Unconventional GMR angular dependence using a compensated ferrimagnet

Abstract

We have designed a GdCo/Cu/NiFe Giant Magnetoresistance (GMR) trilayer, the magnetoresistance of which does not always depend on the angle between the magnetisations of the electrodes. Using a GdCo ferrimagnetic alloy close to compensation, it was possible to experimentally reach the spin flop field Bsf of the ferrimagnetic layer. Below Bsf , the ferrimagnetic layer behaves as a ferromagnetic layer, however above Bsf,the ferrimagnetic sublattice magnetisations are no longer antiparallel and rotate 90◦ away from the GdCo layer total magnetisation, i.e. 90◦ away from the applied field. The GMR responds to the angle between Co and NiFe magnetisations and not to the angle between GdCo and NiFe magnetisations. Such a structure allows to study the change of sign of the GdCo spin polarisation as a function of temperature, and details of the GdCo magnetisation when usual magnetometry is difficult. FeNi can be seen as a sensor for the in-plane component of Co sublattice magnetisation, whereas extraordinary Hall effect measurements give a complementary image of the perpendicular component of the Co magnetisation.