电脉冲对多晶La0.7Ca0.3MnO3比热的影响

对电脉冲诱导的不同电阻态下La_0.7Ca_0.3MnO₃样品的比热进行了研究.实验结果表明,电脉冲导致La_0.7Ca_0.3MnO₃样品比热随电阻状态发生可逆变化.比热随电阻状态的减小而减小.低温比热拟合及不同电阻状态下的比热差与温度关系说明,声子对比热的贡献不随电阻状态变化,磁性和载流子对比热的贡献是导致La_0.7Ca_0.3MnO₃样品比热变化的原因.电脉冲诱导O离子沿一维扩展性缺陷的电化学迁移,导致材料中局部区域的O离子浓度发生变化.O离子浓度的变化导致载流子浓度的变化,同时载流子浓度的变化将使得低温下磁性耦合强度发生变化,从而导致比热发生变化. 关键词： 0.7Ca_0.3MnO₃')" href="#">La_0.7Ca_0.3MnO₃ 比热 氧离子迁移     

La0.33Pr0.34Ca0.33MnO3薄膜的应变效应

采用脉冲激光沉积法分别在(100)LaAlO₃和(100)SrTiO₃基片上生长了La_0.33Pr_0.34Ca_0.33MnO₃薄膜,并通过磁测量和电输运测量对生长在不同基片上的La_0.33Pr_0.34Ca_0.33MnO₃薄膜的物性进行了研究.结果表明,基片和薄膜之间的压应力导致La_{关键词： 钙钛矿锰氧化物 相分离 电荷有序}     

Mn位W掺杂对La0.3Ca0.7MnO3体系磁结构的影响

通过对La_0.3Ca_0.7Mn_1-xW_xO₃(x=0.00，0.04，0.08，0.12，0.15)多晶样品M-T曲线、M-H曲线及ESR谱的测量，研究了Mn位W掺杂对电荷有序体系La_0.3Ca_0.7MnO₃磁结构的影响.结果表明，当掺杂量为0.00≤x≤0.08时，体系存在电荷有序(CO)相，AFM/CO态共存于相变温度以下，电荷有序温度T_CO随着W掺杂量的增加而增加；x＝0.04时，样品在低温下为FM相与AFM/CO相共存，在CO相建立前、后均有FM从PM中分离出来；当x≥0.12时，CO态融化，在极低温度下存在顺磁-铁磁(PM-FM)相变. 关键词： 磁结构 电荷有序 融化 Mn位掺杂     

La0.8-xCa0.2MnO3纳米颗粒的居里温度与磁热效应

采用溶胶凝胶法制备了系列La_0.8-xCa_0.2MnO₃多晶样品,用X射线衍射分析确定了样品的钙钛矿结构,用透射电子显微镜观察了样品的形貌及粒径分布情况,用PAR155型振动样品磁强计测量了样品的磁性随外场和温度的变化,确定样品的居里温度并计算了各样品的磁熵变.磁测量及计算结果表明制备的各样品的居里温度在180—260K的范围内且随焙烧温度和La³⁺离子空位浓度的不同而变化,不同温度焙烧的样品均有较大的磁熵变值,其中1100℃焙烧的La_0.77Ca_0.2MnO₃,多晶样品在240.5K,H=1.0T的外场下的磁熵变达3.76J/kg·K,对实验结果做了定性的分析.该材料具有较高的居里温度和较大的磁熵变,所需外场强度适中,电阻率高,性能稳定,适合做高温磁制冷材料. 关键词： 钙钛矿 居里温度 磁热效应     

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     

La1-xCaxMnO3的巨磁电阻效应

通过测量La_2/3Ca_1/3MnO₃，La_0.6Ca_0.4MnO₃单相多晶样品的磁电阻、电阻与温度的依赖关系，发现在La_1-xCa_xMnO₃体系中随x的变化，其磁电阻峰和电阻峰都发生了位移．作者认为体系中Mn⁴⁺含量是受Ca含量调制的，正是Mn⁴⁺含量的变化，导致磁性 关键词：     

脱氧La0.5Ca0.5MnO3样品的铁磁-反铁磁转变和电阻率变化

研究了氧空位对La_0.5Ca_0.5MnO₃ （LCMO）多晶块材的电输运和磁性质的影响. 随着氧空位的增加, 样品在高温段的电阻率一直增加, 并满足绝热小极化子模型, 而低温段的电阻率先下降后上升, 并出现明显的dR/dT>0的行为, 直至最后变为绝缘的. 氧空位的增加抑止了反铁磁相的出现, 使得脱氧的LCMO样品不发生反铁磁转变, 进一步增加氧空位则会抑制铁磁相. 关键词： 0.5Ca_0.5MnO₃')" href="#">La_0.5Ca_0.5MnO₃ 反铁磁相变 铁磁相变 脱氧     

