Direct observation of ferromagnetic exchange by magnetic force microscopy

In this study, the exchange coupling at the interface of two different magnetic materials remains is investigated by using an original local technique combining lithography with scanning probe microscopy. A home-made magnetic force microscopy device that allows measurements in perpendicular magnetic field up to 1.8 T, operating in noncontact mode is used. The sample used is a bilayer of thin layer of nickel on a very thick yttrium-iron garnet layer (YIG), with strong perpendicular anisotropy and developing stripes of alternate magnetization. The nickel top-layer is prepared to obtain several 400 nm-wide Hall crosses by means of electronic lithography. From the measurements done, the YIG magnetic configuration is determined in which the dark regions correspond to the minority magnetic domains, whose magnetization is opposite to the external field and to the moment of the scanning tip. Hall effect measurements of nickel show that an extraordinary Hall effect depending on the perpendicular component of magnetization is predominant. A strong coupling between garnet and nickel is observed, leading to a measurable rotation of the magnetization planar component just by displacement of the YIG domains underneath.