用内耗技术研究La0.7Pb0.3MnO3巨磁电阻材料

用低频扭摆法在多功能内耗仪上测量了巨磁电阻材料La0.7Pb0.3MnO3(LPMO)的温度内耗谱和弹性模量。结果表明，内耗峰位与测量频率无关，并且峰高与频率成反比，弹性模量在对应的内耗峰处有明显的转折，内耗峰表现为相变峰的特征，结合电阻和磁化率的测试，解释了内耗和电阻-温度曲线的双峰现象，高温内耗峰和高温电阻峰与居里温度有很好的对应，来源于顺磁半导体向铁磁金属的转变，低温内耗峰和磁化率的单调下降来源于铁磁相分离过程。而较大的低温电阻峰部分来源于相分离过程。     

SrRuO3铁磁性能研究

测量了烧结SrRuO3的零场冷却和加场冷却磁化强度和交流磁化率。随外加直流偏场的增加,交流磁化率与温度关系曲线向低温方向移动。继续增加直流偏场,一个峰分裂为两个峰,其中低温下的峰继续向低温方向移动,另一个高温处的峰却向高温方向移动。在不同频率下测量零偏场下的交流磁化率,随频率增加峰值减小,但峰的位置并没有像以前报道得那样向高温方向移动。可见,烧结SrRuO3并没有显示自旋玻璃行为,而是普通铁磁体。     

La_2Mo_(2-x)W_xO_9的氧离子扩散和相变机制研究

采用介电弛豫和内耗技术,并结合XRD技术研究了新型氧离子导体La2Mo2-xWxO9(x=0、0.25、0.5、0.75、1.0、1.2、1.4)的氧离子扩散行为和相变机制.结果表明,在La2Mo2-xWxO9的介电损耗-温度谱和内耗-温度谱上均出现了2个损耗峰,其中低温峰对应氧离子的扩散弛豫过程,高温峰对应氧离子/空位分布的静态无序态到动态无序态的转变.     

纳米铝晶界内耗峰的连续变温研究

采用低温球磨结合热压烧结技术制备了块体纳米铝晶体材料,在多功能内耗仪上采用受迫振动连续变温(25～450℃)的方式测量了纳米铝材料在低频率(0.2～3.0Hz)条件下的内耗和相对动力学模量。结果表明,纳米铝晶体材料受迫振动时,在升温和降温的内耗-温度谱上均出现一个内耗峰;伴随着内耗峰的出现,相对动力学模量明显下降;随着离散振动频率的增加,内耗峰峰位向高温方向移动,说明该内耗峰具有弛豫性;根据Arrhenius关系,得到该内耗峰的激活能为(2.21±0.04)×10-19 J、指数前因子τ0为10-14 s,表明该内耗峰是晶界内耗峰,其机制为铝原子在铝/铝晶界的自扩散。     

La0.4Ca0.6Mn0.98Ru0.02O3中的电致电阻效应研究

单相多晶锰氧化物样品La_0.4Ca_0.6Mn_0.98Ru_0.02O₃具有典型的电子相分离基态, 即低温下铁磁金属相与电荷有序绝缘相共存并竞争, 这为我们研究电致电阻效应提供了很好的模型。本论文基于此目的, 采用固相合成法制备了La_0.4Ca_0.6Mn_0.98Ru_0.02O₃的多晶陶瓷样品, 用X射线衍射证明了样品是单相, 并详细测量了样品的电致电阻效应。实验结果表明, 该体系的电阻率能够在大范围内被输入电流调控, 获得的电致电阻效应高达80%以上。此外, 低温下该样品的电阻态可以通过改变电流输入强度进行连续转换, 反映了样品基态在电流的激发下发生了改变。通过讨论, 我们认为这些观测到的物理效应起源于输入电流提升了样品中的铁磁金属相的稳定性。     

