Co替代La2／3Ca1／3Mn0．9Co0．1O3输运特性和双峰效应的研究

系统研究了Mn位替代的La2/3Ca2/3Mn0.9Co0.1O3的电输运特性和磁诱导行为,这里替代Co离子的多价态对应有多重的自旋态变化.发现了存在于Co替代La2/3Ca1/3Mn0.9Co0.1O3电输运行为上的双峰效应,具体表现为除了居里温度Tc附近电阻-温度曲线的金属-绝缘(M-I)转变行为(TMIH约为182K)以外,在低温区域电输运行为出现二次性的类M-I转变现象(TMIL乱约为124K).通过对外加磁场下输运特性的研究表明,随外加磁场的增加,电阻-温度曲线整体呈现降低行为,即表现为负的磁电阻效应;同时,高温区峰值温度TMIH向高温区移动,这与传统CMR锰氧化物M-I转变特性完全一致;而低温峰值温度了TMIL则几乎不变,即TMIL表现出某种磁场无关的特征.结合样品磁特性测量结果,我们发现,除了场冷(FC)和零场冷(ZFC)曲线的分叉现象可能证明Co替代诱导Mn位的磁无序而导致体系出现不均匀的铁磁/反铁磁(FM/AFM)团簇外,在低温峰值温度TMIL乱附近没有观察到磁性异常变化.对照对Cu和Cr等相关体系的类似研究,证明这种低温二次M-I转变可能与Co替代导致体系的氧缺陷有关.     

多自旋态离子Co替代诱导La2/3Ca1/3MnO3体系的输运反常

利用具有多自旋态的Co离子进行Mn位替代,制备了La_2/3Ca_1/3Mn_1-xCo_xO₃ (0≤x≤0.15) 系列样品并研究了体系的结构和输运特性.结果表明,在替代范围内,样品呈现很好的单相结构,各晶格参数随替代量的增大而减小;Co替代导致体系出现电输运反常,具体表现为在居里温度T_C以下电阻-温度曲线的二次金属-绝缘转 关键词： Mn位替代 双峰现象 自旋结构 磁电阻效应     

多自旋态离子Co替代诱导La2/3Ca1/3MnO3体系的输运反常

利用具有多自旋态的Co离子进行Mn位替代，制备了La_2/3Ca_1/3Mn_1-xCo_xO₃ (0≤x≤0.15) 系列样品并研究了体系的结构和输运特性.结果表明，在替代范围内，样品呈现很好的单相结构，各晶格参数随替代量的增大而减小；Co替代导致体系出现电输运反常，具体表现为在居里温度T_C以下电阻-温度曲线的二次金属-绝缘转     

(La1-xCex)2/3Ca1/3MnO3体系的输运性质和反常磁特性研究

本文报告了对Ce掺杂锰氧化物(La1-xCex)2/3Ca1/3MnO3 (x=0～1.0)系列样品的输运特性和反常磁特性的研究结果.实验表明,Ce掺杂对Tc有明显的抑制作用,整体上电阻率随Ce掺杂含量增加而上升,在外加磁场时表现出极大的磁电阻效应.磁化强度随温度变化的曲线出现了两个转变,高温处对应于Mn离子磁矩的铁磁金属转变,低温处的转变则对应于Ce离子磁矩自旋有序排列的形成.表明Ce掺杂引起样品中铁磁双交换作用和反铁磁超交换作用之间的竞争,Ce离子与Mn离子有很强的相互作用.随Ce掺杂含量的增加,铁磁有序转变温度下降,而反铁磁有序转变温度则向高温处移动,铁磁区域明显减小.     

Y替代La2/3Ca1/3MnO3体系的结构与输运行为

系统研究了(La1-xYx)2/3Ca1/3MnO3(0.0≤x≤0.3)体系的结构和输运行为.结果表明，实验样品具有很好的单相结构，随Y掺杂浓度的增加，金属—绝缘体（M—I）转变温度T-MI向低温区移动，对应的峰值电阻率ρp升高，对x=0.3样品，较未替代样品（x=0.0）增幅达8个数量级.在外加磁场下，材料表现出很强的磁电阻效应.同时，从实验结果出发，直接给出了输运特性与晶体结构之间的关联，并从双交换模型和可变程跃迁理论出发，对实验结果进行了初步讨论. 关键词： La2/3Ca1/3MnO3锰氧化物 Y替代 晶体结构 输运行为     

La_(0.67)Ca_(0.33)Mn_(1-x)Cr_xO_3(0.0≤x≤0.15)中异常的CMR效应

我们研究了Mn位掺Cr体系La0 .67Ca0 .33Mn1-xCrxO3( 0 .0≤x≤ 0 .15)的磁性和磁电阻性质 ,发现Cr掺杂对居里温度Tc 影响不明显 ,但是导致了反常的电输运和磁电阻行为 ,随着Cr含量的增加 ,起始电阻率峰向低温移动 ,同时电阻率出现一个附加峰 .在磁场作用下 ,两个电阻率峰被强烈压缩 ,导致CMR效应中出现双峰 ,并且CMR效应的温区被极大地拓宽 ,从低温直到接近室温 .这个结果表明 :Mn位元素替代是调节CMR效应的有效方法 .     

