La2/3(CaXBa1-X)1/3MnO3材料的绝缘体-金属转变与晶界效应

用固相反应法制备了La2/3(CaxBa1-x)1/3MnO3(x=0.00,0.40,0.45,0.55,0.60,1.00)六种多晶CMR材料,并测量了在77K～350K范围内零磁场和0.4T外磁场下的电阻率。这些样品都出现了双电阻峰结构,采用Mott转变表达式ρ～exp(T0/T)1/4拟合了实验数据,结果表明高温峰的转变是绝缘体-金属(I-M)转变,而低温峰不是I-M转变峰,用自旋极化电子隧道效应和局域化模型解释了电阻和磁电阻行为。此外,在x=0.4～0.6掺杂范围内样品呈现出异常的电阻行为。     

La2/3Ca1/3MnO3/xTiO2复合磁性材料的磁电特性研究

采用溶胶-凝胶法制备La2/3Ca1/3MnO3纳米级磁性颗粒和TiO2胶体,按一定比例制成La2/3Ca1/3MnO3/xTiO2(其中x取0、0.1、0.2、0.3、0.4)复合磁电阻材料。用XRD谱确定样品的晶体结构。采用振动样品磁强计测量了样品的磁滞回线和场冷却磁化强度随温度的变化行为;采用标准四引线法测量样品在恒定磁场下的电阻率随温度的变化。实验测量结果表明体系的磁化强度、剩磁、矫顽力随TiO2含量的变化具有相似的规律,具有室温的居里温度;并且当磁场强度小于0.4T时,磁电阻与磁场平方成正比关系;当大于0.6T时,磁电阻与磁场成线性关系。     

CuO复合对La0.80Sr0.05K0.15MnO3电输运和磁电阻的影响

用固相反应法制备了(1-x)La0.80Sr0.05K0.15MnO3/xCuO(x=0.00,0.04,0.08,0.10,0.20,0.30)二相复合体。通过X射线衍射图谱、扫描电子显微镜照片检测结构,表明形成完好的钙钛矿相和CuO相,CuO分布在钙钛矿颗粒之间,形成对钙钛颗粒的包覆。通过测量零场和加场下的电阻率-温度曲线以及磁电阻-温度曲线,研究电输运性质及磁电阻效应,结果表明,CuO复合对La0.80Sr0.05K0.15MnO3电输运和磁电阻效应影响很大。在高温区出现磁电阻峰,表现出本征磁电阻特征,在低温区随温度降低磁电阻持续增大,表现出由界面引起的隧穿磁电阻特征,总的磁电阻是本征磁电阻与隧穿磁电阻的迭加,在中间温区出现磁电阻的温度稳定性。     

La_(0.80)Sr_(0.05)Na_(0.15)MnO_3/xCuO复合体电输运机制及磁电阻的温度稳定性

用固相反应法制备了La0.80Sr0.05Na0.15MnO3/xCuO(x=0,0.05,0.10,0.15,0.20,0.30)二相复合体系,通过X射线衍射(XRD)图谱、电阻率-温度(ρ-T)曲线、ρ-T拟合曲线、磁电阻-温度(MR-T)曲线,研究了样品的电输运机制及磁电阻的温度稳定性。结果表明,在低温类金属区,电输运机制是自旋电子受单磁子散射作用,用ρ-T2拟合得很好;在较高温区MR-T曲线出现"压峰效应",是因为界面效应产生的非本征磁电阻占优势,本征磁电阻与非本征磁电阻迭加产生的磁电阻随温度变化不大;特别是x=0.05的样品,0.8 T磁场下在240 K~320K温区,磁电阻保持(9.9±0.4)%基本不随温度变化,在室温附近△T=80 K温区磁电阻的温度稳定性很好。     

