Anisotropic magnetoresistance (AMR) of cobalt: hcp-Co vs. fcc-Co

Abstract

In spite of the numerous works devoted to studying the magnetoresistance of Co metal, very diverging results have been reported in the literature on the magnitude of the anisotropic magnetoresistance (AMR) ratio of Co samples mostly without detailed structural characterization. Therefore, the main purpose of the present work was to establish if the crystal structure of Co has an effect on the AMR ratio. With the help of structural studies by X-ray diffraction (XRD) and transmission electron microscopy, fully hcp-Co and predominantly fcc–Co polycrystalline foils were produced by electrodeposition and their magnetoresistance curves MR(H) were measured at room temperature in magnetic fields up to H = 8 kOe. The MR(H) curves indicated much lower saturation fields for fcc-Co than for hcp-Co, in good agreement with the significantly larger magnetocrystalline anisotropy of the hcp phase. These findings were supported also by the measured magnetization isotherms. The coercive field and MR(H) peak position data indicated a magnetically softer behavior of the fcc-Co phase than that of the hcp-Co phase, in agreement with literature findings. Finally, it was established that the AMR ratio is about +1.2 % for hcp-Co whereas it is about +1.9 % for the predominantly fcc-Co samples. By having an estimated volume fraction of the hcp-Co phase in the latter samples from the XRD studies, we could assess an AMR ratio of about 2.2 % for pure fcc-Co. It is the first time that the AMR ratio has been determined separately for the two crystalline phases of Co and the finding that AMR(fcc–Co) ≈ 1.8 AMR(hcp-Co) is a hitherto unknown result. Finally, our AMR results for the two phases of Co metal are discussed in the light of recent progress on the microscopic mechanisms of the AMR effect.