Enhancement of antiferromagnetic coupling in magnetic multilayers by low energy ion beam substrate nanopatterning

Abstract

Ion beam irradiation has been shown to be an interesting tool for tailoring the magnetic properties of thin films and multilayers. The modified properties include magnetic anisotropy, interlayer exchange coupling, exchange bias, magnetic domain structure and magnetization reversal. In this work, new results are shown concerning the enhancement, by one order of magnitude, of the antiferromagnetic coupling strength in amorphous CoSi/Si multilayers by irradiating Si(100) substrates with 1 keV Ar(+) ions. The ion beam exposure induces an increase of the substrate roughness, from 0.07 to 0.88 nm, which enhances antiferromagnetic coupling in the magnetic multilayers grown on top. One possible mechanism governing this enhancement is discussed, related to the formation of magnetic/non-magnetic regions where dipolar interactions could stabilize the antiferromagnetic alignment. The presence of non-magnetic regions is suggested by the observed trend to superparamagnetism, and is expected since the Curie temperature of the amorphous CoSi alloy used is slightly above but very close to room temperature. Accordingly, small fluctuations in the local composition, leading to an enrichment of Si, would produce non-magnetic regions enabling dipolar interactions to take place. Furthermore, the ion beam induced increase of roughness makes surface diffusion of the atoms arriving at the sample difficult, favoring the formation of local non-magnetic inhomogeneities. Finally, the role of other possible mechanisms to enhance antiferromagnetic coupling is also briefly discussed.

Magnetic Exchange Bias and Its Ion Beam Induced Enhancement in a Nanoscale Si/Co/Si System

“…Small local variation in compositions could produce Si rich non-magnetic regions where surface magnetic poles, and the corresponding AFM dipolar interactions, may appear. AFM coupling in a Co x Si 1-x layer has been observed and explained in terms of formation of magnetic/non-magnetic regions as a consequence of local fluctuations of the alloy composition [44]. In our case this local fluctuations in composition is likely to be responsible for the AFM order in the Co x Si 1-x layers.…”

Asymmetric grazing incidence small angle x-ray scattering and anisotropic domain wall motion in obliquely grown nanocrystalline Co films

“…This technique is a powerful tool for the morphology characterization of objects over a wide range of sizes, from micrometer down to nanometer [14], and with strong advantages including its nondestructive character, its averaging capability, and the possibility of studying buried objects thanks to a large x-ray penetration depth. Some previous works have explored the grazing incidence diffuse scattering in obliquely grown multilayers [15], and, more recently, GISAXS has been used to study the morphology of different obliquely grown systems such as amorphous Co-Si multilayers [16], and polycrystalline TiO 2 [17,18], Ni [13], and Au [19] thin films with columnar grains of only a few nanometers' lateral size and up to several hundreds of nanometers thick. In these systems, an interesting effect has been reported consisting of the asymmetric distribution of the in-plane scattered intensity on each side of the specular plane.…”