La_(0.67)Ca_(0.33-x)Ba_xMnO_3的电输运性质反常及磁电阻温度稳定性

用固相反应法制备La_(0.67)Ca_(0.33-x)Ba_xMn O_3(x=0,0.10,0.15,0.20,0.25)系列样品,烧结温度为1200℃。通过X射线衍射(XRD)图谱,扫描电子显微镜(SEM)照片、零场和加场下的电阻率-温度(ρ-T)曲线,磁电阻-温度(MR-T)曲线,研究Ba~(2+)替代Ca~(2+)的La_(0.67)Ca_(0.33-x)Ba_xMnO_3的电输运性质及磁电阻温度稳定性。结果表明:随Ba~(2+)替代量的改变晶体结构发生变化,x=0的样品为立方结构,x=0.10的样品为正交结构,x=0.15,0.20,0.30的样品为六方结构。x=0.10样品的电输运性质出现反常,它的电阻率不仅比未掺Ba的La_(0.67)Ca_(0.33)Mn O_3高出60倍,也比高掺Ba样品的电阻率高出几倍,随Ba~(2+)替代量的增加电阻率逐渐下降,用掺Ba引起的晶格结构变化及Mn O_6八面体畸变进行解释;高掺Ba样品实现了室温附近磁电阻温度稳定性,x=0.15的样品在187~246 K温区磁电阻为(8.91±0.42)%,x=0.20的样品在230~290 K温区磁电阻为(6.07±0.32)%,x=0.25的样品在320 K以下直至50 K温区磁电阻均大于14%。在0.8 T磁场下在室温附近产生磁电阻的温度稳定性有利于MR效应的实际应用。磁电阻温度稳定性机制是,钙钛矿的磁电阻是体相内双交换作用产生的本征磁电阻与界面自旋相关散射或自旋极化隧穿产生的低场磁电阻的叠加,制备样品时控制烧结温度(1200℃),尽量提高低场磁电阻,两种磁电阻叠加产生磁电阻温度稳定性。     

二相复合体La_(2/3)Ca_(1/3)MnO_3/xCuO的界面效应

用固相反应法制备二相复合体La2/3Ca1/3Mn O3/x Cu O(x=0,0.04,0.08,0.12,0.20,0.30)。用X射线衍射(XRD)图谱、扫描电子显微镜(SEM)照片检测样品的结构,通过零场冷和加场冷的电阻率-温度(ρ-T)曲线、磁电阻-温度(MR-T)曲线研究界面效应对电输运性质及磁电阻的影响。结果表明:样品形成完好的钙钛矿相和Cu O相;复合样品的颗粒比较均匀,颗粒大小约2μm,界面比较清晰;复合样品的ρ-T曲线出现双峰,x=0.04的样品电输运性质出现反常,它的电阻率既小于x=0的电阻率,也小于x=0.08的电阻率;复合样品的MR-T曲线也出现双峰,x=0.20,0.30的样品,0.8 T磁场下在50~250 K温区磁电阻不小于13.4%,实现了大温区且有较高磁电阻,有利于MR的实际应用。用影响电输运的多种因素的竞争很好地解释了界面效应。     

钙钛矿锰氧化物(La_(0.9)Eu_(0.1))_(4/3)Sr_(5/3)Mn_2O_7的磁性和电输运性的研究

采用传统固相法制备钙钛矿锰氧化物(La_(0.9)Eu_(0.1))_(4/3)Sr_(5/3)Mn_2O_7多晶样品。通过XRD表明,样品(La_(0.9)Eu_(0.1))_(4/3)Sr_(5/3)Mn_2O_7呈现单相性良好;通过对磁化强度随温度变化(M-T)、磁化强度在不同温度下随磁场变化曲线(M-H)和电子自旋共振ESR谱线表明,随着温度的升高,样品先后经历了三维长程铁磁有序转变点(T_C~(3D)≈75 K)和二维短程铁磁有序转变点(T_C~(2D)≈227 K),在T_C~(3D)与T_C~(2D)之间,系统处于铁磁与顺磁混合态;当温度达到360 K系统进入纯顺磁态,另外样品在低温区(T_C~(3D)以下)出现类团簇自旋玻璃特征。通过磁电阻随温度变化的ρ-T曲线,在T≈84 K附近,样品出现最大磁电阻。而且通过对(La_(0.9)Eu_(0.1))_(4/3)Sr_(5/3)Mn_2O_7的ρ-T曲线拟合,得到样品在高温区域的导电方式为热激活模型。     

