S. Ma

Giant magnetocaloric effect in ε-(Mn0.83Fe0.17)3.25Ge antiferromagnet

The magnetic phase transition and magnetocaloric effect are investigated on e-(Mn0.83Fe0.17)3.25Ge compound. A large positive magnetic entropy change ΔSM is observed, which is accompanied with a field-induced metamagnetic transition from a collinear to a triangular antiferromagnetic configuration in this antiferromagnetic compound. The maximum value of ΔSM is 11.6J∕kgK at 93K for a magnetic field change of 7T. The study on systems with antiferromagnetic phases may open an important field in searching new materials for magnetic refrigeration.

Large room-temperature magnetocaloric effects in Fe0.8Mn1.5As

In Fe0.8Mn1.5As compound, an external magnetic field induces a metamagnetic transition from an antiferromagnetic phase to a ferrimagnetic phase above Ts=285K, leading to large magnetocaloric effects around room temperature. Instead of showing inverse magnetocaloric effects, the sign of the entropy change ΔSM in the compound is unexpectedly negative, revealing a different mechanism. The maximum value of ΔSM is 6.2J∕kgK at 287.5K for a magnetic field change of 5T. The study on systems with antiferromagnetism-related metamagnetic transitions may open an important field in searching good materials for room-temperature magnetic refrigeration.

Large cryogenic magnetocaloric effect of DyCo2 nanoparticles without encapsulation

The structure and formation of nanoparticles without encapsulation of the intermetallic compound DyCo2 were investigated by using x-ray diffraction and high-resolution transmission electron microscopy. The DyCo2 nanoparticles are stable in air without any shell protection. A large magnetic-entropy change of 13.2Jkg−1K−1 was found at 7.5K in an applied-field change from 1to7T, which is ascribed to the large magnetic moment density and the weak interaction energy in the nanoparticles. Such oxidation-resistant rare-earth transition-metal compound nanoparticles with large cryogenic magnetocaloric effect are useful for refrigeration applications at low temperatures.

Oxygen vacancy formation, crystal structures, and magnetic properties of three SrMnO3−δ films

The crystal structures and magnetic properties of the 40 nm brownmillerite SrMnO2.5 film, perovskite SrMnO3-δ film, and mixed-phase film have been systematically investigated. The features of the oxygen vacancy ordering superstructure in the brownmillerite SrMnO2.5 film are observed from HRSTEM as follows: the dark stripes with a periodicity of four (110) planes of the cubic perovskite appearing at an angle of 45° with the substrate-film interface and extra reflection spots in fast Fourier transformation patterns along the (001) plane. When annealing the brownmillerite SrMnO2.5 film under higher oxygen pressure, the top portion undergoes structure transition into perovskite SrMnO3-δ as seen in the mixed-phase film consisting of the perovskite SrMnO3-δ phase dominating at the top part and the brownmillerite SrMnO2.5 phase dominating at the bottom part. The magnetic properties and Mn valences of the brownmillerite SrMnO2.5 film indicate that this film, similar to the bulk, is antiferromagnetic with TN at 37...

Inverted polymer solar cells with enhanced fill factor by inserting the potassium stearate interfacial modification layer

A thin potassium stearate (KSt) film combined with an optimized ZnO film was introduced to improve the fill factor (FF) of highly efficient inverted polymer solar cells (PSCs). Atomic force microscopy and contact angle measurements were used to show that the introduction of KSt did not change the morphology of interlayer. On the contrary, it is beneficial for the spread of the active layer on the interlayer. The origin of enhanced FF was systematically studied by the ideal current-voltage model for a single heterojunction solar cell and electrochemical impedance spectroscopy. On the basis of the data analysis, the reduced charge recombination loss was responsible for this improved FF. At last, when KSt was replaced by sodium stearate (NaSt), the similar experiment phenomenon was observed. This indicates that inserting a metallic stearate modified layer is a promising strategy to enhance inverted PSCs performance.

