In this work we investigate the strain state in Ni films belonging to [Ni(tNi)/Cu(tCu)] × N superlattices (with tNi = 2 nm, 3 nm, tCu from 0 to 3 nm and N between 1 and 6) by Extended X-ray Absorption Fine Structure at the Ni K-edge. The strain of these Ni blocks can be up to 80% larger than that obtained for single thin films with the same content of Ni. The dependence of the strain on N indicates that its relaxation is not progressive with N but gets blocked for certain N values. Also, the replacement of a single Ni layer by Cu, at the middle of the Ni block, results in a significant increment of the strain of the nickel layers, which manifests the relevant effect of the introduction of copper. The large strain values observed suggest the presence of blocking mechanisms acting on the propagation of dislocations. We argue that this observation is related to repulsive interactions between dislocations enhanced by nonhomogeneos stress due to presence of the copper layers separating the Ni blocks.

Understanding and controlling macroscopic quantities directly associated with a random field, such as the coercive field H-c and dislocations in magnetic materials, is important for many applications that include films with perpendicular magnetic anisotropy. Here, using a model system with perpendicular magnetic anisotropy, Cu/Ni/Cu, we show that H-c in double 4 nm thick Ni films is about 0.65 times the value obtained for the single Ni film, a fact that can be understood if the propagation of misfit dislocations is not transmitted to the second magnetic block, implying that its statistical distribution remains unchanged whereas the magnetic driving force increases with the nickel thickness. This interpretation is based on the direct measurement of the in-plane and out-of-plane lattice parameters of the Ni blocks by the extended x-ray absorption fine structure method, a chemically selective technique tuned to probe exclusively the environment of the Ni atoms. With this finding and applying the rigid domain walls model, the H-c ratio between double and single Ni films is calculated, yielding a value of 0.71.

The role of the strain state in epitaxial (001)-oriented Cu/Ni(14 nm)/Cu rings is investigated using a combination of magnetic force microscopy and finite-element calculations. Rings with an external diameter of 3 and 2 mu m and linewidth W larger than 400 nm show two different structures: domains with magnetization oriented in the radial direction exist at the inner and outer radius, separated by an area in the interior of the ring consisting of stripe domains with perpendicular magnetization. The former is the sole magnetic structure observed for W < 400 nm. Micromagnetic calculations on narrow-linewidth structures indicate that the radial domain-wall structure consists of elliptical Bloch lines with a shorter and longer length along the tangential and radial directions, respectively. Finite-element calculations show that the anisotropic relaxation of the in-plane strain is larger at the ring inner and outer edges than in the interior part of the ring and accounts for the reorientation of the magnetization direction.

We present epitaxial structures made of twin nickel blocks with perpendicular magnetic anisotropy separated by a copper layer which, for some values of this interleaving layer, show domain structures with four levels of contrast in magnetic force microscopy images. This manifold domain structure implies that the magnetization in the Ni blocks, in addition to the parallel orientation, undergoes a non-collinear configuration with respect to each other. To explain this result we consider a magnetoelastic domain structure with M in the plane that can elude the clamping done by the substrate with an average strain of −42 × 10−6 (≈70% of the bulk value). Thus, the out-of-plane anisotropy is balanced and a biquadratic exchange coupling can stabilize the non-collinear domain configurations between the Ni blocks.