Pressure Assisted Crystallization of MnAl Thin Films
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Pressure Assisted Crystallization of MnAl Thin Films Gregory A. Fischer1 and M. Lea Rudee Graduate Program in Materials Science/Center for Magnetic Recording Research University of California at San Diego, La Jolla CA. 92093-0401 Vitali F. Nesterenko and Sastry Indrakanti Graduate Program in Materials Science/Department of Mechanical and Aerospace Engineering University of California at San Diego, La Jolla CA. 92093-0411 1
Current Address: U.S. Army Research Laboratory, 2800Powder Mill Rd., Attn: AMSRL-SE-SS, Adelphi, MD. 20783-1197 ASTRACT The effect of hot isostatic pressure processing (HIP) on MnAl films has been compared to vacuum annealing for the purpose of obtaining substantial amounts of tau phase MnAl in films under 200 nm. Films were deposited by dc sputtering from both MnAlNiC an MnAl targets. As-deposited films were nearly amorphous. Post deposition annealing in vacuum produced only small amounts of the ferromagnetic tau-phase in films thinner than 200 nm. In all instances, regardless of substrate and sputtering target, the use of HIP in place of vacuum annealing increased the degree of crystallinity of the samples when compared to those annealed in vacuum. For the 100 nm samples deposited from the MnAlNiC target, these changes in crystallinity were accompanied by changes in the M-H loops of the samples. MnAlNiC HIP samples had improved magnetic properties compared to those of equal thickness annealed in vacuum. The 100 nm HIP sample sputtered from the MnAl target also showed an increase in moment, though the changes were not as dramatic as those seen in the samples sputtered from the MnAlNiC target. The 50 nm films from both targets also showed a change in crystallinity when compared to vacuum annealed samples. These films, unlike the 100 nm films, had ferromagnetic properties that were no better than those of the vacuum annealed samples. This suggests that while the 2 kbar of pressure used in this study assists in the formation of tau-phase in 100 nm films, the appropriate pressure for forming tau-phase in 50 nm films is yet to be determined. INTRODUCTION A variety of techniques including dc magnetron sputtering, high vacuum evaporation, high temperature vapor deposition, and molecular-beam epitaxy have been used to try to synthesize ferromagnetic τ-phase MnAl films [1-7]. Two fairly recent studies have shown that one can obtain nearly pure 8000 Å τ-phase films on glass substrates possessing values of HC and MS of 3 kOe and 420 emu/cc, respectively [8, 9]. While this is the first incidence of obtaining pure τphase films without the use of a template or heterostructure, these films are too thick for use as longitudinal recording media. In these two studies the as-deposited films consisted of nonmagnetic, hexagonal ε-phase. Annealing at 410 oC for 30 m in vacuum transformed the εphase into τ-phase. More recently[10, 11] it has been demonstrated that as-deposited MnAlC
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thin films are nearly amorphous and that, upon annealing, it is difficult to form appreciable amounts of τ-phase when film thi
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