La0.4Ca0.6Mn1-xCrxO3（x=0.00,0.08）的磁电性质研究

用固相反应法制备了La0.4Ca0.6Mn1-xCrxO3（x=0.00,0.08）多晶样品,通过XRD谱、M-T曲线、ρ~T曲线、Lnρ-T-14曲线,研究Cr^3＋替代Mn3＋对La0.4Ca0.6MnO3电荷有序相及电输运性质的影响.实验结果表明：Cr^3＋替代Mn3＋对La0.4Ca0.6MnO3电荷有序相影响...     

57Fe Mössbauer study of La0.67Ca0.33Mn1 − x Fe x O3 CMR system

The structural changes of near-equiatomic α-FeCr alloys, ground in a vibratory mill in vacuum and in argon, were followed as a function of milling time. An amorphous phase forms in both cases but at a much faster rate when milling in argon than when milling in vacuum. Amorphisation by ball-milling of α-FeCr alloys is deduced to be an intrinsic phenomenon which is however speeded-up by oxygen. The amorphous phase crystallizes into a bcc Cr-rich phase and a bcc Fe-rich phase when annealed for short times.     

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＂.     

Growth and optical properties of Cr~（3＋）-doped CdWO_4 single crystals

A high-quality Cr 3+:CdWO4 single crystal at a size of approximatelyΦ25×80 mm is grown using the Bridgman method with CdO,WO3,and Cr2O3 as raw materials and their molar ratio of 100:100:0.5.The temperature gradient of solid-liquid interface at growth is approximately 50?C/cm and the growth rate is 0.05 mm/h.The X-ray diffraction(XRD),absorption,excitation,and emission spectra of different parts of the as-grown and O2-annealed crystals are investigated.Two strong broad optical absorption bands of about 472 and 708 nm are observed,and they are associated with the transitions 4 A2→ 4 T1 and 4 A2→ 4 T2.The weak 4 T2→ 2 E transition(the R-line)at 632 nm is also observed.The crystal-field parameter Dq and the Racah parameters B and C are estimated to be 1 412.4,776.8,and 3 427.6 cm? 1,respectively,according to the absorption spectra and crystal-splitting theory.A broadband fluorescence at about 1 000 nm due to 4 T2→ 4 A2 transition is produced by exciting the samples at 675 nm.After being annealed in an O2 atmosphere,the crystals become more transparent,while the effective light absorption of Cr 3+ ions is evidently enhanced and the emission intensity is also strengthened due to the reduction of oxygen vacancies in the CdWO4 crystal after annealing.     

Electrical and magnetic properties of La0.67Ba0.33Mn1−x (Me) x O3 perovskite manganites: case of manganese substituted by trivalent (Me = Cr) and tetravalent (Me = Ti) elements

The effects of non-magnetic Ti⁴⁺ substitution on the structural, electrical and magnetic properties of La_0.67Ba_0.33Mn_1?x Ti _x O₃ (0≤x≤0.1) are investigated and compared to those existing in La_0.67Ba_0.33Mn_1?x Cr _x O₃ (magnetic Cr³⁺). The structural refinement by the Rietveld method revealed that Ti-doped samples crystallize in the cubic lattice with space group $\mathrm{Pm}\bar{3}\mathrm{m}$ , while samples with Cr crystallize in the hexagonal setting of the rhombohedral $\mathrm{R}\bar{3}\mathrm{C}$ space group for identical contents of dopant. The most relevant structural features are an increase of the lattice parameters, of the cell volume and of the inter-ionic distances with increasing Ti doping level. Both series of samples show a decrease of the paramagnetic–ferromagnetic transition temperature when the amount of chromium or titanium increases. Transport measurements show that when increasing the metal doping, the resistivity increases whereas the metallic behavior of the parent compound La_0.67Ba_0.33MnO₃ is destroyed. For a substitution higher than 5 at.% of Ti and 10 at.% of Cr, the samples exhibit a semiconducting behavior in the whole range of temperature, for which the electronic transport can be explained by variable range hopping and/or small polaron hopping models.     

Large magnetic entropy change and magnetic properties in La （Fel－xMnx）ll.TSil.3Hy compounds

Magnetic properties and magnetic entropy change in La(Fe_{1-x}Mn_x)_{11.7}Si_{1.3}H_y compounds have been investigated. A significant increase of the Curie temperature T_C and a small increase of the saturation magnetizations μ_S have been observed after the introduction of interstitial H, which caused a slight volume expansion. The first-order field-induced itinerant-electron metamagnetic (IEM) transition remains and brings about a large magnetic entropy change around room temperatures for the compounds. The maximal magnetic entropy change is about 23.4, 17.7 and 15.9J/kg·K under a magnetic field change from 0 to 5T for x=0.01, 0.02 and 0.03, respectively. Therefore, the compounds appear to be potential candidates for magnetic refrigerants around room temperatures.     

