XRR spectra of the Ni/NiO multilayers. The symbols correspond to the experimental data, whereas the solid lines are the fittings of the experimental data obtained with the help of the GenX code. The spectra have been vertically shifted for clarity. The results from the fittings are summarized in Table I.
Magnetization hysteresis loops recorded by SQUID at 5 K with the external field applied perpendicular (circles) and parallel (squares) to the film plane for 4 multilayers with tNi = 6.1 nm and 6 repetitions (a), 2.5 nm and 13 repetitions (b), 2.1 nm and 15 repetitions (c), and 1 nm and 21 repetitions (d).
Temperature-dependent magnetization hysteresis loops with the external filed applied perpendicular (circles) and parallel (squares) to the film plane for a Ni/NiO multilayer with tNi = 2.5 nm. The number of repetitions is 13.
Temperature-dependent normalized magnetizations (symbols) for a series of Ni/NiO multilayers as indicated. The lines are the results of Bloch-law fits in the spin-wave regime. For comparison, values for bulk Ni have been introduced from Ref. 20. The number of repetitions from the largest to smallest tNi is: 6, 8, 9, 13, and 21, respectively.
Temperature-dependent normalized uniaxial anisotropies (closed symbols) and normalized magnetizations to the Γ power (open symbols) for representative Ni/NiO multilayers as indicated. The Γ values were determined by the coincidence of the MN Γ to the anisotropy data. The number of repetitions is 6 for tNi = 6.1 nm, 13 for tNi = 2.5 nm, and 9 for tNi = 4.2 nm.
Kt over t plot of the Ni/NiO multilayers at T = 5, 150, and 300 K. The lines correspond to the linear fit of the experimental data points (closed symbols) at each temperature. The slope of the curves and the intercept of them with the y axis allow us to calculate the Kv and Ks values, respectively. The error bar of Kv and Ks values is ∼ 10%. The open symbols represent experimental data points for the 3 films with thin Ni layers, which fail to the fits.
(a) TEM image showing the morphology of a Ni/NiO well-ordered multilayer having 10 repetitions and nominal thickness tNi = 3.5 nm. (b) SAD pattern obtained from the interfacial area of Si/SiO2/multilayer, revealing the nanocrystalline structure of the Ni layers. Diffraction spots attributed to Si are also denoted in the image, corresponding to its  zone axis. (c) Higher magnification image of an area close to the interface between Ni/NiO and the Si substrate, depicting the nanograin architecture of the Ni layers and the amorphous NiO spacers.
Strain analysis results obtained by GPA in individual Ni grains of the multilayer with 10 repetitions and nominal thickness tNi = 3.5 nm (a) HRTEM image depicting the individual grain atomic structure, (b) phase image created using the 111 spatial frequencies of the two grains circled, (c) corresponding strain map calculated by the phase images. The strain profile across the top Ni grain is plotted in (d).
(a) TEM image from a Ni/NiO multilayer with t Ni < 2.5 nm illustrating an amorphous morphology throughout the whole epilayer. This is further confirmed by the SAD pattern in (b), obtained from the interface between Ni/NiO and the Si substrate where only spots corresponding to  projection of Si are included. (c) HRTEM image obtained from the Ni/NiO interfacial region, revealing the existence of only a few Ni nanograins embedded in the amorphous Ni/NiO matrix. The grains do not have a specific orientation with respect to the Si substrate. (d) Higher magnification TEM image illustrating the columnar island morphology of the Ni/NiO epilayer. Black arrows depict the boundaries between the distinct columns throughout the whole film.
Data for the series of Ni/NiO multilayers whose XRR patterns are included in Fig. 1. The values were obtained via the use of GenX code for simulation. The simulated thickness of the layers, as well as the simulated r.m.s. roughness are expressed in nm. The unrealistic values of roughness as the thickness decreases is related to the transition from continuous to discontinuous Ni layers as we will show in Sec. IV.
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