Mn位W掺杂对La0.3Ca0.7MnO3体系磁结构的影响

通过对La0.3Ca0.7Mn1-xWxO3(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La0.3Ca0.7MnO3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TCO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.     

La0.3Ca0.7Mn1－xVxO3体系的有序相和再入型自旋玻璃行为研究

通过对La_0.3Ca_0.7Mn_1－xV_xO₃(x=0.05,0.10,0.134,0.20)体系的M-T曲线、ρ-T曲线、ESR谱的测量,研究了Mn位掺V对La_0.3Ca_0.7MnO₃体系电荷序和自旋序的影响.结果表明,当0.05≤x≤0.134时, 体系存在电荷有序(CO)相,其自旋序随温度降低发生顺磁(PM)-电荷有序(CO)-反铁磁(AFM)变化.当x＝0.20时,CO相逐步融化,在40 K发生自旋玻璃转变,表现出再入型的自旋玻璃行为,低温下的基态存在着多种复杂的磁相互作用之间的竞争机理. 关键词： 电荷有序 自旋序 相分离 再入型自旋玻璃行为     

La0.3Ca0.7Mn1-xWxO3体系电荷有序的建立与融化

用标准的固相反应法制备了La0.3Ca0.7Mn1-xWxO3(0.00≤X≤0.15)多晶样品.通过对样品磁化强度-温度(M-T)曲线、磁化强度-磁场强度(MH)曲线及ESR谱的测量,研究了Mn位W掺杂对La0.3Ca0.7Mn1-xWxO3体系的磁相变的影响.结果表明,随着W掺杂量的增加,体系磁相变发生了复杂的变化过程.当掺杂量为0.00≤X≤0.08时,体系存在电荷有序(CO)相,反铁磁(AFM)/C0态共存于相变温度以下,电荷有序温度(Tco)随着W掺杂量的增加而增加.x≥0.12时,体系电荷有序态逐渐减弱并融化,在极低温度下存在顺磁-铁磁(PM-FM)相变.     

Mn位W掺杂对La0.3Ca0.7MnO3电荷有序相的破坏及磁性质影响

采用固相反应法制备了样品La0.3Ca0.7Mn1-xWxO3(x=0.00,0.12,0.15).通过测量样品的M～T曲线、M～H曲线和ESR曲线,研究了Mn位W掺杂对La0.3Ca0.7MnO3电荷有序相的破坏及磁性质的影响.结果表明:当x=0.00时,在262K时形成电荷有序相(CO相);当T>262K时,表现为顺磁;当T<190K时,表现为长程反铁磁(在AFM本底中存在少量FM成分),AFM/CO态共存;从262K(TCO)到190K,随温度降低在电荷有序态下从顺磁向反铁磁转变.当x≈0.12时,在170K～180K残留CO相,而当x=0.15时,CO相消失,体系存在一个CO态融化过程,在极低温下存在顺磁铁磁(PMFM)相变.     

Bi0.5Ca0.5Mn1-xCoxO3体系中的电荷有序和相分离

利用固相反应法制备了Bi_0.5Ca_0.5Mn_1-xCo_xO₃(0≤x≤0.12)系列多晶样品.研究了Co掺杂对Bi_0.5Ca_0.5MnO₃电荷有序的影响.结果表明,Co掺杂导致电荷有序相逐渐融化、铁磁相互作用的增强;当x≥0.08时,电荷有序转变峰完全消失,但残留的反铁磁电荷有 关键词： 钙钛矿锰氧化物 电荷有序 团簇玻璃 相分离     

钙钛矿型稀土锰氧化物La2/3Ca1/3MnO3中Mn位的Ti替代效应

研究了钙钛矿型锰氧化物La_2/3Ca_1/3Mn_1-xTi_xO₃(0≤x≤0.3)的结构、磁性和输运性质.发现Ti替代Mn强烈地抑制了La_2/3Ca_1/3MnO₃的铁磁性和金属电导,并很大地提高了磁电阻值.在低掺杂情况下(x≤0.04),1%的Mn被Ti替代,居里温度T_C和金属-绝缘体转变温度T_p分别平均下降了31和26.5K.当x=0.06时,铁磁态过渡为团簇玻璃态,并在x=0.20时完全变为自旋玻璃态.指出由于Ti的掺入而引起的磁稀释作用以及局域晶格畸变是产生上述结果的主要原因. 关键词：     

