GMR and Structure in Sputtered Co 90 Fe 10 /Ag Multilayers
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t. Res. Soc. Symp. Proc. Vol. 403 0 1996 Materials Research Society
room temperature.
Layered-GGMR 10
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10
20
30
40
50
Nominal Ag thickness (A)
60
DGMF
Fig. 1. MR magnitudes in as-deposited [7059 glassf][a 50 (CogFje 0 Y A/Ag X A)x20/Ta 120 A] samples.
III. EXPERIMENTAL RESULTS Figure 1 shows a comprehensive survey of the MR magnitudes in asdeposited films as a function of both CoFe and Ag layer The nominal and thicknesses. measured film thickness values will The be discussed shortly. dependence of the MR behavior on the individual layer thicknesses is distinct MR behaviors clear. identified corresponding to can be Two the two regions outlined with solid lines. The region labeled 'Layeredsamples GGMR' designates displaying granular-type MR behavior (broad MR profiles with very low hysteresis) [6,14,15]. These samples are structurally similar to phase-separated single layer GGMR films (commonly formed by co-sputtering) except that they are essentially artificially stratified GGMR films. The region 'DGMR' designates labeled samples that display relatively easily saturated as-deposited MR which on annealing, behave like the NiFe/Ag 'discontinuous'
multilayers of Hylton, et al. [16]. These films are described as consisting of ferromagnetic platelets formed by the breakup of complete or continuous as-deposited ferromagnetic layers by the annealing induced columnar grain boundary diffusion of Ag. Weak negative exchange coupling between platelets in adjacent layers has been proposed as the mechanism for the highlyfield sensitive MR [16]. The 10 A nominal CoFe samples enclosed by the dotted line act as a transition between the two behaviors: as-deposited, the MR profiles are similar to the thicker CoFe-layered samples with Ag thicknesses beyond -30 A; that is, the profiles appear as though annealing should result in DGMR behavior; however, annealing results in layered-GGMR-type profiles that have very broad MR profiles. Note the absence of GMR for the thicker CoFe samples for Ag thicknesses from -5-35 A where conventional AFM coupling GMR would be expected. Multilayer film growth in the CoFe/Ag system with ultrathin CoFe layers is associated with CoFe cluster formation during growth instead of complete layering. Since the surface free energy of Ag is much lower than CoFe [17], the CoFe layers can be expected to grow on Ag as separate islands (Volmer-Weber growth mode [18]) that eventually coalesce once several atomic layers of CoFe have been deposited. The Ag layers in turn would tend to wet the CoFe and grow via a layer by layer growth mode (Frank-van der Merwe [19]). It has recently been reported that Co on Ag is continuous only for Co thicknesses > -8 A [7] and > -10 A [9]. Figure 2 shows schematics of the multilayer structure as a function of increasing CoFe thickness and annealing conditions. Abo
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