(La1-xYx)2/3Ca1/3MnO3(x=0～0.3)体系的磁特性与平均稀土离子尺寸效应的研究

研究了(La1-xYx)2/3Ca1/3MnO3(0.0≤x≤0.3)体系的磁特性,给出了磁转变温度Tc与La位平均离子尺寸之间的关联.结果表明,随Y掺杂浓度的增加,金属-绝缘转变(M-I)温度TMI向低温区移动,对应的峰值电阻率ρp升高,对x=0.3样品而言,较未替代样品(x=0.0)增幅达8个数量级之多,指出由于Y的掺入而致使La位平均离子半径()减少以及局域晶格结构畸变引起的.磁性测量表明了在x>0.1的高浓度区域,随着x的增加,反铁磁性相互作用逐渐增强,导致x=0.2和x=0.3的样品在35K左右出现了自旋玻璃态.     

Mn位Ru替代对Pr0.5Ca0.5MnO3电磁特性及磁电阻的影响

通过对Mn位Ru替代体系Pr0.5Cao.5Mn1-xRuxO3(0.00≤x≤0.09)的磁性及电输运机制进行研究,发现Ru元素的替代对居里温度Tc、绝缘体-金属相变温度TIM有明显影响,导致了异常电输运及磁电阻行为的出现;随着Ru含量的增加,Tc及TIM均向高温区移动,在外加磁场作用下,电阻率峰值被强烈压缩,从而导...     

Mo低掺杂La_(0.67)Sr_(0.33)MnO_3的磁电特性及室温磁电阻增强(英文)

采用固相反应法成功制备了Mo低掺杂La0.67Sr0.33Mn1-xMoxOc(x=0,0.02)单相多晶样品,系统研究了样品的磁性、电输运特性和磁电阻效应.发现Mo低掺杂对La0.67Sr0.33MnO3 的居里温度影响很小,但在La0.67Sr0.33MnO3铁磁基态背景下诱导出团簇自旋玻璃态行为.在低温区(0.1相似文献   

单晶NdMnO3的比热研究

研究了低温下NdMnO₃单晶的比热随温度和磁场的变化(2K≤T≤200K，0T≤H≤8T ).对应于 Mn磁矩亚晶格的A型反铁磁(A-AF)相变，零场下的比热曲线在85K附近出现尖锐的λ形峰，随 着磁场的增加，此λ峰降低展宽而且平滑变化，这与此温度附近磁化强度的变化规律一致. 与磁有序相变相关的熵变约为理论值的26％，这可能是由于磁有序涨落延续在较大温区造成 的.在20K以下，比热曲线出现了明显的肩膀形状的Schottky反常，其峰值随着磁场的增加而 逐渐向高温移动.考虑了低温下比热的各种贡献，根据Nd³⁺位有效分子场(H _mf) 引起的Nd^3＋基态双重态(GSD)劈裂对上述现象进行了解释.通过对2K≤T≤2 0K，0Ｔ≤ H≤8T范围内比热数据的拟合，得到了样品的GSD劈裂，德拜温度和A-AF自旋波劲度系数以及 它们对磁场的依赖关系.发现GdFeO₃型八面体旋转引起的A-AF结构中Mn磁矩亚晶 格的铁磁成分可能是H_mf的来源. 关键词： 比热 Schottky反常 反铁磁相变     

