Nd1-xSrxMnO3中掺杂浓度对电脉冲诱导电阻转变效应的影响

采用两线测量模式对固相烧结方法制备的Nd_1-xA_xMnO₃ (A= Ba, Ca, Sr,x= 0-0.9) 陶瓷样品电脉冲诱导电阻转变(EPIR)效应和I-V特性进行了测量. 结果表明, 与Nd_0.7Sr_0.3MnO₃一样, 相同浓度掺杂的Nd_0.7Ba_0.3MnO₃和Nd_0.7Ca_0.3MnO₃ 样品也能诱发稳定的室温EPIR效应. 进一步对Nd_1-xSr_xMnO₃系列样品的EPIR研究表明, 这种界面相关的EPIR效应与样品中电子或空穴掺杂浓度密切相关, 在半掺杂 (x= 0.5)附近, 样品与电极接触界面能诱发稳定的EPIR效应. 然而, 随掺杂浓度的进一步增大或降低, EPIR效应逐渐出现减弱、不明显到完全消失的过程. 产生这种现象的原因可能与锰氧化物中由于掺杂浓度差异所导致的界面缺陷在不同极性脉冲激励下重新分布而产生的内电场强弱有关.     

层状钙钛矿La1.3Sr1.7Mn2-xCuxO7的磁性及电特性

通过固相反应烧结法成功制备了层状钙钛矿La_1.3Sr_1.7Mn_2-xCu_xO₇多晶，主要研究了其磁电特性.结果表明，样品为Sr₃Ti₂O₇型钙钛矿结构.随着温度的降低，其磁性经历了一个很复杂的转变过程.当x=0时，在T^*=231K出现二维短程铁磁有序，在<     

Tb0.3Dy0.7－xPrx(Fe0.9Al0.1)1.95合金的磁致伸缩、自旋重取向和穆斯堡尔谱研究

系统研究了室温下Tb_0.3Dy_0.7-xPr_x(Fe_0.9Al_0.1)_1.95 (x=0，0.1，0.2，0.25，0.3，0.35)合金中稀土元素Pr替代Dy对晶体结构、磁致伸缩、各向异性和自旋重取向的影响. 结果发现，x≤0.1时，Tb_0.3Dy_0.7-xPr_x(Fe_0.9Al_0.1)_1.95完全保持MgCl₂立方Laves相结构，0.1<x≤0.3，有杂相出现并且随Pr替代量逐渐增多；晶格常数a随Pr含量x的增加缓慢增大. 磁致伸缩测量发现，随着替代量x的增多磁致伸缩减小；x>0.2时超磁致伸缩效应消失. 然而，x=0.1时合金的磁致伸缩略大于没有替代的，而且磁致伸缩随磁场更易趋于饱和，说明Pr替代有助于降低磁晶各向异性. 内禀磁致伸缩λ₁₁₁随Pr替代量x的增加接近线性增加. 由相对磁化率随温度的变化关系可以看出，自旋重取向温度随Pr替代量的增多呈先增后降趋势，在x=0.1处出现极大值. 穆斯堡尔效应表明，随Pr含量的增加Tb_0.3Dy_0.7-xPr_x(Fe_0.9Al_0.1)_1.95合金中易磁化轴可能在{110}面上绕主对称轴作微小转动，发生自旋重取向. 与Al元素替代效应相比，Pr替代Dy对自旋重取向的影响相对较小.     

Co掺杂对铁磁金属La0.8Sr0.2MnO3磁电阻影响机理

通过对La_0.8Sr_0.2Mn_1-yCo_yO₃(y≤02)饱和磁矩和输运的测量,研究了Co对La_0.8Sr_0.2MnO₃的磁电阻影响机制.结果表明,在La_0.8Sr_0.2Mn_1-yCo_yO₃（y≤02）中Co³⁺离子是低自旋态.由于Mn³⁺—O—Co³⁺—O—Mn³⁺类型的磁交换与Mn³⁺-Mn⁴⁺离子间双交换作用相比较弱,Curie温度T_C附近的磁电阻随着Co掺杂量的增加而降低.与此相反,由于Co²⁺离子与e_g巡游电子的反铁磁交换耦合作用,低温区间的磁电阻随着Co掺杂量的增加而升高.     