La0.3Ca0.7Mn1-xWxO3(x=0.04,0.08,0.12)体系的电子自旋共振谱研究

用固相反应法制备了La0.3Ca0.7Mn1-xWxO3(x=0.04,0.08,0.12)多晶材料,测量了样品的p～T曲线及ESR谱,对ESR谱的归一化强度和线宽进行了研究.结果表明:当x≤0.08时,ρ～T曲线出现弯折点,体系出现电荷有序(CO)相,体系随温度降低发生顺磁(PM)-电荷有序(CO)-反铁磁(AFM)变化;当x=0.12时,ρ～T曲线没有出现弯折点,CO相融化,自旋序主要是顺磁(PM),但存在CO相的残留.x≤0.08样品的线宽在高于Tco的温区几乎不随温度变化,体系处在顺磁态,铁磁关联较弱;在低于Tco的温区,电子的局域化程度增强,线宽随温度的降低而增加.     

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}     

磁场对La2－xSrxCuO4单晶热导的影响研究

报道了系列欠掺杂La_2-xSr_xCuO₄（x=0063,0070,0 090,0110,0125）单晶的零场和加磁场情况下ab面和c方向的热导率与温度的关系曲线，测量 温度范围从2到45K. 研究发现ab面和c方向的热导率都受到磁场的压制. 而且在磁场 的作用下，热导压制率随温度变化的关系和场致反铁磁有序的增强与温度的依赖关系有高度 相似性. 认为磁场引起的ab面的热导压制主要是电子热导的变化所致，而c方向的 压制则可能主要来源于声子热导的变化，它们均可能与磁场诱导下欠掺杂La_2-x Sr_xCuO₄的某种电荷有序和磁有序的增强密切相关. 关键词： 热导 电荷有序 磁有序     

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：     

Improvement of refrigerant capacity of La0.7Ca0.3MnO3 material with a few percent Co doping

The influence of first and second order magnetic phase transitions on the magnetocaloric effect (MCE) and refrigerant capacity or relative cooling power (RCP) of La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.95Co_0.05O₃ materials has been investigated. Large low-field-induced magnetic entropy changes are observed in La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.95Co_0.05O₃ materials. The La_0.7Ca_0.3MnO₃ material experiences a large entropy change with a first-order magnetic phase transition at the Curie temperature, T_C. On the other hand, La_0.7Ca_0.3Mn_0.95Co_0.05O₃ displays a smaller entropy change with a second order phase transition. While a first-order magnetic transition material induces a larger MCE (7.528 J/kg K at 5 T) at T_C, this is limited to a narrow temperature range, resulting in a relatively small RCP (218 J/kg), while the Co-doped second-order magnetic transition material induces a smaller MCE (7.14 J/kg K for 5 T), but it is spread over a broader temperature range, resulting in a larger RCP (308 J/kg). The maximum magnetoresistance (MR, defined as ρ(0)/ρ(H)-1) under a field of 5 T is about 206% and 333% for La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.95Co_0.05O₃, respectively. The refrigeration capacity (RCP) is enhanced in La_0.7Ca_0.3Mn_0.95Co_0.05O₃ (by about 41%) due to small changes from Co doping. The magnetocaloric features of these materials at lower magnetic fields (MCE=3.163 for La_0.7Ca_0.3Mn_0.95Co_0.05O₃ and 4.63 J/kg K for La_0.7Ca_0.3MnO₃ at 1 T), and the high RCP and MR can provide some ideas for exploring novel magnetic refrigerants that can operate with permanent magnets rather than superconducting ones as the magnetic field source.     

Charge order and spin order in Nd0.5Sr0.5Mn1? x (Ga x , Ti x )O3 system

Magnetic properties of Nd0.5Sr0.5Mn1-x(Gax,Tix)O3 system (0.04≤x≤0.4) were investigated through magnetization and electron spin resonance (ESR) measurements. It was observed that a small amount of Ti substitution for Mn will destroy the charge-ordering (CO) phase completely and induce the cluster-spin-glass phase in the system, which displays a procedure of collapse of CO and of an enhancement of spin ordering (SO) phase. In contrast, the Ga substitution for Mn induces a melting of CO phase in the system. It was observed that with substitution the CO phase is suppressed gradually and the remanent CO phase is retained all the while, and withal, there is a co-existence of AFM CO phase and FM SO at low temperature. In addition, an abrupt rise of magnetization was observed in M-Tcurves. We attributed this abnormal phenomenon to a transition from canted AFM SO to FM SO in CO region.     

