Effects of orientation transition on exchange anisotropy of Co∕NiMn films by biorientation epitaxial Cu∕Au∕Cu underlayers

A biorientation structure of Co∕NiMn films, composed of columnar (001) and (111) epitaxial grains, was developed by using Cu∕Au∕Cu underlayers on Si substrates. When the Au layer is thinner than 7.5Å, Co∕NiMn films are epitaxial (001) and show exchange fields of 50Oe for 150ÅCo. When the Au layer is thicker than 7.5Å, parts of the Co∕NiMn films change to the (111) orientation, and the (111) volume fraction increases with increasing Au thickness. The exchange field of Co∕NiMn can be manipulated by altering the relative amount of epitaxial (001) and (111) grains and the exchange field reaches 150Oe, an enhancement of 300%, by altering the orientation from (001) to (111). The formation of (111) grains in the biorientation films may decrease (001) crystallite diameter or introduce multidomains in the NiMn films, which plausibly increase uncompensated spins or frustrated spins in NiMn films, leading to the increase in the exchange fields.