(K0.45Na0.55)NbO3无铅压电晶体的生长形态与介电性能研究

采用高温熔融法制备出了尺寸达5 mm×3 mm×1 mm的(K_0.45Na_0.55)NbO₃(KNN)无铅铁电晶体. XRD测试结果表明KNN晶体结构为纯的钙钛矿正交相结构,晶体的显露面为〈001〉结晶面. SEM显微结构分析表明晶体沿[001]方向呈现层状生长台阶,采用负离子配位多面体生长基元模型解释了晶体层状台阶的生长机理. 研究了晶体样品在室温至500 ℃温度范围内的介电性能,两个介电异常峰出现在240和405 ℃,分别对应正交铁电-四方铁电以及四方铁电-立方顺电相相变温度. 采用修正后的居里外斯定律研究了KNN晶体的介电弛豫特性,结果表明KNN晶体的介电弛豫特性接近于普通铁电体特征. 关键词： 0.45Na_0.55)NbO₃晶体')" href="#">(K_0.45Na_0.55)NbO₃晶体 无铅 晶体结构 介电性能     

Ti/Pr0.7Ca0.3MnO3/La0.67Sr0.33MnO3/Pt异质结电致电阻效应的改善

采用脉冲激光沉积技术制备了Ti/Pr_0.7Ca_0.3MnO₃/Pt和Ti/Pr_0.7Ca_0.3MnO₃/La_0.67Sr_0.33MnO₃/Pt异质结并研究了La_0.67Sr_0.33MnO₃功能插层对异质结电致电阻特性的影响. 实验结果表明La_0.67Sr_0.33MnO₃功能层的引入有效提高了器件的电阻转变特性,尤其是电阻转变率和疲劳性得到了极大的改善. 对La_0.67Sr_0.33MnO₃插层改善电致电阻转变特性的机理进行了定性的分析. 关键词： 电致电阻效应 电阻转变比率 疲劳特性     

La0.4FeCo3Sb12晶体结构的x射线和电子衍射表征

用粉末x射线衍射结合Rietveld解析方法和电子衍射研究了一种n型La_0.4FeCo3Sb₁₂化合物的晶体结构和演化.二元Skutterudite化合物的晶体结构 中存在一半径为01980nm的结构间隙,可以填入稀土类原子,稀土原子填入后主要引起晶格中Sb原子位置的变 化,稀土原子相对于其他原子具有较大的热振动参数.化合物的电子衍射花样具有体心立方 晶系的消光规律,晶面的衍射强度与粉末x射线衍射的结果完全一致,有序结构引起了个别 强衍射斑点. 关键词： 填充式skutterudite化合物 晶体结构 Rietveld方法 电子衍射     

Magnetic phase coexistence in Tb- and Sr-doped La0.7Ca0.3MnO3 manganite:A temperature-dependent neutron diffraction study

We report the results of the temperature-dependent neutron diffraction measurements on the nearly half-doped (La_0.325Tb_0.125)(Ca_0.3Sr_0.25)MnO₃ manganite sample. The simultaneous doping of magnetic Tb³⁺ and divalent Sr²⁺ in the La_0.7Ca_0.3MnO₃ system results into a large A-site size disorder. Rietveld refinement of neutron diffraction data reveal that the single phase sample crystallizes in a distorted orthorhombic structure. Increased 〈r_A〉 value affects the transport behavior that results into an insulating-like behavior of the sample. Under application of 1 T field sample exhibit insulating-like behavior while insulator-metal transition (T_IM) is exhibited under 5 and 8 T fields. Variable range hoping (VRH) mechanism of charge carriers is exhibited in the insulating region. Field cooled and zero field cooled magnetization measurement shows the Curie temperature (T_C)~47 K. The refinement of the ND data collected at various temperatures below 300 K shows that there is no structural phase transition in the compound. Around 100 K, a magnetic peak appears at lower angle that can be ascribed to the presence of the A-type antiferromagnetic (AFM) phase. Two more peaks are observed around 50 K at lower angles that can be fitted in CE-type antiferromagnetic phase. Splitting of the peaks at lower temperatures is the signature of orbital ordering in the presently studied nearly half-doped manganite system. Results of the detailed structural analysis of the temperature-dependent ND measurements on (LaTb)_0.45(CaSr)_0.55MnO₃ sample has been discussed in the light of coexisting A-type and CE-type antiferromagnetic phases present in the sample at low temperature.     

