Bulk properties of the UCoGe Kondo-like system

We report on extensive experimental investigations of a single crystal of the orthorhombic uranium compound UCoGe. Bulk measurements on as-grown and annealed single crystals, recording magnetization, magnetic susceptibility, electrical resistivity, magnetoresistivity, thermopower, thermal conductivity and heat capacity data do not reproduce the previously reported coexistence of ferromagnetism with superconductivity. The latter phenomenon was only observed for the annealed sample at T _SC = 0.65 K. New observations show a crossover at around 13 K, visible in thermal and transport measurements as well as the coherent state around 50 K, signaled by a wide knee in ρ(T). Above this temperature, UCoGe exhibits a single-ion Kondo-like effect. The magnetoresistivity of the annealed single crystal increases negatively down to 4.2 K, reaching as a large value about ?27% at a field of 8 T. The latter may be interpreted in terms of fairly strong magnetic fluctuations existing in UCoGe at low temperatures.     

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr1.6V0.4Se4

Structural, electrical and magnetic measurements, as well as electron spin resonance (ESR) spectra, were used to characterise the single-crystalline CuCr_1.6V_0.4Se₄ spinel and study the correlation between the negative magnetoresistance effect and magnon excitations. We established the ferromagnetic order below the Curie temperature T _C ≈ 193 K, a p-type semiconducting behaviour, the ESR change from paramagnetic to ferromagnetic resonance at T _C, a large ESR linewidth value and its temperature dependence in the paramagnetic region. Electrical studies revealed negative magnetoresistance, which can be enhanced with increasing magnetic field and decreasing temperature, while a detailed thermopower analysis showed magnon excitations at low temperatures. Spin–phonon coupling is explained within the framework of a complex model of paramagnetic relaxation processes as a several-stage relaxation process in which the V³⁺ ions, the exchange subsystem and conduction electron subsystem act as the intermediate reservoirs.     

Field-induced insulator-metal-insulator transitions in low-energy H2- ion-irradiated epitaxial La2/3Cal/3MnO3 thin films

Epitaxial La2/3Cal/3MnO3 thin films grown on LaA103 （001） substrates were irradiated with low-energy 120-keV H＋ ions over doses ranging from 1012 ions/cm2 to 1017 ions/cm2. The irradiation suppresses the intrinsic insulator-metal （I-M） transition temperature and increases the resistance by reducing the crystallographic symmetry of the films. No irradiation-induced columnar defects were observed in any of the samples. The specific film irradiated at a critical dose around 8 x 1015 ions/cm2 is in a threshold state of the electric insulator where the I-M transition is absent. In an external field of 4 T or higher, the I-M transition is restored and thus an enormous magnetoresistance is observed, while a negative temperature coefficient resumes as the temperature is reduced further. Magnetic relaxation behavior is confirmed in this and other heavily irradiated samples. The results are interpreted in terms of the displacement of oxygen atoms provoked by ion irradiation and the resulting magnetic glassy state, which can be driven into a phase coexistence of metallic ferromagnetic droplets and the insulating glass matrix in a magnetic field.     

Tunneling through two single molecular magnets in series arrangement junction in parallel/antiparallel situations

We investigate the spin-dependent tunneling transport in a heterostructure with two single molecular magnets (SMMs). The tunneling magnetoresistance (TMR) and negative differential conductance due to the strong resonant tunneling in the junction are demonstrated by the master equation approach. At low bias voltage, the device presents low/high resistant states with the initial states of the single molecular magnets parallel/antiparallel. Strong Coulomb repulsive interaction suppresses the current greatly in antiparallel situation. At high voltage, the middle system containing two SMMs tends to be non-polarized, and acts like ordinary quantum dots.     

Magnetocaloric and magnetoresistance properties of La2/3Sr1/3Mn1−xCoxO3 compounds

Structural, magnetic, magnetoresistance and magnetocaloric studies on La_2/3Sr_1/3Mn_1−xCo_xO₃ compounds were reported. The samples were prepared by the conventional ceramic method. X-ray analysis showed the presence of one phase only, in all studied samples. From electrical resistance measurements it was found that the samples show large negative magnetoresistance behavior. The magnetic measurements were performed in a large temperature range, 4.2–750 K and external magnetic fields up to 5 T. The adiabatic magnetic entropy changes, |ΔS|, were determined from magnetization data. Large magnetocaloric effect (MCE) has been obtained in all studied samples.     

共沉淀法制备纳米多晶La2／3Sr1／3MnO3及其低场磁电阻性质

利用共沉淀法制备了纳米多晶La2/3Sr1/3MnO3粉体,并研究了相应烧结陶瓷的相结构、显微形貌、电性质以及磁电阻效应.样品中存在高密的晶界,对电子的疏运过程产生明显的散射作用,从而使得样品具有较大的电阻值,并且只有当烧结温度大于800℃时才出现金属绝缘体温度相变.样品的磁电阻是低场磁电阻效应,源于晶界处电子的自旋极化隧穿.烧结温度越低的样品,其磁电阻越大.     

磁场下合成Fe3O4粉体的隧道磁阻

通过在半金属Fe₃O₄合成过程中外加磁场的方法,改变样品粒子的表面结晶状态和晶格缺陷,研究了由此引起的Fe₃O₄输运性质的变化.合成的Fe₃O₄粉体的主要导电机理均为自旋极化隧穿和高阶跃迁电导,电阻随温度升高成指数降低,电阻与电压显示了非线形相关性,磁阻与磁场的关系为蝴蝶形,是典型的隧道磁阻特征.与没有外加磁场时合成的样品比较,外加磁场合成的样品显示了更低的电阻和更高的磁阻. 关键词： 磁阻 隧穿 表面 晶格缺陷     

铁磁性锰氧化物掺杂的ZnO压敏电阻性能研究

利用通常的电子陶瓷制备工艺制备了铁磁性锰氧化物La_07Sr_03MnO₃掺杂的ZnO陶瓷. 晶界处存在La_07Sr_03MnO₃(LSMO)和LaMnO₃(LMO)两种杂相. 样品中绝缘相LMO的含量显著影响着样品的电学性能. 掺杂后的样品仍具有一定的铁磁性. 在样品上施加磁场后,样品电阻值增加,表现为正磁电阻性质. 正磁电阻的出现,是由于磁场的存在 关键词： ZnO 压敏电阻 锰氧化物 正磁电阻     

非磁性半导体异常磁电阻效应的有效介质理论

有效介质理论自洽方程被用来研究非磁性半导体材料的异常磁电阻效应. 通过建立两组分无序电导网络,同时引入组分的霍尔效应,计算了体系的有效电导张量随磁场和组分浓度的变化关系.结果表明,当两种组分具有不同类型的载流子时,体系中各组分零场电阻的差异将导致在垂直磁场和平行磁场方向上产生异常的磁电阻效应.这些宏观的磁电阻效应来源于体系中的非均匀性,并与组分的几何逾渗结构具有明显的关联. 关键词： 异常磁电阻效应 非磁性半导体 有效介质近似