Annealing effects on the structure and magnetic properties of Fe–C granular films

Abstract

Fe–C granular films with different Fe volume fractionxv were fabricated using a DC facing-target sputtering system at room temperature andsubsequently annealed at different temperatures. X-ray diffraction and selected areaelectron diffraction analyses indicate that as-deposited and low-temperature annealed () samples are composed of amorphous Fe and C, and highertemperature annealing makes the amorphous Fe transform toα-Fe, which is also confirmed by high-resolution transmission electron microscopy.Magnetic measurements indicate that at room temperature the as-depositedFe–C (xv = 58)granular films are superparamagnetic and annealed ones are ferromagnetic. The coercivity of 100 nm thickFe–C (xv = 58) granular films increases with annealing temperature (for 1 h) and time (at450 °C). The coercivity ofthe 100 nm thick Fe–C (xv = 58) samples annealed at temperatures ranging from 400 to500 °C decreases linearly with measuring temperatureT, signalling a domain wall motion mechanism. For the samples annealed at550 °C, the change of in-planecoercivity with T satisfies the relation , reflecting that this system behaves better as a set of Stoner–Wohlfarth particles. It wasalso found that there exists a critical thickness ( nm) for the 450 °C annealed(for 1 h) Fe–C (xv = 58) granular films with thickness in the range 100–200 nm, below and above which themagnetization reversal is dominated by domain wall motion and by Stoner–Wohlfarthrotation, respectively.