相分离Eu0.5Sr0.5MnO3体系中的场诱导磁相变及输运特性研究

研究了半掺杂相分离锰氧化物Eu0.5Sr0.5MnO3样品的结构和电磁输运特性.在半掺杂情况下,该样品呈O′型正交结构,表明样品存在典型的Jahn-Teller畸变;在75 K附近样品的顺磁/反铁磁背景中开始出现铁磁相,在更低的温度42 K,4000 A/m磁场下M-T的场冷曲线和零场冷曲线出现明显分岔,样品的交流磁化率实部随温度的变化曲线中也在42 K观察到尖峰的出现,表现出团簇玻璃行为.在无外加磁场下该样品在整个测量温区均呈现绝缘体型导电行为,而在外加磁场1.6×106 A/m下出现磁场诱导的绝缘体-金属 (I-M)转变,其电输运特性符合可变程跳跃模型;研究表明,半掺杂Eu0.5Sr0.5MnO3样品的基态存在多种复杂而丰富的磁相互作用之间的竞争机理,其研究将为强关联锰氧化物体系物理机理的理解提供丰富的实验资料.     

Sr掺杂Eu1-xSrxMnO3体系的核磁共振和磁结构研究

对Eu_1-xSr_xMnO₃ (x=0.4, 0.5, 0.6, 0.7) 体系的磁结构进行了系统的研究. 通过核磁共振实验, 磁化测量, 并结合电输运测量结果表明, Sr的掺入使得 EuMnO₃反铁磁母相中出现铁磁相. 铁磁相和反铁磁相的竞争导致样品在低温下的自旋玻璃行为.分析认为, Eu_0.4Sr_0.6MnO₃和Eu_0.3Sr_0.7MnO₃的磁结构在低温下呈现更加复杂的特征, 主要源于铁磁团簇的形成以及无序相的存在.     

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     

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     

半掺杂Sm0.5Ca0.5MnO3体系的电荷有序和再入型自旋玻璃行为

研究了半掺杂锰氧化物Sm_0.5Ca_0.5MnO₃体系的结构、输运和磁特性，结果表明，在半掺杂情况下，该体系呈现O′类正交结构，表明体系存在典型的Jahn-Teller效应畸变；输运结果在整个测量温区均呈现半导体导电行为，没有出现金属－绝缘体(M-I)转变和CMR效应；电荷有序转变发生在T=270K左右，反铁磁转变温度出现在200K附近，且表现出典型的再入型自旋玻璃(spin-glass)行为，自旋玻璃转变温度T_SG在4 关键词： 自旋玻璃 电荷有序 负磁化现象 多相竞争     

An investigation of phase separation by magnetic force microscopy in La0.45Sr0.55MnO3-δ(δ≈0.01)

We have found phase separation in La_0.45Sr_0.55MnO_3-δ (LSMO) by means of electron spin resonance, magnetic force microscopy (MFM) and magnetic measurements. Ferromagnetic and antiferromagnetic phases can coexist at low temperatures, and ferromagnetic and paramagnetic phases coexist when the temperature lies between the Néel and Curie temperatures. The size and shape of the ferromagnetic phases (the minority phases) was first observed directly from MFM images. It is suggested that the phase separation in LSMO is not the charge segregation type, but an electroneutral type due perhaps to the nonuniform distribution of oxygen vacancies.     

La0.82Te0.18MnO3薄膜的输运特性和光诱导效应

用脉冲激光沉积法制备了非金属Te掺杂的钙钛矿锰氧化物La_0.82Te_0.18MnO₃单晶薄膜.该薄膜从83 K升温至373 K过程中发生金属-绝缘体相变，转变点温度为283 K.其电阻率在T<T_MI时符合电子-电子、电子-磁振子散射公式；在T>T_MI时为小极化子输运.薄膜在低温段连续激光(波长为532 nm，40 mW)作用下电阻率显著增大，电阻变化率在253 K达到最大值51.1%,该变化率远大于相同条件下的空穴掺杂材料；在高温段产生了较小的光电导，电阻变化率小于10%.这些现象主要与激光激励下自旋系统和小极化子的变化有关.La_0.82Te_0.18MnO₃薄膜在激光诱导下具有明显的与自旋相关的弛豫现象.激光开始作用时薄膜电阻率随时间的变化符合指数关系. 关键词： 0.82Te_0.18MnO₃薄膜')" href="#">La_0.82Te_0.18MnO₃薄膜 光诱导 输运特性 电子掺杂     