锰位铁掺杂La0.67Sr0.33FexMn1-xO3薄膜的光诱导效应

分别采用溶胶-凝胶法和脉冲激光沉积的方法制备了La_067Sr_033Fe_xMn_1-xO₃(x=0, 005, 010, 015)系列块材和薄膜，研究了Fe部分替代对La_067Sr_033Fe_xMn_1-xO₃薄膜     

Pr1-xSrxMnO3体系价带的光电子能谱研究

对于x从0.0到0.4之间变化的Pr_1-xSr_xMnO₃多晶块体体系所进行的价带光电子能谱实验显示,在Fermi边和Fermi边以下～12eV范围内出现的能带态密度随掺杂量x有一个显著的变化.对这些现象以Pr_1-xSr_xMnO₃体系在基态下发生电荷转移为出发点进行了讨论.提出了随掺杂量x的非线性电荷转移为出发点进行了讨论.提出了随掺杂量x的非线性电荷转移机制和存在由电荷转移导致的二级相变的可能性.     

Ce掺杂SrMnO3的电子结构和磁性的第一性原理研究

用含超软赝势平面波方法的广义梯度近似第一性原理对Ce掺杂SrMnO₃的电子结构和磁性进行了相关研究. 不同的Mn-O键长说明MnO₆八面体发生了强的Jahn-Teller扭曲, 并伴随着晶体构型由立方体(Pm3m)转变为四方晶系(I4/mcm), 同时,Jahn-Teller扭曲也稳定了C型反铁磁基态构型. 电子结构结果表明SrMnO₃和Sr_1-xCe_xMnO₃(x=0.12     

Eu掺杂TbMnO3多晶材料的介电性质

采用固相反应法制备了Tb_0.8Eu_0.2MnO₃多晶材料.对样品的X射线衍射（XRD）分析表明Eu³⁺固溶于TbMnO₃中.测量了样品在低温(100 K ≤T≤ 300 K)和低频下(200 Hz≤f≤100 kHz)的复介电性质.在此温度区间内发现了两个介电弛豫峰.经分析认为低温峰（T≈170 K）起源于局域载流子漂移引起的偶极子极化效应,而高温峰（T≈290 K）则是由离子电导产生的边界和界面层的电容效应引起的.电阻率的测量显示在低温下（T≈230 K）存在明显的导电机制转变.     

不同氮流量制备Mn3CuNx薄膜及其电、磁输运性质的研究

采用对靶磁控溅射方法在单晶Si(100)基片上制备了反钙钛矿结构的Mn₃CuN_x薄膜.通过控制制备过程中的反应气体氮气(N₂) 流量(N₂/Ar+N₂), 研究了氮含量对Mn₃CuN_x薄膜结构及物理性能的影响.分别利用X射线衍射仪、俄歇电子能谱、 原子力显微镜、X射线光电子能谱、物理性能测试系统和超导量子干涉仪, 对所制备薄膜的晶体结构、成分、表面形貌和电、磁输运性质进行了测试.结果表明:制备的薄膜均为反钙钛矿立方结构,且沿 (200) 晶面择优生长.随着氮含量的增大,薄膜表面粗糙度和颗粒度尺寸逐渐增大, 导致电阻率增加.氮含量对薄膜的电输运性质没有影响,所有薄膜电阻率均随着温度的降低逐渐增大, 呈现半导体型导电行为,这与对应的块体材料结果相反.Mn₃CuN_x薄膜随着测试温度的增大发生了 亚铁磁到顺磁的磁转变,且N含量的增大降低了磁有序转变温度,主要是由于N缺陷对Mn₆N八面体结构中 磁交换作用的影响所致.     

Tb1－xDyxFe2－y-Fe掺杂BaTiO3多层膜中的磁电耦合

用溶胶-凝胶法制备了Fe掺杂BaTiO₃粉体，在1350℃下烧结成圆片状多晶样品，并与Tb_1-xDy_xFe_2-y胶合成磁电(ME)双层膜或三层膜.实验分析表明Fe：BaTiO₃依然是四方相钙钛矿结构，但是居里温度及相变潜热均略低于纯净BaTiO₃.研究了Tb_1-xDy_xFe_2-y-Fe∶BaTiO₃双层膜和Tb_1-xDy_xFe_2-y-Fe∶BaTiO₃-Tb_1-xDy_xFe_2-y三层膜的ME效应.在2.8×10⁴A/m的磁场下，两者的横向ME电压系数均达其峰值，分别为6.225和26.25mV·(A·m^-1)^-1·cm^-1.并且，用掺杂BaTiO₃制备的双层膜和三层膜的横向ME电压系数均为相同条件下用纯净BaTiO₃制备的双层膜和三层膜的横向ME电压系数的1.5倍.另外由于不含铅，锆等有害物质，符合环保要求，因此采用掺杂BaTiO₃制备的磁电效应器件具有深入研究和应用价值.     