(1-x)La0.7Ca0.3MnO3/xAg磁电阻的温度稳定性

用固相反应法将Ag引入La0.7Ca0.3MnO3颗粒表面,形成(1-x)La0.7Ca0.3MnO3/xAg(x=0.00,0.10,0.20,0.25,0.30,0.35)。通过XRD谱、扫描电子显微镜(SEM)照片及SEM能谱(EDS)、ρ-T曲线、ρ-T拟合曲线、MR-T曲线,研究电输运性质及MR温度稳定性。结果表明:随复Ag量增大,电阻率急剧下降,高复Ag(x≥0.3)的电阻率比低复Ag(x≤0.10)的电阻率下降2个数量级;发现x=0.00,0.10用ρ-T2公式拟合比较合理,是单磁子散射作用;x=0.20,0.25用ρ-T2.5公式拟合比较合理,是自旋波散射作用;x=0.30,0.35,分别在97,93 K以上用ρ-T4.5公式拟合比较合理,说明是电子-磁子散射作用;所有复Ag样品,在250～20 K温区MR表现出很好的温度稳性,特别是x=0.10的样品,在220～150 K温区MR保持(13.8±0.2)%。提出磁电阻稳定性的机制:Ag与钙钛矿颗粒形成电输运二元通道,钙钛矿颗粒通道产生的低场磁电阻与Ag通道产生的正常磁电阻共同作用,产生MR温度稳定性。     

Cu的Mn位掺杂对La0.7Ca0.3MnO3磁电传输特性的影响

采用溶胶凝胶法制备了系列多晶CMR材料La0.7Ca0.3Mn1-xCuxO3(x=10%,14%,15%,16%,18%,20%).在77 K～350 K温度范围内测量了样品的电阻率及磁电阻大小.实验结果发现,系列样品在整个测温范围内均表现出绝缘相行为,未出现金属相到绝缘相的M-I转变.对系列样品的ρ-T曲线进行Inρ-T-(1/4)关系拟合,发现掺杂量为x=10%、14%两个样品在整个温区内的Inρ-T-(1/4)关系基本呈线性,掺杂量为15%、16%、18%及20%四个样品在较高温区的Inρ-T-(1/4)关系亦基本呈线性.采用Mott等人提出的小极化子VRH模型进行了解释.实验中还发现,系列样品的磁电阻在整个测温范围内基本呈现隧穿磁电阻行为,基于我们采用Monte Carlo模拟方法提出的隧穿磁电阻模型,对样品的磁电阻行为进行了解释.     

Tb3NiSi2合金磁相变与磁热性能研究

依据X射线衍射(XRD)与等温磁化曲线和等磁场变温磁化曲线,主要研究了Tb₃NiSi₂合金相结构与磁性相变和磁热性能。XRD表明,采用800℃保温14天,然后炉冷至室温的热处理方法制备的R3NiSi2(R=Tb,Dy,Ho,Er)合金中,主相均为Gd3NiSi2型正交结构(空间群:Pnma,No.62)相,但杂相R5Si3含量存在差异,其规律是从Er到Tb,含量依次减少,Tb₃NiSi₂合金样品基本为一个单相,其相应晶格常数分别为a=1.1240(8)nm,b=0.41009(8)nm,c=1.12058(1)nm。等温磁化曲线显示在50~300 K温度范围内,Tb₃NiSi₂合金仅展现出铁磁-顺磁相变,并没有在130,82,66,53 K等观察到相关文献报道的多重的反常反铁磁态-铁磁态(AFM-FM)相变。0.01 T磁场下的磁化强度对温度求导曲线(d M/d T)和0~2 T磁场下的Arrott图结果证实合金铁磁-顺磁二级磁相变居里温度(Tc)=88 K。居里外斯定理拟合表明合金中Tb³⁺粒子的有效磁矩为9.90μB(μB为玻尔磁子),同期望值μeff/Tb³⁺=g(J(J+1))1/2=9.72μB基本一致。在磁热性方面,Tb₃NiSi₂合金在0~2 T磁场范围内,低场响应性较差,铁磁态分子的有效磁矩远低于顺磁分子有效磁矩,最大磁熵变(-ΔSM_max)为3.2 J·kg^-1·K^-1;在对应的半高宽温跨(δTFWHM)=35.5K范围内,相对制冷量为113 J·kg^-1。     

Anisotropic Magnetoresistance of Perovskite La_(2/3)Ca_(1/3)MnO_3/YBa_2Cu_4O_8/La_(2/3)Ca_(1/3)MnO_3 Trilayer Films