Abnormal magnetic ordering and ferromagnetism in perovskite ScMnO3 film

Bulk multiferroic ScMnO3 is the stable hexagonal phase, and it is very difficult to prepare its perovskite orthorhombic phase even under high pressure. We fabricated the orthorhombic ScMnO3 thin film by pulsed laser deposition through suitable substrate LaAlO3 and found that nano-scale twin-like domains are naturally formed in the thin film. Magnetic properties of the orthorhombic ScMnO3 thin films show that, besides normal antiferromagnetic ordering at 47 K, an anomalous magnetic transition occurs at 27 K for 60 nm film and at 36 K for 150 nm film only along the c-axis, which is absent in the ab-plane. Moreover, the second magnetic transition for both films is suppressed when the applied field increases from 1 kOe to 10 kOe. In addition, the ferromagnetism shows up in both films at 10 K, and saturation magnetization increases dramatically in 60 nm film compared with 150 nm film. We propose that the second magnetic transition might be more of lattice strain effect and also related to magnetism-induced fer...

Polarization and charge-transfer effect on the transport properties in two-dimensional electron gases/LaNiO3 heterostructure

The film thickness dependent transport properties of the LaNiO3 (LNO) layer epitaxially grown on LaAlO3/SrTiO3 (LAO) 2-dimensional electronic gas (2DEG) have been investigated. The ultrathin LNO films grown on the 2DEG have a sheet resistance below the values of h/e2 in all temperature ranges. The electron density is enhanced by more than one order of magnitude by capping LNO films. X-ray photoelectron spectroscopy shows that the interface undergoes unambiguous charge transfer and electronic reconstruction, leading to modulation doping of such atomically engineered complex oxide heterointerfaces. The polar-catastrophe of the 2DEG is directly linked to the electronic structure and transport properties of the LNO. The transport properties can be well modulated by the thickness of the LAO in the 2DEG, and the data can be well fitted with the polar-catastrophe scenario. These results suggest a general approach to tunable functional films in oxide heterostructures with the 2DEG.

Improving the efficiency of inverted organic solar cells by introducing ferrocenedicarboxylic acid between an ITO/ZnO interlayer

In this study, ferrocenedicarboxylic acid (FDA) has been introduced between an ITO electrode and ZnO interlayer to improve the performance of inverted polymer solar cells. The highest power conversion efficiency (PCE) of 9.06% is achieved among the measurements. Besides, the average PCE of FDA/ZnO based devices is observed with 11.9% enhancement (8.73% vs. 7.80%) compared to ZnO-only devices. Electrical characterization, surface morphology, wetting properties, as well as exciton generation rate and dissociation probability were investigated to understand the impact of FDA insertion on the interfacial properties. It was found that exciton dissociation efficiency and charge collection efficiency were significantly improved after inserting FDA, while the surface morphology, average roughness and water contact angle of the ZnO film were not changed. It was thought that FDA connected to the ITO electrode and ZnO film because of its carboxyl groups, which lead to a compact interfacial contact and reduced charge recombination. In addition, the devices based on the FDA/ZnO interlayers displayed improved stability in the argon-filled glove box without any encapsulation for about 1000 h compared to the ZnO-only devices. This study provides a new idea to introduce materials with functional groups between ITO/metal oxides interfaces to achieve more efficient charge collection and device performance.

Competing magnetic orders in La 0.67 Ca 0.33 MnO 3 /SrMnO 3 multilayers film

Epitaxial films of multilayers of La0.67Ca0.33MnO3/SrMnO3 (LCMO/SMO) and the reference single layer have been deposited by pulsed laser deposition on SrTiO3 (001) substrate allowing us to perform an exhaustive study of the magnetic coupling at the interface of G-type compensated antiferromagnetic (AFM) and double-exchange ferromagnetic (FM). An unexpected exchange bias (EB) effect at the compensated AFM interface under application of an in-plane cooling has been found. The LCMO/SMO multilayers exhibit an exchange bias field of 123 Oe, which vanishes as the temperature rises above 80 K Spin-cluster-like FM /AFM clusters have been deduced to exist at the interface due to the competing types of magnetic order at the interface. A small magnetization has been found above the Curie temperature of the pure LCMO thin films when applied a small field. We speculate that the short-range high-temperature FM order of the Mn3+ and Mn4+ moments above the Curie temperature at the interface gives rise to the magnetic regions that pin the FM LCMO layer as the temperature decreases.