Physical properties of BaMg1/3Nb2/3O3-BaCo1/3Nb2/3O3 solid solutions

Structural, electric and magnetic properties of Ba₃Mg_1−xCo_xNb₂O₉ based dielectric ceramic compounds have been studied. The samples, prepared by a solid state reaction method, were characterised by X-ray powder diffraction (XRPD), electron microscopy (SEM), dielectric (ε(T)) and magnetic measurements (χ⁻¹(T)). The XRPD analyses showed that the crystal structure of these compounds does change by the increase of substitution degree, passing from a superstructure hexagonal-type, (no. 164), space group (SG) to a simple structure cubic-type, (no. 221), SG. However, the evolution of the elementary unit cell lattice parameter can be followed and it exhibit a linear increasing tendency with increase in the substitution, indicating the existence of a solid solution through out the investigated range of substitution (0-1). The microstructure analysis shows a variation in the grain size and also the porosity of the samples with the degree of substitution. The results are in good agreement with that of dielectric measurements, which also showed that the dielectric constant (ε) increases with the increase of cobalt content. The magnetic characterization of cobalt substituted samples showed an antiferromagnetic type super-exchange interaction between these magnetic ions. At the same time, the values of effective magnetic momentum (μ_eff) are close to the value that corresponds to Co²⁺ free ions. The study highlights the possibility of modelling these materials by substitutions, in order to improve properties of negative-positive-zero (NPO) type dielectric applications.     

La0.67Ca0.33MnO3薄膜皮秒光电效应

在1064 nm波长脉冲激光（脉宽25 ps）的照射下，钙钛矿氧化物薄膜La0.67Ca0.33MnO3/SrTiO3具有超快光电效应，对激光脉冲显示ps量级的响应时间，上升沿响应时间300 ps，半高宽700 ps，同时，对激光能量的响应灵敏度为500 mV/mJ。     

Giant dielectric relaxation in SrTiO3-SrMg1/3Nb2/3O3 and SrTiO3-SrSc1/2Ta1/2O3 solid solutions

Ceramic samples of (1−x)SrTiO₃-xSrMg_1/3Nb_2/3O₃ and (1−x)SrTiO₃-xSrSc_1/2Ta_1/2O₃ were prepared, and their dielectric properties were studied at x=0.005–0.15 and 0.01–0.1, respectively, at frequencies 10 Hz–1 MHz and at temperatures 4.2–350 K. A giant dielectric relaxation was observed in the temperature range 150–300 K, and not so strong but well-developed relaxation was found in the temperature range 20–90 K. The activation energy U and the relaxation time τ₀ were determined to be 0.21–0.3 eV and from 10⁻¹¹ to 10⁻¹² s for the high-temperature relaxation and 0.01–0.02 eV and 10⁻⁸–10⁻¹⁰ s for the low-temperature relaxation, respectively. The additional local charge compensation of the heterovalent impurities Mg²⁺ and Nb⁵⁺ (or Sc³⁺ and Ta⁵⁺) by free charge carriers or the host ion vacancies is suggested to be the underlying physical mechanism of the relaxation phenomena. On the basis of this mechanism, the Maxwell-Wagner model and the model of reorienting dipole centers Mg²⁺ (or Sc³⁺) associated with the oxygen vacancy are proposed to explain the high-temperature relaxation with some arguments in favor of the latter model. The polaron-like model with the Nb⁵⁺-Ti³⁺ center is suggested as the origin of the low-temperature relaxation. The reasons for the absence of ferroelectric phase transitions in the solid solutions under study are also discussed. From Fizika Tverdogo Tela, Vol. 44, No. 11, 2002, pp. 1948–1957. Original English Text Copyright ? 2002 by Lemanov, Sotnikov, Smirnova, Weihnacht. This article was submitted by the authors in English.     

Temperature-dependent photoluminescence in La2/3Ca1/3MnO3

Visible photoluminescence and its temperature dependence of La_2/3Ca_1/3MnO₃ in the temperature range 138-293 K were measured. It was observed that the main broad band centered at ∼1.77 eV with the shoulders at ∼1.57 and ∼1.90 eV existed in the entire temperature range. It can be well fitted by three Gaussian curves B₁, B₂ and B₃ centered at ∼1.52, ∼1.75 and ∼1.92 eV, respectively. The intensities of the peak B₁ and B₂ vary as temperature increases. In the entire temperature range, the intensity of B₁ increases with increasing temperature, whereas that of B₂ decreases. The photoluminescence mechanisms for La_2/3Ca_1/3MnO₃ are presented based on the electronic structures formed by the interactions among spin, charge and lattice, in which B₁ was identified with the charge transfer excitation of an electron from the lower Jahn-Teller split e_g level of a Mn³⁺ ion to the e_g level of an adjacent Mn⁴⁺ ion, B₂ is assigned to the transition between the spin up and spin down e_g bands separated by Hund's coupling energy E_J and B₃ is attributed to the transition, determined by the crystal field energy E_C, between a t_2g core electron of Mn³⁺ to the spin up e_g bands of Mn⁴⁺ by a dipole allowed charge transfer process.     