顺丁橡胶α峰在交联前后变频温度谱的特征

在不同频率下系统地测量了顺丁橡胶在交联前后α内耗峰峰参数的两种随频率变化完全相反的表面行为，并得到交联度与α峰松弛强度的定性关系。给出了初步的物理图象，关于α峰高温端对应模量的上升过程被证实的ＰＢＤ的二次结晶过程。     

Mn-Fe基合金的反铁磁与高阻尼

采用动力学分析（DMA）和透射电子显微分析（TEM）等实验方法,研究了含锰45．9％－86．4％（原子分数）范围Mn-Fe-(Cu)合金的反铁磁转变,马氏体相变及高阻尼特性。当锰含量超过71％（原子分数）时合金在变温过程中发生顺磁－反铁磁转变引起模量的剧烈变化。在尼尔点（TN)以下的反铁磁状态－200℃温区内出现一个10^-1数量级内耗的高阻尼区。随着含锰量的增加该区内逐渐显示出两个分立的内耗峰。确定了其中高温端的内耗峰为马氏体相变的贡献而低温端的内耗峰则纯属孪晶界的驰豫型内耗。文中测定了驰豫过程的激活能,讨论了合金呈现高阻尼与两种相变的关系。     

CaCu3Ti4O12中的晶界弛豫

研究了CaCu3Ti4O12在低温低频下的内耗曲线特征,发现金属中来源于晶界的内耗规律也适合CaCu3Ti4O12材料.测量了晶界弛豫激活能的大小和不同温度下该材料的特征弛豫时间.与常温的情况相比,低温下CaCu3Ti4O12的特征弛豫时间明显增加.分析表明:这种弛豫时间的增加来源于在畴区上极化弛豫的动态慢化效应.     

A位复合铁电陶瓷Bi0.5(Na0.82K0.18)0.5TiO3-BiCrO3的介电弛豫

采用固相反应法制备了A位复合铁电陶瓷(1-x)Bi_0.5(Na_0.82K_0.18)_0.5TiO₃-xBiCrO₃(BNKT-BCx). 研究了该陶瓷在室温至500℃温度范围内的介电性能. 结果表明该陶瓷的介电温谱存在两个介电反常峰和一个介电损耗峰, 低温介电反常峰温度附近具有明显的介电常数频率依赖性, 但居里峰随频率增加基本不变, 与典型弛豫铁电体的特征不同. 将弛豫铁电体分为本征弛豫和非本征弛豫铁电体, 通过分析极化前和极化后陶瓷的介电温谱, 发现该体系低温介电反常峰温度附近的介电频率依赖性为空间电荷和缺陷偶极子极化引起的非本征弛豫.     

纳米SnO_2固体的相变内耗和模量

系统测量了不同颗粒尺度纳米 SnO_2固体的内耗和模量,发现60℃附近存在一个内耗峰,对应的模量出现异常.烧结温度升高,峰高变化不明显,模量异常减弱.当烧结温度高于400℃时,模量曲线在-15℃附近出现异常,这种模量异常随烧结温度升高而加强,对应的内耗曲线出现一小峰.初步认为,60℃附近内耗峰和模量异常与纳米固体中的结构再有序有关,而-15℃处的模量异常为结构相变引起.     

The low frequency internal friction and electric magnetic properties in La0.7Pb0.3MnO3 bulk materials

The polycrystalline samples of La_0.7Pb_0.3MnO₃ synthesized by the sol–gel method were studied by the low frequency internal friction measurements and the resistance and ac susceptibility measurements. There are two internal friction peaks at 258 and 198 K, respectively, the peak positions of which do not shift with frequency but the peak heights of which decrease with increasing frequencies. The modulus softening is evident at the corresponding temperatures. Corresponding to the two internal friction peaks, there are two resistance peaks, and a sharp rise and a following continuous decrease of ac susceptibility. It is suggested that the high temperature peaks correspond to the paramagnetic (PM) to ferromagnetic (FM) transition, while the low temperature peaks originate from the process of an anti-ferromagnetic phase (AFM) separating from the ferromagnetic matrix (phase separation) in polycrystalline materials. The peak positions of internal friction and resistance will move towards higher temperature when the sample was annealed in flowing oxygen, and the low temperature peaks become smaller.     

