Comparison of hard magnetic electrodes for magneto-electronics by magnetic force microscopy

Abstract

Both spin valve sensor elements and magnetic tunnel junctions usually consist of a magnetically fixed hard magnetic layer and a soft magnetic counter electrode which can rotate freely in an external driving field. In order to yield reliable sensor signals, the hard magnetic electrode should be magnetically rigid against small fields, i.e., must not be influenced during switching of the sensing electrode. Furthermore, in order to provide a homogeneous switching behavior, the electrodes should show uniformity in their magnetic properties, especially if scaled and patterned down to sub-micron lateral sizes. In this paper, magnetic force microscopy is used to compare the magnetic properties of different hard magnetic layer stacks commonly used in magneto-electronic device technology, i.e. an artificial antiferrimagnet (CoFe/Ru/CoFe and Co/Cu/Co) and exchange biased stacks (MnIr/CoFe/Ru/CoFe). The domain pattern of a polycrystalline artificial ferrimagnet (AFi) shows a strong magnetization ripple, i.e., local variations of the magnetization directions, which are maintained in patterned submicron elements. These statistical local fluctuations can be quantitatively correlated with a reduction of the tunneling magnetoresistance (TMR) of 3%–5% in the minor loop. In addition for small area junctions, the ripple can cause different switching fields in neighboring elements. An additional exchange biasing by a thin natural antiferromagnet is shown to rigidly pin the local magnetization fluctuations of the AFi. The process used for the preparation of this system, however, creates 360° domain walls, which again deteriorate the TMR signal.