Tailoring the magnetic anisotropy, magnetization reversal, and anisotropic magnetoresistance of Ni films by ion sputtering

Abstract

We studied surface morphology induced changes of magnetic anisotropy, magnetization reversal, and symmetry of the anisotropic magnetoresistance (AMR) in ion sputtered Ni films grown on MgO (001). Grazing-incidence ion sputtering generally develops anisotropic surface roughness of the Ni films, i.e., nanometer wide ripples parallel to the ion beam direction, giving rise to uniaxial magnetic anisotropy with the easy axis along the ion beam direction. The formed ripples act as domain wall nucleation and pinning sites during magnetization reversal, while two-jump domain wall motion dominates in the as-grown Ni films. More importantly, the azimuthal angular dependence of the AMR indicates a superposition of twofold symmetry and fourfold symmetry. By relying on grazing-incidence ion sputtering along specific crystallographic directions, we are able to tailor the relative weight of twofold and fourfold symmetry of AMR. We demonstrate that in contrast to the bulk case, the symmetry of the AMR becomes also sensitive to the surface morphology in thin films, which is in particular relevant for the design of magnetotransport based sensors.

Evolution of magnetic anisotropy in cobalt film on nanopatterned silicon substrate studied in situ using MOKE

“…1,2 In a more controlled manner, uniaxial magnetic anisotropy can be generated by periodic nanopatterning of the surfaces of the substrate [3][4][5][6] or of the film itself. [7][8][9] Periodic nanopatterns can be produced in a variety of ways, for example, by i) ion erosion 3,10,11 , ii) nano lithography 12,13 iii) appropriate annealing of sapphire substrate 14 etc. Ion beam erosion is a versatile technique by which nanoripples with controlled ripple wavelength and amplitude can be generated and this technique has been used in many studies to produce controlled UMA in thin films of a variety of materials like cobalt 3,4,15,16 , Fe 17 , permalloy 3,18 etc.…”

In-Situ Investigation of Ion Beam-Induced Crossover in Uniaxial Magnetic Anisotropy of Polycrystalline Fe Films

“…In this regard, the oblique incidence ion beam erosion (IBE) process has been demonstrated to be a handy and powerful tool to induce self-organized nanoscale morphological structures on the substrates ,, as well as 2D thin-film surfaces. ,, In the literature, the oblique incidence IBE process has been used extensively to form nanometer-scale ripple patterns on the surfaces. Depending on surface temperature and ion incidence angles, theses ripples are found to be aligned parallel or perpendicular with respect to the IBE direction. , Magnetic thin films with such a microstructure reveal a pronounced UMA with an easy axis of magnetization along the direction of ripples , on the surface. Even when a magnetic film deposited on ion beam induced nanorippled substrates, UMA was found due to the formation of replicated correlated ripple structures in the film. , Considerable research has already been conducted in this area; in general, the origin of UMA is being understood in terms of the model proposed by Schlömann, where the stray dipolar fields generated by rippled morphology are responsible for UMA in the magnetic films.…”

“…The main difficulty faced in doing the same is the extraction of unambiguous information about the ion beam-induced structural and surface morphological changes simultaneously in conjunction with UMA. So far, most of the studies in the literature were performed ex-situ by preparing a series of ion beam-sputtered thin film samples; ,,,, therefore, it is difficult to keep deposition as well as ion beam parameters identical in different samples to compare UMA. Moreover, as soon as the thin films are exposed to the ambient, its surface gets oxidized, and therefore, genuine effects of the ion beam on the surface structure cannot be obtained.…”

“…Depending on surface temperature and ion incidence angles, theses ripples are found to be aligned parallel or perpendicular with respect to the IBE direction. 23,24 Magnetic thin films with such a microstructure reveal a pronounced UMA with an easy axis of magnetization along the direction of ripples 15,25 on the surface. Even when a magnetic film deposited on ion beam induced nanorippled substrates, UMA was found due to the formation of replicated correlated ripple structures in the film.…”

Morphology induced large magnetic anisotropy in obliquely grown nanostructured thin film on nanopatterned substrate

“…Interfacial morphology and symmetry breaking effect at the nanoscale strongly influence magnetic response of a material such as magnetic anisotropy [1], coercivity [2], magnetization reversal process [3], hysteresis [4], magneto-resistance [5]. In particular, magnetic anisotropy that relies on the preferred orientation of the spin alignment [6] of a thin film structure have extensively finds its application in the spintronics industry [7], perpendicular spin valves [8], magnetic tunnel junction devices [9] as well as other applications in magnetoelectronics [10], high-density storage media [11], and magnetic sensors [12].…”