交换偏置的影响.对退火270℃，5h后的NiFe/PtMn双层膜磁性的研究表明,PtMn织构强弱对交换偏置场的影响不明显，而PtMn的晶粒尺寸是影响交换偏置场的主要因素，PtMn颗粒的相干长度在11.3nm左右时得到了较大的交换偏置场. 关键词： NiFe/PtMn双层膜 交换偏置场     

一维钻石链反铁磁Ising模型磁化的模拟

采用Monte Carlo模拟方法对自旋为1/2的一维钻石链反铁磁Ising系统的磁化行为进行研究.在这个系统中,反铁磁的交换作用和三角结构导致存在自旋阻挫.重点研究不同的反铁磁自旋交换作用对系统磁行为和自旋构型的影响.模拟磁化曲线中M=M_S/3磁化平台,得到平台宽度随不同的自旋交换相互作用强弱的变化关系,以及出现磁化平台时格点的自旋构型;给出亚稳态存在的条件及亚稳态时的微观构型.研究磁滞回线随温度的变化关系.结果表明,随着温度的升高,磁滞回线逐渐减小,最终消失.     

非磁性掺杂下的铁磁/反铁磁系统中的交换偏置和矫顽场

采用Monte Carlo方法,分别讨论了在铁磁/反铁磁双层膜和铁磁/反铁磁单层混合膜中,掺入非磁性物质后,掺杂浓度对交换偏置以及矫顽场的影响.计算结果表明:随着掺杂浓度的增大,双层膜和单层膜交换偏置都有先增大后减小的现象,而其矫顽场则先减小后增大.在相同掺杂浓度下,对随机掺杂和规则掺杂两种不同掺杂方式的结果比较发现:铁磁/反铁磁双层膜中,规则掺杂下产生的交换偏置和矫顽场都得到了增强;对于单层混合膜,随机掺杂下的交换偏置更强,规则掺杂下的矫顽场更大.研究发现对于双层膜规则掺杂可明显地导致其磁滞回线的不对称性,说明铁磁/反铁磁系统中磁滞回线的不对称性与界面自旋微结构密切相关.     

Large coercivity and small exchange bias in Mn3O4 / MnO nanoparticles

Core/shell structured Mn₃O₄/MnO nanoparticles were prepared by arc evaporating metallic manganese in air. These nanoparticles show unconventional exchange bias phenomena, in which the Curie temperature of the ferrimagnetic Mn₃O₄ is lower than the Néel temperature of the antiferromagnetic MnO. The exchange bias field in Mn₃O₄/MnO nanoparticles is much smaller than that in Mn₃O₄/Mn nanoparticles, due to the weak interfacial exchange coupling. The coercivity of the Mn₃O₄ phase in nanoscale is almost three times greater than that of the bulk Mn₃O₄.     

Size-dependent exchange bias in single phase Mn_3O_4 nanoparticles

Glassy magnetic behavior and exchange bias phenomena are observed in single phase Mn_3O_4 nanoparticles.Dynamics scaling analysis of the ac susceptibility and the Henkel plot indicate that the observed glassy behavior at low temperature can be understood by taking into account the intrinsic behavior of the individual particles consisting of a ferrimagnetic(FIM) core and a spin-glass surface layer.Field-cooled magnetization hysteresis loops display both horizontal and vertical shifts.Dependence of the exchange bias field(H_E) on the cooling field shows an almost undamped feature up to 70 kOe,indicating the stable exchange bias state in Mn_3O_4.H_E increases as the particle size decreases due to the higher surface/volume ratio.The occurrence of the exchange bias can be attributed to the pinning effect of the frozen spin-glass surface layer upon the FIM core.     

Dependence of exchange bias on core/shell relative dimension in ferromagnetic/antiferromagnetic nanoparticles

We employ a modified Metropolis Monte Carlo simulation to study the effect of bimagnetic core/shell relative dimension on exchange bias in ferromagnetic/antiferromagnetic nanoparticles. The exchange bias field is inversely proportional to the ferromagnetic shell thickness in the antiferromagnetic (core)/ferromagnetic (shell) nanoparticles, while in the nanoparticles with an opposite core/shell structure the exchange bias behavior is complex and distinguished in different ranges of the ferromagnetic core radius. The work elucidates unambiguously how the core and shell dimensions optimize the exchange bias in nanoparticles.     

