Size-dependent scaling of perpendicular exchange bias in magnetic nanostructures

Abstract

Size-dependent scaling is studied in exchange biased nanostructures obtained by depositing ͓Pt/ Co͔ 3 / IrMn multilayers with perpendicular magnetization onto an array of monodisperse polystyrene nanospheres with a diameter ranging from 58 nm to 320 nm. We find that the exchange bias field increases with the inverse of the sphere diameter when the IrMn layer is single domain. This scaling behavior, which is favorable for further miniaturization of magnetic devices, can be understood within the context of a random field model by a statistical distribution of Mn spins at the Co/ IrMn interface. DOI: 10.1103/PhysRevB.75.012413 PACS number͑s͒: 75.50.Ee, 61.46.Df, 75.70.Ϫi, 75.75.ϩa Fifty years ago, Meiklejohn and Bean discovered a new form of magnetic anisotropy.1 Their discovery, called exchange bias, was a unidirectional shift of the hysteresis loop along the magnetic field axis for ferromagnetic ͑FM͒ materials in close contact with an antiferromagnet ͑AFM͒. Due to its technological applications, exchange bias coupling has generated intense experimental 2 and theoretical research efforts.3-7 Nowadays, FM/AFM bilayers are incorporated in magnetic thin film devices such as magnetic spin-valve sensors, magnetic tunnel junction read heads for hard-disk drives, and magnetic random access memories. For continuous FM/AFM bilayers, variations of the exchange bias field with AFM grain size, 5 AFM domain size, 8 and AFM film thickness 9-11 have been measured, illustrating the theoretically predicted importance of the AFM domain size.Although there are many reports on exchange biased nanostructures, 12-16 mainly with a ferromagnetic in-plane easy axis, very few of them address in depth the effects of miniaturization on the exchange bias properties, where unique effects will set in at some critical length scales; and even the trends are controversial. While most of the nanosystems exhibit a decrease in exchange bias with size reduction, it has been recently reported for sub-100 nm NiFe-IrMn square dots, that depending on the thickness of the AFM layer, the dots could either have larger or smaller bias field than the respective continuous films.14 In addition, many studies focus on structures with dimensions larger than the AFM domain size or compare properties of single-size nanostructures with those of continuous films. Hence, a detailed understanding of size-dependent scaling of exchange bias in magnetic nanostructures that confine the AFM to a singledomain state is still lacking.Here, we present a conclusive account of the effects of size reduction on exchange bias. In our experiments we use an original self-assembly method to systematically vary the FM/AFM structure size and AFM layer thickness. Measurements on these structures reveal a statistical enhancement of the exchange bias field in magnetic nanostructures. This enhancement, which is explained within the context of a random field model, follows a simple scaling law.To fabricate the nanostructures, we employed dense arrays of monodisperse spherical polystyrene p...

Scaling behavior of the exchange-bias training effect

“…27,28,[32][33][34] The inverse FM thickness dependence reveals the interface nature of the EB effect and reflects the origin of EB as a competition between the Zeeman energy of the FM layer and AF/FM interface coupling energy. It is the detailed microscopic understanding of the latter which is still under debate.…”

“…7,[22][23][24] Recently size effects involved in the EB phenomenon have been extensively studied. [25][26][27][28] This includes the dependence of the EB on the AF and FM film thicknesses as well as size effects induced by lateral structuring of the FM and AF components of EB heterostructures. Various characteristic length scales influencing the EB have been identified.…”

Exchange bias of Co/CoO multilayers deposited on nanosphere array

“…In the curved film, the interfaces are increased, which makes the magnetic properties complex. Compared to the multilayer on flat substrate, few works focuses on the study of exchange coupling effect of the multilayer nanostructure on the curved substrate [12][13][14]. Compared to the bilayer, the multilayer on curved substrate not only increases interface area of the FM/AFM layer, but also topography of interface varies along with the surface, which leads to the change of magnetic properties.…”

Reply to “Comment on ‘Size-dependent scaling of perpendicular exchange bias in magnetic nanostructures’ ”

“…We mention a few dissimilarities between the two systems that might account for a different behavior at room temperature. In a recent paper 1 we employed dense arrays of monodisperse spherical polystyrene particles to fabricate ͓Pt͑2 nm͒ / Co͑0.5 nm͔͒ 3 / IrMn ͑t in nm͒/Pt͑2 nm͒ multilayers with a lateral diameter ͑d͒ varying from 58 to 320 nm. These nanostructures were characterized at room temperature using magneto-optical Kerr effect magnetometry and magnetic force microscopy.…”

“…4 in Ref. 1 showing experimental data for nanostructure with four different sizes. Under these conditions the AFM domains are confined by the dimension of the magnetic nanocaps, i.e., the IrMn film is single domain and D AFM Ϸ d. Our results therefore agree with Malozomoff's random field model predicting an inverse proportionality between the magnitude of the bias field and the AFM domain size.…”