An electron paramagnetic resonance study of phase segregation in Nd0.5Sr0.5MnO3

We present results of an electron paramagnetic resonance (EPR) study of Nd_1−xSr_xMnO₃ with x=0.5 across the paramagnetic to ferromagnetic, insulator to metal transition at 260 K (T_c) and the antiferromagnetic, charge ordering transition (T_N=T_co) at 150 K. The results are compared with those on Nd_0.45Sr_0.55MnO₃ which undergoes a transition to a homogeneous A-type antiferromagnetic phase at T_N=230 K and on La_0.77Ca_0.23MnO₃ which undergoes a transition to coexisting ferromagnetic metallic and ferromagnetic insulating phases. For x=0.5, the EPR signals below T_c consist of two Lorentzian components attributable to the coexistence of two phases. From the analysis of the temperature dependence of the resonant fields and intensities, we conclude that in the mixed phase ferromagnetic and A-type antiferromagnetic (AFM) phases coexist. The x=0.55 compound shows a single Lorentzian throughout the temperature range. The signal persists for a few degrees below T_N. The behaviour of the A-type AFM phase is contrasted with that of the two ferromagnetic phases present in La_0.77Ca_0.23MnO₃. The comparison of behaviour of A-type AFM signal observed in both Nd_0.5Sr_0.5MnO₃ and Nd_0.45Sr_0.55MnO₃ with the two FM phases of La_0.77Ca_0.23MnO₃, vis-à-vis the shift of resonances with respect to the paramagnetic phases and the behaviour of EPR intensity as a function of temperature conclusively prove that the Nd_0.5Sr_0.5MnO₃ undergoes phase separation into A-type AFM and FM phases.     

Epitaxial growth La0.67Ca0.33MnO3 thin film by radio frequency sputtering method

Perfect epitaxial growth of La_0.67Ca_0.33MnO₃ (LCMO) thin film has been achieved on (1 0 0) LaAlO₃ (LAO) single crystal substrate by radio frequency sputtering method. X-ray diffraction (XRD) and electron diffraction analysis indicates that La_0.67Ca_0.33MnO₃ film grows epitaxially on LaAlO₃ along [1 0 0] direction of the substrate. The resistivity variation with temperature of the film shows a sharp metal to semiconductor transition peak around 253 K, which is close to that of the target. The magnetoresistance (MR) also reveals high quality epitaxy film characteristic at low temperatures and near the metal to semiconductor transition temperature.     

Spin glass behavior in hole- and electron-doped bismuth manganite single crystals

The hole- and electron-doped bismuth manganites Bi_0.55Ca_0.45MnO₃, Bi_0.55Sr_0.45MnO₃ and Bi_0.95Ce_0.05MnO₃ single crystals were grown using the flux-growth method. Their structural, magnetic and electrical transport properties have been compared studied. All samples show spin-glass magnetic behavior at low temperatures. In the immediate temperature region, an antiferromagnetic transition at T_A and a charge-ordering state at T_CO are observed for Bi_0.55Ca_0.45MnO₃ crystal, whereas only an antiferromagnetic transition exists in Bi_0.95Ce_0.05MnO₃ and Bi_0.55Sr_0.45MnO₃ crystals. Bi_0.55Ca_0.45MnO₃ and Bi_0.55Sr_0.45MnO₃ samples show semiconducting transport behavior in the whole studied temperature range, whereas Bi_0.95Ce_0.05MnO₃ is an insulator at room temperature. In addition, near T_CO a positive magnetoresistance as large as 70.7% is observed for Bi_0.55Ca_0.45MnO₃ sample under 5 T applied magnetic field. The obtained results may originate from the rotation of the polarized Bi-6s² lone pair electrons in the magnetic field.     

Lattice distortion-induced phase transformations in La1 − y Pr y MnO3 + δ manganites

The structural and magnetic phase transformations that occur in the system of self-doped La_{1 ? y} Pr _y MnO_{3 + δ} (δ ≈ 0.1, 0 ≤ y ≤ 1) manganites in the temperature range 4.2–300 K are studied by X-ray diffraction and measuring the temperature and field dependences of dc magnetization. The low-temperature magnetic phase transformations induced by the substitution of Pr for La correlate well with the structural phase transformations at T = 300 K, which indicates a strong coupling of the electronic and magnetic subsystems of La_{1 ? y} Pr _y MnO_{3 + δ} manganites with the crystal lattice. The anomalies of the magnetic and structural properties detected in this work in the form of peaks and inflection points in the concentration dependences of the magnetization and lattice parameters of the pseudocubic phase of La_{1 ? y} Pr _y MnO_{3 + δ} (0.1 ≤ y ≤ 0.7) in the temperature range 4.2–300 K are explained in terms of the existing concepts of the effect of Fermi surface nesting on the renormalization of the density of states and the hole dispersion near E _F in the presence of a strong coupling of holes with low-frequency optical phonons, which results in their transformation into quasiparticles. The narrow peak in the magnetization curve M(y) of La_{1 ? y} Pr _y MnO_{3 + δ} that is detected near y = 0.3 at T = 4.2 K is assumed to correspond to the peak of coherence of quasiparticles with a low energy of coupling with the crystal lattice near E _F, which was found earlier in the photoelectron emission spectra of manganites. The disappearance of the narrow magnetization peak with increasing Pr concentration is explained by the transition of charge carriers from the mode of “light” holes weakly coupled to one of the soft phonons to the mode of “heavy” holes strongly coupled to several phonons. The transition between phases with strongly different effective quasiparticle masses proceeds jumpwise; that is, it has features of the metal-insulator Mott transition and is accompanied by the transition from 3D to 2D quasiparticle motion in planes ab.     