La2/3Ca1/3MnO3/Eu2CuO4/La2/3Ca1/3MnO3磁性隧道结的制备与表征

采用磁控溅射，紫外线光刻和离子束刻蚀制备了La_2/3Ca_1/3MnO₃/Eu₂CuO₄/La_2/3Ca_1/3MnO₃磁性隧道结.通过对获得的磁性隧道结的I-V特性测量，发现非线性的I-V特性，显示结样品的隧穿特性.有趣的是发现在电极材料La_2/3Ca_1/3MnO₃的金属-绝缘体转变温度(Tp)以下，I-V曲线出现一个跳变.随着温度降低，开始出现跳变的临界电流增大，但是跳变都发生在同样的电压下～209mV.当电流增大或减小在跳变点附近出现回滞.这一跳变只发生在铁磁金属态，表明这是一个磁性相关联的效应，可能对应一种新的磁性开关过程.虽然，目前对这一现象背后的物理机理还不清楚，但是，这一现象有可能在未来自旋电子学器件方面具有潜在的应用价值. 关键词： 庞磁电阻 磁性隧道结 开关效应     

Polarization characteristics of the relaxor ceramics of multicomponent system with PbNb2/3Zn1/3O3, PbNb2/3Mg1/3O3, and PbTiO3

The phenomenon of dielectric hysteresis in the relaxor ceramics of multicomponent system with PbNb_2/3Zn1/₃O₃, PbNb_2/3Mg_1/3O₃, PbTiO₃ has been studied. The influence of different modifiers on polarization characteristics is examined and the correlations of these properties with the phase and chemical composition of the ceramics are investigated. The results obtained can be used to predict relaxor properties.     

Spin-polarized tunneling in nanostructured La2/3Sr1/3MnO3

We present magnetic and transport properties of nanocrystalline La_2/3Sr_1/3MnO₃ powders prepared by a gel-combustion method using citric acid as the fuel. The coercive magnetic field H_c is significantly different to the field H_c^* for which the magnetoresistance (MR) is maximum. The MR at low fields (LFMR) exhibits a power-law dependence with magnetization, MR∝Mⁿ, with n=2.5–3.3 for temperatures ranging from 5 to 200 K. The results are discussed in terms of a distribution of particle size in our sample.     

E-T phase diagrams of a solid solution of the multicomponent PbZn1/3Nb2/3O3-PbMg1/3Nb2/3O3-PbNi1/3Nb2/3O3-PbTiO3 system near the morphotropic phase boundary

The specific features of the dielectric and pyroelectric responses of a solid solution in the barium-doped multicomponent yPbZn_1/3Nb_2/3O₃-mPbMg_1/3Nb_2/3O₃-nPbNi_1/3Nb_2/3O₃-xPbTiO₃ system of the composition y = 0.0982, m = 0.4541, n = 0.1477, and x = 0.3 near the morphotropic phase boundary have been investigated. It has been assumed that the maxima revealed in the dependences of the reversible permittivity on the electric field strength with both the forward and backward changes in the field (E ₌) are associated with the induced phase transition. Based on the experimental results, the E-T phase diagrams are constructed in the temperature range from ?100 to 150°C for different temperature-field regimes: (i) variation in the electric field E ₌ at a fixed temperature of the sample and (ii) variation in the temperature of the sample at a constant value of E ₌. It has been found that there is a singular point in the E-T phase diagram and that, in the vicinity of this point, the dielectric and pyroelectric responses of the studied ceramics exhibit specific features.     

Electric and magnetic behaviour in double doped La2/3＋4x/3Sr1/3-4x/3Mn1-xMgxO3

The double-doped La2/3＋4x/3Sr1/3-4x/3Mn1-xMgxO3 samples with fixed Mn^3＋/Mn^4＋ ratio equal to 2/1 are investigated by means of magnetism and transport measurements. Phase separation is observed at temperature higher than T^onset c for x = 0.10 and 0.15. For x = 0.10, rather strong phase separation induces drastic magnetic random potential and results in the localization of carriers. Thus, the varlable-range hopping process dominates. For other samples, there is no or only weak phase separation above T^onset c. Thus, thermal activation mechanism is responsible for the high temperature transport behaviour. For x = 0.20 and 0.25, unexpected AFM behaviour is observed at low temperature. All these results are well understood by considering the special role of the ＂double-doping＂.     

对靶溅射技术制备的La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜中Tc的降低

本文用对靶溅射技术制备了La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜.与YBCO单层薄膜相比,由于超导/铁磁系统中的磁性邻近效应,三层薄膜表现出较低的超导转变温度.薄膜的R～T测量曲线显示出超磁阻(CMR)效应和超导转变,预示着超导和铁磁特性共存于LCMO/YBCO/LCMO三文治结构.