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We analyzed the protective ability of chemical vapor deposition (CVD) graphene domains against corrosion of Cu surfaces. Fresh graphene domains of various shapes were ideal corrosion-inhibiting layers. However, obvious corrosion was found within graphene domains exposed to the air for over a week. Our work demonstrates that the opportunities for corrosion of CVD graphene were provided by wrinkles but not others, such as Cu grain boundaries and graphene domain boundaries, which are always believed the primary factor for inferior quality of the CVD graphene at present.

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 375 K. However, the strained tetragonal perovskite SrMnO3-δ film exhibits ferromagnetic behavior with Curie temperature at 75 K and a saturation magnetization of 2.5μB/Mn at 2 K. Moreover, the top perovskite SrMnO3-δ phase of the mixed-phase film also exhibits ferromagnetic behavior evidenced by the existence of the exchange bias. We propose that the ferromagnetic properties in both the perovskite SrMnO3-δ film and the top perovskite SrMnO3-δ phase in the mixed-phase film originate from Mn3+–Mn4+ double exchange coupling. However, the formation of Mn3+ differs for the two samples in that it is caused by oxygen vacancies in the former and the distribution of oxygen content across the film during annealing in the latter.

We investigate the effect of strain on the phenomenon of charge-order melting, that is the transformation of a charge-ordered insulating state to a metallic state under the influence of a magnetic field ͑the melting field͒ in thin films of Pr 0.7 Ca 0.3 MnO 3 grown on various substrates. We find that unstrained films grown on SrLaGaO 4 behave quite similar to bulk material, but that strained films grown on SrTiO 3 and NdGaO 3 show hugely increased melting fields. Strain relaxation by postannealing again leads to bulklike behavior. In this material the antiferromagnetic charge-order phase can coexist with a ferromagnetic insulating state. Magnetization measurements, where we demonstrate the presence of exchange bias effects, show that this is also the case in the strained films. We argue that the phase mixture in the strained films is more difficult to melt than in the unstrained case.

Magnetization behavior across a metamagnetic transition from an antiferromagnetic state to a ferrimagnetic state is investigated in detail for compound Mn 2 Sb 0.95 Sn 0.05 . The study clearly brings out various generic features associated with a first-order transition, viz., the appearance of hysteresis and the coexistence of magnetic phases. We also observe that the magnetization versus field butterfly loops occurs, while the virgin curve lies outside the envelope magnetization curve. The electronic specific-heat coefficient at low temperatures increases with increasing applied magnetic field, after the field is larger than the critical transition field. This is direct evidence of the formation of a super-zone gap that yields the change of density of electric states and further proves that the large magnetoresistance effect in intermetallic compounds is originated from the reconstruction of Fermi surface due to the collapse of the super-zone gap after the metamagnetic transition.

Articles you may be interested inWe have investigated the relation between defect structure and charge order melting in thin films of epitaxial Pr 0.5 Ca 0.5 MnO 3 ͑PCMO͒, grown under strain on SrTiO 3 . We compared the behavior of an 80 nm film grown in one deposition step at 840°C with the behavior of a film grown in two steps.In the two-step case, a thin PCMO layer of 10 nm was deposited at 120°C, followed by 70 nm deposited at 840°C. The increase of the growth temperature leads to complete crystallization of the first layer and the lattice constants of the two-step grown film indicate that tensile strain is still present. On the other hand, a magnetic field of only 5 T is required to melt the charge-order state in the two-step grown film, which is a much lower than the value for the normally grown film. This appears to be connected to a larger amount of threading dislocations present in the first ͑recrystallized͒ layer.

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