title: Exchange bias of Ni nanoparticles embedded in an antiferromagnetic IrMn matrix.
abstract: The magnetic properties of Ni nanoparticles (Ni-NPs) embedded in an antiferromagnetic IrMn matrix were investigated. The Ni-NPs of 8.4 nm mean diameter were synthesized by inert gas aggregation. In a second processing step, the Ni-NPs were in situ embedded in IrMn films or SiOx films under ultrahigh vacuum (UHV) conditions. Findings showed that Ni-NPs embedded in IrMn have an exchange bias field HEB = 821 Oe at 10 K, and 50 Oe at 300 K. The extracted value of the exchange energy density is 0.06 mJ m(-2) at 10 K, which is in good accordance with the results from multilayered thin film systems. The Ni-NPs embedded in SiOx did not show exchange bias. As expected for this particle size, they are superparamagnetic at T = 300 K. A direct comparison of the Ni-NPs embedded in IrMn or SiOx reveals an increase of the blocking temperature from 210 K to around 400 K. The coercivity of the Ni-NPs exchange coupled to the IrMn matrix at 10 K is 8 times larger than the value for Ni-NPs embedded in SiOx. We studied time-dependent remanent magnetization at different temperatures. The relaxation behavior is described by a magnetic viscosity model which reflects a rather flat distribution of energy barriers. Furthermore, we investigated the effects of different field cooling processes on the magnetic properties of the embedded Ni-NPs. Exchange bias values fit to model calculations which correlate the contribution of the antiferromagnetic IrMn matrix to its grain size.
authors.affiliation: Institut für Mikro- und Nanomaterialien, Universität Ulm, D-89069 Ulm, Germany.
authors.forename: Balati
authors.initials: B
authors.lastname: Kuerbanjiang
authors.forename: Ulf
authors.initials: U
authors.lastname: Wiedwald
authors.forename: Felix
authors.initials: F
authors.lastname: Haering
authors.forename: Johannes
authors.initials: J
authors.lastname: Biskupek
authors.forename: Ute
authors.initials: U
authors.lastname: Kaiser
authors.forename: Paul
authors.initials: P
authors.lastname: Ziemann
authors.forename: Ulrich
authors.initials: U
authors.lastname: Herr