Pd/Ag掺杂的La0.67(Ca0.65Ba0.35)0.33MnO3体系电特性和磁电阻增强效应

系统研究了xAg-La_0.67(Ca_0.65Ba_0.35)_0.33MnO₃和xPd-La_0.67(Ca_0.65Ba_0.35)_0.33MnO₃(xAg-LCBMO和xPd-LCBMO)两种复合体系的电特性和磁电阻特性. 结果发现,Pd和Ag的掺杂都引起电阻率的大幅降低和峰值电阻率温度的升高,这主要源于晶粒边界/表面良导体金属晶粒的析出. 另外,Pd和Ag的掺杂都引起室温磁电阻的大幅增强. 尤其是27%摩尔比的Ag掺杂诱导了高达70%的室温磁电阻,几乎是未掺杂母体LCBMO的10倍,而27%摩尔比的Pd掺杂诱导产生了更高的磁电阻,约170%. 磁电阻的大幅增强,与良导体金属掺杂引起的样品电阻率的降低有关. 另一方面,晶粒表面/边界Mn离子与Pd离子接近诱导Pd离子的自旋极化对磁电阻的增强起了重要的促进作用.     

Pr0.5Ca0.5MnO3/Si异质结输运特性和整流特性研究

利用固相反应法制备Pr_0.5Ca_0.5MnO₃(PCMO)靶材,并采用脉冲激光沉积法(PLD)在n型Si(111)基片上沉积PCMO外延薄膜,研究了薄膜的输运特性及薄膜与Si衬底形成异质结的整流特性.结果表明:在80—300 K温度范围内,PCMO薄膜的阻温关系符合变程跳跃模型,随着温度的升高表现出从一维到二维再到三维的转变,分析认为可能源于轨道自由度对电子输运特性的影响.PCMO/Si异质结在磁场作用下整流特性得到改善,且在正向电流方向出 关键词： 0.5Ca_0.5MnO₃')" href="#">Pr_0.5Ca_0.5MnO₃ Si 整流 正磁电阻     

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     

Influence of epitaxial growth on phase competition in Pr0.5Sr0.5MnO3 films

A series of Pr_0.5Sr_0.5MnO₃ (PSMO) films with various thickness were epitaxially grown on substrates of (0 0 1)-oriented (LaAlO₃)_0.3(SrAl_0.5Ta_0.5O₃)_0.7 (LSAT), LaAlO₃ (LAO) and SrTiO₃ (STO), and (0 1 1)-oriented STO using pulse laser deposition. Influence of epitaxial growth on phase competition was investigated. A ferromagnetic metal to antiferromagnetic insulator (FMM-AFI) transition upon cooling is present in both largely compressed situations deposited on LAO (0 0 1) and tensile cases deposited on STO (0 0 1) but absent in little strained films grown on LSAT (0 0 1), indicating that the antiferromagnetic insulating state is favored by strains. On the other hand, the 400 nm films deposited on (0 1 1)-oriented STO as well as LAO substrates show FMM-AFI transition. These results reveal that both the orientation of epitaxial growth and substrate-induced strain affect the FMM-AFI transition.     

Glassy-ferromagnetic behavior in Eu0.5Sr0.5CoO3

Polycrystalline perovskite cobalt oxide Eu_0.5Sr_0.5CoO₃ was prepared by the conventional solid-state reaction method. X-ray powder patterns indicated the prepared samples are pure, cubic perovskite structure (Pm3?m), and with no evidence of any secondary phases. The dc magnetization and ac susceptibility measurements were carried out to investigate the magnetic properties of the sample, and which indicated that cluster-glasses properties are suppressed with the increasing of the coercive field. We denied the possibility of spin-glasses and the existence of the Hopkinson effect in Eu_0.5Sr_0.5CoO₃ through the temperature-dependent ac susceptibility measurements, and explained the magnetic behavior of Eu_0.5Sr_0.5CoO₃ with the competition between magnetic anisotropy and the external magnetic field.     

