Size dependence of the exchange bias field in NiO/Ni nanostructures

Abstract

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...

Nonlinear spin wave magnetization of solution synthesized Ni nanoparticles

“…The exchange bias field ⌬H C is found to vary from 80 to 700 Oe depending on the type of Ni/ NiO nanostructure. 10,27 In the present work ⌬H C is found to be Ͻ2 Oe at all temperatures, 5 to 300 K, clearly indicating that NiO is nonmagnetic in the present work and does not couple magnetically to the core Ni crystallites. The thin nonmagnetic NiO layer, however, cannot fully suppress the magnetic interactions, exchange and dipolar, between the Ni particles.…”

Exchange bias effects in ferromagnetic wires

“…Earlier works on magnetic wires were mostly concentrated on size dependence and orientation dependence on field [2,15,16]. Fraune et al [17] have investigated the size dependence of exchange bias in NiO/Ni nanostructures. They observed that for NiO/Ni wires narrower than 3 mm, the exchange bias field significantly depends on the wire width.…”

Loop bifurcation and magnetization rotation in exchange-biasedNi∕FeF2

“…For example, it has been shown in FM/AF nanostructures where the FM domain size is reduced through nanopatterning that exchange bias can be significantly altered. 8 In contrast, there have been limited experimental studies on the AF domain size dependence of exchange bias, 9 largely due to the difficulty in characterizing and manipulating AF domains in FM/AF systems. [10][11][12] The domain sizes may also affect the shape of the hysteresis loop.…”