Giant Low-Field Magnetoresistance in Polycrystalline La_(2/3)Ca_(1/3)Mn_(1-x)Cu_x O_3 Sintered at 1100 ℃

Themixed valenceperovskitemanganiteRE1-yAyMnO3(RE =rareearth ,A =alkalineearth)systemisoneofthemoststudiedtopicsduetoitsunusualmagneticandtransportproperties ,especiallythecolossalmagnetoresistance (CMR)effect[1,2 ] .CMRisfoundonamagneticfieldscaleofseveralteslas ,whichisnotveryappealingforapplications .Accord ingly ,manyresearchgroupsfocusedontheinvestiga tionofextrinsicmagnetoresistanceeffectsfoundinvar iousmagneticoxides[3,4 ] ,sincethesepromisedalargemagnetoresistanceratioinlowfields …     

Giant Low-Field Magnetoresistance in Polycrystalline La2/3Ca1/3Mn1-xCuxO3 Sintered at 1100 ℃

The La2/3Ca1/3Mn1-xCuxO3 (0≤x＜6%) samples were prepared by sol-gel method. Transport properties, conductive mechanism and low-field magnetoresistance (MR0) were investigated. It is found that conductive mechanism is described by Mott′s variable-range hopping and the MR0 near the insulator-metal transition temperature (Tp) increases from 2% to 80% with x from 0 to 4% for a low field of 0.3 T. The experiment also shows that the intergrain magnetoresistance is largely improved. The results indicate that a proper amount of Cu doping can substantially improve magnetoresistance effects.     

La_((1+x)/3)Ba_((2-x)/3)Mn_xCu_(1-x)O_3(0.8≤x≤1)晶体结构和输运性质

用固态反应法制备了名义组分为 L a(1 + x) /3Ba(2 - x) /3Mnx Cu1 - x O3(0 .8≤ x≤ 1) 的系列试样。试样结构的单相性经粉末 X射线衍射分析而得以证实 ,且随 Cu的掺杂其结构由未掺杂时的立方对称性转变为正交对称性。输运实验表明 :所有试样均经历了半导体到金属的转变 ,但转变温度 TP随掺杂量增加而逐渐移向低温端 ;外加磁场大大增加了 TP附近的导电性 ,导致 TP附近最大磁电阻效应的出现     

La2/3(Ca,Ba)1/3MnO3系统的顺磁共振研究

报道了对La2/3Ca1/3MnO3和La2/3Ba1/3MnO3 同温度下的电子顺磁共振（EPR）实验研究，当温度高于某一特征温度Tmin时，共振线宽随温度降低而线性变窄；当温度低于Tmin时，共振线宽表现出反常展宽。基于“瓶颈（bottleneck)”自旋－晶格弛豫模型我们对这种行为进行了讨论，并且指出A位离子平均半径强烈影响着自旋－晶格交换作用和自旋－自旋相互作用的强弱。     

La2/3(Ca,Ba)1/3MnO3系统的顺磁共振研究

报道了对La_2/3Ca_1/3MnO₃和La_2/3Ba_1/3MnO₃系统不同温度下的电子顺磁共振(EPR)实验研究,当温度高于某一特征温度Tmin时,共振线宽随温度降低而线性变窄;当温度低于Tmin时,共振线宽表现出反常展宽.基于“瓶颈(bottleneck)”自旋-晶格弛豫模型我们对这种行为进行了讨论,并且指出A位离子平均半径强烈影响着自旋-晶格交换作用和自旋-自旋相互作用的强弱.     

Plasma-Etching Enhanced Mechanical Polishing for CVD Diamond Films

Chemically vapor deposited diamond films were etched at different parameters using oxygen plasma produced by a DC （direct current） glow discharge and then polished by a modified mechanical polishing device. Scanning electron microscope, atomic force microscope and Raman spectrometer were used to evaluate the surface states of diamond films before and after polishing. It was found that a moderate plasma etching would produce a lot of etch pits and amorphous carbon on the top surface of diamond film. As a result, the quality and the efficiency of mechanical polishing have been enhanced remarkably.     

Evidence of decisive effect of crystal-field splitting in spin-state transition

Based on the first-principle calculations for 3D Hofmann-like spin-crossover (SCO) compound [Fe(C4H4N2){Pt(CN)4}], the discrepancy of transition mechanism is clarified with quantitatively distinguishable evidence of second order phase transition. It shows that the stretch around 0.2 ? of Fe-N bond length leads to the continuous structure expansion, as the energy splitting ΔEHL between low-spin and high-spin states reduces from 2.554 2 eV to -0.327 8 eV, and the crystal-field splitting (CFS) is reduced from 1.845 8 eV to 0.420 8 eV meanwhile. A physics image relating the calculations results with CFS in the frame of ligand-field theory is presented, which manifests that CFS is a necessary parameter to be introduced directly in the theory of spin-state transition.     

La0.67Ca0.33MnO3/MgO复合体系中的电输运和磁电阻效应

用化学方法制备了（1-x）La0．67Ca0．33MnO3（LCMO）／xMgO颗粒复合体系．通过选择不同的前期预烧温度（Ts1）控制LCMO原粉的颗粒大小．研究表明，MgO的含量和LCMO的前期预烧温度对复合体系的电输运和磁电阻效应有显著的影响．对瓦1=1100℃的样品，仅在纯LCMO和x=1mol％的复合样品中可以观察到绝缘体金属转变．而对Ts1=900℃的样品，在x=7mol％的复合样品中还可以观察到绝缘体金属转变，其最大低场磁电阻（H=0．3T）从纯LCMO的5％增加到27％．SEM形貌分析表明MgO含量增加或LCMO前期预烧温度升高都使复合体系中LCMO颗粒之间的联结性减弱．运用自旋极化隧穿机理，对实验结果进行了讨论．