Magnetic Materials and Devices Group

Magnetic Thin Films

Project Staff:

C.A. Ross, E. Friend, in collaboration with R.C. O'Handley

Sponsors:

National Science Foundation

Previous researchers have investigated the magnetic properties of multilayer thin-film sandwiches of copper/nickel/ copper epitaxially grown on silicon. In this system, the magnetic properties of the nickel layer change drastically as the thickness (and therefore the amount of strain) of the nickel layer varies, and it does so over a thickness range that is uniquely broad. This effect is due to both the large magnetoelastic energy and to the surface magnetocrystalline anisotropy energy of the nickel film. Our research focuses on understanding the effects of patterning these Cu/Ni/Cu films into nanoscale lines. Once patterned, the strained films can relax at the edges, with the strain relief being a function of line width and thickness. The ultimate aims are to elucidate the relationship between stress and magnetic properties in nanoscale features, to understand and control the effects of stress on magnetic nanostructures, and to exploit this effect to achieve desired magnetic properties. This understanding is a key part of the development of future magnetic recording technologies such as patterned recording media and read/write heads.
Magnetic characterization of Cu/Ni/Cu nanolines has shown that, contrary to the expectations from shape anisotropy, the preferred magnetization direction is transverse to the line direction in the sample plane. This is due to strain relief in the nanoline across its width. Simulations of strain in these structures have shed further light on the relation between the nanoline cross-section aspect ratio and strain relief, showing that strain is relaxed at the edges of the lines over a distance of about 1.5 times the film thickness. Therefore narrower lines show a greater magnetic anisotropy parallel to their width. Strain relief is even greater in the case of overetch into the copper substrate. Magnetic anisotropy data is being analyzed to understand how the net anisotropy of the nanolines agrees with theoretical models.

Fig. 1. Electron micrograph of an array of 200 nm wide lines patterned into a Cu 200 nm /Ni 6.9nm/Cu 5nm film deposited epitaxially on a Si (100) substrate. The easy axis is in plane, perpendicular to the lines, due to strain relaxation caused by patterning.