NiO/Ni wires have been investigated as a function of their width in order to investigate the size dependence of exchange bias. The samples have been prepared by e-beam lithography and ion milling of ion beam sputtered thin films. For NiO/Ni wires narrower than 3 m, the exchange bias field significantly depends on the wire width. A NiO/Ni film shows an exchange bias field of Ϫ78 Oe whereas the exchange bias field of wires narrower than 200 nm is reduced to approximately Ϫ40 Oe. The coercive field of the NiO/Ni film is 28 Oe and increases to 210 Oe for the narrowest wires. The decrease of the exchange bias field for the narrowest wires is consistent with a recent microscopic model of exchange bias where the appearance of a unidirectional anisotropy in ferromagnet/antiferromagnet bilayers has been attributed to the presence of antiferromagnetic domains in the bulk of the antiferromagnet. A possible onset of a transition from a multidomain to a single-domain state of the antiferromagnet as a function of the NiO/Ni wire width seems to be the origin for the observed decrease of the exchange bias field for narrow wires.The exchange bias ͑EB͒ effect occurs due to the exchange coupling at antiferromagnet͑AFM͒/ferromagnet͑FM͒ interfaces leading to a shift of the magnetic hysteresis loop along the field axis.1 This shift of the hysteresis loop can be established either by cooling the AFM/FM bilayers in a magnetic saturation field below the Néel temperature T N of the AFM or by depositing the bilayers in an external magnetic field.2 The exchange bias effect has been used over several decades and more recently for pinning the magnetization of one of the two electrodes in magnetoresistive devices ͑giant magnetoresistance multilayers 3 and tunnel junctions 4,5͒. Although the exchange bias effect has already been intensively exploited in micron-and submicron-sized magnetoelectronic devices, its microscopic origin is not yet fully understood.A recent experiment on Co/CoO bilayers in conjunction with a Monte Carlo simulation study has shown that the dilution of the antiferromagnet CoO with nonmagnetic impurities ͑e.g., Co 1Ϫx Mg x O͒ or defects ͑e.g., Co 1Ϫy O͒ in its volume part leads to the formation of antiferromagnetic volume domains. 6 The formation of antiferromagnetic domain walls leads to a small surplus magnetization at the AFM/FM interface which couples to the FM and results in a unidirectional anisotropy. Hence, the antiferromagnetic domains are the microscopic origin of exchange bias. This result is complementary to a previous approach attributing exchange bias to the formation of antiferromagnetic domains with domain walls ͑DW͒ perpendicular to the AFM/FM interface in the presence of only interface roughness.7 From these models one has to conclude that the exchange bias of AFM/FM bilayers vanishes when the AFM becomes single domain. From a systematic investigation of a possible finite size effect of exchange bias the microscopic role of the DW formation can be elucidated as well as the lower limit of the extension of...

We have carried out two-port network analyzer ferromagnetic resonance measurements on a coplanar waveguide. We present a detailed description on how to calculate from the raw measurement data a value proportional to the complex susceptibility and permittivity of the ferromagnetic material. Necessary corrections for errors due to imprecise sample placement on the waveguide and the sample dimensions are presented. Evaluated data up to 15 GHz are provided for two model samples: a 40 nm Co80Fe20 layer showing a large linewidth (≈900 MHz) and a 40 nm Co72Fe18B10 layer yielding a small linewidth (≈360 MHz). Using these experimental data the presented evaluation scheme based on all four scattering parameters is then compared to commonly used approximate evaluation schemes relying on only one S parameter. These approximate methods show close agreement for the ferromagnetic resonance frequencies (the relative error is below 1%). However, the resonance linewidths show a relative error that can reach 10% in comparison with the presented evaluation method.

Spin tunnel junctions (CoFe/Al2O3/CoFe/MnIr) were fabricated with tunneling magnetoresistance (TMR) of 39%–41% after anneal at 300 °C, decreasing to 4%–6% after anneal at 410 °C. Junction resistance decreases from (0.8–1.6) to (0.5–0.8) M Ω μm2 during anneal. The pinned-layer moment decreases by 44% after anneal at 435 °C, but the free-layer moment does not change. The current–voltage characteristics change significantly and become asymmetric above 300 °C. Rutherford backscattering analysis (RBS) shows that above 300 °C, strong interdiffusion starts at the CoFe/MnIr interface with Mn moving into CoFe, causing the electrode moment to decrease. Mn eventually reaches the Al2O3/CoFe interface contributing to the TMR decrease. RBS analysis of a separate CoFe/Al2O3/CoFe structure shows only minor structural changes at the CoFe/Al2O3 interfaces after anneal at 435 °C, possibly leading to a second mechanism for the loss of interface polarization and TMR.

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We provide a state-of-the-art review of the main strategies for the enhancement of analytical performance of sensors using nanomaterials, particularly nanowires and carbon-based materials. We emphasize the way to overcome the problem of device-todevice variation. We discuss the study of the influence of nanomaterial characteristics, sensor dimensions and operational conditions on sensing performance, and the application of appropriate calibration models. ª

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