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Copper / Nickel Multilayers
The Copper/Nickel multilayer system is of interest to mechanical engineers as a system for studying the strength of materials. SPM techniques have been used by several groups to study the physical features of nanoscale indents and deduce information about the materials deformation modes and mechanisms.
Another aspect of these particular multilayer materials is their magnetic properties, which we were also able to study by carrying out MFM in conjunction with the AFM studies of the indent features.
Figure 1 shows the derivative of the AFM topography data (left) and the corresponding MFM data (right) for a 15 micron by 15 micron scan centered on a nanoindent placed in a Cu/Ni multilayer structure with 25 angstrom layer thicknesses. The topography data is easier to see as a derivative since the total height was large. It is possible to see pile up around the indent and perhaps some slip lines. The MFM image shows that the indent and the region including the pile up material have had their structure modified significantly and the strong magnetic signature is lost. The rest of the film shows the maze like domains and indicate a compressive in plane stress leading to tension out of the plane of the film and a magnetoelastic easy axis (see below). The maze domain structures form to lower the total energy of the film by reducing the net magnetization (at the expense of having some domains anti-aligned with the easy axis).
Figure 2 shows the derivative of the AFM topography data (left) and the corresponding MFM data (right) for a 15 micron by 15 micron scan centered on a nanoindent placed in a Cu/Ni multilayer structure with 500 angstrom layer thicknesses. These images are similar to those in Fig. 1, but now we can see subsurface cracks revealed in the magnetic structure of the indent plus pile up region. These may actually be in the silicon substrate. The maze domain width has narrowed for this multilayer, indicating actually higher strain in these structures. There is more demagnetization (denser maze structure) as a result.
Stress/strain and magnetic structure
A material’s magnetic structure is determined by the configuration of spins which minimizes the total magnetic energy. In thin films without external fields, this includes contributions from shape and stress anisotropies, and magnetostatic and magnetic exchange effects. The anisotropies determine the easy magnetization direction and the final magnetic structure is a compromise between shape anisotropy aligning the magnetization (M) with the longest axis, stress anisotropy aligning M along the direction of largest stress, and demagnetizing effects breaking large domains into smaller ones and setting up closure domains to contain the flux. Domain structures will be discussed below. Shape anisotropy effects can be large due to the high aspect ratio of thin films. Lattice mismatches in the structure induce stress anisotropy and can have the strongest effect, manifested through the magnetoelastic energy,
to lowest order, where lambda is the material dependent magnetostriction constant and the gammas are the direction cosines of M with respect to the principal stress axes. For positive lambda materials, M will align along the direction of most positive (or least negative) stress. In plastic deformation, the final stress state is a superposition of the stress due to the damage structures with residual stress, which occurs on unloading.
Domain formation minimizes the net magnetization by breaking it up into smaller domains having different orientations (demagnetization) and closure domains keeping flux within the material. Demagnetization segregates spins into oppositely aligned domains separated by 180° domain walls. These fine domains have a net overall attraction for each other and lower the total energy. They also satisfy the magnetoelastic energy since they point along (parallel or antiparallel to) the easy axis. Any excessive stray field flux is reduced by in-plane closure domains, separated from the vertical domains by 90? walls.
The actual observed domain structure is often complicated by defects, microstructure, and the material’s previous exposure to fields. In general, increasing the number of domains lowers the total energy. However, smaller domains mean more domain walls. Despite these complications, conclusions can be drawn from MFM data since they reproduce general features of stress-induced domain structures observed and understood from previous experiments on various materials using a variety of techniques, including Bitter-pattern optical and Lorentz-force scanning electron microscopies. For in-plane magnetization, the general trend is for the domains to segregate into bands of alternating orientation in the surface direction normal to the easy axis. For out of plane magnetization, the magnetization breaks into bands, but meanders in the surface plane unless some other aspect influences its direction.
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Related Links
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Structure/Property Relations, MST-8