Soft Thin Films for Flux Concentrators

In this paper, we describe the development of multilayer (ML) thin films suitable for flux concentrators (FC). We prepared ML films having magnetic thicknesses n middot t_F in the range of 400 Aringto 3000 Aringby ion beam deposition (IBD). The films exhibit Ruderman-Kittel Kasuya-Yosida (RKKY) long range antiferromagnetic (AF) coupling through very thin Ru interlayers. We prepared two types of samples: (CoFeBt_F/Rut_Ru)xn and (NiFet_F/Rut_Ru)xn, where CoFeB is (Co₇₀Fe₃₀)_80at%B_20at% and NiFe is Ni₈₁Fe₁₉. The former has good thermal stability and high magnetic moment while the latter, previously reported, has poor thermal stability and low spontaneous magnetic moment. We discuss the magnetic properties of the films in terms of the remanent magnetic moment M_r and saturation field H_s. Despite having higher spontaneous magnetic moment of 1200 emu/cm³, (CoFeB/Ru)xn films with adequate magnetic properties for sensor-in-gap devices with FC could not be prepared. By contrast, samples consisting of NiFe350 Aring/(Ru7 Aring /NiFe350 Aring)x7 exhibited suitable magnetic properties. We lithographically patterned the films into FC and test structures to investigate their magnetization processes by anisotropic magnetoresistance (MR) measurements. The MR curves indicate magnetization processes by uniform rotation and show absence of hysteresis.