Permalloy thin films exchange-coupled to arrays of stencilled cobalt islands
Patterned magnetic thin films are of interest due to the potential for new technological applications and also because of their modified physical behaviour as the lateral dimensions are reduced. The functionality of patterned films can be intelligently designed by combining materials with different magnetic properties. In particular, bilayers comprising a magnetically hard and a magnetically soft film (often referred to as exchange-spring systems) provide useful model systems for optimization of exchange-coupled hard-soft nanocomposites, which are promising as high performance magnets because they achieve a high energy product through the combination of a high coercivity in the hard phase and a high magnetization at saturation in the soft phase. We have extended the concept of a patterned bilayer system, developing a method employing stencil masks to fabricate a continuous soft film of permalloy (Py) exchange-coupled to an array of magnetically harder cobalt (Co) islands. This planar geometry was selected to allow us to study exchange-spring systems with Neél instead of Bloch magnetic domain walls, which may result in modified exchange spring behavior. The stencil lithography (shadow mask deposition) method was employed because it allows deposition of the continuous, patterned and cap layers without breaking the vacuum, providing an undisturbed, clean interface which results in a reproducible, direct exchange-coupling between the two ferromagnetic layers. The arrays of Co islands, 1 or 2 nm-thick, were deposited on continuous Py (Ni[80%]Fe[20%]) thin films, 2 or 5 nm-thick, by electron-beam evaporation onto silicon wafer substrates. Following deposition of the continuous Py layer, a stencil mask with 1 by 1 mm2 silicon nitride membranes (Si3N4), each containing a 800 by 800 um2 regular array of holes, was employed in-situ for patterning of the Co layers. Magnetic hysteresis loops were obtained from magneto-optical Kerr effect measurements. In addition, the magnetic spin configurations during magnetization reversal were studied with photoemission electron microscopy (PEEM) at the Swiss Light Source, Paul Scherrer Institut and interesting domain structures were observed. The presence of the Co islands results in a spatial modulation of the magnetic properties of the soft Py continuous film, allowing the positioning of domain walls at the island boundaries. The detailed magnetic behavior depends critically on the magnetic properties of the films and for these Py/Co bilayers, the behavior was found to be highly dependent on the film thickness. These systems extend the possibilities for engineering magnetic thin film properties and could be of interest for positioning of domain walls in devices based on domain wall logic and domain wall magnetoresistance.
Record created on 2006-09-19, modified on 2016-08-08