Sr掺杂Eu1-xSrxMnO3体系的磁相变以及输运特性研究

对Eu_1-xSr_xMnO₃ (ESMO, x=0—1)体系的结构和磁性进行了系统的研究,结果表明Sr的掺入使EuMnO₃反铁磁母体的磁结构发生巨大的变化.通过磁化和电输运测量,深入探讨了高掺杂浓度Eu_0.4Sr_0.6MnO₃和Eu_0.3Sr_0.7MnO_{3关键词： 1-x}Sr_xMnO₃体系')" href="#">Eu_1-xSr_xMnO₃体系 Sr掺杂 可变程跳跃模型     

Magnetic and magnetocaloric properties of Pr0.6−xEuxSr0.4MnO3 manganese oxides

Studies of the structural, magnetic and magnetocaloric properties of polycrystalline Pr_0.6−xEu_xSr_0.4MnO₃ (0≤x≤0.15) perovskite manganites were carried out. Substitution for praseodymium with europium, with smaller ionic radius, induces local distortion in the 〈Mn–O–Mn〉 bonds and consequently causes a random distribution in the magnetic exchange interactions. The competition between magnetic interactions leads to the appearance of an inhomogeneous magnetic state in our samples. Pr_0.6−xEu_xSr_0.4MnO₃ (x=0, 0.05, 0.1 and 0.15) polycrystalline samples were prepared using the solid–solid reaction method at high temperature. The compounds yielded are single phase and crystallize in the orthorhombic system with the Pnma space group. The substitution of Eu for Pr leads to a decrease of the Curie temperature T_C from 303 K for x=0.00 to 260 K for x=0.15. All of our compounds exhibit a large magnetic entropy change with a maximum around 2.2 J/kg K under a magnetic applied field change of 2 T for all compounds.     

Experimental evidence for a magnetic two-phase state in manganites

It is found that samples of manganites La_0.9Sr_0.1MnO₃ (single crystal), Eu_0.7A_0.3MnO₃ (A=Ca, Sr; ceramics), and La_0.1Pr_0.6Ca_0.3MnO₃ and La_0.84Sr_0.16MnO₃ (thin epitaxial films) that are either field-cooled (in a magnetic field) or zero-field-cooled differ in low-temperature magnetization, and the hysteresis loop of field-cooled samples exhibits a displacement. This displacement signifies that a ferro-antiferromagnetic state occurs in these samples. The exchange integral J～10^?6 eV is calculated from this displacement, which describes the exchange Mn-O-Mn coupling through the interface ferromagnetic droplet-antiferromagnetic matrix. The magnetoresistance and volume magnetostriction of La_1?x Sr_xMnO₃ single crystals exhibit similar dependences on x, temperature, and the magnetic field in the vicinity of the Curie point, which points to the fact that these dependences are due to the same reason, namely, the occurrence of a magnetic two-phase ferro-antiferromagnetic state caused by strong s-d exchange.     

Magnetic and electric properties of layered perovskites Nd2−2xSr1+2xMn2O7 (x=0.3–0.5)

Magnetic and electric properties of layered perovskites Nd_2−2xSr_1+2xMn₂O₇ (x=0.3, 0.4 and 0.5) are sensitive to the doping content x. The sample with x=0.5 is antiferromagnetic (AFM) and insulating. On decreasing x, the AFM ordering is suppressed and a canted AFM or weak ferromagnetic (FM) ordering appears, and the resistivity decreases. The sample with x=0.4 still shows insulating behavior, but a metal–insulator transition is observed for x=0.3. By suggesting the presence of a competition between AFM super-exchange interaction and FM double-exchange interaction, the doping dependence of magnetic and electric properties can be understood.     

Phase transitions induced by a strong magnetic field in electron-doped manganites

The magnetic and magnetoelastic properties of single crystals of electron-doped rare-earth manganites La_1?x Sr _x MnO₃ are studied. Phase transitions from the A-type antiferromagnetic phase to the C-type anti-ferromagnetic phase in a strong magnetic field are revealed in La_1?x Sr _x MnO₃ manganites with a strontium content x = 0.65. A similar phase transition is observed in manganites with a strontium content x = 0.8, at which the La_0.2Sr_0.8MnO₃ manganite is assumed to transform from the C-type antiferromagnetic phase to the G-type antiferromagnetic phase.     

