Microstructure and Gmr in (111) Sputter-Deposited Co/Cu Multilayers
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ABSTRACT This paper presents the results of giant magnetoresistance (GMR), magnetic and microstructural investigations of sputter-deposited Co/Cu multilayers. The multilayers were designed to be at the second maximum of the oscillatory exchange coupling and were deposited onto a range of ion beam irradiated (111) Si substrates giving a transition from a random orientation to a highly oriented (111) texture. The randomly oriented multilayers exhibited mixed coupling and -20% GMR, which had components arising from irreversible and reversible magnetization changes, whereas the highly oriented (111) multilayers were almost completely ferromagnetically coupled and showed very little GMR.
INTRODUCTION A topic of interest in the GMR literature has been the apparent difference in behaviour between MBE grown, epitaxial (111) Co/Cu multilayers and polycrystalline sputter-deposited systems. The latter [1,2,3] give large GMR values and oscillatory coupling with two or three GMR maxima at the spacer thickness, tc, of -0.9, 2.0 and 3nm. Early measurements [4] on MBE structures did not show oscillatory coupling, nor indeed much evidence of any antiferromagnetic coupling. Explanations for this difference have included the proposal [4] that a relatively small proportion of (100) oriented material was in fact responsible for the large effects in sputtered samples. The fact that multilayers sputter-deposited on iron buffer layers showed both large GMR and a lack of (111) texture gave some support to that reasoning [5]. The situation is somewhat clearer now, and (11) MBE multilayers grown with teu at the first maximum have shown good GMR with 30-80% at 4.2K and >20% at room temperature [6,7,8,9]. The strength of antiferromagnetic coupling, however, appears to vary in the first maximum between investigations [8, 10] and, surprisingly, the second and subsequent maxima are sometimes not observed [6,8] but sometimes are [11]. A recent [12] explanation for these different findings can be linked to the common observation [e.g. 2,7,8] that multilayers structured at the first antiferromagnetic maximum in the oscillation, in both MBE and sputtered samples, show large GMR but also a relatively large fraction of ferromagnetic coupling. It is suggested in [12] that the areal ferromagnetic fraction, linked to coupling across spacer "islands" increases with the order of the antiferromagnetic coupling maximum and can completely mask the higher order maxima. For spacer "islands" below a critical size ferromagnetic order persists despite antiferromagnetic coupling. In this paper we present some results on the magnetoresistive, magnetic and structural properties of sputter-deposited Co/Cu that link GMR with texture and orientation. EXPERIMENT The Co/Cu multilayers were fabricated with the structure 16x(lnm Co/2nm Cu) by sputter365 Mat. Res. Soc. Symp. Proc. Vol. 384 ©1995 Materials Research Society
0 The samples deposition on to nominally room temperature, i.e. 30i-10 C, (111) Si substrates. were prepared in a UHV-compatible system at a pressure of
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