Polycrystalline Cr-doped ZnO films are prepared by the co-sputtering method. Diamagnetism is observed in the conductive samples deposited in pure Ar. However, ferromagnetism is found in films with the same Cr dopant prepared under different oxygen partial pressures. The magnetization shows a strong dependence on the Cr concentration and, especially, on oxygen pressure. It is found that native point defects, which can be adjusted by the oxygen partial pressure during deposition, play a crucial role in the observed magnetic behaviors. The obtained ferromagnetism can be described by the dopant-donor/acceptor hybridization model, which associates exchange interaction with shallow-bound carriers. These results may help to understand the wide range of experimentally determined magnetic moments and its changes with different metal types and concentrations prepared by different groups and methods.
Structural and magnetic properties of as-deposited and 550°C annealed Fe–C granular films have been investigated. It is found that the as-deposited samples are composed of amorphous Fe granules dispersed in an amorphous C matrix, and transformed to α(bcc)∕γ(fcc)-Fe and graphitelike C after being annealed at 550°C for one hour. The morphologies of the as-deposited samples are composition dependent, and the phase segregation between Fe and C phases can be seen by high-resolution transmission electron microscopy. Magnetic measurements reveal that the as-deposited samples are superparamagnetic at Fe composition of xv=49,58,65, and soft ferromagnetic at xv=77,84. The in-plane coercivity of the annealed specimens decreases with increasing xv, which is attributed to the enhanced intra-particle interaction, as evidenced by the long-range domain structures observed using magnetic force microscope images. The magnetic reversal mechanism deviates from ideal domain wall motion mode and approaches Stoner–Wohlfarth rotational mode with the decrease of Fe volume fraction.
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