Synthesis and characterization of graphene nanoplatelet-La0.7Ca0.3MnO3 composites

ABSTRACT

La_0.7Ca_0.3MnO₃ perovskite and its composites with graphene nanoplatelet (GNP) were prepared using a wet chemical method. The structural, magnetic and magnetocaloric properties of La_0.7Ca_0.3MnO₃: GNP composites were investigated to determine the effect of GNPs. The results of XRD analysis show that the synthesised powders can be almost indexed to pure phase orthorhombic La_0.7Ca_0.3MnO_3. The magnetic measurements demonstrate that 0.7 and 1% GNP amounts cause an increase in the Curie temperature (T_C), and for larger amounts of GNP, the T_C monotonically decreases, except for the sample with 10% GNP. The results obtained from the Arrott plots show that the magnetic phase transition of the samples transforms from the first to second order with increasing GNP amount. The changes in the magnetocaloric properties are interpreted in terms of perovskite phase formations via structural analysis. The amounts of graphene nanoplatelets in the oxide powders are correlated with the observed magnetocaloric properties. The best magnetocaloric performance with the maximum magnetic entropy change of 3.99 Jkg^?1K^?1 and refrigeration capacity of 90 Jkg^?1 was obtained at a 2?T magnetic field.     

Specific heat of Sm0.55Sr0.45MnO3 manganite in magnetic fields up to 15 T: An anomalous critical behavior of the ferromagnet in magnetic field and the observation of a tricritical point

The specific heat of Sm_0.55Sr_0.45MnO₃ manganite is measured in the vicinity of the ferromagnetic phase transition in strong magnetic fields up to 15 T. An anomalous critical behavior of the Sm_0.55Sr_0.45MnO₃ ferromagnet in magnetic field is predicted and experimentally observed. The anomalous behavior manifests itself in that, in magnetic fields up to 4 T, the field favors fluctuations and the specific-heat anomaly increases with the field and sharpens, becoming λ-shaped. In fields above 4 T, the behavior becomes classical: the field suppresses the fluctuations and the specific-heat peak is smeared out. The hysteresis of the transition temperature measured in the heating and cooling runs is about 15 K in zero magnetic field. As the field increases, it narrows gradually and vanishes completely when the field reaches 4 T. The results of the experiments are explained in terms of the competition between the hysteresis and the fluctuations of the magnetic order parameter. The H-T phase diagram of Sm_0.55Sr_0.45MnO₃ certainly indicates that, at 4 T, Sm_0.55Sr_0.45MnO₃ has a tricritical point, at which the ferromagnetic and paramagnetic phases of Sm_0.55Sr_0.45MnO₃ are leveled.     

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.     

Behavior of the atomic and magnetic structure of La0.35Pr0.35Ca0.30MnO3 at a metal-insulator phase transition

The evolution of the structural and magnetic properties of the CMR (colossal-magnetoresistance) compound La_0.35Pr_0.35Ca_0.30MnO₃ as the temperature changes from 10 to 293 K is investigated by means of neutron diffraction. It is shown that the changes in the transport and magnetic properties are directly related with the rearrangement of the atomic structure. A phase transition to the metallic state occurs together with simultaneous ferromagnetic ordering of the manganese moments and is accompanied by a jump in volume. The static distortions of the oxygen octahedra which are observed to occur prior to the magnetic phase transition and which are practically absent at room temperature and in the FM phase attest to the orbital ordering of oxygen atoms on the bonds, with freezing-in of the Jahn-Teller modes. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 672–677 (10 May 1998)     

Anomalies in the magnetic and magnetoelastic properties of Nd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3</Subscript> single crystals in strong magnetic fields

Measurements of the magnetic and magnetoelastic properties of Nd_0.5Ca_0.5MnO₃, Nd_0.55Ca_0.45MnO₃, and Nd_0.6Ca_0.4MnO₃ single crystals have revealed correlations between the observed anomalies in magnetization and magnetostriction associated with suppression of the antiferromagnetic phase and charge ordering by a strong magnetic field followed by transition to the ferromagnetic state. H-T phase diagrams are constructed.