Electron spin resonance and AC susceptibility studies on La0.9Pb0.1MnO3 single crystals

Phase separation has been recognized as important properties of manganite to create colossal magnetoresistance (CMR). Frequency dependence of AC susceptibility and electron spin resonance (ESR) studies confirm the phase separated glassy magnetism in the La_0.9Pb_0.1MnO₃ crystals. ESR spectra exhibit a gradual narrowing of the asymmetric FMR signals upon increasing the temperature and approaching the Curie temperature (T_c). The temperature dependence of line width (ΔH_pp), g-factor, double integrated (I) intensity of the resonance signals and frequency dependence peak temperatures are discussed in this paper to explain inhomogeneity in the La_0.9Pb_0.1MnO₃ single crystals.     

Magnetoimpedance effect at various temperatures for manganite La0.7Ca0.3MnO3−δ

In the present work, the AC magnetoimpedance effect in La_0.7Ca_0.3MnO_3−δ at various temperatures are investigated. The peak of the metal–insulator transition occurs in the temperature dependence of impedance. Negative magnetoimpedance effect in the La_0.7Ca_0.3MnO_3−δ is obtained at frequencies f≤10 MHz. In the magnetoimpedance effect of manganites, the magnetic field not only decreases the permeability μ_t, but also reduces the resistivity ρ by aligning the local spins and varying the transfer integral t_ij. The AC magnetoimpedance participated by the DC colossal magnetoresistance (CMR) in manganites, should be connected with the combined effects of double exchange interaction, electron–phonon coupling and skin effect.     

A study of internal friction in polypropylene (PP) filled with nanometer-scale CaCO3 particles

The internal friction and relative elastic modulus of polypropylene (PP) filled with nanometer-scale calcium carbonate (nm-CaCO₃) particles in different contents (0, 4, 7, and 15 vol.%) are measured in the temperature range 150–400 K with a torsion pendulum. The peak associated with the glass transition and a small peak (′ peak) associated with the pre-melting process in crystalline parts of PP was observed around 290 and 370 K, respectively. At temperatures lower than 270 K, no peaks were observed. With increasing content of nm-CaCO₃ particles, the apparent activation energy of the peak decreases, and after passing a minimum of 4.7 eV at 4 vol.% of nm-CaCO₃, it increases. In contrary to this behavior, the peak temperature has a maximum of 289 K at the same filler fraction. Correspondingly, the highest tensile and flexural strength of PP were obtained around this content. These observations may be understood through the influences of fillers on the degree of crystallization of PP and on the mobility of molecules of PP.     

Percolation mechanism for colossal magnetoresistance

We propose a new mechanism to explain colossal magnetoresistance. The explanation assumes that the materials displaying colossal magnetoresistance are halfmetallic and proposes that the effect is a critical phenomenon, which is intimately connected with the ferromagnetic-to-paramagnetic phase transition present in these materials. The proposed mechanism is a percolation mechanism; the behavior of the resistance is described using a resistor network. An analysis of the percolation phase diagram and Monte Carlo calculations on the resistor network show a full qualitative correspondence with the experimentally observed features of colossal magnetoresistance.     

