Magnetoresistance, micromagnetism, and domain-wall scattering in epitaxial hcp Co films

Large negative magnetoresistance ~MR! observed in transport measurements of hcp Co films with stripe domains were recently reported and interpreted in terms of domain-wall ~DW! scattering mechanism. Here detailed MR measurements, magnetic force microscopy, and micromagnetic calculations are combined to elucidate the origin of MR in this material. The large negative room-temperature MR reported previously is shown to be due to ferromagnetic resistivity anisotropy. Measurements of the resistivity for currents parallel ~CIW! and perpendicular to DW’s ~CPW! have been conducted as a function of temperature. Low-temperature results show that any intrinsic effect of DW’s scattering on MR of this material is very small compared to the anisotropic MR. The effect of magnetic domain walls ~DW’s! on the transport properties of thin films and nanostructures is a topic of great current interest. Recent experimental research has extended early studies of iron single crystals 1,2 to nanofabricated thin-film structures of 3d transition metals 3‐5 and transition-metal alloys. 6,7 This topic has been approached from a number of viewpoints. In nanowires an experimental goal has been to use magnetoresistance ~MR! to investigate DW nucleation and dynamics in search of evidence for macroscopic quantum phenomena. Conductance fluctuations and MR hysteresis observed at low temperature in nanowires of Ni, Fe, and Co ~Refs. 8 and 9! have stimulated theoretical work on the effect of DW’s on quantum transport in mesoscopic ferromagnetic conductors. 10,11 In thin films and microstructures with stripe domains, experiments have focused on understanding the basic mechanisms of DW scattering of conduction electrons. Specifically, large negative MR observed at room temperature in hcp Co thin films with stripe domains were recently reported and interpreted in terms of a giant DW scattering contribution to the resistivity. 4 Independently, and to understand this result, a mechanism of DW scattering was proposed which invokes the two channel model of conduction in ferromagnets and spin dependent electron scattering—a starting point for understanding the phenomena of giant MR ~GMR!. 12 Within this model DW’s increase resistivity because they mix the minority and majority spin channels and thus partially eliminate the short circuit provided by the lower resistivity spin channel in the magnetically homogeneous ferromagnet.