Magneticanisotropyhasbeenfoundinmanyex oticmaterials,suchasmetallicsuperlatticeswithgi antmagnetoresistanceeffectsandthehightempera turesuperconductingoxideYBa2Cu3O7 δ[1～4].The anisotropycouldfindnovelusesinnewdeviceappli cations.Agoodexampleisthehig…     

Giant Low-Field Magnetoresistance in Polycrystalline La2/3Ca1/3Mn1-xCuxO3 Sintered at 1100 ℃

The La2/3Ca1/3Mn1-xCuxO3 (0≤x＜6%) samples were prepared by sol-gel method. Transport properties, conductive mechanism and low-field magnetoresistance (MR0) were investigated. It is found that conductive mechanism is described by Mott′s variable-range hopping and the MR0 near the insulator-metal transition temperature (Tp) increases from 2% to 80% with x from 0 to 4% for a low field of 0.3 T. The experiment also shows that the intergrain magnetoresistance is largely improved. The results indicate that a proper amount of Cu doping can substantially improve magnetoresistance effects.     

La_(0.5)Sm_(0.2)Sr_(0.3)MnO_3/Ag_x二相复合体的结构及磁电阻

用固相反应法制备La0.5Sm0.2Sr0.3MnO3/(Ag2O)x/2(x=0.00,0.04,0.08,0.25,0.30)样品,通过X射线衍射谱线(XRD),扫描电子显微镜(SEM)照片及SEM能谱(EDS),ρ-T曲线,研究样品的输运行为及磁电阻效应。结果表明:少量掺杂时Ag全部挥发。掺杂量较多时,挥发后多余的Ag主要以金属态包覆在母体颗粒的表面,使体系形成两相复合体。掺Ag为30%摩尔比时,样品的电阻率较低掺杂样品的电阻率降低一个数量级,在300K、0.5T磁场下,磁电阻明显增强,达到9.4%,这与颗粒母体界面结构的改善有关,也与材料电阻率的降低有关。     

Ag掺杂对La0.8Ca0.2MnO3电输运性质及磁电阻的影响

用固相反应法制备La1-x(Ca1-yAgy)xMnO3(y=0.0,0.2,0.4,0.6,0.8)系列样品,通过X射线衍射(xRD)谱,电阻率-温度(p-T)曲线,磁电阻-温度(MR-T)曲线,研究了在A位同时掺入一价、二价元素而保持Mn3+/Mn4+比值(摩尔比n(A)/n(B))不变的La1-x(Ca1-yAgy)xMnO3体系A位离子半径＜rA＞及A位离子的无序度σ2对电输运性质及磁电阻的影响.结果表明:所有样品的绝缘体,金属相变温度基本不变,用A位离子半径＜rA＞及A位离子的无序度对电输运性质影响的竞争给予解释;在0.8T磁场下,y=0.0样品在133～26K温区MR基本保持23％以上,y=0.6样品在209 ～ 131 K温跨区MR都在23％以上,在如此宽温区产生如此大的MR有利于MR的实际应用;MR的温度稳定性的机制是本征磁电阻与隧穿磁电阻竞争的结果.     

Transverse magnetoresistance peculiarities of thermoelectric Lu-doped Bi2Te3 compound due to strong electrical disorder

The n-type thermoelectric Bi_1.9Lu_0.1Te₃ was prepared by microwave-solvothermal method and spark plasma sintering. The magnetic field and temperature dependences of transverse magnetoresistance measured within temperature 2–200 K interval allow finding the peculiarities characteristic for strongly disordered and inhomogeneous semiconductors. The first peculiarity is due to appearance of linear-in-magnetic field contribution to the total magnetoresistance reflected in a crossover from quadratic magnetoresistance at low magnetic fields to linear magnetoresistance at high magnetic fields. The linear magnetoresistance can result from the Hall resistance picked up from macroscopically distorted current paths due to local variations in stoichiometry of the compound studied. The second peculiarity is that both linear magnetoresistance magnitude and crossover field are functions of carrier mobility which is in agreement with the Parish and Littlewood model developed for disordered and inhomogeneous semiconductors. An increase in the mobility due to a decrease in temperature is accompanied by an increase in the magnetoresistance magnitude and a decrease in the crossover field. Finally, the third peculiarity is related to the remarkable deviation of the total magnetoresistance measured at various temperatures from the Kohler's rule. Presence of strong inhomogeneity and disorder in the Bi_1.9Lu_0.1Te₃ structure concluded from the magnetoresistance peculiarities can be responsible for the remarkable reduction in the total thermal conductivity of this compound.     

