Domain structure of epitaxial Co films with perpendicular anisotropy

Abstract

Epitaxial hcp Cobalt films with pronounced c-axis texture have been prepared by pulsed lased deposition (PLD) either directly onto Al2O3 (0001) single crystal substrates or with an intermediate Ruthenium buffer layer. The crystal structure and epitaxial growth relation was studied by XRD, pole figure measurements and reciprocal space mapping. Detailed VSM analysis shows that the perpendicular anisotropy of these highly textured Co films reaches the magnetocrystalline anisotropy of hcp-Co single crystal material. Films were prepared with thickness t of 20 nm < t < 100 nm to study the crossover from in-plane magnetization to out-of-plane magnetization in detail. The analysis of the periodic domain pattern observed by magnetic force microscopy allows to determine the critical minimum thickness below which the domains adopt a pure in-plane orientation. Above the critical thickness the width of the stripe domains is evaluated as a function of the film thickness and compared with domain theory. Especially the discrepancies at smallest film thicknesses show that the system is in an intermediate state between in-plane and out-of-plane domains, which is not described by existing analytical domain models

Modulation of the properties of thin ferromagnetic films with an externally applied electric field in ferromagnetic/piezoelectric/ferromagnetic hybrids

“…An extra reason for working with Co layers with thickness in the range of 30-50 nm comes from the fact that changes in the magnetic anisotropy of shape origin should be fully effective here; for very thin films (below 30 nm), an in-plane magnetic anisotropy dominates, while for sufficiently thick films (above 50 nm) an out-of-plane magnetic anisotropy is established. [23][24][25] Unfortunately, the requisite of using Co thickness in the range of tens of nanometers meets a strong conflict originating from the morphology of the PE crystals employed here; the as-prepared PMN-PT single crystals have a mean surface roughness in the range of many hundred nanometers as this is evidenced by detailed AFM data (see below). Thus, Co outer layers with thickness of only a few tens of nanometers would probably be ineffective electrodes for voltage delivery since their comparatively small thickness (in respect to the PMN-PT surface roughness) cannot ensure the homogeneous covering of the PMN-PT crystal.…”

“…23 Referring to the coercive field H c , we believe that the observed modulation is motivated by two candidate mechanisms: (a) by the pinning of already existing domain walls that can be effectively enhanced either by the surface/interface roughness or "bulk" static disorder that are newly introduced by the developed strain 26-29 and (b) by the modulation of domain walls population that inevitably should accompany the modulation of magnetic domain size due to the deformation of the FM film thickness motivated by the induced strain (the magnetic domain size w is proportional to the square root of the film thickness d, that is w $ d 1/2 ). [23][24][25] Referring to the saturation magnetization m sat , we believe that the observed modulation can be motivated by the change of the magnetic anisotropy due to the change of the shape anisotropy either at the local or global level. For Co it is well known that, except for the magnetocrystalline anisotropy, another mechanism of magnetic anisotropy stems from the dimensional restrictions due to the reduction of the layer thickness (shape anisotropy).…”

“…Typically, Co films below the critical thickness 30-50 nm exhibit in-plane magnetization processes, while above 30-50 nm they attain out-of-plane magnetic configuration. 23,24 Thus, by working in the "grey area" of 30-50 nm where the magnetic shape anisotropy of the FM layers is not robust we are probably able to modulate it due to the strain introduced by the PE crystal that can induce shape deformation at the local and/or global level. Modulation of the magnetic anisotropy from in-plane to out-of-plane can result in changes in the saturation magnetization that is measured along the external magnetic field (applied in-plane).…”

Investigation of thermally evaporated nanocrystalline thin cobalt films

“…Moreover, the analysis of the relative intensities of the observed rings exhibits the presence of (0002) texture where the hexagonal axis is preferentially oriented perpendicularly to the film surface [31]. Nevertheless, to get a deeper insight into the crystallographic orientation of the films, it is necessary to apply more sensitive techniques, such as X-ray diffraction (XRD) Θ-2Θ spectra, [12,28,29], the X-ray Schulz pole figures [12,32], or the electron backscattered diffraction (EBSD) method [33,34]. These measurements are planned in near future.…”

“…The obtained films are usually nanocrystalline in nature and possess specific, improved mechanical, physical, magnetic, and chemical properties in comparison with conventional microcrystalline counterparts. Depending on the film thickness as well as the preparation method and conditions used, cobalt thin films exhibit a wide range of morphological and magnetic properties, and especially various magnetic domain structures with inplane and outofplane magnetization [10][11][12][13]. …”