Influence of mechanical strain on essential characteristics of GMR structures

Abstract

and lubygsavba.sk Giant magnetoresistance (GMR) ofCo and Fe-Co based structures with Cu and Au spacers were e-beam evaporated onto Si wafers. The thickness oflayers was obtainedfrom the simulation ofX-ray reflectivity spectra. The GMR ratio was between 3.3 and 5.6 %. The effect of strain upon samples was studied in a bending configuration. The diferent dependences of coercivity (Hc) vs. strain were found. E.g. for sample with Co(5)/Au(2.2)/Co(2) core structure (where numbers denote thickness in nm) Hc increases with increasing compressive stress, whereas for sample with Co(O.5)/Cu(3)/Co(5) core structure it increases with tensile stress. The relative change of GMR ratio vs. loading in the strain interval + 280x]0-6 is 1-2 % near to the point of inflection of GMR vs. H curve (3.6 kA/m) for the second sample. The structures can be further optimized and applied in sensors ofmechanical quantities. IntroductionThe giant magnetoresistance effect (GMR) was discovered nearly 20 years ago [1, 2]. Nowadays GMR-based products are commercially available and research moved into diverse areas, e.g. combining standard GMR with magnetostriction [3][4][5]. This way combined sensors of mechanical quantities, like deformation, pressure or displacement are produced. In these sensors the magnetization orientation in magnetostrictive material is influenced by stress (strain), i.e. inverse magnetostriction is effective [3]. The measured quantity is detected due to the change of GMR in external magnetic field. The effects of stress (strain) on MR were studied also in other papers. GMR sensor response onto magnetoelastic stress is described in [6], strain induced GMR changes in CoFe/Cu structures are presented in [7]. Also tunnel MR structures with strain gauge factor 300 -600 were studied [8].The early GMR structures relied on a very thin spacer layers in multilayers where the magnetic coupling provides antiferromagnetic alignment of magnetic layers at zero field. A simpler arrangement is the spin-valve (SV) [9], where two magnetic layers are separated by a non-magnetic spacer and magnetization of one layer is pinned so that it cannot be rotated in a moderate field. In [3][4][5] SV strain sensors with one layer pinned by antiferromagnet layer were studied.Pseudo SV structures with pinning of one layer due to different coercivities were also reported [10]. Sometimes they don't show the true asymmetric SV behaviour. This type of strain sensors based on Co layers is studied in this work. The idea is to make the structure more simple, avoiding generally used magnetic alloys. Magnetostriction in Co layers is well understood, being -6 x 10-5 for pure element [11] (constriction in magnetic field), whereas in alloys it is mostly positive (elongation).