Ren-Fu Yang

Intrinsic exchange bias effect in phase-separated La0.82Sr0.18CoO3 single crystal

We report a study of the exchange bias of a high quality perovskite cobaltite La(0.82)Sr(0.18)CoO(3) single crystal with spin glass (SG) like nature. According to the dependence of exchange bias field on the cooling field, an antiferromagnetic (AFM) interfacial exchange coupling constant at the spin-glass region J(i) approximate to -0.68 meV was found. The size of the FM regions t(FM) approximate to 9 nm was estimated in the spin-glass system from the inverse proportion of exchange bias field H(EB) to the size of the FM regions.

Endurance improvement of resistance switching behaviors in the La0.7Ca0.3MnO3 film based devices with Ag–Al alloy top electrodes

The resistance switching characteristics of the La0.7Ca0.3MnO3-based devices with the top electrodes of Ag, Ag–Al alloys with the atomic ratios of Ag:Al=2:1 (2AgAl) and Ag:Al=1:2 (Ag2Al), and Al have been investigated. The device with 2AgAl top electrode shows excellent endurance, where more than 1000 cycles of reproducible current-voltage hysteresis with stable high and low resistance states have been observed. Based on Auger electron spectroscopy measurement and the detailed investigation of current-voltage curves of these devices, it is suggested that the oxygen affinity of the metal electrode, which is determined by the chemical component of Ag and Al, has an important influence on the interface structure and the resistance switching endurance. The present work provides a possible way for the improvement of the resistance switching endurance by modulating oxygen affinity of the electrode.

Observation of a Griffiths-like phase in bilayered manganites

The authors report the observation of a Griffiths-like phase in La2−2xSr1+2xMn2O7 (x=0.30, 0.33, and 0.40) and (La0.8Eu0.2)2−2xSr1+2xMn2O7 (x=0.33) single crystals by means of electron spin resonance, magnetic susceptibility, and magnetoresistance measurements. In the temperature region TC<T<350K, the resonance signal consists of a ferromagnetic resonance line and a paramagnetic resonance line, which suggests that the system is not in a pure paramagnetic phase. The ferromagnetic resonance signal disappears and the magnetic susceptibility starts to deviate from the Curie-Weiss law at the same temperature T*≈350K, independent of doping level and anisotropy. These results indicate that a Griffiths-like phase exists at TC<T<T* in bilayered manganites, which is due to intrinsic inhomogeneity caused by quenched disorder.

Electron spin resonance study of spin correlations in charge-ordered La2−2xSr1+2xMn2O7(x=0.6)

Electron spin resonance (ESR) studies have been performed on a bilayered manganite La2−2xSr1+2xMn2O7(x=0.6) single crystal. The charge ordering (CO) and antiferromagnetic (AFM) transitions can be well identified from the ESR spectra in both the ab plane and c axis directions. A coexistence of paramagnetic resonance signal and low-field AFM resonance signal was observed below the CO temperature TCO. The analysis of the ESR parameters as a function of temperature suggests the existence of weak ferromagnetic (FM) correlations in the paramagnetic state and AFM correlations below TCO. The competition of FM and AFM correlations results in a broad peak in ESR intensity. Our results also show that the intensity of AFM correlations increases with decreasing temperature as the system enters from the CO state into the A-type AFM ground state. Therefore, the evolution and anisotropy of spin correlations can be fully mapped by ESR study.

Role of wide bandgap host in the degradation of blue phosphorescent organic light-emitting diodes

Accelerated reliability tests of blue phosphorescent organic light-emitting diodes (OLEDs) comprising bis[(4,6-difluorophenyl)pyridinato-N,C2](picolinato)-iridium(III) (FIrpic) doped in four different wide bandgap hosts were conducted. The half-life of the OLEDs stressed under a high current density of 100 mA/cm2 varied in a wide range, revealing an important role of the host. Pulsed current stressing with a 1% duty cycle was performed to suppress self-heating, but only extended the lifetime by 2–3.2×. For blue OLEDs with a host favoring hole transport, current stressing caused a shift of the recombination zone toward the anode, turning the emission color to greenish blue. These results suggest that device degradation was mainly caused by charge-trapping defects generated within a narrow zone close to the electron-transport layer. It is expected that the lifetime of blue phosphorescent OLEDs can be effectively extended by selecting an appropriate host which has good stability, enables efficient charge inj...

Evolution of the inter-layer coupling in bilayered manganites revealed by ferromagnetic resonance spectra

Ferromagnetic resonance has been used to investigate the inter-layer coupling in a bilayered manganite La(2-2x)Sr(1+2x)Mn(2)O(7) (x = 0.38) single crystal. The coexistence of a ferromagnetic resonance line and a paramagnetic resonance line was observed over a broad temperature range. Antiferromagnetic coupling between the adjacent MnO(2) bilayers can be identified from the observation of an optical mode in the resonance spectra. Analysis of the temperature dependence of the resonance field and intensity reveals the evolution of the inter-layer coupling as a function of temperature. Our study suggests that ferromagnetic resonance provides a useful method for investigating the inter-layer coupling in bilayered manganites.

Origin of glassy magnetic behaviour in the bilayered manganite (La0.5Nd0.5)1.2Sr1.8Mn2O7

We report a detailed study of the glassy magnetic behaviour in a phase-separated (La0.5Nd0.5)1.2Sr1.8Mn2O7 single crystal. The frequency dependence of the ac susceptibility, as well as ageing and memory effects observed in the sample, are similar to the glassy behaviour in spin glasses. However, the evolution of ac susceptibility with superposed dc fields is contrary to what is expected for a classical spin-glass phase. Instead, it can be well understood in terms of the coexistence of a spin-glass-like phase and an assembly of magnetic clusters in a phase-separated state. Therefore, our study clarifies the origin of glassy behaviour in the bilayered manganite (La0.5Nd0.5)1.2Sr1.8Mn2O7. (Some figures in this article are in colour only in the electronic version)