Magnetic and Magnetoresistance Properties in （La0.67Ca0.33MnO3）x／（ZrO2）1-x Composite System

Inrecentyears,muchattentionhasbeenpaidto therare earthdopedmanganitesduetotheirextraor dinarymagneticandtransportproperties.Onone hand,itisbecausetheirabundantelectronicstructure andmagneticproperties[1];ontheotherhand,they exhibitcolossalmagnetoresistan…     

Magnetic Coupling in La0.67 Ca0.33 MnO3／La0.67 Sr0.33 CoO3／La0.67 Ca0.33MnO3 Trilayer Films

The perovskite La0.67 Ca0.33 MnO3/La0.67 Sr0.33 CoO3/La0.67 Ca0.33 MnO3 trilayers were fabricated by a facing-target sputtering technique and their magnetotransport properties were investigated. The magnetoresistance is dependent on spacer thickness and dramatically decreases when La0.67 Sr0.33 CoO3 layer is thick enough because of its short-circuiting effect. Different from La0.67 Ca0.33 MnO3 single layer, trilayer films with thin La0.67 Sr0.33 CoO3 spacer have the enhanced metal-semiconductor transition temperature (TMS) of La0.67 Ca0.33 MnO3 layers. The magnetic coercivity Hc shows a nonmonotonic behavior with changing the spacer layer thickness at 230 K. The waist-like hysteresis indicates that there is an indirect exchange coupling between the top and bottom Lao.67 Ca0.33 MnO3 layers across the spacer La0.67 Sr0.33 CoO3 layer.     

改善钙钛矿材料室温下磁电阻效应的有效途径

将Ag2O与La0.67Ca0.25Sr0.08MnO3均匀混合经高温烧结后形成La0.67Ca0.25Sr0.08MnO3/xAg两相复合体系。随Ag掺入量的增加,样品的电阻率明显下降,磁化强度有小量下降,TC及电阻率的峰值温度TP没有明显变化。304K温度下,从x=0.25样品中得到最大磁电阻效应,磁电阻比约为41%,分别是La0.67Ca0.25Sr0.08MnO3和La0.67Sr0.33MnO3样品的1.6及10倍。磁电阻的本征效应和传导电子在界面处的自旋相关散射作用是产生室温下增强磁电阻效应的主要原因。结果说明,利用离子的掺杂效应调整居里温度到室温附近,再结合金属/钙钛矿复合体系界面属性丰富的特性来制备庞磁电阻材料是提高室温磁电阻有效的途径,这对应用研究具有十分重要的意义。     

Fex-C1-x/Si颗粒膜的巨磁电阻效应

利用激光脉冲沉积（PLD）方法制备了沉积于硅基片上的掺杂过渡金属的非晶碳膜结构Fex-C1-x/Si。Fex-C1-x/Si的磁电阻（MR）可正可负，随温度而变化。当温度T〈258K时，Fe0．011-C0.989／Si的MR为负值；当258K〈T〈340K时，该材料的MR为正值，在室温磁场为1T时，该材料的正MR可以大于20％。且在不同的温度范围中，该材料的MR和外加磁场的依存关系呈现出不同的特点：在T=280和300K时，当磁场小于1T时，MR随磁场的增加而快速增加，之后随磁场的继续增加MR增加开始变得缓慢；在T=350K时，MR近似以磁场的B^1.5。的规律变化；而在T=30K时，MR为负值且其大小随磁场的增加而减小。利用双通道模型对该MR效应进行了初步解释。     

Large Magnetic Entropy Effect in La2／3Ca1／3MnO3

The magnetocaloric effect in the colossal magnetoresistance material La2/3Ca1/3MnO3 was studied. From the measurements of temperature dependence of magnetization in various magnetic fields, the large magnetic entropy change associated with the ferromagnetic-paramagnetie transition was discovered. This result suggests that perovskite manganites are suitable candidates as working substance in magnetie refrigeration technology.