Large-scale synthesis and magnetic properties of cubic CoO nanoparticles

We present the synthesis, microstructural and magnetic characterization of cubic CoO nanoparticles with well-controlled size and shape. The as-synthesized CoO nanoparticles are stable because of the organic coating that occurred in situ. The Néel temperature is 225 and 280 K for the 42 and 74 nm CoO particles, respectively. The CoO nanoparticles exhibit anomalous magnetic properties, such as large moments, coercivities and loop shifts. These results provide evidence for the formation of spin compensated random system in CoO. The structurally distorted and magnetically disordered surface layer ferromagnetic phase played an important role in the magnetic behavior of CoO nanoparticles. The smaller is the particle size, the stronger is the contribution of the ferromagnetic phase and the more is the surface layer helpful to enhance the observed coercivity and the exchange bias.     

Structure and magnetic properties of Fe/Fe oxide clusters

Fe clusters have been synthesised in ultra-high-vacuum chamber using a gas-stabilized cluster aggregation method that ensures good control of the cluster size and naturally oxidized in order to obtain Fe/Fe oxide core-shell nanoparticles. The morphology of an individual nanoparticle, as revealed by transmission electron microscopy, consists of a Fe core of an average diameter of 4.4 nm surrounded by an oxide shell of uniform thickness of about 1.2 nm in average. The nanoparticles may be assimilated with a ferro-/antiferromagnetic (FM/AF) system. The morpho-structural features have been correlated with magnetic measurements on the core-shell nanoparticles. A significant exchange bias effect has been measured, when the sample was field-cooled under an applied field of 3 T. As the morphology of core-shell nanoclusters is much more complicated than in FM/AF bilayers of regular thickness due to the particular geometry of the coronal AF layer, the shape and surface anisotropy have to be taken into account for a correct interpretation of the magnetic data.     

Coexistence of exchange bias and magnetization pinning in the MnO(x)/GaMnAs system

Coexistence of exchange bias (H(E)) and magnetization (M) shift was observed in as-grown and field-annealed MnO(x)/Ga(0.95)Mn(0.05)As bilayers. It was found that H(E) initially decreases with the annealing time t(a) and then increases when t(a) > 30 min, while the M shift remains almost unchanged with t(a). X-ray photoelectron spectroscopy (XPS) analysis reveals that MnO(x) is composed of MnO and Mn(3)O(4), and the volume amount ratio of Mn(3)O(4) to MnO increases with increasing t(a). A simple model based on a uniform MnO-Mn(3)O(4) interface with constant 'pinned' uncompensated interfacial spins is proposed to account for the observed exchange-biased phenomena in the bilayers.     

Size and Shape Effect of Gold Nanoparticles in “Far‐Field” Surface Plasmon Resonance

The “far‐field” surface plasmon resonance (FSPR) of metal nanoparticles, which have built a facile way to emission enhancement of red, green, blue, and white with nice reproducibility, has big potential application in solution‐processed organic light‐emitting diodes (OLEDs). According to the theory of the “far‐field” effect, the reflectivity of the metal surface and the phase shift at the reflection play an important role in enhancing ratio, which strongly relate to the size and shape of nanoparticles. In this work, gold nanospheres with different sizes and nanorods are synthesized in order to determine the size and shape effect of FSPR. The results demonstrate that the one with higher reflectivity in a certain range induces a better emission enhancement in the luminous efficiency and the maximum brightness. The nanoparticles with bigger sizes and shape of rods have higher reflectivity, which is consistent with the simulation based on FSPR effect. The phase shifts of different nanoparticles are optimized by the distance between gold nanoparticles and emitters. The metal NPs with a high reflectivity and the applicable phase shift will have big potential for the emission enhancement in OLEDs.     

Microstructural and magnetic properties of passivated Co nanoparticle films

Co nanoparticle films were prepared by plasma–gas-condensation-type particle beam deposition system. High-resolution transmission electron microscopy images show that the Co nanoparticles have a very narrow size distribution with an average diameter of 20 nm, and each of the Co nanoparticles is covered with an 3 nm layer of CoO. Hysteresis loops of the films after field-cooling in a 5 T magnetic field are greatly shifted, which can be attributed to the exchange bias effect caused by the interfacial exchange coupling between the CoO shell and the Co core. The zero field cooled films show several prominent properties, such as a quite large coercive field, a small remanence and their abnormal dependences on temperature. All these observations can be attributed to the existence of an exchange bias effect within each single Co nanoparticle even without a field-cooling process.