Research on charge-ordering state in La0.5Sr0.5MnO2.88 and La0.5Sr0.5Mn0.5Ti0.5O3 systems

The structural, magnetic and transport properties of La_0.5Sr_0.5MnO_2.88 and La_0.5Sr_0.5Mn_0.5Ti_0.5O₃ samples have been investigated systematically. Indeed, this series has been considered to understand the influence of physical parameters such as oxygen deficiency and titanium doping effect in undoped La_0.5Sr_0.5MnO₃ sample. Ceramic material based on La_0.5Sr_0.5MnO_2.88 exhibits interesting behaviours of charge-ordering (CO), ferromagnetic (FM) states and a good conductivity down to the lowest temperatures. The substitution of Ti for Mn destroyed drastically the CO, damaged the motion of itinerant e_g electrons and changed the local parameters of perovskite cell. A change of the structure from tetragonal to rhombohedral symmetry is observed causing a weakening of double-exchange interaction. The experiment results show that the suppression of the CO is sensitive to the variety of Mn³⁺/Mn⁴⁺ ratio. In a field of 8 T at 10 K, FM and CO phase can be evaluated to be ∼20:80 according to the μ_exp/μ_cal ratio for La_0.5Sr_0.5MnO_2.88, whereas the CO state is suppressed for La_0.5Sr_0.5Mn_0.5Ti_0.5O₃ sample, FM and anti-ferromagnetic (AFM) phase are coexisted and evaluated to be ∼54:46, respectively.     

The effect of Gd-doping on the charge ordering state of Bi0.3−xGdxCa0.7MnO3 (0≤x≤0.30)

The effect of Gd-doping on the charge ordering (CO) state in perovskite-type manganates Bi_0.3−xGd_xCa_0.7MnO₃ with x=0, 0.02, 0.05, 0.1, 0.3 has been investigated by transport and magnetic property measurements. It is found that CO temperature (T_CO) and antiferromagnetic (AFM) ordering temperature T_N occurring below T_CO decrease obviously with increasing Gd-doping level. Accompanying the variation of T_CO, the increased magnetization and the decreased resistivity are observed. In addition, the increased magnetic inhomogeneity has been also observed in the samples based on the difference between the zero-field-cooling (ZFC) magnetization M_ZFC and field-cooling (FC) magnetization M_FC, which is ascribed to the competition between ferromagnetic (FM) phase induced by Gd-doping and CO AFM phase. The experimental results indicate that the Bi³⁺ lone pair electron with 6s² character plays a dominating role on the CO state of Bi_0.3Ca_0.7MnO₃.     

Crossover of critical behavior in La0.7Ca0.3Mn1−xTixO3

Critical behavior of Ti-doped La_0.7Ca_0.3MnO₃ ceramics are studied using magnetization methods. The results show that the paramagnetic–ferromagnetic transition is first order for the undoped sample, then for low-doping samples the mean-field model is suitable, and finally a 3D Heisenberg model is satisfied for higher doping samples. These findings demonstrate that the critical behavior of the magnetic transition for manganites is sensitive to Mn-site doping.     

Electrical and magnetic behavior of La0.7Ca0.3MnO3/La0.7Sr0.2Ca0.1MnO3 composites

The electrical transport properties and the magnetoresistance of La_0.7Ca_0.3MnO₃/La_0.7Sr_0.2Ca_0.1MnO₃ composites are investigated as a function of sintering temperature. On the basis of an analysis by X-ray powder diffraction and scanning electron microscopy we suggest that raising the sintering temperature enhanced the interfacial reaction and creates interfacial phases at the boundaries of the La_0.7Ca_0.3MnO₃ and La_0.7Sr_0.2Ca_0.1MnO₃. Results also show that in 3 kOe, and at the Curie temperature, the magnetoresistance value of 14% was observed for the composite sintered at 1300 °C. Based on 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 experimental resistivity—temperature data from 50-300 K and find that the activation barrier decreases as temperature is increased.     

A位的Gd掺杂对La0.7Sr0.3MnO3体系磁电性质的影响

通过实验研究了La_0.7-xGd_xSr_0.3MnO₃(x= 0.00，0.10，0.15，0.20，0.30 ，0.40，0.50，0.60，0.70)体系的M-T曲线、M-H曲线、红外光谱、拉曼光谱、ρ-T曲线和M R-T曲线.实验结果表明：随着Gd掺杂的增加，体系从长程铁磁有序向自旋团簇玻璃态和反铁 磁状态转变，高掺杂时的输运性质在其磁背景下发生异常.Gd掺杂引起的磁结构变化和额外 的磁性耦合将导致庞磁电阻效应. 关键词： 磁结构 输运行为 庞磁电阻效应