Relaxation peaks associated with the oxygen-ion diffusion in La2−xBixMo2O9 oxide-ion conductors

The effects of bismuth doping on the oxygen-ion diffusion in oxide-ion conductors La_2−xBi_xMo₂O₉ (x=0.05, 0.1, and 0.15) have been studied by both internal friction and dielectric relaxation techniques. Two internal friction peaks of relaxation type (P₁ and P₂ peak) were observed at a measurement frequency of 4 Hz around 380 and 430 K, respectively. As for the dielectric measurement, a prominent dielectric relaxation peak (P_d) was found in all the Bi-doped samples around 700 K at a measurement frequency of 50 kHz, which actually consists of two sub-peaks (denoted as P_d1 and P_d2 peak). With increasing Bi-doping content, two peaks shift to higher temperature and decrease in height, while the activation energy of both peaks increases. The main reason was interpreted as the introduction of the lone-pair electrons of bismuth, which tends to block the diffusion of oxygen ion.     

The nonlinear influence of an electric field on phase transitions in ferromagnetic semiconductors: Lanthanum manganite

We investigate nonequilibrium processes of self-heating induced by electric current in ferromagnetic semiconductors exhibiting colossal magnetoresistance (CMR) in the vicinity of the Curie temperature. The heat balance equation is solved taking into consideration localized states that appear as a result of scattering from magnetic inhomogeneities and are characterized by a percolation threshold proportional to the amplitude of spin fluctuations. The appearance of N-shaped current-voltage characteristics and hysteresis in the dependence of magnetization on electric potential difference, which are caused by the emergence of a “hot” (with respect to internal temperature) semiconductor paramagnetic phase, is revealed in the steady-state regime. The possibility of suppression of the effect of colossal magnetoresistance with increasing potential difference is indicated. The onset of self-oscillation of current and magnetization with decreasing transverse dimensions of the sample is demonstrated.     

(La2/3Ca1/3)(Mn(3-x)/3)Fex/3)O3体系磁电阻行为的研究

通过系统地测量（La_2/3Ca_1/3）（Mn_{（3-x）／3}Fe_x／3）O₃（x＝0、0.1、0．2、0．3的体系样品的电阻率-温度关系以及一定温度下磁电阻率与磁场的关系,发现随x的变化其磁电阻率峰和电阻率峰均发生位移,磁电阻率峰值增大,并伴生磁电阻率峰展宽效应．作者认为由于Fe的替代,引起体系中Mn³⁺／Mn⁴⁺比率及磁矩的变化,加之外场对磁有序结构的调制作用,从而影响了Mn³⁺－O^－Mn⁴⁺的双交换作用,最终导致磁电阻行为发生变化．     

Effect of Ca2PbO4 on the formation of the (2223) phase in the Bi---Pb---Sr---Ca---Cu---O system

The effect of excess Ca₂PbO₄ on the superconducting properties of Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O_yB_x (B ≡ Ca₂PbO₄) is investigated through X-ray diffraction, resistivity and a.c. magnetic susceptibility measurements. The Xpray diffraction results show that the volume friction of the low T_c (221s) phase decreases and that of (2223) phase increases with the addition of Ca₂PbO₄. For x = 0.6 and 1.5, only the (2223) phase exists. At higher Ca₂PbO₄ concentrations, i.e. for x = 3.0, the low T_c phase again appears along with the high T_c phase. Diffraction peaks appear at d = 2.02, 2.814 and 2.85, which are characteristic peaks of Ca₂PbO₄, and their intensities increase with increasing x, indicating that Ca₂PbO₄ exists in the lattice in the same form. However, the T_c(0) values decreased gradually from 107 to 98 K with increasing Ca₂PbO₄ concentration. Compared with the undoped sample, the width of the transition δT_c is broad for impurity-added samples and reaches a maximum value for x = 3.0. Pure and Ca₂PbO₄-doped samples showed two peaks in ξ″ vs. T curves and two steps in ξ′ vs. T curves. The first peak is close to the transition temperature and corresponds to the midpoint of the first step in the ξ′ curve. The second peak appears below 98 K and this peak maximum corresponds to the midpoint of the second step. The position of both sets of peaks remained almost the same up to x = 1.5 and shifted towards lower temperatures thereafter. The role of Ca₂PbO₄ in the growth of the (2223) phase and the mechanism of energy loss are discussed.