Structure and magnetic properties of Ni films obtained by pulsed laser ablation deposition
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ulsed laser ablation deposition was used to fabricate Ni films. The structure of the as-deposited films is hexagonal (a ⳱ 0.267 nm and c ⳱ 0.435 nm), and its resistivity is nearly one order of magnitude larger than the resistivity of bulk Ni. No spontaneous magnetization was observed in the as-deposited samples. Annealing above 200 °C induced an irreversible structural transformation from the metastable hexagonal phase to the stable face-centered-cubic (fcc) one. Parallel to this transformation, the magnetic moment of the annealed Ni samples increased, and their resistivity decreased. On annealing at 440 °C, the structural transformation from the hexagonal-close-packed to the fcc phase was completed, and the magnetic moment, Curie temperature, and electrical resistivity of the fcc Ni films were found similar to the ones observed in bulk Ni. A direct correlation between the volume fraction of the fcc Ni phase in our films and their magnetic moment was established.
I. INTRODUCTION
During the recent years there has been a growing interest in the study of metastable phases of the ferromagnetic transition metals since new phenomena are observed in them. For instance, the thermally stable structural phase of Co, at room temperature, is the hexagonal-close-packed phase. However, the reduction in the size of Co crystals to the nanometer range induces the formation of the face-centered-cubic (fcc) phase at room temperature.1,2 Also, the body-centered-cubic (bcc) phase may be observed when Co is deposited on GaAs substrates.3 On the other side, although the stable phase at room temperature of Fe is bcc, the fcc phase was reported to be observed when the previous element was deposited on single-crystal copper substrates.4 Related to Ni, this ferromagnetic element exhibits a fcc phase at room temperature. However the existence of hcp Ni phase was debated for a long time. It was reported that heat treatments of Ni in a CO atmosphere at 170 °C led to the formation of a nonmagnetic hexagonal structure. When Ni samples were further heated to 250 °C, a structural transformation to the fcc phase took place.5 In this case, the presence of contaminants like C and N could explain the formation of a hexagonal phase.6 Nevertheless, more recent works have reported the existence of Ni hcp phase in samples obtained by sputtering,7 pulsed plasma evaporation,8 or by reduction of Ni complexes with boron or aluminum hydrides.9 In these previous works, the presence of Ni3C-type compounds was discarded after chemical analyses or x-ray diffraction measurements. J. Mater. Res., Vol. 17, No. 8, Aug 2002
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In our present work we report the formation of nonmagnetic hexagonal Ni films by the pulsed laser ablation technique. Heat treatments at 440 °C produced transformation to a magnetic fcc Ni structure, similar to the stable structure of bulk Ni. In the intermediate range of annealing temperatures, a mixture of the fcc and hcp phases was present in our samples. II. EXPERIMENTAL
The pulsed laser a
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