A Hexagonal Close-packed Phase in Nickel Films S. M. Zharkov, V. S. Zhigalov, and G. I. Frolov Kirenskii Institute of Physics, Siberian Division, Russian Academy of Sciences, Akademgorodok, Krasnoyarsk, 660036 Russia Received August 15, 1995 INTRODUCTION In recent years. the problem of formation of meta- stable pha<;es in thin films of 3d transition metals has taken the ever growing attention of investigators [1-3]. It is known that crvstal structures formed in thin films are not typical of bulk alloys. This opens the way for producing new materials with. frequently, anomalous properties, which is of great practical and theoretical interest. Molecular-beam epitaxy is the main technique for the preparation of such materials. This method, however, has an evident disadvantage: the metastable phases only exist in ultra thin layers because the fonna- tion of the phases is detennined by the effect of single- crystal substrates. Atomic deposition techniques pro- vide very high effective quenching rates and allow one to avoid the above-mentioned disadvantage. EXPERIMENTAL In this work, nickel films were produced by pulse plasma evaporation in a vacuum of 2 x I0- 6 Torr. The films were deposited on cover glasses at room t,rmper- ature. The pulse rate of deposition was 10 4 -IOS Als; the pulse duration was I0-4 s; the pulse frequency was 1-10 Hz. The thickness of the investigated films was 500 A. The saturation magnetization Ms of the films was measured by torsional anisometer; the accuracy of the measurements was 2.6 x I0- 3 dvnlcm 3 • The micro- structure and phase composition ·of the films were investigated by transmission electron microscopy using a PREM-200 electron microscope. We studied struc- tural transformations and magnetic properties of the films upon multistep annealings using 1-h holdings at each fixed temperature in a range of 50-500°C in a vac- uum of 2 x 1 Q-6 Torr. RESULTS AND DISCUSSION Figures 1 a and 1 b show electron diffraction patterns taken from the nickel films. As can be derived from the electron diffraction pattern (Fig. la), the as-deposited films have a hexagonal close-packed (hcp) structure with the lattice parameters a = 2.64 ± 0.01 A. and c = 4.33 ± 0.01 A.; the size of microcrystallites is 200-250 A.. Upon annealing below 200°C, the crystal structure of the films did not change. After annealing at 250°C, maxima belonging to the face-centered cubic structure (fee) typical of bulk nickel was observed in the electron diffraction pattern (Fig. lb) along with the maxima belonging to the hcp structure. After annealing at 300°C, only the maxima belonging to the fee phase are found in the electron diffraction pattern (Fig. le). Fur- ther annealings increased the size of microcrystallites and did not change the crystal structure. After anneal- ing at 500°C, the typical size of microcrystallites was found to be 0.2-0.4 )!m. Figures 2a and 2b show the magnetization and elec- trical resistivity of the films as a function of the anneal- ing temperature. At room temperature and in fields up to 17 kOe, the as-sputtered films have zero magnetiza- tion (within the accuracy of the torsional anisometer). After annealing at 250°C, the films acquire a magnetic moment; after annealing at 300°C, the saturation mag- netization Ms was close to the saturation magnetization of bulk nickel and was not changed upon further annealings. The resistivity of the films increases with increasing size of the microcrystallites. As literature data show, nickel films with the hcp structure and lattice parameters close to those obtained by us were first produced by Bublik and Pines [4] using thermal vacuum evaporation at a high rate of deposition (unfortunately, the rate was not given). 'fhe hcp struc- ture was observed in the films of 20--30 A in thickness; a mixture of the hcp and fee phases was found with increasing thickness of the films to 50 A. Only the fee phase was found in films thicker than 50 A.. The mag- netic properties of these films were not measured. In [5], single-crystal nickel films with an hcp struc- ture (a = 2.50 and c = 3.98 A) were produced by elec- trodeposition on an hcp cobalt single crystal (a= 2.50 and c = 4.07 A). The thickness of the films was greater than 0.1 )!m. The magnetic anisotropy constant of the films was less than that of pure cobalt. However, based on these data, it is difficult to unambiguously detemrine the magnetic properties of the nickel films with the hcp structure because of the presence of cobalt. More recently, the hcp phase was found in nickel films pro- duced by ion-plasma deposition on substrates cooled to 77 K [6]. The as-sputtered films had amorphous struc- ture; upon annealing at 490 K, the films crystallized into an hcp structure with lattice parameters a = 2.5 and c = 4.0 A. According to [6], both amorphous and crys- talline films with an hcp structure were paramagnetic up to 77 K. Upon annealing at 730 K, an fee phase 328