Giant and Anisotropic Magnetoresistance in CoFe-Cu Granular Alloys: The Role of the Ferromagnetic Concentration

Abstract

CoFe-Cu granular films with compositions ranging from 0.11-0.34 of ferromagnetic (FM) atomic concentration were prepared by rf sputtering. As-cast samples were rapidly annealed at various temperatures up to 750 °C to promote the segregation of FM particles within the metallic matrix. X-ray diffraction, transmission electron microscopy and atomic/magnetic force microscopy were used to determine the microstructure of the samples. Magnetic and transport properties suggested that this family of samples can be classified into three groups: (i) below about 20% at. of FM content, all as-cast and annealed samples, display the typical features of granular solids constituted by the random distribution of nanometric CoFe particles within a Cu matrix, and the maximum magnetoresistivity is about 20% at low temperature (giant magnetoresistivity -GMR-); (ii) For as-cast samples with 20 to 30% at. of FM content, magnetoresistivity and magnetisation display both a complex bimodal behaviour and large metastable effects associated with the interparticle interactions, which stabilise a magnetic microstructure well below the volume percolation threshold, as already shown in CoFe-AgCu granular alloys. Thus, magnetoresistivity is very low (1-3%) due to large magnetic correlations; and (iii) above about 30% at. of FM content (and below the volume percolation threshold), as-cast samples display both anisotropic magnetoresistance (AMR) and GMR, as shown in Co-Ag alloys. However, annealing leads to the formation of large magnetic particles, which result in a transition from the AMR-GMR regime to the expected GMR regime, with a maximum GMR of about 7%.