We investigated a hematite α-Fe2O3/permalloy Ni80Fe20 bilayer film where the antiferromagnetic layer consisted of small hematite grains in the 2 to 16 nm range. A pronounced exchange bias effect occurred below the blocking temperature of 40 K. The magnitude of exchange bias was enhanced relative to reports for identical compounds in large grain, epitaxial films. However, the blocking temperature was dramatically reduced. As the Néel temperature of bulk α-Fe2O3 is known to be very high (860 K), we attribute the lowtemperature onset of exchange bias to the well-known finite-size effect which suppresses the Morin transition for nanostructured hematite. Polarized neutron reflectometry was used to place an upper limit on the concentration and length scale of a layer of uncompensated moments at the antiferromagnetic interface. The data were found to be consistent with an induced magnetic region at the antiferromagnetic interface of 0.5-1.0 μB per Fe atom within a depth of 1-2 nm. The field dependence of the neutron spin-flip signal and spin asymmetry was analyzed in the biased state, and the first and second magnetic reversal were found to occur by asymmetric mechanisms. For the fully trained permalloy loop, reversal occurred symmetrically at both coercive fields by an in-plane spin rotation of ferromagnetic domains. We investigated a hematite α-Fe 2 O 3 /permalloy Ni 80 Fe 20 bilayer film where the antiferromagnetic layer consisted of small hematite grains in the 2 to 16 nm range. A pronounced exchange bias effect occurred below the blocking temperature of 40 K. The magnitude of exchange bias was enhanced relative to reports for identical compounds in large grain, epitaxial films. However, the blocking temperature was dramatically reduced. As the Néel temperature of bulk α-Fe 2 O 3 is known to be very high (860 K), we attribute the low-temperature onset of exchange bias to the well-known finite-size effect which suppresses the Morin transition for nanostructured hematite. Polarized neutron reflectometry was used to place an upper limit on the concentration and length scale of a layer of uncompensated moments at the antiferromagnetic interface. The data were found to be consistent with an induced magnetic region at the antiferromagnetic interface of 0.5-1.0 μ B per Fe atom within a depth of 1-2 nm. The field dependence of the neutron spin-flip signal and spin asymmetry was analyzed in the biased state, and the first and second magnetic reversal were found to occur by asymmetric mechanisms. For the fully trained permalloy loop, reversal occurred symmetrically at both coercive fields by an in-plane spin rotation of ferromagnetic domains.

By ascertaining NiO surface roughness in a Ni80Fe20/NiO film system, we were able to correlate the effects of altered interface roughness from low-energy ion-beam bombardment of the NiO layer and the different thermal instabilities in the NiO nanocrystallites. From experiment and by modelling the temperature dependence of the exchange bias field and coercivity, we have found that reducing the interface roughness and changing the interface texture from an irregular to striped conformation enhanced the exchange coupling strength. Our results were in good agreement with recent simulations using the domain state model that incorporated interface mixing. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3697405

A Ni80Fe20/(Ni,Fe)O thin film exhibits a positive exchange bias when cooled in a zero field and a negative exchange bias when field cooled. With transmission electron microscopy and electron energy loss spectrometry, the composition and magnetic structure has been ascertained and a distribution of magnetization easy axes about the interface extrapolated. The results indicate that the positive exchange bias is from antiferromagnetic interface moments perpendicular to their ferromagnetic counterparts. With field cooling the alignment is put into a parallel configuration resulting in a negative exchange bias.

Nanocomposite films of Ni80Fe20/NixFe1−xO were prepared by a dual ion-beam deposition technique. The structural and magnetic properties of nanocomposite films fabricated with oxygen content in the deposition assist beam ranging from 0% to 55% were studied. The dependence of the resistivity on oxygen percent shows that the compositions with exchange-enhanced coercivity are close to a percolation threshold. A strong temperature dependence of coercivity Hc and exchange bias field Hex is found in these composite films. Films prepared with 46% O2 in the assist beam exhibit an enhanced Hc relative to Permalloy (Ni80Fe20) and a characteristic shifted hysteresis loop indicative of exchange coupling between the constituent metal and oxide phases. At T=10 K, films prepared with 44% O2 in the assist beam have an exchange shift Hex∼−225 Oe with a blocking temperature TB∼100 K that reflects the low Néel temperatures of FeO-rich NixFe1−xO solid solutions.

The exchange bias effects of NiFe/Cr-oxide bilayers were studied. Results have shown that NiFe/Cr-oxide bilayers exhibited an exchange bias loop shift when field cooled to 5 K. A strong linear dependence of ferromagnetic NiFe and antiferromagnetic Cr2O3 thicknesses on the exchange bias field H-ex was observed. The largest interfacial exchange energy E-int similar to 5.4x10(-2) erg/cm(2) was found in bilayers with the thickest Cr2O3 layer indicating that stronger interface exchange coupling is enabled by thicker Cr2O3 layers. In addition, H-ex decreased linearly with increasing %O-2/Ar ratio, reflecting that ion-beam bombardment tends to degrade the Cr2O3 surface spin structures. We also find that annealing the Cr-oxide layer yields both a structural phase transformation and improved crystallinity, giving rise to stronger exchange bias behavior. Further, the coexistence of in-plane as well as out-of-plane exchange biases was observed in a NiFe/annealed Cr2O3/Al2O3(0001) bilayer. This clearly indicates that by using the single crystal Al2O3(0001) substrate together with a rapid thermal annealing process, the antiferromagnet Cr2O3 spins are tilted toward the out-of-plane direction and thus exhibit this unusual exchange bias behavior

The effects of interfacial coupling at the boundary of ferromagnetic and antiferromagnetic components in a nanoscale columnar-structured thin film of Ni 80 Fe 20 / CoO have been examined. Field-cooling the film results in very different temperature dependences of the enhanced coercivity and exchange-bias shift of the hysteresis loop. The exchange-bias temperature dependence is well described by thermal fluctuations of the interfacial spins while the coercivity temperature dependence indicates that single-domain-like columns are being coherently rotated by the thermal fluctuations of the interface spins. Furthermore, only a portion of the spins in the antiferromagnetic layer seem to be associated with the spin coupling that results in exchange bias. X-ray magnetic resonant scattering measurements show clearly the presence of canted Co interfacial moments that provide a local field which enables exchange bias at a significantly higher temperature than the onset of an enhanced coercivity.

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This paper describes the photovoltaic performance of dye-sensitized solar cells (DSSCs) containing graphene-incorporated counter electrodes (CEs). The location and thickness of graphene in CEs are optimized to improve the photovoltaic performance of DSSCs, compared with typical Pt CEs. The DSSC, with a Pt/few-layer graphene (FLG) CE, achieved 8% in short-circuit current density and 13% in power conversion efficiency (PCE). Electrochemical impedance spectroscopy shows that the DSSC, with a Pt/FLG CE, exhibits a series resistance lower than that with a Pt CE. The lower series resistance is attributed to the contact resistance at the interface of platinum and fluorine doped tin oxide. The contact resistance is reduced by the formation of the thin platinum-carbon composite layer. It is demonstrated that the consumption of Pt could be reduced with a Pt/FLG CE. However, graphene/Pt CEs resulted in a slow charge-transfer process and consequently a worse photovoltaic performance of DSSCs. V