Magnetic and electrical properties of weakly doped manganese-deficient La1 − x Ca x Mn1 − z O3 manganites

Samples of La_{1 ? x} Ca _x Mn_{1 ? z} O_{3 + δ} (x = 0.05?0.15) with deficient manganese and excess oxygen δ do not pass into a metallic state and have low spin ordering temperatures T _C at acceptor Mn⁴⁺ concentrations near the percolation threshold. These results are explained by carrier localization in clusters near cation vacancies. A break in the carrier transport chain Mn-O-Mn in the form of absent manganese favors cluster formation and decreases the double exchange energy and T _C of the samples. Closeness to the percolation threshold results in strong (more than four orders of magnitude) changes in the electrical resistivity in a magnetic field. The changes in the cluster sizes with the temperature and the magnetic field that are determined from the magnetotransport properties are satisfactorily described in the model of phase separation into small-radius metallic droplets in a dielectric paramagnetic and an antiferromagnetic matrices.     

Structure and magnetic properties of Osn （n=11～22） clusters

The structure and magnetic properties of Osn （n=11～22） clusters are systematically studied by using density functional theory （DFT）. For each size, the average binding energy per atom, the second-order differences of total energies and the highest occupied molecular orbital （HOMO）–the lowest unoccupied molecular orbital （LUMO） gaps are calculated to analyze the stability of the cluster. The structures of Os14 and Os18 clusters are based on a close-packed hexagonal structure, and they have maximum stabilities, so n=14, 18 are the magic numbers. The 5d electrons play a dominant role in the chemical reaction of Osn clusters. The magnetic moments of Osn clusters are quenched around n=12, and when n=18～22 the value approximates to zero, due to the difference of electron transfer.     

钙钛矿锰氧化物La_(2/3)Sr_(1/3)Fe_xMn_(1-x)O_3的结构与磁性研究

本文利用溶胶-凝胶法制备了名义成分为La_(2/3)Sr_(1/3)Fe_xMn_(1-x)O_3(x=0.0,0.1,0.2,0.3,0.5)的系列样品,样品先后经过773,873,1073 K热处理,热处理时采用缓慢升温方式,X射线衍射分析表明,该系列样品均为单相钙钛矿结构,空间群为R3c,利用X'Pert HighScore Plus软件计算了样品的晶粒尺寸、晶格常数、晶胞体积及键长、键角,利用物理性能测量系统测量了样品的磁性,发现样品在10K的磁矩随掺杂量的增加而减小,但存在两个明显不同的变化区域:从x=0到x=0.2时,平均每个分子的磁矩从2.72μB迅速下降到0.33μB,居里温度从327 K下降到95 K,下降了232 K;而从x=0.2到x=0.5时,平均每个分子的磁矩从0.33μB缓慢下降到0.05μB,居里温度从95K下降到46K,只下降了49K,我们认为Fe与Mn离子磁矩反平行是样品磁矩随Fe掺杂量增加而下降的原因之一。     

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}     

Magnetotransport properties of La0.67Ca0.33MnO3//La0.67Sr0.33MnO3 bilayers

The perovskite bilayers La0.67Ca0.33MnO3 （LCMO） （100 nm） / La0.67Sr0.33MnO3（LSMO） （100 nm） and LSMO （100 nm） / LCMO （100 nm） are fabricated by a facing-target sputtering technique. Their transport and magnetic properties are investigated. It is found that the transport properties between them are different obviously due to distinguishable structures, and the different lattice strains in both films result in the difference of metal-to-insulator transition. Only single-step magnetization loop appears in our bilayers from 5K to 320K, and the coercive force of LSMO/LCMO varies irregularly with a minimum ～ 2387A/m which is lower than that of LCMO and LSMO single layer films. The behaviour is explained by some magnetic coupling.     

La0.67Ca0.33MnO3薄膜中的矫顽力随膜厚与衬底的变化

系统研究了衬底为SrTiO3和LaAlO3上的La0.67Ca0.33MnO3薄膜中的矫顽力随厚度和应变的变化。结构分析表明薄膜为（001）方向织构，而且薄膜中的晶粒尺寸随着薄膜厚度的减小而减小。磁测量表明矫顽力先随着膜厚的减小而增加，在t=10-25nm附近到达一极大值。随后，矫顽力随厚度的减小而降低。还得出矫顽力的大小与测量方向有关：t≥25nm (t≤10nm)时，难磁化方向的矫顽力大于（小于）易磁化方向的矫顽力。据此，我们提出：在厚膜（t≥25nm）中，矫顽力变化由畴壁钉扎机制决定；在超薄膜（t≤10nm）中，则与磁畴的形核机制有关。根据t= 5、10、25、400nm的LCMO/STO薄膜的初始磁化曲线，以及t=5，50nm的LCMO/LAO薄膜的小磁滞回线的测量，我们对薄膜中矫顽力机制作了验证，并且还讨论了钉扎和形核机制发生的非均匀区的尺寸。     