Electron spin resonance study of the Ba-doping manganite Nd0.5Sr0.5MnO3

We have performed magnetization measurements and electron spin resonance （ESR） on polycrystalline manganites of Nd0.5Sr0.5-xBaxMnO3 （x = 0.1）. Phase separation and phase transitions are observed from the susceptibility and the ESR spectra data. Between 260 K （～ Tc） and 185 K （～ TN）, the system coexists of the paramagnetic phase and the ferromagnetic （FM） phase. Between 185 K and 140 K, the system coexists of the FM phase and the antiferromagnetic （AFM） phase. These results indicate that the system has a very complex magnetic state due to the origin of the instability stemming from manganite Nd0.5Sr0.4Ba0.1MnO3 by partially substituting the larger Ba^2＋ ions for the smaller Sr^2＋ ions.     

ENHANCEMENT OF FERROMAGNETIC CLUSTER INDUCED BY MAGNETIC FIELD IN THE PHASE-SEPARATED La0.5Ca0.5MnO3

Magnetic and transport properties of La_0.5Ca_0.5MnO₃ have been investigated by measuring the magnetization and resistance in zero-field-cooled (ZFC) and field-cooled (FC) modes. Conspicuously irreversible behaviors of magnetization/resistance in the two different modes were observed below the charge ordering transition temperature (T_CO). The ZFC and FC magnetizations at 5K, as functions of the magnetic field, coincide for μ₀H≤1T. Afterwards, the ZFC magnetization tends to an approximate constant, but the FC one increases linearly with increasing field. There exists an excellent correspondence between magnetization and resistance below T_CO. All the results suggest that the ferromagnetic clusters embed in the charge-ordered matrix. The phenomenon of ferromagnetic clusters growing up easily in the FC procedure has been interpreted according to the model of thermally activated two-level system.     

Phase separation and abnormal transport behaviours in La0.7-xGdxSr0.3MnO3 system

The magnetic and transport behaviours of the La_{0.7-x}Gd_xSr_{0.3}MnO_3 (0≤x≤0.70) system are investigated. The experimental results indicate that with increasing Gd doping content, the magnetism of the system changes from the long-range ferromagnetic order state to the cluster-spin glass state, then to the antiferromagnetic (AFM) state. It is interesting that the phase separation appears at x=0.30 and 0.40 and disappears for x≥0.50 where the AFM state occurs. At high doping content, the transport behaviours exhibit abnormality, e.g. there are two temperature ranges in which the ρ－T curves can be well fitted by a variable-range hopping (VRH) model. We suggest that the VRH does not come from the hopping of carriers between clusters, but from the different magnetic backgrounds in the clusters.     

Revival of metastable behavior in phase separated La0.5Ca0.5MnO3+δ

The La_xCa_1−xMnO_3+δ compositions close to charge ordering (x∼0.5) show a gradual relaxation from a metallic/ferromagnetic state to an insulating/antiferromagnetic state with thermal cycling. Here, we report on the magnetic relaxation in the metastable state and also the revival of the metastable state (in a relaxed sample) due to high temperature thermal treatment. We also show the changes in the magnetization and the thermoelectric power as the revived metastable state is cycled. We find that the changes in the thermoelectric power extend well into the region above the charge ordering temperatures. This suggests that the micro-structural changes accompanying the thermal cycling leave their imprint in the paramagnetic insulating state as well.     

Disorder-order phase transition induced by size effect in bulk La2/3Sr1/3MnO3

Bulk La_2/3Sr_1/3MnO₃ ceramic samples prepared by thermal decomposition are investigated using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). An abnormal phenomenon, where three kinds of La_2/3Sr_1/3MnO₃ phases with different structures and the same composition coexist in the same grain, has been observed. Besides the stable rhombohedral majority phase, the two other phases are a simple cubic structure with a=0.389 nm and a new hexagonal structure with a=0.544 nm, c=0.668 nm. The simple cubic phase is a residual phase of high-temperature due to the size effect and bondage of twin boundary. Image simulations have suggested that the new hexagonal phase is the La-Sr ordered structure with space group , which is converted from the disordered simple cubic phase. The formation mechanism of the ordered phase is explained from volume energy and interface energy considerations.