Critical behavior and magnetic relaxation dynamics of Nd0.4Sr0.6MnO3 nanoparticles

Detailed DC and AC magnetic properties of chemically synthesized Nd_0.4Sr_0.6MnO₃ with different particle size (down to 27?nm) have been studied in details. We have found ferromagnetic state in the nanoparticles, whereas the bulk Nd_0.4Sr_0.6MnO₃ is known to be an A-type antiferromagnet. A Griffiths-like phase has also been identified in the nanoparticles. Further, critical behaviour of the nanoparticles has been studied around the second-order ferromagnetic-paramagnetic transition region (|(T?T _C)/T _C|???0.04) in terms of modified Arrott plot, Kouvel–Fisher plot and critical isotherm analysis. The estimated critical exponents (β, γ, δ) are quite different from those predicted according to three-dimensional mean-field, Heisenberg and Ising models. This signifies a quite unusual nature of the size-induced ferromagnetic state in Nd_0.4Sr_0.6MnO₃. The nanoparticles are found to be interacting and do not behave like ideal superparamagnet. Interestingly, we find spin glass like slow relaxation of magnetization, aging and memory effect in the nanometric samples. These phenomena have been attributed to very broad distribution of relaxation time as well as to inter-particle interaction. Experimentally, we have found out that the dynamics of the nanoparticle systems can be best described by hierarchical model of spin glasses.     

Phase separation induced by oxygen isotope substitution in manganites of the Sm<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3</Subscript> system

The effect of ¹⁶O → ¹⁸O isotope substitution on the electrical and magnetic properties of the manganite system Sm_1?xSr_xMnO₃ has been studied. It is shown that oxygen isotope substitution brings about a substantial change in the phase diagram in the intermediate region 0.4<x<0.6 between the ferromagnetic metal and antiferromagnetic insulator regions and induces phase separation and transformation of the ground metallic into insulating state for x=0.475 and 0.5. The specific features of the metal-insulator transitions in the Sm-Sr system and the nature of the low-temperature phase are discussed.     

Magnetocaloric effect in La0.65−xEuxSr0.35MnO3

The magnetocaloric properties for the Eu-doped La_0.65?xEu_xSr_0.35MnO₃ samples with x = 0.05, 0.15, 0.20, and 0.30 upon 0.05T magnetic field have been investigated. It is found that the Eu doping in this system decreases the magnetocaloric properties lightly. Moreover, the results of Eu doping clearly indicate that the magnetocaloric effect in this system is tunable, which is beneficial for manipulating magnetocaloric refrigeration that occurs in various temperature ranges. This makes the La_0.65?xEu_xSr_0.35MnO₃ samples potential candidates for practical applications. A complete characterization of the magnetic properties of this material aids to the understanding required for the technological exploitation of such materials, and it suggests La_0.65?xEu_xSr_0.35MnO₃ perovskite as the promising magnetic refrigerant.     

Structural, magnetic and magnetotransport behavior of La0.7SrxCa0.3−xMnO3 compounds

A systematic investigation of the structural, magnetic and electrical properties of a series of nanocrystalline La_0.7Sr_xCa_0.3−xMnO₃ materials, prepared by high energy ball milling method and then annealed at 900 °C has been undertaken. The analysis of the XRD data using the Win-metric software shows an increase in the unit cell volume with increasing Sr ion concentration. The La_0.7Sr_xCa_0.3−xMnO₃ compounds undergo a structural orthorhombic-to-monoclinic transition at x=0.15. Electric and magnetic measurements show that both the Curie temperature and the insulator-to-metal transition temperature increase from 259 K and 253 K correspondingly for La_0.7Ca_0.3MnO₃ (x=0) to 353 K and 282 K, respectively, for La_0.7Sr_0.3MnO₃ (x=0.3). It is argued that the larger radius of Sr²⁺ ion than that of Ca²⁺ is the reason to strengthen the double-exchange interaction and to give rise to the observed increase of transition temperatures. Using the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the resistivity versus temperature data measured in the range of 50-320 K and found that the activation barrier decreased with the raising Sr²⁺ ion concentration.