La0.67Ca0.33MnO3单晶中Fe的掺杂提高本征磁电阻率和激活能的研究

La_0.67Ca_0.33MnO₃ and La_0.67Ca_0.33Mn_0.96Fe0.04O₃单晶的磁性和磁电阻特性。由于缺少晶粒间界，磁场对于自旋涨落的抑制作用更加明显，所以单晶中的磁电阻率比多晶的提高了两个数量级。通过Fe的掺杂，在40×10⁵A/m的磁场下，磁电阻率从3400%进一步提高到了17600%。通过对居里温度以上电阻率的小极化子模型拟和的结果，我们看到随着Fe的掺杂，激活能被提高。磁极化子激活能的增大可以更加局域电子，这可能是在Fe掺杂单晶中本征磁电阻率进一步提高的主要原因。     

Ru掺杂La0.7Ca0.2Ba0.1Mn1-xRuxO3的磁性和输运性质

利用固相反应法制备了Ru掺杂La0.7Ca0.2Ba0.1Mn1-xRuxO3(x=0～0.06)的多晶样品,探讨了Ru掺杂对体系结构,输运性质以及磁电阻的影响.多晶X射线衍射证实所有样品均保持简单立方钙钛矿结构.通过零场冷却(ZFC)和加场冷却(FC)下的磁化曲线的测量发现随温度降低样品发生了顺磁(PM)到铁磁(FM)的相变,且样品的居里温度(Tc)随Ru掺杂发生了显著的变化,从x=0.00时的306.7K,下降到x=0.02时的294.3K,紧接着又上升到x=0.04时的302.4K.测得居里温度明显高于La0.7Ca0.2Ba0.1Mn1-xMoxO3体系,而且其磁性也大为增强.由零场和外加磁场H=1T测量得到样品的ρ～T曲线表明随温度降低样品同时发生了从绝缘体到金属的转变,绝缘体-金属转变温度低于相应的居里温度.适量的Ru掺杂降低了样品的电阻率,增强了低温时的磁电阻.     

Double-peak resistivity transport properties of La0.67Ca0.33MnO3 ceramics

Polycrystalline samples of La_0.67Ca_0.33MnO₃ were prepared by solid-state reactions by varying the pelletization force and the sintering temperature. Lowering the sintering temperature gave rise to smaller grains and increased the overall resistivity of the ceramic. Partial melting was observed in the ceramics sintered at higher temperatures (1400-1500 °C). Additionally, these ceramics showed two distinct resistivity peaks. The resistivity peak near the magnetic transition (T_C) was sharp, whereas the second peak was a broad one observed below T_C.     

用激光分子束外延在玻璃衬底上生长La0.67Sr0.33MnO3薄膜

采用激光分子束外延技术,在玻璃衬底上制备了La_067Sr_033MnO₃ （LSMO） 薄膜,X射线衍射测量结果表明在玻璃衬底上生长的LSMO薄膜沿c轴择优取向生长.在外磁场3 T和88, 220, 300 K条件下,LSMO薄膜的磁电阻变化率分别达到-378%, -268%和-607%.实验结果表明,在廉价的玻璃衬底上制备大面积和具有应用价值的锰氧化物薄膜是可行的. 关键词： 锰氧化物薄膜 玻璃衬底 外延生长     

BaTiO_3/La_(0.67)Sr_(0.33)MnO_(3-δ)复合薄膜的磁致电极化和磁介电特性研究

采用脉冲激光沉积法制备了BaTiO_3(BTO)与缺氧的铁磁绝缘态La_(0.67)Sr_(0.33)MnO_(3-δ)(LSMO)构成的磁电复合薄膜,研究了20—300K温度区间内磁场对电极化特性和介电特性的影响.研究发现,施加磁场使得电滞回线的剩余极化强度和矫顽场均增大,其变化率峰值分别为111.9%和89.6%,峰值温度分别为40K和60K.异质结具有显著的磁介电效应,在测量温度区间内,磁场使得介电常数增大,介电损耗减小.在0.8T场强下,介电常数的最大磁致变化率出现在60K,达到了300%,而介电损耗也在此温度实现了最大变化,减小为零场时的50.9%.该磁电复合薄膜的磁致电极化和磁介电特性的极值均出现在LSMO层的磁电阻峰值温度附近,这说明磁场对电滞回线和介电参数的调制应该源自电荷相关的耦合作用.其可能的机理是磁场使得锰氧化物中的Mn离子局域磁矩趋于有序排列,并通过自旋-轨道耦合以及界面效应间接影响了BTO的电极化特性.研究结果对于多铁器件的开发和应用具有重要意义.